effects of sodium salts on pressor reactivity in salt...

541

Effects of Sodium Salts on Pressor Reactivity inSalt-Sensitive Men

Arya M. Sharma, Sabine Schattenfroh, Hans-Michael Thiede,Wolfgang Oelkers, and Armin Distler

Blood pressure in patients with essential hypertension is raised by sodium chloride but not by nonchloridesodium salts. Although a high sodium chloride diet is known to augment the pressor response tonorepinephrine and angiotensin II, the effect of nonchloride sodium salts on pressor responsiveness hasnot been studied so far. To examine whether sodium chloride and nonchloride sodium salts evoke differentpressor responses to these agonists, we performed graded norepinephrine and angiotensin II infusions insalt-sensitive (u=7) and salt-resistant (it=8) normotensive subjects. The subjects were given a low saltdiet (20 mmol/day) for 3 weeks, to which a supplement of 200 mmol sodium per day, provided as eithersodium chloride or sodium citrate, or a placebo was added for 1 week each. We found that, althoughsodium chloride raised mean arterial blood pressure in the salt-sensitive subjects (p<0.005), sodiumcitrate did not However, under both sodium salts pressor response to norepinephrine and angiotensin IIwas significantly greater than under placebo (p<0.02). Furthermore, with both sodium salts, pressorresponse in the salt-sensitive subjects was greater than in the salt-resistant subjects (p<0.01). This studythus demonstrates that, although blood pressure in salt-sensitive individuals is raised by sodium chlorideonly, both sodium chloride and sodium citrate evoke similar increases in pressor response to norepi-nephrine and angiotensin H. Since pressor response increased with both sodium salts but resting bloodpressure increased only with sodium chloride, enhanced pressor responsiveness alone cannot account forthe sodium chloride-induced rise in resting blood pressure. (Hypertension 1992;l<h541-548)

KEY WORDS • salt-sensitive hypertension • sodium chloride • catecholamines • vascularreactivity • renin-angiotensin-aldosterone system

Aenhanced pressor response to endogenous va-soactive substances including norepinephrineand angiotensin II has been found in patients

with essential hypertension1-12 and in genetically predis-posed normotensive subjects,111314 and this responsehas been assumed to play a major role in the pathogen-esis of essential hypertension. A high dietary sodiumchloride intake increases the pressor response to bothnorepinephrine15 and angiotensin II,16 and this couldcontribute to the salt-induced blood pressure rise insalt-sensitive individuals. Although the blood pressure-raising effect of salt has previously been thought to bemediated by the sodium ion alone, increasing evidenceindicates that it depends on the administration of bothsodium and chloride (see Reference 17 for review).Thus, in patients with essential hypertension, a blood

From the Department of Internal Medicine, Division of Ne-phrology (A.M.S., S.S., H.-M.T., A.D.) and the Division of Endo-crinology (W.O.), Universitatsklinikum Steglitz, Free Universityof Berlin, Berlin, FRG.

Results were presented in part at the 4th Scientific Meeting ofthe American Society of Hypertension, New York, 1989, and the4th European Meeting on Hypertension, Milan, Italy, 1989.

Supported in part by research grants from the Deutsche For-schungsgemeinschaft (Oe 47/9-1) and Staatl. Mineralbrunnen,Siemens Erben, Mainz, FRG.

Address for correspondence: Dr. Arya M. Sharma, Medizin-ische Klinik, Klinikum Steglitz, Free University of Berlin, Hinden-burgdamm 30, D-1000 Berlin 45, FRG.

Received July 11, 1991; accepted in revised form February 10,1992.

pressure rise was induced by the administration ofsodium chloride as a supplement to a low salt diet butnot by comparable amounts of sodium provided aseither the bicarbonate,18-19 citrate,20 or phosphate21 salt.

Whether this difference between the blood pressureeffects of sodium chloride and nonchloride sodium saltsis due to their evoking different pressor responses tonorepinephrine and angiotensin II has not been studiedpreviously. Therefore, we studied the effect of thedietary intake of sodium chloride or sodium citrate onthe pressor response to these substances in salt-sensitivenormotensive subjects with positive family histories ofhypertension. We also compared the effects of bothsodium salts on plasma levels and urinary excretion ofcatecholamines, plasma levels of renin activity, angio-tensin II and aldosterone, and the response of renalplasma flow to angiotensin II infusions in these individ-uals. Salt-resistant normotensive individuals with nega-tive family histories of hypertension served as controlsubjects. We selected genetically predisposed nor-motensive subjects for our study because these subjectsoffer the unique opportunity of studying pathogenicmechanisms believed to be related to hypertensionwithout the confounding presence of overt hypertensionand its sequelae.

Methods

The protocol of the study was approved by the EthicsCommittee of our hospital. All participants gave their

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542 Hypertension Vol 19, No 6, Part 1 June 1992

informed consent. The study was performed in anambulatory setting. All meals were prepared in thehospital kitchen.

SubjectsHealthy male volunteers (20-31 years old) were

studied after a routine physical and laboratory exami-nation to rule out hypertension, hyperlipidemia, diabe-tes mellitus, and hepatic or renal disease. Only subjectswith a diastolic blood pressure below 85 mm Hg and asystolic pressure below 140 mm Hg were included. Pa-rental histories on hypertension were obtained by directpersonal communication with the family physicians.Subjects with at least one parent under treatment forhypertension were regarded as having a positive familyhistory of hypertension.

Assessment of Salt Sensitivity

Salt sensitivity was assessed, as described previous-ly,22 in a total of 34 subjects before the commencementof the study. In brief, subjects were given a standardizeddiet containing 20 mmol sodium chloride, 60 mmolpotassium, and 20 mmol calcium per day for 14 days,adding either placebo or a supplement of 200 mmolNaCl/day for 1 week each. Total caloric intake wasestimated so as to keep body weight constant. Thesubjects were advised to drink about 2 1 water per day.On the morning of the seventh day of each period, aftera 30-minute rest, blood pressure was measured in therecumbent subject over 1 hour at 2-minute intervalswith an automatic device (Tonoprint, Speidel andKeller, Jungingen, FRG). The standard error of themean for a single 60 -minute period ranged between 0.54and 0.95 mm Hg. Salt sensitivity as in previous stud-ies13'22-23 was defined as a significant drop in meanarterial pressure greater than 3 mm Hg, calculated asthe difference between the 30 blood pressure readingsduring the high and low salt periods (/?<0.05). We havepreviously shown salt sensitivity thus denned to be awell reproducible phenomenon in normotensiveindividuals.22

Study ProtocolTen salt-sensitive subjects with positive family histo-

ries and 10 salt-resistant subjects with negative familyhistories of hypertension agreed to participate in thestudy. In addition to the low salt diet described above,subjects were given, as a dairy supplement, 40 capsulescontaining either sodium chloride or sodium citrate (5mmol sodium per capsule) or a placebo for 7 days each.The sequence of the three regimens (placebo, sodiumchloride, and sodium citrate) was randomized accordingto a Latin square design. The resultant daily intake of220 mmol during the high sodium periods exceeds theaverage sodium intake in West European societies byroughly 25%, but is still well within the normal range.24

Compliance was assessed throughout the study by mea-suring daily 24-hour urinary sodium, chloride, andpotassium excretion. Creatinine excretion served as ameasure for the accuracy of urine collection. Subjectswere considered compliant when sodium and chlorideexcretion was below 35 mmol/24 hr during the last 3days of the low sodium chloride period, above 180mmol/24 hr during the last 3 days of the high salt period,

and when sodium excretion was higher than 180mmol/24 hr and chloride excretion was lower than 35mmol/24 hr during the last 3 days of the sodium citrateperiod.

On the sixth day of each period, after 30 minutes ofrest and insertion of a venous Teflon cannula, bloodpressure was measured as described above. Then, after3 hours at supine rest, a venous blood sample was drawnfrom an antecubital vein for determination of sodium,chloride, potassium, calcium, renin activity, angiotensinII, aldosterone, and catecholamines. In addition, anarterial blood sample was drawn from the radial arteryfor assessment of the acid base and blood gas status, theresults of which have been reported previously.23 Pres-sor response to intravenous norepinephrine infusionwas then assessed by obtaining dose-response curvesfor norepinephrine (Arterenol, Hoechst, FRG) as de-scribed previously.6 Norepinephrine was infused indoses of 0.05, 0.1, and 0.2 jig/kg/min. Each dose wasinfused for 10 minutes. Blood pressure recorded everyminute reached a new plateau after 3-5 minutes at eachdose, and the means of the last five readings were usedto calculate the representative blood pressure responseto each dose.

On the seventh day of each period the effect ofgraded intravenous infusions of angiotensin II (Hyper-tensin, CIBA-GEIGY) on blood pressure, renal plasmaflow, and aldosterone secretion was determined. Renalplasma flow was measured as the clearance of para-aminohippurate (PAH) (Nephrotest, BAG, Lich, FRG)as described by Schnurr et al25 and corrected for 1.73 m2

body surface area. Angiotensin II was infused in dosesof 2,4, and 8 ng/kg/min. The 60-minute infusion of eachdose was followed by drawing blood samples for mea-surement of aldosterone and PAH. Blood pressure wasmeasured at 5-minute intervals during the infusionperiods, and the means of the last six readings wereused to calculate the pressor response to each angioten-sin II dose.

Laboratory Procedures

Plasma and urinary electrolytes were measured bystandard laboratory techniques. Plasma renin activity,angiotensin II, and aldosterone were assayed as de-scribed previously.26 Plasma and urinary catecholamineswere measured by reversed-phase, high-performance liq-uid chromatography (HPLC) with electrochemical detec-tion (BCJM 1, ERC, Alteglofsheim, FRG) and by apply-ing a slight modification of the method of Frayn andMaycock.27 In brief, after the addition of the internalstandard (3,4-dihydroxy-benzylamine, Sigma, Munich,FRG) the heparinized plasma or urine sample wasadjusted to pH 6.5 and passed over a weak cationexchanger column (Biorex 70 100 mesh, Bio Rad,Munich, FRG). After washing with water, the catechol-amines were eluted with 3 M Nad. Eluate pH wasadjusted to 8.5 in the presence of sodium metabisulfite(antioxidant). The amines were then further extracted bybinding to acid-washed aluminum oxide. After washingand elution with 0.1 M perchloric acid, an aliquot wasinjected into the HPLC system, where the amines wereseparated by passing through a steel column (120x4 mm)packed with spherisorb ODS II (3 /im) (Knauer, Berlin,FRG). The mobile phase consisted of a sodium citrate



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Sharma et al Sodium and Pressor Response 543

TABLE 1. Physical, Plasma, and Urine Variables in Salt-Resistant and Salt-Sensitive Normotensire Subjects Duringthe Administration of Placebo, Sodium Chloride, and Sodium Citrate as a Supplement to a Low Salt Diet

Variables

Physical variablesSystolic BP (mm Hg)Diastolic BP (mm Hg)Mean BP (mm Hg)Heart rate (bpm)Weight (kg)

Plasma variablesSodium (mmol/1)Chloride (mmol/1)Potassium (mmol/1)Calcium (mmol/1)

Urine variablesVolume (ml/24 hr)Sodium (mmol/24 hr)Chloride (mmol/24 hr)Potassium (mmol/24 hr)Calcium (mmol/24 hr)

Placebo

108.2±2.964.3±2.979.1 ±2.658.0+2.7753±3.6

139.0±0.798.1 + 1.33.9±0.12.3±0.1

l,500±21821.5±4.923.8±3.449.6±3.33.3+1.0

Salt-resistant (n=8)

Sodiumchloride

105.9±3.565.5 ±2.279.0±2.357_5±1.776.0±3.5

141.1±1.0102.8±0.8*

4.1 ±0.12.3±0.1

l,850±119216.5+15.4*214.6+18.3*58.9±6.24.1 + 1.0

Sodiumcitrate

106.1±3.065.2±3.078.9±2.058.0±2.875.6±3.7

141.9+1.799.5±0.7§3.8±0.12.3±0.1

1,812+121192.9±12.6'21.1±4.1#48.9±2.22.1±0.2#

Placebo

102.7±3.365.0±2.977.6±2.760.1±1.272.0 ±1.2

140.0±0.6101.3 + 1.3

4.4±0.112.4±0.1

1,614±35219.9+5.823.0±4.352.1±4.93.6±1.2

Salt-sensitive (n

Sodiumchloride

107.2±3.071.4±2.7*83.2±2.2*57.5 + 1.472.4±1.3

141.2±0.7102.9±0.8

4.5±0.22.4 ±0.1

1,921 ±298222.4±10.2*247.3 ±15.3*51.1+3.54.9±0.9

=7)

Sodiumcitrate

105J±2.968.5±2.0

80.7±1.8t58.3 + 1.173.0 ±1.2

141.3±0.999.6+0.6114.0±0.12.3±0.1

1,771±253195.9±22.0*20.7±2.3#44.7±3.22.1±0.6#

Values are mean±SEM. BP, blood pressure.+p<0.01 and *p<0.005 vs. placebo.tp<0.05, ||p<0.016, §/><0.01, and #/><0.005 vs. sodium chloride.tp<0.005 vs. salt-resistant.

buffer (pH 4.5) containing 10% methanol and octane-sulfonic acid (100 mg/1). The flow rate was set at 0.3ml/min. The detection potential at the glassy carbonelectrode versus the Ag/AgCl reference electrode was setat 800 mV. All samples were measured in duplicate.Recovery ranged between 60% and 70% for all amines.The detection limit was below 5 pg. Interassay andintra-assay variance was below 5%.

Statistics

Statistical analysis was performed using the SPSS/PC+

software package (SPSS Inc., Chicago, 111.). The saltsensitivity of an individual subject was tested by usingthe independent t test to detect significant differencesbetween the 30 blood pressure readings taken duringthe high salt diet and those during the low salt diet.Subsequently, the means of the 30 blood pressurereadings as well as all other variables under the differentregimens were compared within each group by thetwo-tailed t test for paired samples, the significancelevel being reduced to p<0.016 to compensate formultiple comparisons (placebo versus sodium chloride,placebo versus sodium citrate, and sodium chlorideversus sodium citrate). Otherwise, statistical signifi-cance was defined asp<0.05. The response to angioten-sin and norepinephrine infusion between the twogroups was compared by two-way analysis of variance(ANOVA) for repeated measures over time under eachregimen. Otherwise, the two-tailed / test for indepen-dent samples was used for comparisons between thesalt-sensitive and salt-resistant groups. Results are pre-sented as mean±SEM.

Results

Two salt-resistant and three salt-sensitive subjectswere excluded from analysis because of poor compli-ance as assessed by urinary sodium and chloride excre-tion. There was no difference in age (25.0±0.4 versus25.6±0.8 years) or body mass index (21.4±0.8 versus22.8±0.9 kg/m2) between the remaining salt-sensitiveand salt-resistant subjects.

Physical, plasma, and urinary variables are shown inTable 1. There was no significant difference in urinaryexcretion of sodium and chloride between the twogroups under the different regimens. A rise in diastolicand mean arterial pressure was induced in the salt-sensitive subjects by sodium chloride (/?<0.005 versusplacebo) but not by sodium citrate (p>0.2). The differ-ent regimens had no effect on blood pressure in thesalt-resistant group. Body weight was not significantlyaffected in either group.

In the salt-sensitive group, the pressor response tograded norepinephrine infusion was higher at all doseswith both high sodium regimens than with the low saltdiet (/?<0.01, ANOVA) (Figure 1). In contrast, pressorresponse to norepinephrine was not significantly af-fected by sodium intake in the salt-resistant group.Although the pressor response to norepinephrine underthe low salt regimen was similar in the two groups, thepressor response to the highest norepinephrine doseunder both high sodium regimens was significantlygreater in the salt-sensitive than in the salt-resistantsubjects (p<0.05, ANOVA). Plasma levels and urinaryexcretion of norepinephrine, epinephrine, and dopa-mine are shown in Table 2. Plasma norepinephrinelevels were lower with both sodium chloride and citrate



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FIGURE 1. Line plot shows change in mean arterial pressure(MABP) during graded norepinephrine (NA) infusion in salt-resistant (n=8) and salt-sensitive (n=7) normotensive subjectsduring administration of placebo, sodium chloride, and sodiumcitrate as a supplement to a low salt diet (mean ±SEM).

than with placebo, although the difference was signifi-cant only in the salt-sensitive group (/><0.01). Therewere no significant treatment effects on plasma epi-nephrine and dopamine. Urinary excretion of catechol-amines was not significantly affected by either regimen.

Under both high sodium chloride and sodium citrate,the salt-sensitive subjects showed a greater overall pres-sor response to angiotensin II than under placebo(Figure 2) (p<0.01, ANOVA). With both sodium salts,pressor response to the lowest angiotensin II dose wasalso augmented in the salt-resistant group (p<0.05,ANOVA). Although the two groups did not differ intheir pressor response to angiotensin II under the lowsalt regimen, under both sodium salts pressor responsewas significantly higher in the salt-sensitive subjectsthan in the salt-resistant group (p<0.05, ANOVA).Under the low salt regimen, there was a comparabledecrease in renal plasma flow (PAH clearance) duringangiotensin II infusion in both groups (Figure 2). How-ever, the angiotensin H-induced reduction in renalplasma flow was augmented under both sodium salts inthe salt-sensitive subjects (p<0.05, ANOVA), but wasnot significantly changed in the salt-resistant individu-als. The plasma aldosterone concentration after angio-

tensin II infusion was lower under the high sodiumregimens in both groups (p<0.01, ANOVA) (Figure 3).Although the two groups did not differ in their adrenalangiotensin II responsiveness under the low salt regi-men, the plasma aldosterone concentration in responseto the angiotensin II infusion tended to be lower in thesalt-resistant than in the salt-sensitive individuals duringboth high sodium regimens. Plasma renin activity, an-giotensin II, and aldosterone levels were significantlylower under both the sodium chloride and the sodiumcitrate regimens compared with placebo in both groups(/?<0.01; Figure 4).

Discussion

The current study demonstrates that in salt-sensitivenormotensive individuals, pressor response to exogenousnorepinephrine and angiotensin II increases under a highdietary intake of sodium administered as either sodiumchloride or sodium citrate. Thus, the enhanced pressorresponse to norepinephrine15 and angiotensin II16 knownto be induced by a high salt intake clearly depends onsodium but not on chloride balance. As opposed tosodium chloride, however, sodium citrate failed to raiseresting blood pressure. The different effects of the twosalts on resting blood pressure can, therefore, not beaccounted for by differences in their effects on the pressorresponse to norepinephrine or angiotensin II. Moreover,our data do not support the assumption that the sodiumchloride-induced rise in resting blood pressure is directlylinked to the concomitant rise in pressor response tonorepinephrine or angiotensin II.

Previous studies have demonstrated that pressor re-sponse to exogenous norepinephrine6-15-28-30 is inverselycorrelated to plasma levels of norepinephrine, and thishas been attributed to a decreased availability of adren-ergic receptors with increasing sympathetic activity andvice versa.6 In the current study, plasma norepinephrinewas significantly reduced under both high sodium regi-mens in the salt-sensitive subjects. One reason for theincreased pressor response to norepinephrine in thesesubjects could therefore be a functional upregulation ofvascular a-adrenergic receptors or postreceptor mecha-nisms after a reduction in sympathetic outflow. Such anadaptation thus need not necessarily have a net effect onresting blood pressure. This assumption is supported by

TABLE 2. Plasma Levels and 24-Hour Urinary Excretion of Catecholamines in Salt-Resistant and Salt-SensitiveNormotenstre Subjects During the Administration of Placebo, Sodium Chloride, and Sodium Citrate as a Supplementto a Low Salt Diet

Variables

Plasma catecholamines

Norepinephrine (pg/ml)

Epinephrine (pg/ml)

Dopamine (pg/ml)

Urinary catecholamine excretion

Norepinephrine (/ig/24 hr)

Epinephrine (jig/24 hr)

Dopsmine (M&/24 hr)

Salt-resistant (n

Placebo

242+21

30.4±4.1

22.1 ±4.4

40.7±3.3

4.2±0.3

241 ±21

Sodiumchloride

2O6±15

26.2±9.1

16.7±1.7

33.7±5.1

5.6+1.0

248±25

=8)

Sodiumcitrate

187±20

28.5±5.9

20.6±2.7

31.6±5.2

3.5 ±0.6

194±20

Placebo

263±31

25.9±7.0

19.1±3.4

38.4±4.6

7.1 ±0.9

210±30

Salt-sensitive (n

Sodiumchloride

184±15'

24.6±11.6

17.3 ±4.4

27.4±3.0

7.4±0.6

199±15

=7)

Sodiumcitrate

160±32*

27.7±6.0

15.9±3.7

31.1±6.0

5.8±0.7

176±15

Values are mean±SEM.*p<0.01 vs. placebo.



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FIGURE 2. Line plots show change inmean arterial blood pressure (MABP)(top panel) and renal plasma flow(bottom panel) during graded angio-tensin II (All) infusion in salt-resistant(n=8) and salt-sensitive (n=7) nor-motensive subjects during administra-tion of placebo, sodium chloride, andsodium citrate as a supplement to a lowsalt diet (mean±SEM).

the finding that, although pressor response was increasedunder sodium citrate in the salt-sensitive group, restingblood pressure did not change significantly. In contrast,in the salt-resistant group, sodium intake did not signif-icantly influence either pressor response to norepineph-rine or plasma norepinephrine levels.

As with the interrelation between the pressor responseto norepinephrine and plasma levels of norepinephrine,the pressor response to angiotensin II has also been shown

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FIGURE 3. Line plots show plasma aldosterone levels duringgraded angiotensin II (All) infusion in salt-resistant (n=8)and salt-sensitive (n=7) normotensive subjects during admin-istration of placebo, sodium chloride, and sodium citrate as asupplement to a low salt diet (mean±SEM).

to correlate inversely with plasma levels of angiotensin II31

and renin activity.6-30-32 In our present study, both sodiumsalts lowered plasma angiotensin II levels and renin activ-ity while enhancing pressor response to angiotensin II inboth groups. During both high sodium regimens, however,pressor response was higher and plasma levels of angio-tensin II and renin activity tended to be lower in thesalt-sensitive than in the salt-resistant subjects. The aug-mented angiotensin II-induced reduction in renal plasmaflow under both sodium salts observed in the salt-sensitivegroup is compatible with an increased sensitivity of therenal vasculature to angiotensin II in these individuals.Since blood pressure in the salt-sensitive group was raisedonly by sodium chloride but not by sodium citrate, ourdata show that the sodium-induced augmentation of vas-cular responsiveness to angiotensin II does not necessarilyresult in a net increase of resting blood pressure.

Although upregulation of receptor sensitivity to nor-epinephrine and angiotensin II as a consequence oflower circulating levels of these agonists under a highsalt diet may be one mechanism leading to the greaterpressor response observed in the salt-sensitive subjects,sodium-induced effects on other factors known to influ-ence pressor response, such as increased cytosolic cal-cium33 or altered baroreceptor reflex function,34 mustlikewise be considered.

In our present study, both sodium salts had a similarsuppressive effect on the angiotensin II-induced releaseof aldosterone. Salt depletion is known to sensitize the



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FIGURE 4. Bar graphs show plasma rerun activity, angiotensinII, and aldosterone levels in salt-resistant (n=8) and salt-sensitive (n=7) normotensive subjects during administration ofplacebo, sodium chloride, and sodium citrate as a supplement toa low salt diet (mean±SEM). Ang I, angiotensin I.

adrenal cortex to angiotensin II,26-35-3* but whether thiseffect is mediated by changes in chloride balance hasnot been studied previously. Our data now show thatchloride depletion per se does not enhance the sensi-tivity of the adrenal cortex to angiotensin II infusion.Therefore, sodium but not chloride depletion seems toaccount for the sensitization of the adrenal cortex toangiotensin II observed during salt restriction. Plasmaaldosterone levels during angiotensin II infusion in ourstudy were slightly but not significantly lower in thesalt-resistant than in the salt-sensitive group under bothhigh sodium diet regimens.

Though a high salt diet has been shown to lowerplasma levels of norepinephrine37-39 and angiotensinjl;35,4o the effect of a nonchloride sodium salt on plasmacatecholamines and angiotensin II levels has not beenstudied in humans previously. Our finding that bothsodium salts have comparable suppressive effects onplasma levels of norepinephrine, renin activity, angio-tensin II, and aldosterone demonstrates that the sym-pathetic nervous system and the renin-angiotensin-aldosterone system are primarily modulated by sodiumand not chloride intake. This is in agreement with datashowing that both sodium chloride and nonchloride

sodium salts suppress plasma renin activity and aldoste-rone levels in hypertensive subjects19-20 as well as in thedeoxycorticosterone-acetate41 and one-kidney, oneclip42 rat models of hypertension. However, there areconflicting data from studies performed in theSpragvie-Dawley rat43-46 and in normal individuals47

suggesting that chloride rather than sodium regulatesrenin secretion. Thus, in normal Sprague-Dawley rats,renin activity was suppressed by sodium chloride addedto a low salt diet but not by various nonchloride sodiumsalts providing equal amounts of sodium.43"46 Similarly,normal individuals on a low sodium diet showed sup-pression of plasma renin activity after acute intravenousinfusion of saline but not after sodium bicarbonate.47

Although the difference between our findings and theeffect of the nonchloride sodium salt on renin activity inthe latter study may be due to renin activity beinginfluenced differently by acute sodium bicarbonateloading than with chronic administration of a non-chloride sodium salt, we cannot provide a ready expla-nation for the discrepancy between our findings andthose obtained from studies in the Sprague-Dawley rat.

It has been suggested that failure to appropriatelysuppress sympathetic activity with a high salt diet maycontribute to salt sensitivity in patients with essentialhypertension.4849 This, however, does not appear tohold true for salt-sensitive normotensive subjects, sincein our salt-sensitive subjects plasma norepinephrinelevels decreased with high salt intake. Thus, failure ofplasma norepinephrine to decrease with high salt intakein patients with salt-sensitive hypertension may reflect aphenomenon secondary to established hypertensionsuch as a resetting of baroreceptor reflexes or sympa-thetic outflow rather than a primary defect.

In this current study, as in a previous study innormotensive individuals performed by Skrabal et al,13

pressor response to norepinephrine under a high saltdiet was greater in salt-sensitive than in salt-resistantsubjects. In addition, we found that systemic and renalvascular response to angiotensin II in the salt-sensitivesubjects was likewise enhanced under the high sodiumdiet (the sodium content of which, however, was stillwithin the normal range). Enhanced pressor response tonorepinephrine1-5-7'10-50 and angiotensin IP-4A12 hasbeen found in the majority of studies performed inpatients with essential hypertension, but some authorshave failed to confirm this.51-54 Differences in the studyprotocol and in the assessment of reactivity to the agentinfused as well as other factors including age, saltintake, and race may account for these inconsistentfindings. Thus, in a recent study33 that failed to find agreater pressor response to norepinephrine in patientswith essential hypertension than in normotensive con-trol subjects, hospitalization before the examinationdecreased diastolic pressure to less than 90 mm Hg onthe day of the study in most patients previously classi-fied as hypertensive in an outpatient setting. In anotherrecent negative study,54 different total doses of norepi-nephrine were administered to the two study groups,and the pressor response was expressed as percentchange in blood pressure per nanomole increase inplasma norepinephrine, an approach very differentfrom the analysis using dose-response curves applied inour and most other studies. Since, as our study indi-cates, enhanced pressor responsiveness to vasoactive



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substances may be present in subgroups of normoten-sive subjects who are salt sensitive, have a family historyof hypertension, or both, an inappropriate choice ofnormotensive control groups may further explain whysome investigators have failed to find hyperreactivity topressor agents in hypertensive subjects compared withnormotensive control subjects.

Structural alterations leading to a thickening of thewalls of resistance arterioles have been proposed toaccount for the hyperreactivity to pressor agents ob-served in hypertensive subjects.55 The observation inthis and other studies111314 of an enhanced pressorresponse to norepinephrine and angiotensin II in nor-motensive individuals predisposed to the developmentof hypertension clearly indicates that enhanced reactiv-ity to these substances may be present before thedevelopment of overt hypertension and, therefore, doesnot result from changes secondary to established hyper-tension. However, structural changes cannot completelybe ruled out as a cause of hyperreactivity since there issome indirect evidence suggesting that such changesmay already be present in normotensive offspring ofpatients with essential hypertension.56 Other mecha-nisms, including an increased o^ft-adrenergic receptorratio,13 a salt-induced rise in cytosolic calcium,33 oraltered baroreceptor reflex function,34-57 that could allbe operative even in the absence of structural changesappear to be more likely to account for the increasedreactivity observed in our salt-sensitive subjects.

In a series of studies by Williams and Hollenberg,52-58

a subset of patients with salt-sensitive hypertensiondesignated "non-modulators" has been described.These patients are characterized by their failure toincrease aldosterone response to angiotensin II duringsalt restriction and failure to increase renal blood flowresponse to angiotensin II when on a high salt diet.These individuals were reported to account for 55-65%of patients with salt-sensitive hypertension.58 In con-trast, in our salt-sensitive normotensive subjects bothadrenal responsiveness and renal blood flow response toangiotensin II were significantly affected by salt intake,and these individuals must therefore be regarded as"modulators." One may therefore speculate that, al-though salt sensitivity in established hypertension maybe associated with "non-modulation," this does not applyto young salt-sensitive normotensive subjects. Weinber-ger and Fineberg59 have recently reported that salt-sensitive normotensive subjects experience an increasein blood pressure as they grow older, and it would be ofinterest to follow up these individuals to observewhether they become non-modulators as they growolder and overt hypertension develops.

Several mechanisms have been suggested to accountfor the different blood pressure effects of sodium chlo-ride and non-chloride sodium salts. As pointed out in arecent review by Boegehold and Kotchen,17 the anionsupplied along with sodium may determine its effect onplasma volume by influencing its distribution amongintracellular and extracellular compartments, thus ac-counting for different blood pressure effects despite asimilar sodium balance. As noted in the present study,sodium bicarbonate and citrate are known to reduceurinary calcium excretion,20-*0 which may likewise affectthe blood pressure response to sodium intake.17 Thealkalizing effect of sodium citrate or bicarbonate may

also contribute to their failure to raise blood pressure insalt-sensitive individuals.23

In summary, the present study demonstrates that, al-though blood pressure in salt-sensitive individuals is raisedby sodium chloride only, both sodium chloride and sodiumcitrate evoke similar increases in the pressor response tonorepinephrine and angiotensin II. This increase in pres-sor response depends on sodium but not on chloridebalance. Since pressor response increases with both so-dium salts but resting blood pressure increases with so-dium chloride only, enhanced pressor responsivenessalone obviously does not account for the salt-induced risein resting blood pressure. Importantly, under a high(though normal) sodium diet, the pressor response to bothnorepinephrine and angiotensin II was higher in salt-sensitive than in salt-resistant subjects. The pathophysio-logical significance of this finding for the possible laterdevelopment of hypertension in these genetically predis-posed individuals remains to be determined.

AcknowledgmentsWe thank Dr. Andreas Kribben, who helped in the study of

some subjects. We also thank Dr. Volker Bahr, Klaus Schlot-ter, Petra Exner, and Birgit Faust for technical assistance inanalyzing the plasma samples and Ulrike Schorr for her help inpreparing the manuscript.

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A M Sharma, S Schattenfroh, H M Thiede, W Oelkers and A DistlerEffects of sodium salts on pressor reactivity in salt-sensitive men.

Print ISSN: 0194-911X. Online ISSN: 1524-4563 Copyright © 1992 American Heart Association, Inc. All rights reserved.

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