Review article

Metabolites, water and mineral exchanges acrossthe rumen wall: mechanisms and regulation*

D Rémond F Meschy R Boivin

1 Station de recherche sur la nutrition des herbivores,Centre Inra de Theix, 63122 Saint-Genès-Champanelle;

2 Laboratoire de nutrition et alimentation, Inra Ina-PG, 78352 Jouy-en-Josas;3 Service de physiologie, ENV de Lyon, 69280 Marcy-l’Étoile, France

(Received 14 February 1995; accepted 16 August 1995)

Summary — In ruminants, the forestomachs, and especially the reticulorumen, have walls withanatomical and histological properties that permit the exchanges of various metabolites, water and min-erals between the rumen contents and the blood. The development of papillae on the walls and the localblood circulation favour these exchanges. They depend to varying degrees on the food supply and theconcentrations of volatile fatty acids (VFA) produced by the microbial catabolism of polysaccharides.The absorption of VFA and ammonia occurs essentially by a process of passive diffusion of their non-ionised form through the epithelial cell membranes. For each of these substances, the existence of atransport system for the ionised forms is also suspected, but its relative importance is unknown. Short-term modifications in the absorption of these two substances are thus primarily determined by varia-tions in their intraruminal concentrations and pH. Other factors may also be implicated, and it is knownin particular that the absorption of ammonia is enhanced when the intraruminal concentration of VFAor the carbon dioxide (C02) level increases. The movement of urea through the wall occurs from theblood towards the rumen content according to the concentration gradient. The main factors liable to influ-ence the transepithelial flux of urea seem to be the blood urea levels and factors that act on the con-tact surface between the blood compartment and the epithelium (C02, VFA). Ruminal ammonia con-centration also affects the net urea transfer across the rumen wall but the mechanisms involved in this

regulation are not clearly understood. The absorption of water through the rumen wall results froman osmotic pressure gradient between the rumen and the plasma. This is modified not only by factorsthat modify the blood flow rates at the wall, but also by electrolyte concentrations. The absorption of min-erals from the rumen has also been demonstrated (Mg, Ca, Na, Cl, K, sulphur and certain metals). Thisoccurs by mechanisms of varying complexity according to the element involved.

rumen wall I volatile fatty acid I ammonia I urea I water I minerals

*

Report presented in the 9th Conference on Nutrition and Feeding of Herbivores, Clermont-Ferrand, France, 16-17 March 1994.

Résumé — Échanges de métabolites, d’eau et de minéraux à travers la paroi du rumen : méca-nismes et régulations. Chez les ruminants, les préestomacs, et plus particulièrement le réticulo-rumen, ont une paroi dont les caractéristiques anatomo-histologiques permettent les échanges dedifférents métabolites, d’eau et de minéraux entre le contenu ruminai et le sang. Le développement despapilles de la paroi et l’importance de l’irrigation sanguine favorisent cette absorption et sont plus oumoins dépendants des apports alimentaires et de la concentration en acides gras volatils produitspar le catabolisme microbien des polysaccharides. L’absorption des acides gras volatils et de l’ammo-niaque s’effectue essentiellement selon un processus de diffusion passive de leur forme non ioniséeà travers les membranes des cellules épithéliales. Pour chacune de ces substances, l’existence d’unsystème de transport pour les formes ionisées est également suspectée, mais on ne connaît pasl’importance relative de cette voie de passage transmembranaire. Les modifications à court terme del’absorption de ces deux molécules sont donc en premier lieu déterminées par des variations deconcentration intraruminales et des variations de pH. D’autres facteurs peuvent également interveniret on sait notamment que l’absorption d’ammoniaque est accrue lorsque la concentration intraruminaleen acides gras volatils ou la teneur de C02 augmente. Le passage d’urée à travers la paroi s’effectuedu sang vers le contenu ruminai en fonction du gradient de concentration. L’urée étant rapidement ettotalement hydrolysée dans le rumen, les principaux facteurs susceptibles de faire varier ce flux tran-sépithélial d’urée semblent être l’urémie et les facteurs agissant sur la surface de contact entre lecompartiment sanguin et l’épithélium (CO2, acides gras volatils). La concentration ruminale en ammo-niaque joue également un rôle dans la régulation du transfert d’urée ; son mode d’action n’est cepen-dant pas clairement déterminé. L’absorption d’eau à travers la paroi du rumen résulte d’un gradient depression osmotique entre le rumen et le plasma ; elle est modifiée, non seulement, par les facteurs quimodifient le débit sanguin au niveau de la paroi, mais également par les concentrations en électrolytes.Les minéraux peuvent également être absorbés à travers la paroi du rumen (Mg, Ca, Na, CI, K, soufreet certains métaux) ; cette absorption s’effectue par des mécanismes plus ou moins complexes etson intensité est variable selon l’élément considéré.

paroi ruminale / acide gras volatil / ammoniaque / urée / eau / minéraux

INTRODUCTION

The development of voluminous forestom-achs, and in particular the development ofthe reticulorumen, gives ruminants a highlycharacteristic digestive physiology. Certainaspects of this digestive physiology are wellknown. Numerous studies have beendevoted to the motricity of the reticuloru-men, its regulation and its effects on diges-tive transit. Likewise, the biochemicalaspects of microbial digestion and its con-sequences for nutrition and metabolism in

ruminants have been thoroughly studied. In ncontrast, the extent to which the reticuloru-men is able to absorb and recycle materialsthrough its wall is less well known. It was

long assumed, as stated by Colin in 1853,that &dquo;the epithelium of the rumen is less per-meable than that of the mouth and oesoph-agus, and is without doubt the main obsta-

de to absorption in the first stomach com-partments&dquo;. Although it is now well estab-

lished that the mucosa of the reticulorumen

possesses an absorptive function, it is onlyin recent years that significant progress hasbeen made in the quantitative evaluation ofthe absorption processes, through the use ofisotopic dilution techniques that allow nitro-gen flow rates to be estimated (Nolan andLeng, 1972), and the development of meth-ods for ruminal blood flow measurement

and ruminal vein catheterisation (Remond etal, 1993c), which make it possible to mea-sure net fluxes of metabolites across the

ruminal wall. The parallel use of in vitro tech-niques (Martens et al, 1978) has generatedessential information for understanding themechanisms involved in these transepithe-lial transport processes.

For certain substances derived frommicrobial metabolism or present in the feed,

the rumen can represent the most impor-tant absorption site in the digestive tract(VFA, ammonia, magnesium, etc). Thequantitative extent of this absorptiondepends primarily on the absorption capac-ity of the rumen, which is determined by thestructure of the epithelium and the amountof exchange surfaces between the diges-tive contents and the blood. It also dependson the mode of transport specific to eachsubstance.

MORPHOLOGICAL AND FUNCTIONALCHARACTERISTICS OF THE RUMENWALL

Structure of the ruminal mucosa

The reticulorumen is a highly voluminouscompartment (80 to 100 L in the cow, 10 to15 L in the sheep). It is subdivided into sev-eral compartments by various folds of itswall, which has the effect of increasing theamount of surface area contacting thedigesta. The surface area of the rumenmucosa is further increased by the pres-ence, over practically the entire inner wall, ofnumerous conical or tongue-shaped papil-lae. The distribution, size and number ofthese papillae depend on the species andalso on diet. In cattle, 80 to 85% of the innersurface are covered with papillae; in sheep,the papillae are irregularly distributed, andthe ruminal pillars and the dorsal sac areusually devoid of papillae, or bear papillae ofsmall size. The differentiation of papillaeobserved in different parts of the rumenappears to be linked to the layering of thedigestive contents in the rumen.

The papillae, as well as the entire innerrumen wall, are covered with a mucousmembrane composed of a keratinised multi-layer epithelium, a richly vascularised con-junctive tissue, crossed by nerve fibres andlymph ducts, and a fine layer of smooth

muscle separating the mucosa from theunderlying tissues.

The epithelium is formed of four cell lay-ers: the stratum basale, the stratumspinosum, the stratum granulosum and thestratum corneum (Steven and Marshall,1970). The cells of the stratum basaleappear to be the most metabolically activesince they contain numerous mitochondriaand free ribosomes. A large proportion ofthe assimilation and metabolism of the sub-stances absorbed from the rumen may thusoccur in this cell layer. It is here that the celldivision takes place. Thereafter, the cellsdifferentiate as they migrate towards thestratum corneum.

According to Steven and Marshall (1970),the fusion of the cell membranes (tight junc-tions) in the stratum granulosum leads toan obliteration of the intercellular spacesand thus causes this cell layer to act as abarrier controlling the movement of materi-als between the blood and the rumen. The

presence of this barrier in the stratum gran-ulosum was not, however, observed by Hen-rikson (1970), and Henrikson and Stacy(1971) later observed that the barrier to dif-fusion through the epithelium of differentmarkers was located in the inner layers ofthe epithelium corneum. According to theseauthors, the selective permeability of theepithelium may be explained by the coat-ing of mucopolysaccharide on the kera-tinised cells, and by the succession of dif-ferent types of membrane junctions(desmosomes, tight junctions) beneath thecornified layer, rather than by an actual bar-rier in the stratum granulosum.

The stratum corneum is made up of ker-atinised cells (the cell nuclei have vanished).The keratinisation of the surface layer pro-tects the mucosa against the abrasive actionof the rumen contents and the penetration ofmicroorganisms. Although intercellularspaces are once more visible in this layer,they are sufficiently narrow to prevent bac-teria from passing through (Steven and Mar-

shall, 1970). The biosynthesis of keratin inthe epithelial cells during their movementfrom the stratum granulosum to the stratumcorneum is accompanied by a decrease incell permeability (Fell and Weekes, 1975).The keratin formed softens, however, tosome extent, on contact with the rumen con-

tents, which should make it more perme-able. The stratum corneum of the epithe-lium is thus a complex system containingboth strongly keratinised cells that offer ahigh resistance to abrasion, and partly ker-atinised mucus-producing cells with absorp-tive capacity (Sydney and Lyford, 1988).

The efficiency of nutrient transport acrossthe epithelium thus depends to a largeextent on the integrity and degree of kera-tinisation of the stratum corneum. This layercan display various anomalies: parakerato-sis, which leads to an incomplete keratini-sation of the cells of the stratum corneum

and a disappearance of the stratum gran-ulosum, or hyperkeratosis, characterisedby excessive thickening of the stratumcorneum, sometimes associated with a

thickening of the stratum granulosum.The thickness of the ruminal epithelium

depends on the rate of cell division in thebasal layer of the epithelium, the speed ofmigration of the cells from the deep layers tothe surface layers, and the rate of cell shed-ding from the stratum corneum. Many stud-ies have been conducted on the regulationof cell proliferation in the epithelium (Galfiet al, 1991 It is known that the mitotic activ-ity of the epithelium can be depressed byfasting, and restored by the subsequent re-feeding (Tamate et al, 1974). In addition, it

is very strongly stimulated by intermittentfeeding (Sakata and Tamate, 1974). Injec-tion of acetate, butyrate and propionate intothe rumen stimulates the proliferation ofepithelial cells in fasting sheep (Sakata andTamate, 1978, 1979). Butyric acid has astronger stimulating effect than either pro-pionic or acetic acid. The replacement of aforage-based diet by one based on con-

centrate also causes a rapid rise in themitotic index of the ruminal epithelium(Goodlad, 1981 Diets that produce anabundant production of VFA with a high pro-portion of butyric acid are considered, there-fore, to be promotors of cell proliferation inthe rumen.

The structure of the epithelium is alsoaffected by the rate of cell shedding fromthe surface layers of the epithelium. Accord-ing to McGavin and Morrill (1976), the grat-ing effect of rough forage is necessary toprevent an excessive accumulation of ker-atinised cells at the surface of the ruminal

mucosa. The abrasive action of the rumen

contents thus plays an important role in therate at which cells are shed from the stratum

corneum. The contribution of adhering bac-teria colonising the surface cells of theepithelium should not, however, be under-estimated. Their action can accelerate the

breakdown of keratinised cells, and therebytheir shedding (Cheng and Costerton, 1980).The distribution of these bacteria at the sur-

face of the ruminal epithelium indicates thatthey are also affected by food abrasion(McCowan et al, 1980). The breakdown ofepithelial tissue by bacteria may thus bemore efficient at locations where the rumen

wall is less affected by physical erosion.

The lumen surface area of the epithe-lium can vary widely according to the num-ber, size and shape of the papillae thatdevelop on the inner surface of the rumen.When the diet is changed, the morphologi-cal adaptations of the ruminal epitheliumare rapid (2 to 3 weeks), and are particularlyevident in the atrium. In adult ruminants, themass of the mucosa increases linearly withthe amount of food ingested daily (Fell andWeekes, 1975). This weight increase isaccompanied by an increase in the length ofthe papillae. The diet can also modify theshape and number of these papillae. A dietrich in concentrate, generally associatedwith high concentrations of VFA in therumen, results in a stronger development

of rumen papillae than is observed with adiet based on forage (Wiegand et ai, 1975).Gabel et al (1987) observed that the outersurface of the ruminal papillae can beincreased by 200 to 400% by a change froma hay-based diet to one containing 64 to90% concentrate.

The inner surface area of the epitheliumalso varies according to the developmentof the mucosa (Tamate and Sakata, 1979).For a ruminal mucosa with well developedpapillae, the basal membrane of the epithe-lium is not flat and parallel to the outer sur-face of the epithelium, but displays irregu-larly-sized folds. These increase the surfacearea of the epithelium-conjunctive tissueinterface and reduce the effective thicknessof the epithelium. This type of interface isassociated with well developed papillaeknown to be very important sites of absorp-tion (Tamate and Sakata, 1979).

In summary, the ruminal epitheliumresponds to an increase in ingested foodquantities, an increase in levels of highlyfermentable substrates in the rumen, and

to any other factor that induces an increasein VFA production (especially butyrate), bya change in the number and shape of thepapillae present on the walls, accompaniedby an increased proliferation of basal layercells, which causes an increase in the sur-face area of the epithelium-conjunctive tis-sue interface. These structural modifica-tions of the epithelium result in an increaseof the luminal surface area of the rumen

wall, and an increase in the exchange sur-face between the capillaries and the epithe-lium, thereby giving the rumen a higherabsorption capacity.

Diets rich in concentrate often result in

hyperkeratosis of the epithelium. The thick-ening of the stratum corneum may result inlowered nutrient absorption rates (Nocek etal, 1980) and the increase in the absorptionsurface area observed with this type of dietwould then act as a corrective to the hyper-keratinisation in order to maintain the

absorptive capacity of the epithelium (Syd-ney and Lyford, 1988). Thorlacius andLodge (1973) even observed an increasein the VFA absorption through the rumenwall despite an apparent hyperkeratinisa-tion.

Irrigation of the rumen wall

The ruminal mucosa is richly vascularisedand presents a complex network of anas-tomosed vessels in the subepithelial con-junctive tissue (Cheetham and Steven,1966). The basal layer of the epithelium is incontact with a dense network of capillaries,and loops of capillaries also run through thefolds at the interface of the epithelium andthe conjunctive tissue of the ruminal papillae.The presence of vascularisation in these

folds can double the exchange surfacebetween the capillaries and the interstitialtissue relative to the lumen surface of the

epithelium (Cheetham and Steven, 1966).The rate of rumen blood flow can vary

widely according to the ruminant’s feedingpattern and diet. Throughout a feeding cycleit may account for 15 to 40% of the bloodflow in the portal vein (Barnes et ai, 1983).For sheep fed one meal per day, the bloodflow rate could multiply by three or fourbetween the beginning and the end of themeal (Barnes et at, 1983; Rémond, 1992).This increase in flow rate during ingestiondepends on the amount of dry matteringested; when the same amount of feed issplit up into 16 daily meals, these flow ratevariations fall to about 35% (R6mond, 1992).The rumen blood flow also increases dur-

ing rumination, but far less markedly(Remond, 1992; Meot and Boivin, 1994). It

also depends on the type of diet, and prob-ably too, on the feeding level. By supple-menting a hay-based diet with starch,Rémond et al (unpublished data) observedan increase in daily ruminal blood flow ofabout 50%.

The injection of labelled microsphereshas shown that the blood flow (per unitweight of tissue) in the ruminal mucosa is 8to 17 times greater than in the parietalsmooth muscles (Engelhardt and Hales,1977; Barnes et al, 1983). During ingestion,the flow rate rises rapidly in the parietal mus-cle layers, concurrent with the increase inthe contractile activity of the rumen. Thesubepithelial blood flow rises more gradu-ally, and peaks only towards the end of foodintake (Barnes et al, 1983). Apparently then,the blood flow in these two parietal areas iscontrolled by distinct mechanisms. While theblood flow in the muscle layers seems to beregulated via parasympathetic innervation,the subepithelial flow appears to be con-trolled locally by the end products of intraru-minal fermentation, independently of inner-vation (Barnes et al, 1986). VFA (especiallybutyric acid) and C02 are known to bringabout a marked rise in the subepithelial bloodflow (Dobson et al, 1971; Dobson, 1979).

Increased blood flow can raise the rate of

absorption of substances through the rumenepithelium by reducing their concentrationsin the interstitial space (washout effect),thereby increasing the concentration gradi-ent between the lumen and blood. This

effect of blood flow on absorption is minor forsubstances that require active transport pro-cesses for absorption (their movementthrough the epithelium is relatively inde-pendent of the concentration gradient), andfor substances whose absorption is limitedby their low transmembrane diffusability. In ncontrast, for substances that diffuse veryrapidly through the epithelium, the absorp-tion rate is closely dependent on the bloodflow rate (Mailman, 1982).

In conclusion to this first section, it is evi-dent that the permeability of the epithelium(degree of keratinisation and thickness ofthe cell layers) and the area of the exchangesurface between the digestive compartment,the epithelium and the blood compartment,can vary markedly according to diet, and

most particularly to the rate of VFA produc-tion. These adaptations of the epitheliumhave a marked impact on the quantitativeextent of transepithelial exchange. Theintensity of these exchanges, particularlyfor those substances that diffuse passivelyacross the epithelium, is also affected bythe subepithelial blood flow and the rumenmotricity, which respectively ensure renewalof the blood and rumen contents in contactwith the epithelium, thereby maintaining ahigh transepithelial gradient.

MECHANISMS AND REGULATION OFTHE TRANSEPITHELIAL MOVEMENTSOF SOME METABOLITES

Volatile fatty acids

The microbial fermentation of polysaccha-rides (cellulose, hemicellulose, starch, etc)in the rumen produces mainly VFA. Depend-ing on the food composition, this produc-tion represents 50 to 75% of the ingestedmetabolisable energy (Siciliano-Jones andMurphy, 1989; Bergman, 1990). The absorp-tion of VFA, therefore, makes a very largecontribution to meeting the ruminant’senergy needs. VFA absorption through therumen wall, first demonstrated by Barcroft etal in 1944, represents 65 to 85% of theintraruminal production (Weston and Hogan,1968; Peters et al, 1990, 1992). The vari-ability of this proportion is chiefly accountedfor by wide-ranging renewal rates for theliquid phase of the rumen contents.

The main VFA present in the rumen areacetic, propionic and butyric acids. Theirmolar proportions range from 70:20:10 witha hay-based diet to 50:35:15 with a con-centrate diet. The VFA are weak acids with

pKa values of 4.75, 4.87 and 4.81 for acetic,propionic and butyric acids, respectively.Calculations, using the Henderson-Hassel-bach relation, indicate that for the pH val-

ues generally observed in the rumen (pH6-7), more than 95% of the VFA will occurin their ionised form. Lowering the intraru-minal pH, and thereby increasing the pro-portion of non-ionised VFA, increases theVFA absorption rate (Dijkstra et al, 1993). Asnon-ionised VFA diffuse much more readilyacross the double lipid layer of the cell mem-branes than their ionised forms, it is gener-ally considered that VFA absorption takesplace mainly by simple diffusion of the non-ionised form across the epithelium of therumen (Bergman, 1990). The absorption ofVFA in their ionised form cannot be ruled

out, however. Using the Ussing chambertechnique, Kramer et al (1994) showed thata proportion of the VFA could be absorbedin their ionised form, and that this absorptioninteracted with that of chloride, which isabsorbed in exchange for intracellular bicar-bonate. A system of electroneutral trans-port in which the VFA ions are exchangedwith intracellular bicarbonate has alreadybeen observed in a teleostean herbivore

(Titus and Ahearn, 1992). The absorption

of VFA across the ruminal epithelium seemsto occur, therefore, by a combination of pas-sive diffusion of the non-ionised form andof facilitated diffusion of the ionised form

(fig 1 The relative extents of these twotransmembrane transport processes are notyet known. According to Titus and Ahearn(1992), for organs such as the rumen, inwhich VFA concentrations are high, the VFAanion transport system would only make asmall contribution to the total VFA absorp-tion, acting more as an intracellular bicar-bonate excreting system.

Making the assumption that VFA aremainly absorbed by diffusion of the non-ionised form, their absorption rate willdepend on the concentration gradient of thisform between the digestive contents andthe epithelium, and between the epitheliumand the blood (the three compartmentsinvolved in the absorption process). Thesegradients are determined by the total VFAconcentration in each compartment, and bythe pH, which governs the equilibriumbetween the ionised and non-ionised forms.

As the concentration of non-ionised VFAin the rumen contents is low, conversion ofthe ionised to the non-ionised form close to

the rumen wall will favour absorption. A ’pHmicroclimate’ has been observed near the

epithelium of the distal colon of the guineapig and rat (Rechkemer et al, 1979; McNeiland Ling, 1980). This local pH is stable andindependent of the pH in the lumen bulkphase, but is close to neutral (pH 6.4-6.9).Even if such a ‘pH microclimate’ exists nearthe rumen wall, the fraction of non-ionisedVFA will still be tiny. Most of the models pos-tulated for VFA absorption across the rumenwall involve protonation of VFA before theycross the apical pole of the epithelial cells.The protons may derive either from thehydration of C02 from microbial fermenta-

tion, yielding HC03 and H+, or from theNa+/H+ exchanger present at the apical poleof the cells (Stevens et al, 1986). A linkbetween the absorption of weak acids andthe Na+/H+ antiport has been observed byGabel and Martens (1991 ).

Inside the epithelial cells, a new equilib-rium between the ionised and non-ionisedforms of the VFA is set up in the local pHconditions. This tends to increase the non-ionised VFA gradient across the apicalmembrane of the cells. This concentration

gradient is accentuated by the intense intra-cellular catabolism of the VFA.

VFA output from the basal pole of theepithelial cells may take place by the sameprocess as the input at the apical pole, thatis, passive diffusion of the non-ionised formand assisted diffusion of the ionised form.

Studies in isolated rumens have shownthat at acid pH (4.5-5.5), the VFA absorptionrate increases with increasing chain length,or acetic < propionique < butyric acid,whereas at a pH close to neutral, theabsorption rates of these three acids arevery similar (Thorlacius and Lodge, 1973;Dijkstra et al, 1993). The absorption ratesof VFA with branched chains also seem tobe lower than those of their straight-chain

isomers (Oshio and Tahata, 1984). Thesedifferences in absorption rates may be dueto solubility differences in the lipid layers ofthe cell membranes, differences in VFA

absorption mechanisms (different degrees ofcarrier involvement) and differences in theextent of VFA metabolism in the epithelialcell (cf review Remond et al, 1995).

Ammonia

The ammonia in the rumen derives mainlyfrom deamination of amino acids released

during the breakdown of food, microbial andendogenous proteins, together with thehydrolysis of endogenous urea and any pre-sent in food. Loss of ammonia from therumen can occur by incorporation into micro-bial proteins or absorption through therumen wall (35 to 65% of the ammonia lossfrom the rumen), or by export from therumen with the digestive contents (10% ofthe loss) (Nolan and Stachiw, 1979; Sid-dons et al, 1985; Obara et ai, 1991 Loss ofammonia by absorption through the rumenwall can thus be quantitatively very high.This absorption was first clearly demon-strated by McDonald in 1948. Since then,numerous studies have been carried out to

try to determine the mechanisms govern-ing this absorption.

Ammonia is a weak base with a pKa of 9(Leng and Nolan, 1984). The Henderson-Hasselbach equation shows that at pH val-ues between 6 and 7, practically all theammonia will be in its ionised form (99.9and 98.7%, respectively), that is, in the formthat will diffuse poorly across the lipid layersof the cell membranes. At pH values nearneutral, the ammonia absorption increaseswith the intraruminal ammonia concentra-tion (Hogan, 1961; B6deker et ai, 1990;Remond et al, 1993b). When the intrarumi-nal pH is lowered (at constant ammoniaconcentration), the ammonia flux across therumen wall is depressed (Hogan, 1961;

Chalmers et al, 1971; Bodeker et al, 1990).At an acid pH, the ammonia absorptionremains stable despite the increase inintraruminal ammonia concentration (Hogan,1961 These findings are generally takenas evidence that ammonia absorptionacross the epithelium of the rumen occursby simple diffusion of the non-ionised form(NH3). The possibility of ammonium ion(NH4+) absorption from the digestive con-tents has also been considered (Hogan,1961; Siddons et al, 1985) but as this formis weakly lipid-soluble, its movement acrossthe membranes of the epithelial cells wouldrequire the assistance of carriers. Accordingto B6deker (1994), a system able to trans-port NH4+ is present in the rumen epithe-lial. The quantitative importance of thisabsorption route remains to be assessed.

In the short term, the absorption of ammo-nia thus depends mainly on the concentra-tion of NH3 near the epithelium. The VFAand the C02 in the rumen can modulate this

dependency and favour ammonia absorp-

tion (B6deker et al, 1992b, Rémond et al,1993b). Several suggestions have beenmade concerning the action of these twovariables. For B6deker et al (1992a, b; fig 2),the interaction between the VFA and the

absorption of ammonia may occur after theapical membrane of the epithelial cells. Sincethe pH inside these cells is close to 7, theVFA absorbed in their non-ionised form will

dissociate and release protons which canbe used to form NH4+ from the absorbed

NH3. This process would result in a decreasein the intracellular NH3 concentration and

thereby favour its absorption. Likewise, theintracellular release of HC03 and H+ from

C02 and H20 by the action of carbonic anhy-drase could serve as a proton source forNH4+ formation (Bbdeker et al, 1992a). Ateach stage in its diffusion across the epithe-lium, the NH3 thus equilibrates with NH4+according to the prevailing pH, right throughto the blood compartment.

The effects of VFA and C02 on ammonia

absorption may also be explained by their

action on the subepithelial blood flow (Dob-son, 1979) and irrigated capillary surfacearea (Thorlacius, 1972). Remond et al(1993b) observed that when butyric acidwas supplied to the rumen contents or whenC02 was blown in, the increase in ammoniaflux across the rumen wall was always lowerthan the increase in ruminal blood flow pro-duced by either of the two treatments. Theincrease in blood flow rate favours the

absorption of substances by reducing theirconcentration in the interstitial fluid steep-ing the capillaries. It is therefore highly likelythat the effect of VFA and C02 on ammonia

absorption involves both biochemical reg-ulation linked to the NH3-NH4+ equilibriumwithin the different cell layers in the epithe-lium, and regulation linked to ammonia elim-ination in the interstitial fluid via capillary cir-culation.

The daily absorption of ammonia fromthe rumen has been recorded in sheepunder various feeding conditions (table I).The quantitative data available in the litera-ture are, however, too sparse to accuratelypredict the daily fluxes from quantities ofingested nitrogen or intraruminal ammoniaconcentrations. In contrast to what the

results of Siddons et al (1985) might sug-gest, it is difficult to establish a linear rela-

tionship between ammonia absorption andNH3 concentrations in the rumen. Given theextent of the modifications to the absorp-tion capacity of the rumen (surface area xpermeability of epithelium) according to diet,particularly when this is rich in rapidly fer-mentable energy sources, it is not surprisingthat under these conditions, the variations in

absorption cannot be wholly explained byvariations in the concentration gradientbetween the rumen and the blood, andhence the variations in intraruminal NH3-

Urea

Since the work of Simmonet et al (1957),numerous studies have shown that blood

urea can diffuse across the rumen epithe-lium. This process is nutritionally beneficialfor the ruminant, since the bacteria presentin the rumen are able to use the urea nitro-

gen to synthesise proteins, the amino acidsof which will subsequently be available forpostruminal absorption. The quantities ofnitrogen recycled in this way vary widely(1 to 9 gN/day in sheep), and may accountfor 5 to 25% of the nitrogen ingested (table I).

The work of Houpt and Houpt (1968)showed that urea transfer across the rumen

wall was linearly related to the rumen-bloodconcentration gradient, and it has been gen-erally accepted that urea crosses the rumi-nal epithelium by simple diffusion. Intraru-minal hydrolysis of urea by bacterial ureasestherefore facilitates the movement of urea

through the rumen wall by maintaining aconcentration gradient favourable to diffu-sion. Hence, it has been shown that the inhi-

bition of urease activity in the rumen causesa decrease in the transepithelial flux of urea(Houpt and Houpt, 1968; Rémond et al,1993b). In addition, the urea flux is depen-dent on the permeability of the epithelium.Since damage to the structure of the stratumcorneum results in a marked increase in

urea transport, urea diffusion seems to be

strongly limited by the low permeability ofthis epithelial layer (Houpt and Houpt, 1968).Modifications to the exchange surfacesbetween the blood and rumen contents mayalso affect the quantitative extent of ureatransport.When the feed is supplemented with a

rapidly fermentable energy source (sucrose,extruded barley, wheat starch), the daily fluxof urea across the rumen wall can be dou-

bled (table I), and the capacity of the epithe-lium to eliminate blood urea (the transep-ithelial urea flux relative to arterial urea flux)can be multiplied by four (Remond et al,unpublished data). As the blood urea leveldecreases when these energy supplementsare added, the increase in urea flux mustbe mainly due to modified epithelial surface

area and permeability, in response to intraru-minal VFA levels. The intraruminal urease

activity increases when the feed is supple-mented with rapidly fermentable carbohy-drates (Cook, 1976); it could thus also be

implicated in the long-term regulation of ureaflux. Kennedy et al (1981) observed anincrease in the number of facultative anaer-obic bacteria adhering to the rumen wallwhen the diet was supplemented with glu-cose. However, a regulating effect of theepithelial urease activity on urea transferstill has not been firmly demonstrated. Inaddition, the intraruminal urease activityalways seems to be in excess relative tothe influx of urea in the rumen (Norton etal, 1982a; Whitelaw et al, 1991). Since theurease activity associated with the epithe-lium is generally greater than that of therumen fluid (Wallace et al, 1979; Rybosovaet al, 1984), its involvement in the long-termregulation of urea flux is controversial.

The transfer of urea across the rumenwall varies in the course of a feeding cycle(Rémond et al, 1993a), and so is evidentlygoverned by a system of short-term regu-lation. Bubbling C02 in the rumen signifi-cantly increases urea flux across the rumenwall (Thorlacius et al, 1971; Engelhardt etal, 1978; Remond et al, 1993b). Likewise,increasing the butyric acid concentration inan isolated pouch of the rumen favours ureatransfer (Engelhardt et al, 1978). The actionof these two intraruminal factors does notinvolve modifications to the ruminal urease

activity (Thorlacius et al, 1971; Remond etal, 1993b). Although C02 and butyric acidstimulate subepithelial blood flow (Dobson,1984), the permeability of the capillary wallsto urea is too low for the blood flow to affecturea diffusion (Landis and Pappenheimer,1963). In addition, according to the resultsof Dobson et al (1971 urea clearanceseems virtually independent of blood flow.VFA and C02 could affect the amount of

epithelial surface irrigated by acting on theclosing of anastomoses between arterial

and venous capillaries, and the opening ofprecapillary sphincter. This suggestion ofa regulation of urea flux by the amount ofexchange surface is supported by theresults of Thorlacius (1972), according towhich C02 and VFA considerably increasethe volume of blood present in the ruminalpapillae.

Increasing intraruminal ammonia con-centration decreases the urea flux acrossthe rumen wall (Engelhardt et al, 1978;Remond et al, 1993b). According to Remondet al (1993b), the ammonia absorption maybe responsible for reducing urea flux. Theeffect of ammonia on urease activity is longterm (Cheng and Wallace, 1979) rather thanshort term, and the mechanism by whichammonia regulates the transepithelial fluxof urea during short-term variations is notyet known. The work of H6rnicke et al

(1972) showed that the ammonia absorp-tion can modify haemodynamics in the rumi-nal mucosa; this could also explain the effectof ammonia on urea transfer.

The main factors acting on urea transferthrough the ruminal epithelium, and theirmodes of action, are summarized in figure 3.Other factors may also be involved in the

regulation of transepithelial urea flux.Increasing osmotic pressure in an isolatedpouch of the rumen with mannitol (Houpt,1970) stimulates urea transfer. However,Rémond et al (1993b) raised the intrarumi-nal osmotic pressure with NaCl injections,and observed no effect on urea flux despitea lowering of the water absorption from therumen. Under normal feeding conditions,the osmotic pressure seems to have little

effect on urea transfer. Hormonal regula-tion of the urea flux has also been consid-ered. According to Houpt (1970), vaso-pressin may modify the permeability of therumen wall to urea. However, Thorlacius etal (1971) observed no modification of theurea clearance in response to a vasopressininjection. The work of Harrop and Phillip-son (1970) and Remond et al (1993b) also

suggests that gastrin might play a role inthe regulation of the urea flux across therumen wall.

Amino acids

The absorption of free amino acids acrossthe rumen wall was first observed byDemaux et al (1961 The permeability ofthis epithelium to amino acids has sincebeen confirmed, with different methods, byCook et al (1965) and Leibholz (1971 a, b).The work of Webb et al (1992) even sug-gests permeability to peptides of a smallsize.

The concentrations of free amino acids inthe rumen are generally very low (Annison,1956; Williams and Cockburn, 1991 Theyvary during the course of a feeding cycle,peaking 1 h after ingestion of a meal (Leib-holz, 1969; Williams and Cockburn, 1991 ).During this period, for diets rich in nitrogen,free amino acid concentrations in the rumencan exceed those in the plasma, resulting inpassive diffusion through the ruminal epithe-lium (Leibholz, 1969). For most diets, the

absorption of free amino acids from therumen is, however, slight, given their verylow levels there.

The concentrations of peptides in therumen are higher than those of free aminoacids. These concentrations vary accord-

ing to the origin of the food proteins, peak-ing about 1 h after feeding (Williams andCockburn, 1991). The possibility of peptideabsorption from the rumen by passive dif-fusion cannot, therefore, be ruled out (Webbet al, 1992).

Vitamins

The work of R6rat et al (1958b) has shownthat the rumen wall is also permeable to B-group vitamins. Permeability to vitamin B1seems extremely low, however (Hbller etal, 1977). Although the vitamin concentra-tions in the rumen are high under normalfeeding conditions, it is unlikely that therumen is a major absorption site for B vita-mins (R6rat et al, 1958a) since these aremainly located inside microorganisms andso are not available for absorption (R6rat

et al, 1958b). With heavy supplementation,this absorption can become significant, asshown for niacin, which is absorbed mainlyas nicotinamide (Erickson et al, 1991 ).

TRANSEPITHELIAL MOVEMENTOF WATER AND MINERALS

Water

There is very little quantitative data con-cerning the net water transfer across therumen wall. From the differences observedbetween inflow through the oesophagus andoutflow through the reticulo-omasal orifice,Warner and Stacy (1968) estimated thatwater absorption from the rumen could varyin the sheep from 50 ml/h at rest to 300 mULduring the hours following water intake.Using the isolated rumen method, combinedwith the use of tritiated water, Willes et al

(1970) observed that for sheep fed cutlucern hay twice daily, this absorptionranged from 300 to 800 mUh, the highestvalues being observed 2 to 3 h after feeding.Comparable results (360 to 720 mUh peak-ing 1 h after feeding) were obtained byRemond et al (1993a) in sheep fed choppeddactylis hay, calculating the water flux fromthe haemoglobin levels in the arterial andvenous blood and the ruminal blood flow.The daily net flux of water during this exper-iment showed an absorption of 10 L/day.For sheep fed continuously with compressedfeed, Faichney and Boston (1985) estimatedthe ruminal water absorption at 5 L/day.Ingestion of compressed feed results in amarkedly lowered mastication time (inges-tion plus rumination), which in turn results ina lower saliva secretion. The differences in

saliva flow between the experiments ofFaichney and Boston (1985) and Remond etal (1993a) probably explain the differencesin water flux observed through the rumenwall.

It is generally agreed that the main factorresponsible for water movement throughthe ruminal epithelium is the osmotic gra-dient between the blood in the subepithe-lial region and the rumen contents. In con-trast, the mechanism by which increasingruminal butyric acid concentration causesan increase in water absorption through therumen wall (Dobson et al, 1976; Remondet al, 1993a) is controversial. The absorptionof butyric acid may increase the rate of waterabsorption across the epithelium by causingan increase in ruminal blood flow (Dobson etal, 1976). Increasing the subepithelial bloodflow, by a washout effect, would, however,tend to lower the concentration of solutesabsorbed in the interstitial fluid and so

reduce the osmotic pressure gradientbetween the digestive contents and theblood. The coupling of the absorption ofsodium with that of VFA (Gabel andMartens, 1991) may also explain the stim-ulating effect of butyric acid on water flow.This effect may thus be linked to modifica-

tions to the osmotic pressure gradient viaregulation of the absorption of electrolytessuch as sodium. By lowering the intrarumi-nal pH with hydrochloric acid, Willes et al(1970) also observed an increase in waterabsorption across the rumen wall. This couldalso be explained by an increased VFAabsorption (linked to the fall in pH) or anincreased absorption of CI- supplied by HCI.On injecting into the rumen two acids(butyric and acetohydroxamic) that can beabsorbed through the rumen wall, increasedwater absorption was observed with or with-out increased blood flow (Remond et al,1993b). Variations in water movementacross the wall thus appear to be more

closely linked to variations in electrolytemovement than to fluctuations in blood flow.

Dobson et al (1970) observed that C02stimulated water re-absorption from the ven-tral sac of the washed isolated rumen. Its

mode of action was discussed by Dobson(1984), who suggested the existence of a

counter-current exchange system that main-tains high concentrations of bicarbonate andC02 in the capillary blood, resulting in a highosmotic pressure. However, Remond et al

(1993b) showed that in a normally filledrumen, increasing the C02 did not signifi-cantly modify the water flux across the wall. l .

It may therefore be concluded that varia-tions in C02 concentration in the rumenhave minimal impact on transepithelial waterflow.

Finally, the rate of water absorption fromthe rumen is liable to vary according to theanimal’s state of hydration. Severalresearchers have shown that in dehydratedruminants, the absorption is paradoxicallymuch lower than in hydrated animals,despite an osmotic gradient that is particu-larly favourable to absorption (Silanikove,1994). However, these observations arecontroversial and have not yet been satis-factorily explained.

Minerals

As in single-stomach animals, the intestine,and particularly the small intestine, waslong considered to be the main or evenexclusive site of mineral absorption fromruminants. Over the last 15 years, this

assumption has been challenged for mag-nesium, for which the forestomachs are themain absoptive site in the gastrointestinaltract, and also for phosphorus, calcium,copper, iron and zinc for which absorptioncan occur before the duodenum (Kirk et al,1994; Rahnema et al, 1994). Most of thequantitative results were obtained from cal-culating ruminal input/duodenal output bal-ance. This type of method has the disad-vantage of giving no indication of the exactabsorptive site (reticulorumen, omasum,abomasum) and does not take into accountthe minerals introduced by the saliva whichcould mask possible absorption from theforestomachs.

The absorption of minerals in the diges-tive tract can involve several pathways.Practically all the mineral elements that arepresent in bioavailable forms can cross thewalls of the digestive tract along an elec-trochemical gradient. This process is usuallynonsaturatable and is not subject to anyphysiological or nutritional regulation. Thistransfer can also be facilitated by transportsystems (Mg, Na, Cl, P). It can also involve

primary (Ca, Na) or secondary (Mg) activetransport involving energy consumption.

Magnesium

Magnesium absorption from the reticuloru-men accounts for about 80% of its absorp-tion from the entire digestive tract (Tomasand Potter, 1976). Many studies have shownthat the efficiency of Mg absorption isstrongly influenced by the physical andchemical conditions prevailing in the rumen.Thus, increasing intraruminal K concentra-tion, or reducing that of Na produces amarked decrease in Mg absorption (reviewsby Fontenot et al, 1989; Leonhard et al,1989). Increasing the phosphate concen-tration favours Mg absorption (Beardsworthet al, 1989b). The results for the effect ofincreasing ammoniacal nitrogen are con-flicting: a decrease, sometimes marked, inMg absorption (Martens and Rayssiguier,1980; Care et al, 1984; Martens et al, 1988)or no effect (Moore et al, 1972; Grings andMales, 1987). The experimental methodsused probably partly account for these dif-ferences; the results of Moore et al (1972)and Grings and Males (1987) were obtainedby the balance method on animals accus-tomed to a nitrogen-rich diet, while the stud-ies showing an effect of NH3 concentrationon Mg absorption were carried out by per-fusing buffer solutions in isolated rumenpouches. The effect of NH3 concentration

on Mg absorption therefore appears to belimited in time and it may occur at the time of

the postprandial ruminal NH3 peak. Theabsence of any effect due to changes in dietmay be linked to the adaptive mechanismsof the epithelium (Martens et al, 1991 ).

A fall in pH causes a drop in Mg absorp-tion, which is less marked, however, in ani-mals that have received a concentrate-richdiet (G5bel et al, 1987). Increasing theintraruminal osmotic pressure from 240 to

367 mosmol/L (Martens, 1985) or from 315 5to 422 mosmol/L (Gabel et al, 1987) doesnot affect the Mg absorption, although it

modifies the transepithelial flux of water.

The available energy in the rumen is a

factor favouring Mg absorption. In particular,the VFA concentration is positively corre-lated with Mg absorption (Martens andRayssiguier, 1980; Martens et al, 1988).Forage supplementation with starch (Thom-son et al, 1984; Giduck and Fontenot, 1987),lactose (Rayssiguier and Poncet, 1980;Giduck and Fontenot, 1987) or glucose

(Giduck and Fontenot, 1987; Giduck et al,1988), causes an appreciable increase inthe ruminal Mg absorption. This effectobserved in vivo is difficult to interpret sincesupplementation with fermentable energysubstrates simultaneously produces modi-fications to pH, and to VFA and NH3 con-

centrations, and modifications to the mor-

phology of the epithelium.In vitro studies of the mechanisms of Mg

absorption have shown that this absorptionresults both from paracellular and transcel-lular diffusion of Mg along an electrochem-ical gradient, and from electroneutral trans-port that may take place via a Mg++/2H+exchanger in the apical membranes(Martens et al, 1991; fig 4). The existence ofan electrogenic diffusion mechanism pro-vides an explanation for the inhibiting effectof K on Mg absorption. In vitro methods,which allow the K concentration and the

transmembrane potential (ddpt) to be varied

independently, have shown that the effect ofK is linked to an increase in the ddpt andnot to a direct K effect (Martens et al, 1987,1991 An increase in intraruminal K mayhave several effects, reducing the para-cellular diffusion of Mg by raising the ddpt,and reducing the transcellular diffusion bylowering ddpt in the apical membranes(Martens et al, 1991). Reduced Mg absorp-tion resulting from a drop in intraruminal pHmay also be explained by a decrease inddpt linked to a decrease in Na/K-ATPaseactivity (Gäbel et al, 1987).

The electoneutral transport of Mg in theapical membranes depends on the intra-cellular proton concentration, itself partlydetermined by the amount of intracellulardissociation of the absorbed VFA and the

hydration of C02 (Martens et al, 1991). Theabsorption of Mg is therefore partly deter-mined by the operation of the Na/K pumpin the basal membranes, which influencesthe ddpt values, and the operation of theelectroneutral exchangers that evacuateintracellular HC03 (Martens et al, 1991). ).

Calcium

The possibility of a net absorption of cal-cium by the ruminal epithelium is now firmlyestablished. The quantitative importance ofthis absorption seems to depend on the con-centration, form and solubility of the Ca, andon the nutrient/Ca interactions.

The net transport of calcium through therumen wall depends on the concentrationof Ca++ in the rumen: net secretion up to

about 1 mmol of Ca++ per L, and net absorp-tion for higher concentrations (H61ler et al,1988a; Beardsworth et al, 1989a). Underphysiological conditions, this concentrationis situated between 1 and 4 mmol/L (Gringsand Males, 1987; Holler et al, 1988a).

In addition, the net absorption of Ca isclosely linked to the concentration of inor-ganic phosphate (Pi) in the rumen (Holler

et al, 1988b; Beardsworth et al, 1989b). Aconcentration of about 15 mmol/L of Pi is

optimal for Ca absorption (Care et ai, 1989).This corresponds to the usual physiologi-cal values in animals with no P deficiency. Incontrast, the absorption of Ca is correlatednegatively with the Mg concentration in therumen; an increase in the Mg concentrationfrom 1 to 5 mmol/L, spanning the normalphysiological range of 2.6 to 3.5 (Grace et al,1988), causes an appreciable decrease inCa absorption (Care et al, 1989).

The absorption of Ca in the digestive tracttakes place simultaneously by diffusion inthe intercellular spaces and by transcellu-lar movement. In the small intestine, the lat-ter makes use of a three-stage process: dif-fusion through the apical membranes,transport inside the cytosol by a vitamin D-dependent Ca binding protein and primaryactive transport in the basal membranes.Less is known about the mechanisms of Ca

absorption from the ruminal epithelium thanabout that of Mg. In vitro studies indicatedthat the net flux of Ca is dependent uponactive Na transport accomplished by theNa/K-ATPase system, and that the transep-ithelial fluxes consisted in both ddpt-depen-dent and ddpt-independent components(Holler et ai, 1988a). Adding 1-25-dihy-droxycholecalciferol increases the netabsorption of Ca without, however, reducingserous-mucous transport in goats (Breves etal, 1989), suggesting, as at intestinal sites,a regulation of active Ca transport by thisvitamin D metabolite.

Sodium and chloride

Large quantities of Na enter the rumen assodium bicarbonate contained in saliva (from3 to 3.5 g/L of Na in animals with no defi-ciency); about 50% of the salivary Na isreabsorbed before the duodenum (Pfefferet al, 1970; Gabel and Martens, 1991 ). Allthe epithelia of the forestomachs displayhigh Na absorptive capacities, but this

capacity seems to be greatest in the rumen.Chloride is also absorbed in the rumen

(Martens and Blum, 1987). The amount of Clabsorbed is about half that of Na (Gabel etal, 1987).

The mechanisms of the preintestinalabsorption of Na and Cl (fig 4) gave rise toa review (Gabel and Martens, 1991 Theabsorption of Na in the rumen takes placeagainst an electrochemical gradient (Gabelet al, 1987). In vivo studies have notdemonstrated any such mechanism for Clinsofar as the electrochemical gradientfavours its passive absorption (Gabel et al,1989). The transfer of Na and Cl across theapical membrane of the epithelial cellsresponds to Na+/H+ and CI-/HC03-exchanges, respectively. The transfer ofNa across the basal membrane involvesan Na/K-ATPase system. The mechanismof the CI- exit remains to be ascertained

(Gabel and Martens, 1991 ). In the rumen,the Na+/H+ exchange may be stimulatedby the absorption of weak acids, whichwould consequently affect the efficiency ofNa absorption.

Phosphorus

For P, most of the work done in vivo in

sheep reports a net secretion of 1 to

15 g/day before the duodenum (Grace etal, 1974; BenGhedalia et al, 1975; Dillonand Scott, 1979; Greene et al, 1983; Wylieet al, 1985). These results vary widelyaccording to the level of feeding and theP return by the saliva. This abundantendogenous return by the saliva can maska net P absorption through the ruminalepithelium (Breves et al, 1988; Yano et al,1991), although this seems minor (Yanoet al, 1991). The absorption of P throughthis epithelium does not make use of activetransport; it may take place via elec-troneutral transport or by simple transcel-lular or paracellular diffusion (Breves et al,1988).

Potassium

The absorption of K in the different com-partments of the ruminant’s stomach isslight; K is absorbed mainly in the smallintestine (Kirk et al, 1994; Rahnema et al,1994). This absorption does occur passively,the quantity absorbed being proportional toconcentration. Inversion of flow can evenoccur if the K concentration is greater in theplasma than in the rumen.

Sulphur

A large fraction of the absorbed S isabsorbed from the rumen, mainly as sul-phides derived from the reduction of foodsulfates, or as inorganic S resulting in par-ticular from the breakdown of sulphur-con-taining amino acids by rumen microorgan-isms, especially sulphate-reducing bacteria.At the usual plasma pH, the sulphides arevery weakly dissociated. Absorption there-fore concerns the nondissociated form

(hydrogen sulphide), which is liposoluble.This absorption is rapid; the half-life of sul-phides in the rumen is 10 to 22 min (Bray,1969). It is a simple diffusion, directly relatedto the sulphide concentration in the medium(Bray and Till, 1975; Doyle and Moir, 1980).The quantity of S absorbed as sulfate can beconsidered as negligible.

Copper, iron and zinc

The in vivo balances indicate that the absorp-tion of Cu, Fe and Zn mainly takes placebefore the duodenum (Kirk et al, 1994). Thequantitative importance of this absorptionthrough the rumen epithelium, and the mech-anisms involved are still unknown.

CONCLUSION

This review shows that the rumen cannot

be reduced to a simple microbial fermenta-

tion compartment, but that it is also the sitefor the exchange of numerous substancesthrough the rumen wall. This exchange cantake the form of either absorption from therumen or, less often, diffusion into the rumenfrom the blood. Numerous studies haveshown that the anatomical, histological andfunctional characteristics of the rumen

mucosa enable these exchanges.The most recent studies have focused

in particular on evaluating the extent of thesetransfers in vivo, and, using in vitro meth-ods, on ascertaining the mechanismsinvolved in these transfers through therumen wall.

Through the absorptive capacities of itswall, the rumen thus plays an important rolein helping to provide some of the ruminant’senergy (absorption of VFA), water and min-eral needs. In addition, the absorption pro-cess ensures a certain homeostasis of the

rumen contents, by preventing excessivebuild-up of VFA and ammonia. This alsoenables the rumen to participate in recy-cling processes of varying complexity, forexample, of urea or certain minerals, throughrumen-saliva cycles.

Finally, it is quite clear that absorptionthrough the rumen wall can constitute agood in vivo and in vitro model for the studyof general mechanisms of digestive absorp-tion.

REFERENCES

Annison EF (1956) Nitrogen metabolism in the sheep.Protein digestion in the rumen. Biochem J 64, 705-714

Barcroft J, McAnally RA, Phillipson AT (1944) Absorptionof volatile acids from the alimentary tracts of thesheep and other animals. J Exp Biol 20. 120-129

Barnes RJ, Comline RS, Dobson A (1983) Changes inthe blood flow to the digestive organs of sheepinduced by feeding. Q J Exp Physio168, 77-88

Barnes RJ, Comline RS, Dobson A (1986) The control ofsplanchnic blood flow. In: Control of digestion andmetabolism in ruminants (LP Milligan, WL Grovum,

A Dobson, eds), Reston Publishing Co, Reston, VA,USA, Chapter 3

Beardsworth LJ, Beardsworth PM, Care AD (1989a)Calcium fluxes across the wall of the ovine reticulo-rumen in vitro. Res Vet Sci 47, 404-405

Beardsworth LJ, Beardsworth PM, Care AD (1989b) Theeffect of phosphate concentration on the absorptionof calcium phosphorus and magnesium from the retic-ulo-rumen of the sheep. BrJ Nutr61, 715-723

Ben-Ghedalia D, Tagari H, Zamwel S, Bondi A (1975)Solubility and net exchange of calcium magnesiumand phosphorus in digesta flowing along the gut ofsheep. Br J Nutr 33, 87-94

Bergman EN (1990) Energy contributions of volatile fattyacids from the gastrointestinal tract in variousspecies. Physiol Rev70, 567-590

Bbdeker D (1994) Participation of NH4’ in ammonia

transport across sheep rumen epithelium. Proc SocNutr Physiol3, 89 9

Bbdeker D, Shen Y, Hbller H (1990) Influence of shortchain fatty acids and HC03-on ammonia absorp-tion through the sheep rumen wall. In: Proceedingsof the Third International Symposium on the Nutritionof Herbivores. MSAP, Malaysia, 36

Bbdeker D, Oppelland G, H61ler H (1992a) Involvementof carbonic anhydrase in ammonia flux across rumenmucosa in vitro. Exp Physiol77, 517-519 9

B6deker D, Shen Y, Kemskowski J, Hbller H (1992b)Influence of short-chain fatty acids on ammoniaabsorption across the rumen wall of sheep. ExpPhysiol77, 369-376

Bray AC (1969) Sulphur metabolism in sheep. II. The

absorption of inorganic sulphate and inorganic sul-phide from the sheep’s rumen. Austr J Agric Res20, 739-749

Bray AC, Till AR (1975) Metabolism of sulphur in the gas-tro-intestinal tract. In: Digestion and metabolism ofruminants (IW McDonald, ACI Warner, eds), Univer-sity of New England, Armindale, Australia, 243-260

Breves G, Hbller H, Packheiser P, Gabel G, Martens H

(1988) Flux of inorganic phosphate across the sheeprumen wall in vivo and in vitro. Q J Exp Physiol73,343-351

Breves G, G!bel G, Pfeffer E, Martens H (1989) Unidi-rectional calcium fluxes across the isolated rumenmucosa of goats as affected by 125-(OH)-D3. ProcNutr Soc 48, 163A

Care AD, Brown RC, Farrar AR, Pickard DW (1984)Magnesium absorption from the digestive tract ofsheep. Q J Exp Physiol69, 577-587

Care AD, Beardsworth LJ, Beardsworth PM, Breves G(1989) The absorption of calcium and phosphatefrom the rumen. Acta Vet Scand 86, 152-158

Chalmers MI, Jaffray AE, White F (1971) Movementsof ammonia following intraruminal administration ofurea or casein. Proc Nutr Soc 30, 7-17 7

Cheetham SE, Steven DH (1966) Vascular supply ofthe absorptive surface of the ruminant stomach.J Physiol 186, 56P P

Cheng KJ, Wallace RJ (1979) The mechanism of pas-sage of endogenous urea through the rumen walland the role of ureolytic epithelial bacteria in the ureaflow. BrJ Nutr42, 553-557

Cheng KJ, Costerton JW (1980) Adherent rumen bac-teria - their role in the digestion of plant material,urea and epithelial cells. In: Digestive physiologyand metabolism in ruminants (Y Ruckebusch,P Thivend, eds), MTP Press, Lancaster, 227-250

Colin G (1853) Physiologie compar6e des animauxdomestiques, tome I. Baillibre, Paris, 667 p

Cook AR (1976) Urease activity in the rumen of sheepand the isolation of ureolytic bacteria. J Gen Micro-biol92, 32-48

Cook RM, Brown RE, Davis CL (1965) Protein

metabolism in the rumen. I. Absorption of glycineand other amino acids. J Dairy Sci 48, 475-483

Demaux G, Le Bars H, Molle J, R6rat A, Simonnet H(1961) Absorption des acides amines au niveau durumen, de l’intestin grble et du cxcum chez le mou-ton. Bull Acad Vet Fr 34, 85-88

Dijkstra J, Boer H, van Bruchem J, Bruining M, Tam-minga S (1993) Absorption of volatile fatty acids fromthe rumen of lactating dairy cows as influenced byvolatile fatty acid concentration, pH and rumen liquidvolume. BrJNutr69, 385-396

Dillon J, Scott D (1979) Digesta flow and mineral absorp-tion in lambs before and after weaning. J Agric Sci92, 289-297

Dobson A (1979) The choice of models relating tritiatedwater absorption to subepithelial blood flow in therumen of sheep. J Physiol297, 111-121

Dobson A (1984) Blood flow and absorption from therumen Q J Exp Physio169, 599-606

Dobson A, Sellers AF, Shaw GT (1970) Absorption ofwater from isolated ventral sac of rumen of the cow.

J Appl Physiol 28, 100-104

Dobson A, Sellers AF, Thorlacius SO (1971) Limitationof diffusion by blood flow through bovine ruminalepithelium. Am J Physiol 220, 1337-1343

Dobson A, Sellers AF, Gatewood VH (1976) Absorptionand exchange of water across rumen epithelium.Am J Physiol231, 1588-1594

Doyle PT, Moir RJ (1980) Sulfur and methioninmetabolism in sheep. IV. Metabolism and absorp-tion in the intestines. Aust J Biol Sci 33, 303-307

von Engelhardt W, Hales JRS (1977) Partition of capil-lary blood flow in rumen, reticulum, and omasum ofsheep. Am J Physiol232, E53-E56

von Engelhardt W, Hinderer S, Wipper E (1978) Fac-tors influencing the endogenous urea-N secretionand utilization in the gastrointestinal tract. In: Rumi-nants digestion and feed evaluation (DF Osbourn, DE

Beever, DJ Thomson, eds), Agriculture ResearchCouncil, London, UK, 4.1-4.12 2

Erickson PS, Murphy MR, McSweeney CS, Trusk AM(1991) Niacin absorption from the rumen. J DairySci 74, 3492-3495

Faichney GJ, Boston RC (1985) Movement of waterwithin the body of sheep fed at maintenance underthermoneutral conditions. Aust J Biol Sci 38, 85-94

Fell BF, Weekes TEC (1975) Food intake as a mediatorof adaptation in the ruminal epithelium. In: Diges-tion and metabolism in the ruminant (IW McDonald,ACI Warner, eds), University of New England, Armi-dale, NSW, 2351, Australia, 101-118 8

Fontenot JP, Allen VG, Bunce GE, Goff JP (1989) Fac-tors influencing magnesium absorption andmetabolism in ruminants. JAnim Sci67, 3445-3455

Gabel G, Martens H (1991) Transport of Na and Clacross the forestomach epithelium: mecanisms andinteractions with short chain fatty acids. In: Pro-

ceedings of the Vllth international symposium onruminant physiology, Sendai 1989, Academic Press,London, UK, 129-147

Gabel G, Martens H, Suendermann M, Galfi P (1987)The effect of diet intraruminal pH and osmolarity onsodium chloride and magnesium absorption fromthe temporarily isolated and washed reticulo-rumenof sheep. Q J Exp Physiol and Cognate Medical Sci-ences72, 501-511 1

Gabel G. Bell M, Martens H (1989) The effect of lowmucosal pH on sodium and chloride movementacross the isolated rumen mucosa of sheep. Q JExp Physiol74, 35-44

Galfi P, Neogrady S, Sakata T (1991) Effects of volatilefatty acids on the epithelial cell proliferation of thedigestive tract and its hormonal mediation. In: Phys-iological aspects of digestion and metabolism inruminants (T Tsuda, Y Sasaki, R Kwashima, eds),Academic Press, San Diego, CA, USA, 49-59

Giduck SA, Fontenot JP (1987) Utilization of magne-sium and others macrominerals in sheep supple-mented with different readily-fermentable carbohy-drates. J Anim Sci 65, 1667-1673

Giduck SA, Fontenot JP, Rahnema SH (1988) Effect ofruminal infusion of glucose volatile fatty acids andhydrochloric acid on mineral metabolism in sheep.J Anim Sci 66, 532-542

Goodlad RA (1981) Some effects of diet on the mitoticindex and the cell cycle of the ruminal epithelium ofsheep. Q J Exp Physiol66, 487-495

Grace ND, Ulyatt MJ, MacRae JC (1974) Quantitativedigestion of fresh herbage by sheep. III. The move-

ment of Mg, Ca, P, K and Na in the digestive tract.J Agric Sci 82, 321-330

Grace ND, Caple IW, Care AD (1988) Studies in sheepon the absorption of magnesium from a low molec-ular weight fraction of the reticulo-rumen contents. BrJ Nutr59, 93-108

Greene LW, Fontenot JP, Webb KE (1983) Site of mag-nesium and other macromineral absorption in steersfed high levels of potassium. J Anim Sci 57, 503-510

Grings EE, Males JR (1987) Effects of potassium onmacromineral absorption in sheep fed wheat straw-based diets. J Anim Sci 64, 872-879

Harrop CJF, Phillipson AT (1970) The effect of diet andpentagastrin on the influx of urea into the rumen ofsheep. Proc Nutr Soc 30, 3A

Henrikson RC (1970) Ultrastructure of ovine ruminalepithelium and localization of sodium in the tissue.J Ultrastruct Res 30, 385-401 1

Henrikson RC, Stacy BD (1971) The barrier to diffusionacross ruminal epithelium: a study by electronmicroscopy using horseradish peroxidase, lan-thanum, and ferritin. J Ultrastruct Res 34, 72-82

Hogan JP (1961) The absorption of ammonia through therumen of the sheep. Aust J Biol Sci 14, 448-460

H61ler H, Fecke M, Schaller K (1977) Permeability tothiamin of the sheep rumen wall in vitro. J Anim Sci44, 158-161

Hbller H, Breves G, Kocabatmaz M, Gerdes H (1988a)Flux of calcium across the sheep rumen wall in vivoand in vitro. 0 J Exp Physiol73, 609-618 8

Holler H, Breves G, Dubberke M (1988b) Flux of inor-ganic phosphate and calcium across the isolatedmucosa of the sheep omasum. J Vet Med Series A35, 709-716 6

H6rnicke H, Ehrlein HJ, Engelhardt Wv (1972) H!mo-dynamische wirkungen intraruminaler ammoniak-gaben bei zeigen. Zentralbl Veterinarmed [A] 19,822-842

Houpt TR (1970) Transfer of urea and ammonia to therumen. In: Physiology of digestion and metabolism inthe ruminant (AT Phillipson, ed), Oriel Press, New-castle-upon-Tyne, UK, 119-131

Houpt TR, Houpt KA (1968) Transfer of urea nitrogenacross the rumen wall. Am J Physiol214, 1296-1303

Kennedy PM, Milligan LP (1980) The degradation andutilisation of endogenous urea in the gastrointestinaltract of ruminants: a review. Can J Anim Sci 60, 205-221

Kennedy PM, Clarke RTJ, Milligan LP (1981) Influencesof dietary sucrose and urea on tranfer of endoge-nous urea to the rumen of sheep and number ofepithelial bacteria. BrJNutr46, 533-541

Kirk DJ, Fontenot JP, Ranhema S (1994) Effects of feed-ing lasolacid and monensin on digestive tract flowand partial absorption of minerals in sheep. JAnimSci 72, 1029-1037

Kramer T, Gürtler H, Gabel G (1994) Interactionsbetween short chain fatty acids, sodium, and chlorideduring their absorption from the reticulorumen ofsheep. Proc Soc Nutr Physiol3, 82

Landis EM, Pappenheimer JR (1963) Exchanges of sub-stances through the capillary walls. In: Handbook ofphysiology (WF Hamilton, ed), American Physio-logical Society, Washington, DC, USA, Vol II

Leibholz J (1969) Effect of diet on the concentration offree amino acids, ammonia and urea in the rumenliquor and blood plasma of the sheep. J Anim Sci29, 628-633

Leibholz J (1971 a) The absorption of amino acids fromthe rumen of the sheep. 1. The loss of amino acidsfrom solutions placed in the washed rumen in vivo.Aust J Agric Res 22, 639-645

Leibholz J (1971 b) The absorption of amino acids fromthe rumen of the sheep. 2. The transfer of histidine,glycine, and ammonia across the rumen epitheliumin vitro. Aust J Agric Res 22, 647-653

Leng RA, Nolan JV (1984) Symposium: Protein nutri-tion of the lactating dairy cows. J Dairy Sci 67, 1072-1089

Leonhard S, Martens H, Gabel G (1989) New aspects ofmagnesium transport in ruminants. Acta Vet Scand86, 146-151

Mailman D (1982) Blood flow and intestinal absorption.Fed Proc 41, 2096-2100

Martens H (1985) Magnesium absorption from the tem-porarily isolated rumen of sheep. The effect of waterabsorption and osmotic pressure. Zentralbl Veterin-armed !32.631-635

Martens H, Rayssiguier Y (1980) Magnesium metabolismand hypomagnesaemia. In: Proceedings of the diges-tive physiology and metabolism in ruminants. AVI IPublishing Co, Westport, CT, USA, 117-466

Martens H, Blum I (1987) Studies on the absorption ofsodium and chloride from the rumen of sheep. CompBioch Physiol [A} 86, 653-656

Martens H, Harmeyer J, Michael H (1978) Magnesiumtransport by isolated rumen epithelium. Res Vet Sci24,161-168

Martens H, Gabel G, Strozyk H (1987) The effect ofpotassium and the transmural potential differenceon magnesium transport across an isolated prepa-ration of sheep rumen epithelium. 0 J Exp Physiol72, 181-188

Martens H, Heggeman G, Regier K (1988) Studies on theeffect of K, Na, NH4’, VFA and COZ on the net

absorption of magnesium from the temporarily iso-lated rumen of heifers J Vet Med 35, 73-80

Martens H, Leonhard S, Gabel G (1991) Minerals anddigestion: exchanges in the digestive tract. In:

Rumen microbial metabolism and ruminant digestion(JP Jouany, ed), INRA, Editions, Paris, France, 192-216

McCowan RP, Cheng KJ, Costerton JW (1980) Adher-ent bacterial population on the bovine rumen wall:distribution patterns of adherent bacteria. Appl Env-iron Microbiol39, 233

McDonald IW (1948) The absorption of ammonia fromthe rumen of the sheep. Biochem J42, 584-587

McGavin MD, Morrill JL (1976) Scanning electronmicroscopy of ruminal papillae in calves fed variousamounts and forms of roughage. Am J Vet Res 37,497-508

McNeil NI, Ling KLE (1980) The mucosal surface pH ofthe large intestine. Gastroenterology78, 1220

M6ot F, Boivin R (1994) Rumen motility and ruminalblood flow in sheep. Proc Soc Nutr Physiol3, 9

Moore WF, Fontenot JP, Webb KE (1972) Effect of formand level of nitrogen on magnesium utilisation.J Anim Sci 35, 1046-1053

Nocek JE, Herbein JH, Polan CE (1980) Influence ofration physical form, ruminal degradable nitrogenand age of rumen epithelial propionate and acetatetransport and some enzyme activities. J Nutr 110,2355-2364

Nolan JV (1975) Quantitative models of nitrogenmetabolism in sheep. In: Digestion and metabolism inthe ruminant (IW McDonald, ACI Warner. eds), Uni-versity of New England, Armidale, Australia, 416-431

Nolan JV, Leng RA (1972) Dynamic aspects of ammo-nia and urea metabolism in sheep. BrJNutr27, 177-194

Nolan JV, Stachiw S (1979) Fermentation and nitrogendynamics in Merino sheep given a low-qualityroughage diet. Br J Nutr 42, 63-80

Nolan JV, Krebs GL, Hennessy DW (1987) Aspects ofprotein nutrition and metabolism in ruminants. In:

Isotope-aided studies on nonprotein nitrogen andagroindustrial byproducts utilization by ruminants.IAEA, Vienna, Austria, 1-17 7

Norton BW, Janes AN, Armstrong DG (1982a) The effectof intraruminal infusions of sodium bicarbonate,ammonium chloride and sodium butyrate on ureametabolism in sheep. BrJNutr48, 265-274

Norton BW, Mackintosh JB, Armstrong DG (1982b) Ureasynthesis and degradation in sheep given pelleted-grass diets containing flaked barley. Br J Nutr 48,249-264

Obara Y, Dellow DW, Nolan JV (1991) Effects of energy-rich supplements on nitrogen kinetics in ruminants.In: Physiological aspects of digestion and metabolismin ruminants (T Tsuda, Y Sasaki, R Kwashima, eds),Academic Press, San Diego, CA, USA, 515-539

Oshio S, Tahata I (1984) Absorption of dissociatedvolatile fatty acids through the rumen wall of sheep.Can J Anim Sci 64 (suppl), 167-168

Peters JP, Shen RYW, Robinson JA, Chester ST (1990)Disappearance and passage of propionic acid fromthe rumen of beef steer. J Anim Sci 68, 3337-3349

Peters JP, Shen RYW, Robinson JA (1992) Disappear-ance of acetic acid from the bovine reticulorumen

at basal and elevated concentrations of acetic acid.

J Anim Sci 70, 1509-15 i 7

Pfeffer E, Thomson A, Armstrong DG (1970) Studieson intestinal digestion in the sheep. 3. Net move-ment of certain inorganic elements in the digestivetract on rations containing different proportions ofhay or rolled barley. BrJ Nutr24, 197-204

Rahnema S, Wu Z, Ohajuruka W, Weis WP, PalmquistDL (1994) Site of mineral absorption in lactatingcows fed high-fat diets. JAnim Sci72, 229-235

Rayssiguier Y, Poncet C (1980) Effect of lactose sup-plement on digestion of lucerne hay by sheep. 11.

Absorption of magnesium and calcium in the stom-ach. J Anim Sci 51, 186-192

Rechkemmer G, Ronnau K, Kuschinsky W, EngelhardtWV (1979) pH-microclimate at the surface of theintestine of guinea-pig and rat. Pfluegers Arch 382,R31 (abstr)

Remond D (1992) Echanges d’azote ur6ique et ammo-niacal a travers la paroi du rumen de mouton : cine-tique, bilan journalier et mécanismes de regulation.These n°467, univ Clermont-Ferrand-11, France

Remond D, Chaise JP, Delval E, Poncet C (1993a) Netflux of metabolites across the rumen wall of sheep fedtwice a day with orchardgrass hay. J Anim Sci 71,2529-2538

Remond D, Chaise JP, Delval E, Poncet C (1993b) Nettransfer of urea and ammonia across the rumen wall

of sheep. J Anim Sci 71, 2785-2792

Remond D, Poncet C, Lefaivre J (1993c) Technical note:Ruminal vein catheterization and continuous bloodflow measurement in ruminal arteries of sheep.J Anim Sci 71, 1276-1280

Remond D, Ortigues I, Jouany JP (1995) Energy sub-strates for the ruminal epithelium. Proc Nutr Soc 54,95-105

R6rat A, Le Bars H, Molle J (1958a) Utilisation d’unem6thode de perfusion pour la mise en evidence deI’absorption des vitamines B chez le mouton nor-malement aliment6. Compt Rend Acad Sci 246,1920-1922

R6rat A, Molle J, Le Bars H (1958b) Mise en evidencechez le mouton de la permbabilit6 du rumen auxvitamines B et conditions de leur absorption a ceniveau. Compt Rend Acad Sci 246, 2051-2054

Rybosova E, Javorsky P, Havassy I, Horsky K (1984)Urease activity of adherent bacteria in the sheeprumen. Physiologia Bohemoslovaca 33, 411-416 6

Sakata T, Tamate H (1974) Effect of the intermittentfeeding on the mitotic index and the ultrastructureof basal cells of the ruminal epithelium in the sheep.Tohoku J Agric Res 27, 133

Sakata T, Tamate H (1978) Rumen epithelial cell prolif-eration accelered by rapid increase in intraruminalbutyrate. J Dairy Sci 61, 1109-1113 3

Sakata T, Tamate H (1979) Rumen epithelium cell pro-liferation accelered by propionate and acetate.J Dairy Sci 62, 49-52

Sellers AF, Stevens CE, Dobson A, McLeod FD (1964)Arterial blood flow to the ruminant stomach. Am J

Physiol207, 371-377

Siciliano-Jones J, Murphy MR (1989) Production ofvolatile fatty acids in the rumen and cecum-colon ofsteers as affected by forage:concentrate and foragephysical form. J Dairy Sci 72, 485-492

Siddons RC, Nolan JV, Beever DE, Macrae JC (1985)Nitrogen digestion and metabolism in sheep con-suming diets containing contrasting forms and levelsof N. Br J Nutr 54, 175-187

Silanikove N (1994) The struggle to maintain hydratationand osmoregulation in animals experiencing severedeshydration and rapid deshydration: the story ofruminants. Exp Physiol79, 281-300

Simmonet H, Le Bars H, Mol[6 J (1957) Le cycle del’ur6e administr6e par voie buccale chez les rumi-nants. Compt Rend Acad Sci 244, 943-945

Steven DH, Marshall AB (1970) Organization of therumen epithelium. In: Physiology of digestion andmetabolism in the ruminant (AT Phillipson, ed), OrielPress, Newcastle-upon-Tyne, UK, 80-100

Stevens CE, Argenzio RA, Roberts MC (1986) Com-parative physiology of the mammalian colon andsuggestions for animal models of human disorders.Clin Gastroenterol 15, 763-786

Sydney J, Lyford J (1988) Growth and development ofthe ruminant digestive system. In: The ruminant ani-mal: digestive physiology and nutrition (DC Church, h ,ed), A Reston Book, Prentice Hall, Canada Inc,Toronto, ON, Canada, 44-63

Tamate H, Sakata T (1979) Epithelium-propria inter-face of ruminant forestomach. Ann Rech Vet 10,482-484

Tamate H, Kikuchi T, Sakata T (1974) Ultrastructuralchanges in the ruminal epithelium after fasting andsubsequent refeeding in the sheep. Tohoku J AgricRes 25, 142-155

Thomson JK, Gelman AL, Jessiman CS (1984) Effectof digestible carbohydrates on the apparent absorp-tion of magnesium by wether lambs. Can J Anim Sci64, 219-220

Thorlacius SO (1972) Effect of steam-volatile fatty acidsand carbon dioxyde on blood content of rumen papil-lae of the cow. Am J Vet Res 33, 427-430

Thorlacius SO, Lodge GA (1973) Absorption of steam-volatile fatty acids from the rumen of the cow as influ-

enced by diet, buffers, and pH. Can J Anim Sci 53,279-288

Thorlacius SO, Dobson A, Sellers AF (1971) Effect ofcarbon dioxyde on urea diffusion through bovineruminal epithelium. Am J Physio1220, 162-170

Titus E, Ahearn GA (1992) Vertebrate gastrointestinal fer-mentation: transport mechanisms for volatile fattyacids. Am J Physiol262, R547-R553

Tomas FM, Potter BJ (1976) The site of magnesiumabsorption from the ruminant stomach. BrJ Nutr36,37-45

Wallace RJ, Cheng KJ, Dinsdale D, 0rskov ER (1979)An independent microbial flora of the epithelium andits role in the ecomicrobiology of the rumen. Nature279, 424-426

Warner ACI, Stacy BD (1968) The fate of water in therumen. 2. Water balances throughout the feedingcycle in sheep. Br J Nutr 22, 389-410 0

Webb KE, Matthews JC, DiRienzo DB (1992) Peptideabsorption: a review of current concepts and futureperspectives. J Anim Sci 70, 3248-3257

Weigand E, Young JW, McGilliard AD (1975) Volatilefatty acids metabolism by rumen mucosa from cattlefed hay or grain. J Dairy Sci 58, 1294-1300

Weston RH, Hogan JP (1968) The digestion of pastureplants by sheep. 1. Ruminal production of volatilefatty acids by sheep offered diets of ryegrass andforage oats. Aust J Agric Res 19, 419-432

Whitelaw FG, Milne JS, Wright SA (1991) Urease (EC3.5.1.5) inhibition in the sheep rumen and its effect onurea and nitrogen metabolism. BrJNutr66, 209-225

Willes RF, Mendel VE, Robblee AR (1970) Water trans-fer from the reticulo-rumen of sheep. J Anim Sci 31,85-91

Williams AP, Cockburn JE (1991) Effect of slowly andrapidly degraded protein sources on the concentra-tions of amino acids and peptides in the rumen ofsteers. J Sci Food Agric 56, 303-314 4

Wylie MJ, Fontenot JP, Greene LW (1985) Absorption ofmagnesium and other macrominerals in sheepinfused with potassium in different parts of the diges-tive tract. J Anim Sci 61, 1219-1229

Yano F, Yano H, Breves G (1991) Calcium and phos-phorus metabolism in ruminants. In: Proceedings ofthe Vllth international symposium on ruminant phys-iology, Sendai !969. Academic Press, London, UK,277-291