256:816-821, 1989. Am J Physiol Regulatory Integrative Comp PhysiolN. Silanikove

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Role of rumen and saliva in the homeostatic response to rehydration in cattle

NISSIM SILANIKOVE Migal-Galilee Technological Centre, Kiryat Shmona 10200, Israel

SILANIKOVE, NISSIM. Role of rumen and saliva in the home- ostatic response to rehydration in cattle. Am. J. Physiol. 256 (Regulatory Integrative Comp. Physiol. 25): R816-R821, 1989.-It has been shown recently that the circulation created by the continuous secretion of voluminous amounts of saliva rich in Na+ to the large store of fluid sequestered in the rumen and its reabsorption from the gut is an integral part of water and Na+ homeostasis in cattle. The role of this system in water and Na+ restitution following acute dehydration and rapid rehydration was studied. Cattle were able to withstand dehy- dration of 18% of their initial mass and to replenish their water losses in one drinking. The water imbibed was first retained in the rumen and slowly released. Rapid expansion (or dilution) of their blood as a result of large influxes of hypotonic water from the rumen was prevented by a parallel increase in the secretion of hypotonic saliva. The accelerated saliva secretion refluxed back to the rumen almost half of the water absorbed. Saliva electrolyte concentration varied simultaneously with an increase or decrease in saliva flow. Na’, HCO;, HPO;, and pH were inversely related to saliva flow rate while Cl and K+ were positively related. It seems that visceral afferent response was involved in activation of salivary flow rate.

secretion; composition; absorption; outflow; acute dehydration

COMPARED WITH most other mammals, where losses of water over 15% of body weight are fatal, ruminants can tolerate larger losses amounting to at least 18% in cattle, 20% in sheep, 25% in camels, and >40% in the desert black Bedouin goat. On rehydration, cattle replace 75 87% and sheep 70% of their weight loss, whereas camels and the black Bedouin goat replenish their entire loss, all of it within 3-10 min (22). Compared with the eryth- rocytes of other mammalian herbivores (Camelidae, Equidae), those of ruminants (cows, sheep, goats) are very susceptible to a lowered tonicity (22); therefore buffering the rate of water absorption following rehydra- tion is vital.

Mammals are divided into two categories of drinkers: those that rapidly replenish lost water and those that do so gradually (1). In addition to ruminants, to the first class belong donkey and dog. To the second class belong the golden hamster, guinea pig, and primates, including humans. The donkey, capable of losing 20% of its mass during dehydration, and dog do not have a rumen; yet their blood is not considerably diluted following rapid replenishment of their water losses (17, 26). However, under mildly hypotonic conditions both cows and sheep (10) were found to absorb water rapidly.

In cattle, camels, sheep, and Bedouin goats, urine flow dropped immediately after drinking to rates that were even lower than those recorded in the dehydrated ani- mals and barely regained predrinking rates even 4 h after drinking (3, 8, 21). Urine output also remained low following rehydration in dogs (26). It seems that mam- mals that are capable of rapidly replenishing lost water are also capable of retaining the newly ingested water despite rapid expansion of their body fluid. Ruminants produce large amounts of saliva roughly equivalent to twice their daily water turnover and to five to six ex- changes of their plasma water (23).

There are two possible explanations for the ability of ruminants to retain water following rapid rehydration. 1) The water is retained in the rumen and slowly released (7, 13, 22). In this case, rumen epithelium should have a capability to prevent water absorption despite a large osmotic gradient 200-300 mosmol/kg. 2) Saliva secretion is accelerated after rapid rehydration so that the rate and composition would change in direct relation to the amount and tonicity absorbed from the gut, returning to the rumen the surplus water absorbed.

The purpose of the present experiment was to test these two hypotheses using beef cattle as experimental animals.

METHODS

Site and animals. The experiments were carried out at the Kare Deshe Beef Cattle Experimental Station, 10 km north of the Sea of Galilee and close to the Jordan River. The animals were kept outside, individually, in yards fully exposed to solar radiation throughout the day. Maximum daily temperatures ranged from 33 to 38°C and relative humidity was not in excess of 45%.

Four beef cows (Simmental crossed with local breed, Bos taurus), neither lactating nor pregnant and weighing 420 & 40 kg, were used. Three to 6 mo before initiation of the experiments, the esophagus was exteriorized to the skin and then fistulated (25). This did not affect normal feeding and drinking and allowed complete diversion of the saliva when the plug of the fistula was opened.

Experimental procedures. Baled medium-quality wheat hay (dry matter 90.1%, crude protein 12.1%, and Na+ 100 meq/kg on a dry-matter basis) was fed before and throughout the experiment. Feed was provided once a day (0700 h) in amounts 15% more than the intake of the previous day.

Water was offered in large drums (90 liters) for 20 min

R816 0363-6119/89 $1.50 Copyright 0 1989 the American Physiological Society

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HOMEOSTATIC RESPONSE TO REHYDRATION R817

in two different trials, each lasting 10 consecutive days: 1) once a day at 0800 h and 2) once every 3 days at 0800 h. Consumption was recorded. The measurement in cows dehydrated for 24 h included only measurement of saliva flow and composition.

Immediately after the termination of drinking, a dose of 250 mg Cr-EDTA in 40 ml of water was introduced through the fistula to the reticulorumen (RR). One and 4 h later the procedure was repeated. Saliva flow rate was measured by diverting the saliva secreted through the unplugged fistula into a graduated cylinder for 5 min. From each collection -8 ml was taken for analysis, and the rest was introduced to the RR through the fistula. Saliva flow rate was measured every 10 min during the 1st h, every 20 min between 1 and 4 h, and every 30 min between 4 and 7 h postdrinking.

Rumen volume at each dosage of Cr-EDTA, water flow to the omasum, and rate of water and Na+ absorption from the RR between dosage times and the relative error were determined as described (25). Na+ and Cr-EDTA concentration in RR fluid and osmolality, pH, Na+, K+, Cl-, HCO;, and HPOT contents in saliva were deter- mined as mentioned (25).

RESULTS

Water ingestion after dehydration. When given access to water after 24 h of dehydration, beef cows consumed -25 l/animal, which was equivalent to the entire mass loss. When given access to water after 72 h of dehydra- tion, 63 l/animal were consumed, which was equivalent to 89% of their mass lost during dehydraton or to 98% recovery of their original mass (Table 1).

Rumen space increased from 23 to 86 liters within 10 min in drinking cows after 72 h of dehydration. A drop from 154 to 63 meq/l (244%) in Na+ concentration of ruminal fluid was also found. Part of the discrepancy between fluid and Na+ dilution could account for the presence of 21 meq/l Na+ in the ingested water.

Water and Na+ balance in RR after rehydration. Water and Na+ balance in the RR was measured after drinking to satiation in animals that were dehydrated to -18% of initial mass (Table 2).

In contrast to the measurements taken in normally hydrated and dehydrated cows (23, 25) in which the measurements were taken at near steady state, the meas- urements in the present experiment differ considerably from steady state. However, since only one variable, saliva flow rate and composition, fluctuated consi .derably and it was frequently measured, the accuracy of the

TABLE 1. Water ingestion and recovery of lost mass in beef cows dehydrated to -6 and 18%

Length of Water

Deprivation, days

Recovery Dehydrated Water Body Mass, Ingestion, % of Mass

% of Initial kg kg lost during

dehydration mass

1 396k15 25t4 100t5 10025 3 349t12 63t5 88t5 98t5

Values are means of: SD.

results in terms of net accumulated changes (Table 2) should not be affected. Nevertheless, water or Na+ ab- sorption at specific times within intervals of measure- ments (especially during the 1st h of measurements) may differ considerably from the average value. Since outflow from the RR and net accumulation of water and Na+ within the RR were relatively small and constant in comparison to salivary changes, the pattern of change in water and Na+ absorption should be quite similar to those of saliva flow (Fig. 1) and Na+ inflow with saliva (product of Figs. 1 and 3), respectively.

The experimental error associated with the measure- ments of water and Na+ (NOF) is relatively large, amounting to -75% during the 1st h, -69% between 1 and 4 h postdrinking, and -54% between 4 and 7 h postdrinking (Table 2). However, since NOF of water and Na+ were considerably lower than inflow (saliva- tion), the amount of water absorbed from the rumen was overestimated by only 5 liters during O-7 h postdrinking and that of Na+ by 366 meq (Table 2). These errors represent only 18% (water) and 20% (Na+) of the total amount calculated to be absorbed from the rumen. In addition, since the water and Na+ that outflow from the rumen are most probably almost completely absorbed at the lower part of the gut, the above discrepancy concern- ing the site of absorption is less important for the esti- mate of the total amounts absorbed.

The average outflow of water from the RR was 0.7 l/h during the 1st h, 1 l/h between 1 and 4 h postdrinking, and 1.4 l/h between 4 and 7 h postdrinking. Net absorp- tion of water from the rumen during the 1st h was 6.3 liters, dropped to 4.5 l/h between 1 and 4 h postdrinking, and was 2.8 l/h between 4 and 7 h postdrinking (Table 2). At the end of the 7-h measuring period, 57% (36 liters) of the imbibed water had been absorbed; 28 liters of the water was absorbed directly through the rumen wall. An additional 7.2 liters were passed out of the rumen and probably most of it was absorbed at lower parts of the gut. However, rumen volume was reduced by only 19.1 liters, since 17 liters were replaced by saliva (Table 2).

Na+ absorption rate, which was almost zero at the termination of dehydration (25), rose to 365 meq during the 1st h postdrinking. Between 1 and 4 h postdrinking Na+ absorption rate was on average 293 meq/h, and between 4 and 7 h postdrinking Na+ absorption rate was 199 meq/h (Table 2). From 4 h postdrinking and later, Na+ was steadily accumulated in ruminal fluid at the rate of -32 meq/h between 1 and 4 postdrinking and 49 meq/h between 4 and 7 h postdrinking.

During the 7 h of measurements, there was an inflow of 2,692 meq of Na+ to the RR with saliva, 611 meq passed to lower parts of the gut, and most of it was probably absorbed. Despite large differences in concen- tration between the rumen and plasma, 1,841 meq were absorbed against concentration gradient, and as a con- sequence of this process only 240 meq accumulated in the RR.

Saliva flow and composition during rehydration. Qual- itatively, the changes in saliva flow rate and composition after drinking to satiety after dehydration of 6% of initial

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R818 HOMEOSTATIC RESPONSE TO REHYDRATION

TABLE 2. Water and Na+ balance in rumen after drinking to satiety in beef cows dehydrated to -18% of initial body mass

Inflow (Saliva) outflow* Net Absorptiont Net Accumulation

O-l h postdrinking

Water, l/h 2.9t0.3 0.7t0.05 (-75) 6.3t0.5 (5.8) -4.1t0.4 Na+, meq/h 406239 44+4(-75) 365+37(332) -3kO.3

l-4 h postdrinking

Water, l/h 2.5t0.3 1.020.6 (-69) 4.5t0.4 (3.8) -3.ot0.4 Na+, meq/h 388t40 73+6(-69) 293+30(250) 32t7

4-7 h postdrinking

Water, l/h 2.2t0.2 1.4t0.07 (-54) 2.8t0.3 (2.0) -2.Ot0.3 Na+, meq/h 374t42 126~~15 (-54) 199225 (131) 4926

Cumulative: O-7 h postdrinking

Water, l/7 h 17.0t1.8 7.920.8 28.2k3.5 (23.2) -19.1k2.5 Na+, meq/7 h 2692+272 611t50 184ltl90 (1475) 240t26

Values are means t SD. * Values in parentheses are % of relative errors calculated with Eq. 3 of Ref. 25. T Values in parentheses are potential deviation due to error in outflow measurement.

t I I I I I I 1

.- 72 hr WC- 24 hr

TIME FOLLOWING REHYDRATION FIG. 1. Saliva flow rate after drinking in cows dehydrated for 24

and 72 h.

mass were similar to those observed drinking after de- hydration to -18% of initial mass. However, the fluctua- tion in saliva flow rate (Fig. 1) and composition (Figs. 2- 8) were considerably more moderate.

Saliva secretion, already slow, declined still further by 50% (from 0.25 to 0.12 l/h) during the first 10 min after termination of drinking in cows dehydrated for 72 h (Fig. 1), and saliva Na+/K+ ratio showed no change. From 10 min postdrinking and on, saliva flow rate increased sharply reaching a peak at 50-60 min postdrinking (Fig. 1) .

DISCUSSION

As previously found in camels (13), goats (8, 22), and wild sheep (27), this study also demonstrated that water imbibed by cattle following a prolonged period of water deprivation is first retained in the RR.

The reduction in the net water outflow from the RR in comparison to the flow recorded in normally hydrated

360

* 340

P

>

6 320 z cn 0 E 300

w

tr i 280 a

2 260

B i 240

a cn

I I I I 1 I 1 1 I 1 I I *

72 --mm 24

z T _---mm w-w

# I

I I 1 I I

20 40 do nin // 4 h $ ’ ’ 8 hr TIME FOLLOWING REHYDRATION

FIG. 2. Saliva osmolality after drinking in cows dehydrated for 24 and 72 h.

animals (25) is consistent with the above-cited results. The slow rate at which fluid leaves the rumen into the lower parts of the gut is related to the low osmolality- Na+ concentration in the rumen, as also noted by Shkol- nik et al. (22). The presence of receptors in the RR that could respond to such changes has been demonstrated (12)

However, the view that the ability of ruminants to drink large amounts of water without apparent ill effects is due to the ability of the rumen wall to prevent the water from entering the blood too rapidly (8, 13, 22) has been disputed at least in cattle in the present work (Table 1) .

On the other hand, the present results were consistent with the hypothesis of Dobson (10) that differences in osmotic pressure between the rumen and blood are the

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HOMEOSTATIC RESPONSE TO REHYDRATION R819

72 hr -v-- 24 hr

-

65 d 0 20 40 60 min. 5 6 7 8hr

100, I I I I I I I /I

I ' ' I 0 20 40 6Omin 5 6 7 8hr

TIME FOLLOWING REHYDRATION

FIG. 5. Saliva HCO; after drinking in cows dehydrated for 24 and 70 1, TIME FOLLOWING REHYDRATION

FIG. 3. Saliva Na+ after drinking in cows dehydrated for 24 and 72 h. I I I I I l I I I

-72hr 4

f 0 12’ i= 72 hr a - u 5 10 -

_-__ ahr I --m_ ___ - -- e

T

?a -c- - --_ I I 1 I I rr I I I I I J*

0 20 40 60 min 5 6 7 8hr

T 1 ME FOLLOWING REHYDRATION 2 I FIG. 6. Saliva HPO, after drinking in cows dehydrated for 24 and

48 h. cd21 1 I a I 11 I N

’ ’ ’ ’ 20 40 60 Illill 5 6 7 8 hr

of 18% of cow’s initial weight was lost until reequilibrium at 7 h postdrinking occurred. By extrapolation from the results of Siebert and MacFarlane (21), it may be as- sumed that -3% (45% of the total lost) of the imbibed water was lost through the urinary route.

Cattle are unable to maintain their plasma volume during dehydration (21, 25). Rapid replenishment of their blood volume is therefore essential for restitution of their normal circulatory function. In the desert Bedouin goats even after higher dehydration rates (25- 40% of body weight), plasma volume at the end of de- hydration remained at a level that may be found in normally hydrated mammals (7,8). In the extreme desert where the Bedouin goat lives, regular periods of water deprivation are encountered. Retention of the imbibed water is essential for these goats, otherwise it will be short in the next dehydration cycle. As implicated by the present results, a larger proportion of the absorbed water is expected to be refluxed back to the gut by desert ruminant species. Apparent movement of urea and elec- trolyte with water to the gut following rehydration has

TIME FOLLOWING REHYDRATION FIG. 4. Saliva K+ after drinking in cows dehydrated for 24 and

72 h.

main driving force for water fluxes across RR epithelium and that ruminants are capable of rapidly absorbing large amounts of water from this pool. Rapid expansion and dilution of plasma as a result of a large influx of hypo- tonic water was prevented by a parallel increase in the secretion of hypotonic saliva, refluxing back to the RR almost half of the water absorbed.

Though all ruminants seem to be capable of retaining most (over 90%) of the imbibed water following rehydra- tion, until reequilibrium of the newly ingested water occurred (3, 8, 13, 27) desert ruminants seem to be more efficient (21). In quickly rehydrated Bedouin goats after dehydration of 2540% of their initial body mass, only 1.4% of the imbibed water was excreted in their urine during a 7-h postdrinking period (8). In the present experiment, on the average, -7% of the drinking water, 12% of the water absorbed from the gut after dehydration

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R820 HOMEOSTATIC RESPONSE TO REHYDRATION

hr 72

- - - - whr

I . T

-k ’ - .

c -- x--- . .

‘f 1 i--I_

10 I I I I I I 6omin

// I 1 I 20 40 5 6 7 !hr

TIME FOLLOWING REHYDRATION

FIG. 7. Saliva Cl- after drinking in cows dehydrated for 24 and 48 h.

r 72 hr

- - -- 24 hr

8.8 I I 1 I 1 I I ‘I I I I I 1

0 20 40 60min 5 6 7 8 hr

TIME FOLLOWING REHYDRATION FIG. 8. Saliva pH after drinking in cows dehydrated for 24 and

48 h.

been noted by Siebert and MacFarlane (21) in their experiments with camels and cattle.

In a previous report (23, 25) it was demonstrated that the volume of water stored in the rumen contributed the major portion (55%) of the total water loss during de- hydration. Utilization of gut water during dehydration involves a considerable increase in Na+ load. Increased natriuresis resulting in negative Na+ balance (19, 26) as well as sequestration of Na+ in the RR (23, 25) seems to be a basic homeostatic mechanism to counter increase in plasma Na+.

Rapid rehydration caused a large change in Na+ dis- tribution. Approximately 86 liters with a concentration of 63 meq/l (-35% of labile Na+ pool) are present in the RR immediately after drinking vs. an estimated 66 liters extracellular space with Na+ concentration of 145 meq/l (25). About 27% of the labile systemic pool (2,692 of 10,150 meq) passed to the rumen with the saliva by 7 h postdrinking. In comparison to the dehydration state, rehydration is a complete reverse of the situation; now each molecule of Na+ becomes important in preventing excessive losses from the extracellular fluid.

The following components of the homeostatic re- sponses are involved in Na+ restitution. 1) Almost com-

plete retention of Na+ excretion in the kidney. This response was demonstrated in cattle and camels (Zl), sheep (3), goats (S), and dogs (26). 2) Reduction in Na+ concentration in saliva (Fig. 3), therefore minimizing its enteric secretion. 3) Activation of Na+ absorption from the RR (Table 2). This response seems to be quite sudden since it was demonstrated that Na+ absorption almost completely ceased in these cows in advanced dehydrated states (25). 4) Almost instantaneous regaining of appe- tite (observed by the author in domestic ruminants- sheep, goats, cattle). Regaining of appetite occurred de- spite RR distension and before any significant change in blood osmolality. Ingestion of feed liberates solutes and generates volatile fatty acids. Osmotic equilibrium be- tween the RR and systemic fluid occur within 2 h in goats with access to feed (4) compared with >8 h in situations in which feed was denied from the goats fol- lowing rehydration (7).

Bianca (2) reported occasional cases of hemolysis in cattle following acute dehydration and rapid rehydration. As implicated by the present results, such responses might be expected in cases of Na+ deficiency and conse- quently lack of Na+ in the RR after rehydration.

The danger from water intoxication is greater than dehydration even in ruminants highly adapted to desert life such as the Bedouin goat. Etzion et al. (11) reported that water-deprived Bedouin goats survived losing 50% of their initial mass after 6 days of dehydration in the desert and then imbibed the entire lost amount at one drinking. However, all goats eventually died from he- molysis. These goats are watered regularly only once every 4 days by the Bedouin. Arriving at a water hole they imbibe volumes of water amounting to 25-45% of their body mass without hemolysis (22). During the additional 2 days of dehydration, the goats probably lost -3 liters. Since they survived, they must have utilized most of the water left in the rumen at the end of 4 days of dehydration (-1.7 liters) (7, 8) and such utilization must also have involved the absorption of Na+ stored in the gut (23-25). Consequently, the water absorbed from goats that were dehydrated for 6 days was practically pure water, which was the cause of death by hemolysis. This interpretation of the Etzion et al. (11) experiment exemplifies dramatically the importance of Na+ seques- tration in the foregut during dehydration in ruminants (23, 25).

The sudden further reduction in saliva flow immedi- ately after drinking in cows that were dehydrated to -18% of their mass may be related to the depressing effect of expansion of the rumen wall on saliva flow (15). However, soon after, the accumulation of large volumes of water absorbed from the rumen to blood apparently caused the reverse of the situation, namely, acceleration of saliva flow.

The initiation of increased saliva secretion may be explained if the role of a visceral sensory afferent input is to provide an “early-warning” signal on overflow of water from the gut. In support of this hypothesis there is convincing functional evidence on volume, osmolality, and Na’ receptors in the liver, which modulates kidney function (20). The liver and the central nervous system

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HOMEOSTATIC RESPONSE TO REHYDRATION R821

(5, 6) were also identified as possible sites influencing saliva flow and composition. Increase in peripheral blood tonicity has been suggested for the reduction in saliva secretion taking place before the end of feed intake in ruminants (5, 6). Induced natriuresis upon elevation of portal Na+ osmolality (20) and reflex renal vasoconstric- tion on portal vein distension (16,20) have been reported.

Address fo r reprint requests: N. Silanikove, c/o E. Maltz, nology Dept., ARO, P.O. Box 6, Bet -Dagan 50250, Israel.

Zootech-

Received 19 May 1987; accepted in final form 4 November 1988.

REFERENCES 18.

1. ADOLPH, E. F. Termination of drinking: satiation. Federation Proc. 41: 2533-2535,1982.

2. BIANCA, W. Effects of dehydration, rehydration and overhydration on the blood and urine of oxen. Br. Vet. J. 126: 121-132,197O.

3. BLAIR-WEST, J. R., A. BOBIK, A. BROOK, M. D. ESLER, A. GIBSON, A. MORRIS, M. J. MCKINLEY, AND P. T. PULLAN. Renin ADH and the kidney: a congress of conundrums. Prog. Biochem. Phar- macol. 17: 20-28, 1980.

4. BROSH, A., B. SNEH, AND A. SHKOLNIK. Effect of severe dehydra- tion and rapid rehydration on the activity of the rumen microbial population of black Bedouin goats. J. Agr. Sci. Lond. 100: 413-421, 1983.

5. CARR, D. H. The regulation of parotid and submandibular salivary secretion in sheep. Q. J. Exp. Physiol. 69: 589-597, 1984.

6. CARTER, R. R., W. C. GROVUM, AND W. W. BIGNELL. Effect of tonicity in the content of the stomach and duodenum and in blood on parotid salivary secretion in sheep (Abstract). Proc. Nutr. Sot. 44: 137A, 1986.

7. CHOSNIAK, I., AND A. SHKOLNIK. The rumen as a protective osmotic mechanism during rapid rehydration in the black Bedouin goat. In: Osmotic and Volume Regulation, edited by E. Skadhughe and C. B. Jurgensen. Copenhagen: Munksgard, 1978, vol. 11. (Alfred Benzon Symp.)

8. CHOSNIAK, I., C. WITTENBERG, J. ROSENFELD, AND A. SHKOLNIK. Rapid rehydration and kidney function in the black Bedouin goat. Physiol. Zool. 57: 573-579, 1984.

9. DENTON, D. A. The effect of Na+ depletion on the Na+:K+ ratio of the parotid saliva of the sheep. J. Physiol. Lond. 13: 516-525, 1956.

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