Chronic blockade of angiotensin during sodium deprivation JOHN E. HALL, ARTHUR C. GUYTON, Department of Physio Logy and Biophysics, of Medicine, Jackson, Mississippi 39216

MANIS J. SMITH, JR., AND THOMAS University of Mississippi School

HALL, JOHN E., ARTHUR C. GUYTON, MANIS J. SMITH, JR., THOMAS G. COLEMAN. Chronic blockade of angiotensin II formation during sodium deprivation. Am. J. Physiol. 237(6): F424-F432, 1979 or Am. J. Physiol.: Renal Fluid Electrolyte Physiol. 6(6): F424-F432, 1979.-The present study was designed to investigate the mechanisms by which the reninangiotensin system (RAS) regulates arterial pressure (AP) and renal function during chronic sodium deprivation. Intravenous infusion of the converting enzyme inhibitor SQ 14225 (14 pg. kg-’ .min-‘) for 8 days in 12 sodium-deficient dogs caused a marked decrease in AP from 90 t 1 to 67 t 2 mmHg and a reduction in glomerular filtration rate (GFR), filtration fraction (FF), and plasma aldosterone concentration (PAC). Despite the fall in AP and GFR, urinary Na excretion and effective renal plasma flow (ERPF) increased above control levels. In four dogs, infusion of aldosterone (200 pg/day) for 8 days during continuous SQ 14225 infusion restored PAC to levels above control, but did not significantly change AP or renal function from the values observed during SQ 14225 infusion alone. However, infusion of angiotensin II (AII) (10 or 20 ng kg-‘. min-‘) for 5-8 days during continuous SQ 14225 infusion almost completely restored AP and renal function to control levels. These data indicate that the RAS plays a major role in regulating AP, renal hemodynamics, and Na excretion during Na deprivation, probably through the direct effects of AI1 rather than through changes in PAC. AND

l

renin-angiotensin system; converting enzyme inhibition; aldosterone; glomerular filtration rate; renal blood flow; sodium excretion; blood pressure

SYSTEM is widely recognized as a potent means of regulating blood pressure (5, 27), but the mechanisms by which it exerts this control chronically have not been fully elucidated. Angiotensin II (AII) is known to be a potent constrictor of peripheral arterioles, and this action undoubtedly plays a major role in the short-term effects of AI1 on blood pressure, especially in conditions such as sodium deprivation or renal artery constriction in which the renin-angiotensin system is activated (2, 24, 27). However, additional mechanisms are probably involved in the chronic effects of AI1 on blood pressure, since infusion of extremely low and initially subpressor dosesof AI1 causes a gradual increase in blood pressure that is more pronounced at high sodium intakes (8, 9). Another action of AI1 that could play a more important role in long-term regulation of blood pressure is to stimulate the secretion of aldosterone (7, 13), which in turn THE RENIN-ANGIOTENSIN

F424

II formation

G. COLEMAN

increases renal tubular reabsorption of sodium and water and extracellular fluid volume. This indirect effect of AI1 on renal excretion, acting through aldosterone, is regarded by many investigators as the primary mechanism by which the renin-angiotensin system chronically regulates renal excretion and extracellular fluid volume. In addition to effects mediated via changes in aldosterone secretion, AI1 also has important direct effects on renal hemodynamics and tubular reabsorption of sodium and water which might also play an important role in long-term blood pressure regulation (15, 22). Several recent studies from our laboratory (20, 28), and from other laboratories as well (26), have provided evidence that AI1 has a potent antinatriuretic and antidiuretic effect on the kidney that is independent of changes in plasma aldosterone concentration. In sodium-deficient dogs, renal arterial infusion of an AI1 antagonist or the angiotensin converting enzyme inhibitor SQ 20881 for 60-90 min greatly increased renal excretion of sodium and water without altering plasma aldosterone concentration, suggesting that during sodium deprivation endogenous production of AI1 is sufficient to cause pronounced antinatriuresis and antidiuresis independently of a change in aldosterone secretion (17,20,28). Furthermore, this direct effect of AI1 on renal excretion, which could be mediated either through changes in renal hemodynamics or tubular reabsorption, may be more important quantitatively than its indirect effects mediated through changes in aldosterone secretion. Although the acute effects of AI1 infusion or blockade on blood pressure and renal function have been clearly documented (20, 21, 25, 26), there is very little information on the quantitative importance of endogenously produced AI1 in the long-term regulation of renal hemodynamics and electrolyte excretion. It is possible that during chronic blockade of AII, other control systems (i.e., aldosterone) may become increasingly important for regulation of renal excretion, and therefore blood pressure, and completely compensate for loss of control by AII. McCaa et al. (29) recently reported that administration of the angiotensin converting enzyme inhibitor SQ 14225 in sodium-depleted dogs resulted in a sustained reduction in blood pressure and plasma aldosterone concentration and that these effects could be reversed by infusion of aldosterone or aldosterone plus angiotensin II along with SQ 14225. However, the relative importance of changes in plasma concentrations of aldosterone and angiotensin II in contributing to the chronic effects of

0363-6127/79/0000-OOOO$Ol.25

Copyright

0 1979 the American

Physiological

Society

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converting enzyme inhibition on the control of renal hemodynamics and electrolyte excretion have not been previously reported. In the present study, we used the angiotensin converting enzyme inhibitor SQ 14225 (2-D-methyl-3-mercaptopropanoyl+proline) to quantitate the role of the reninangiotensin system in the long-term control of renal hemodynamics, electrolyte excretion, and arterial pressure during sodium deprivation. Since AI1 blockade also decreases plasma aldosterone concentration (13)) we attempted to separate the effects of reduced plasma aldosterone concentration from the direct effects of AI1 blockade by infusing aldosterone along with SQ 14225 at rates calculated to restore plasma aldosterone concentration to levels observed prior to converting enzyme inhibition. To determine whether the effects of SQ 14225 on blood pressure and renal function could be reversed with exogenous AII, we also infused AI1 at rates of 10 or 20 ng. kg-’ . mine1 along with SQ 14225 for several days and compared the steady-state levels of blood pressure, plasma aldosterone concentration, renal hemodynamics, and electrolyte excretion with the values measured before infusion of SQ 14225 in sodium-deficient dogs. The rates of AI1 infusion used in this study were calculated to raise plasma concentrations of AI1 to levels similar to those occurring during sodium deprivation before infusion of SQ 14225. METHODS

All experiments were conducted in conscious male mongrel dogs. Polyvinyl catheters were implanted in the femoral arteries and veins under aseptic conditions, and the dogs were permitted to recover for at least 3 wk before control measurements were made. Antibiotics were administered daily and rectal temperatures were measured to insure that the dogs were afebrile at the time of the study. During the recovery period, as well as during the control and experimental periods, the dogs were fed a sodium-deficient diet (h/d, Riviana Foods, Inc.) which provided less than 5 meq sodium and approximately 45 meq potassium per day. Free access to distilled water was permitted at all times. Measurements of electrolyte and water balance, 24 h recording of mean arterial blood pressure, and continuous intravenous infusions were performed as previously described (6). Briefly, each dog was housed in a metabolic cage in a quiet air conditioned room with a 12-h light cycle and fitted with a harness that contained a Statham pressure transducer mounted at heart level. One of the femoral artery catheters was filled with heparinized saline (1,000 U/ml) and connected to the pressure transducer for continuous recording of arterial pressure, 24 h a day, on a Grass polygraph (model 7D); the mean pressure for each hour of recording was determined and used to calculate the daily average arterial pressure for each dog. One of the femoral venous catheters was connected to a roller pump (Sage Instruments, model 375A), which was used to infuse various solutions continuously 24 h a day. All solutions were pumped through a disposable Millipore filter (Cathivex, Millipore Corp.) to prevent contaminants and bacteria from passing into the venous infusion catheters. The infusion tubing and wires

F425 from the transducer were brought out of the top of the cage through a flexible tubing that was attached to the harness; this apparatus permitted the dogs to move freely in the cage but not to turn completely around. Before the control period, each dog was trained to lie quietly while blood samples were drawn from a chronic femoral artery catheter and studies of renal function were performed. In all experiments, collection of blood samples and measurements of renal function were begun at approximately 8 A.M. each day, about 16-18 h after the last feeding. During the control period, which lasted at least 7 days, the dogs were infused intravenously with a sterile solution of 5% dextrose and water at a rate of 100 ml/day to maintain the patency of the venous infusion catheter. Following the control period, the angiotensin converting enzyme inhibitor SQ 14225 was infused intravenously into 12 sodium-deficient dogs at a rate of 14 pg.kg-‘. min-’ in 100 ml/day of the 5% dextrose vehicle solution. The infusion of SQ 14225 was continued for 8 days while blood pressure was measured continuously, and 24-h urinary excretion of sodium, potassium, and water, and 24-h intake of water were measured daily. Glomerular filtration rate (GFR), effective renal plasma flow (ERPF), plasma aldosterone concentration (PAC), plasma protein concentration, and plasma electrolyte concentrations were measured at least 2 times during the control period, and at least 3 times during the 8 days of SQ 14225 infusion. The effectiveness of the SQ 14225 infusion in blocking AI1 formation was assessed by injection of 4 pg of angiotensin I (approximately 200 rig/kg body wt). Before infusion of SQ 14225, injection of angiotensin I elicited a 20- to 35-mmHg increase in mean arterial pressure; after 8 days of SQ 14225 infusion, intravenous injection of 4 pg of angiotensin I caused no significant change in mean arterial pressure. To investigate the importance of decreases in plasma aldosterone concentration in contributing to the blood pressure and renal effects of SQ 14225, an infusion of daldosterone (CIBA Pharmaceutical Co.) was begun on the eighth day of SQ 14225 infusion and continued for an additional 8 days in four dogs. The infusion rate of aldosterone (200 pg/day) was calculated to increase plasma aldosterone concentration to levels at least as high as those measured in sodium-deficient dogs before infusion of SQ 14225. After 8 days of aldosterone plus SQ 14225, the aldosterone infusion was stopped and SQ 14225 infusion was continued for an additional 8 days. Then [Asp’&#]angiotensin II (CIBA Pharmaceutical Co.) was infused at a rate of 10 ng kg-’ emin-’ along with SQ 14225 for 8 days. The total volume infused was maintained at 100 ml/day and the total sodium intake was 5 meq/day in all experiments. In another group of four sodium-deficient dogs, SQ 14225 was infused at a rate of 14 pg. kg-’ emin-’ for 8 days, and then angiotensin II was infused at a rate of 20 ngokg%nin-’ along with SQ 14225 for 5 additional days. The rate of infusion and sodium intake was maintained at 100 nil/day and 5 meq/day, respectively, in all experiments. Glomerular filtration rate and effective renal plasma flow were estimated from the clearances of [‘251]iothalal

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F426

HALL,

GUYTON,

mate (Glofil, Abbott Laboratories) and [ ‘“‘Iliodohippurate (Hippuran, Mallinckrodt Nuclear), respectively, by the method of Hall et al. (18). Plasma and urine sodium and potassium concentrations were determined by flame photometry (Instrumentation Laboratory, IL 343). Plasma renin activity was measured by radioimmunoassay of angiotensin I using a modification of the method of Haber et al. (16). Plasma aldosterone and cortisol concentrations were measured using radioimmunoassay procedures ( ‘251-aldosterone radioimmunoassay, Diagnostic Products; 1’51-cortiso1 radioimmunoassay, New England Nuclear). Control data were compared with experimental data using Dunnett’s t test for multiple comparisons (10). Statistical significance was considered to be P c 0.05. All data in the text, tables, and figures are expressed as means t SE unless otherwise indicated.

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[ Sa-l4,225ll4~g/kg/minl~

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RESULTS

Effects of SQ 14225 in sodium-deficient dogs. Plasma renin activity in sodium-deficient dogs averaged 3.90 t 0.48 ng angiotensin 1. ml-l . h-‘. In normal sodium-replete dogs, plasma renin activity averaged approximately 0.40.7 ng angiotensin I l ml-’ oh-’ in previous studies in our laboratory (8, 22, 28). The changes in mean arterial pressure, glomerular filtration rate, and effective renal plasma flow observed during 8 days of SQ 14225 infusion (14 pg. kg-’ min-‘) in sodium-deficient dogs are illustrated in Fig. 1. Mean arterial pressure averaged 90 t 1 mmHg during the 5 day control period and decreased to 73 t 2 mmHg after 1 day of SQ 14225infusion; blood pressure then decreased further and stabilized at 67-68 mmHg after 5 days of SQ 14225 infusion. Glomerular filtration rate decreased to 86 t 4% of the control value after 2 days of SQ 14225 infusion, and remained at 88-90s of the control value throughout the 8 days of SQ 14225 infusion. Despite the reduction in GFR and the large decrease in arterial pressure, ERPF increased to 120 t 4% of the control value on the first day, and remained elevated by 19-21s during 8 days of SQ 14225 infusion. Therefore, the filtration fraction (GFR/ERPF) fell markedly during SQ 14225 infusion in sodium-deficient dogs, averaging 0.39 t 0.01 during the control period and decreasing to 71-75s of the control value during chronic infusion of SQ 14225. The changes in urinary sodium and potassium excretion observed during chronic infusion of SQ 14225 in sodium-deficient dogs are shown in Fig. 2. Urinary sodium excretion increased from a control value of 1.75 t 0.19 to 3.10 t 0.40 meq/day after 1 day of SQ 14225 infusion, and remained slightly elevated throughout the 8 days of converting enzyme inhibition. However, urinary potassium excretion was not altered significantly during SQ 14225 infusion. The changes in plasma aldosterone, sodium, and potassium concentrations that occurred in sodium-deficient dogs during infusion of SQ 14225 are shown in Fig. 3. Plasma aldosterone concentration decreased gradually from a control value of 64 t 6 to 41 t 5 ng/dl after 7 days of SQ 14225 infusion. Plasma cortisol concentration

3

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TIME (days) FIG. 1. Effects of chronic intravenous infusion of SQ 14225 on mean arterial pressure (MAP), glomerular filtration rate (GFR), and effective renal plasma flow (ERPF) in sodium-deficient dogs. Values are means +. SE. 1 SQ-14,225(14Fg/kg/min) 4.0 r

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FIG.

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averaged 1.4 t 0.1 pg/dl during the control period and increased slightly to 2.1 t 0.1 pg/dl after 8 days of SQ 14225 infusion. Plasma sodium concentration decreased significantly during chronic infusion of SQ 14225, averaging 141.3 t 0.5 during the control period and 137.1 t 0.5 meq/liter after 8 days of SQ 14225 infusion. However, plasma potassium concentration was not significantly changed by infusion of SQ 14225. Effects of SQ 14225plus aldosterone, or SQ 14225plus angiotensin II in sodium-deficient dogs. The changes in mean arterial pressure, effective renal plasma flow, and glomerular filtration rate that occurred during infusion of aldosterone (200 pg/day) or angiotensin II (10 ng + kg -’ emin-‘) along with SQ 14225 are shown in Fig. 4. After 8 days of SQ 14225infusion, mean arterial pressure decreased from a control value of 102 t 5 to 71 t 6

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CHRONIC

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TIME [days] 5, Effects of chronic intravenous infusion of SQ 14225, SQ 14225 plus aldosterone, and SQ 14225 plus angiotensin II on plasma aldosterone (PAC), cortisol (PCC), sodium (PNJ, and potassium (PK) concentrations in sodium-deficient dogs. Values are means + SE.

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infusion, plasma aldosterone fell to 26.8 t 2.4 ng/dl, and after 8 days of SQ 14225 plus aldosterone (200 pg/day), ERPF plasma aldosterone concentration averaged 60.2 t 5.7 1% control]’ ‘O ng/dl. Accordingly, even though plasma aldosterone con100 centration was restored to levels actually above those measured in sodium-depleted dogs before SQ 14225 infusion, the arterial pressure and renal hemodynamic re100 sponsesto SQ 14225 infusion were not altered. Likewise, 6fR go infusion of aldosterone did not correct the hyponatremia 1% 80 caused by infusion of SQ 14225 (Fig. 5) and did not significantly decrease average urinary sodium excretion 70 (2.20 t 0.31 meq/day during aldosterone plus SQ 14225 TIME [days] versus 2.92 t 0.69 meq/day during SQ 14225 infusion). FIG. 4. Effects of chronic intravenous infusion of SQ 14225, SQ Urinary potassium excretion also did not change signifi14225 plus aldosterone, and SQ 14225 plus angiotensin II on mean cantly during aldosterone replacement, averaging 39.4 t arterial pressure (MAP), effective renal plasma flow (ERPF), and glomerular filtration rate (GFR) in sodium-deficient dogs. Values are 5.4 meq/day during SQ 14225 infusion, and 40.7 t 4.7 means t SE. meq/day during infusion of SQ 14225 plus aldosterone. When the infusion of aldosterone was stopped and SQ mmHg, ERPF increased to 119 t 8% of control, and GFR 14225 infusion was continued for 8 more days, plasma decreased to 78 t 8% of control. When aldosterone was aldosterone concentration decreased to 24.5 t 7.2 ng/dl infused at a rate of’200 lug/day along with SQ 14225 for (Fig. 5). However, blood pressure, ERPF, GFR, and 8 days, blood pressure, ERPF, and GFR did not change plasma electrolyte concentrations were not significantly significantly from the levels measured after 8 days of SQ different from the values measured during infusion of SQ 14225 infusion alone. However, this infusion of aldoster14225 plus aldosterone, or during the initial period of one increased plasma aldosterone concentration to levels infusion of SQ 14225 alone (Figs. 4 and 5). Urinary above those observed in sodium-deficient dogs prior to sodium and potassium excretion, however, tended to infusion of SQ 14225 (Fig. 5). During the control period decrease progressively with time, probably due to the (after 3-4 wk on a sodium-deficient diet), plasma aldos- maintenance of the low sodium intake and the initial loss terone averaged 44.5 t 4.5 ng/dl; after 8 days of SQ 14225 of sodium in the first few days of SQ 14225 infusion. 120

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F428

HALL,

During the &day period when SQ 14225 infusion was continued but aldosterone infusion was stopped, urinary sodium and potassium excretion averaged 1.93 t 0.29 and 31.8 t 2.3 meq/day, respectively. To determine whether the blood pressure, renal, and adrenal effects of SQ 14225could be reversed by restoring circulating levels of AII, an infusion of AII was subsequently initiated at a rate of 10 ng kg-‘. min-’ while the SQ 14225infusion was maintained (Figs. 4 and 5). Arterial pressure increased from 70 t 5 mmHg during SQ 14225 infusion to 92 t 6 mmHg during SQ 14225 plus AII; this level of arterial pressure was only slightly below the level observed prior to infusion of SQ 14225 (102 t 5 mmHg). Infusion of AI1 decreased effective renal plasma flow to a level not significantly different from that observed before infusion of SQ 14225. Despite the reduction in effective renal plasma flow, GFR increased from 78 t 5% of the control value during SQ 14225 infusion to 89 t 5% of the control value after 8 days of SQ 14225 plus AI1 infusion. Therefore, infusion of SQ 14225plus AI1 caused a marked increase in the filtration fraction above the level observed during SQ 14225infusion alone. Infusion of SQ 14225 plus AI1 (10 ng kg-’ min-‘) for 8 days also increased plasma aldosterone concentration from 24.5 t 7.2 during SQ 14225 to 39.5 t 8.4 ng/dl. Plasma cortisol, sodium, and potassium concentrations were not significantly different during SQ 14225 plus AI1 than during SQ 14225 infusion alone. There was also a small but significant decrease in average urinary sodium excretion to 1.41 t 0.12 meq/day during SQ 14225 plus AII, but potassium excretion did not change significantly, averaging 37.4 t 5.0 meq/day. In a separate series of experiments, AI1 was infused at a rate of 20 ng.kg-’ emin-’ for 5 days along with SQ 14225 (Figs. 6 and 7) after periods for control observations and

GUYTON,

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SMITH,

AND

COLEMAN

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TIME [days) FIG. 6. Effects of chronic intravenous 14225 plus angiotensin II on mean arterial

filtration rate dium-deficient

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infusion of SQ 14225 and SQ pressure (MAP), glomerular (GFR), and effective renal plasma flow (ERPF) in sodogs. Values are means k SE.

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TIME [days) FIG. 7. Effects of chronic intravenous infusion of SQ 14225 14225 plus angiotensin II on plasma aldosterone (PAC), cortisol

sodium SE.

(P&,

and potassium

(PK) concentrations.

Values

and SQ (PCC), are means +,

infusion of SQ 14225 alone. In these experiments, as in our previous series, infusion of SQ 14225 alone caused a marked reduction in mean arterial pressure, GFR, and plasma aldosterone concentration, while increasing effective renal plasma flow. Urinary sodium excretion increased from 1.7 t 0.2 meq/day during the control period to an average of 3.8 t 0.6 meq/day during SQ 14225 infusion. Potassium excretion did not change markedly, averaging 42.6 t 3.0 during the control period and 51.1 t 3.1 meq/day during SQ 14225 infusion. Infusion of SQ 14225 plus AI1 (20 ng kg-’ min-‘) increased blood pressure and plasma aldosterone concentration to levels slightly above control and decreased effective renal plasma flow to levels slightly below control. Even though effective renal plasma flow was greatly reduced by AI1 infusion, GFR increased slightly above the level observed during SQ 14225 infusion; thus the filtration fraction was not different from that measured during the control period. Urinary sodium excretion decreased to 1.8 t 0.2 meq/day during SQ 14225 plus AI1 infusion, while urinary potassium excretion did not change significantly, averaging 46.6 t 5.0 meq/day. l

loo MAP ImmM

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SO-14,225(14~g/kg/mm)

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DISCUSSION

Blockade of angiotensin II formation with SQ 14225 in conscious sodium-depleted dogs caused a rapid reduction in blood pressure followed by a slower decline that plateaued in 4-6 days. Similar findings in sodium-depleted rats have recently been reported by Bengis et al. (2). The present study provides additional documentation in intact conscious dogs of the importance of the renin-angiotensin system in the long-term regulation of blood pressure during chronic sodium deprivation. But more importantly, the present study provides quantitative infor-

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mation about the mechanisms by which the renin-angiotensin system controls blood pressure and renal function during chronic changes in sodium intake. Data from this study suggest that the changes in aldosterone secreti .on which occur as a result of increased formation of AI1 do not play a major role in regulating blood pressure and renal function during chronic sodium deprivation when compared to the effects of AI1 mediated by other mechanisms. Chronic blockade of AI1 formation with SQ 14225 in sodium-deficient dogs caused a substantial reduction in plasma aldosterone concentration as welI as a reduction in blood pressure and an increase in renal excretion of sodium. However, when aldosterone was infused with the SQ 14225 infusion for 8 days to restore plasma aldosterone concentration to levels higher than those observed prior to converting enzyme inhibition, blood pressure remained unchanged from the hypotensive levels observed during infusion of SQ 14225 alone. Furthermore, restoration of plasma aldosterone concentration to levels above control did not significantly alter effective renal plasma flow, GFR, urinary sodium excretion, or plasma electrolyte concentrations. Thus, data from the present study provide no evidence that the effects of angiotensin converting enzyme inhibition on blood pressure and renal function during sodium deprivation are mediated to a major extent by changes in aldosterone secretion. These findings differ somewhat from those of McCaa et al. (29) who reported that infusion of aldosterone (10 pg. kg-’ day-.‘) completely reversed the effect of SQ 14225 on blood pressure in sodium-deficient dogs, even though urinary sodium excretion was not altered by aldosterone replacement. An explanation for this discrepancy is not readily apparent, but may be related to differences in experimental procedure. In the present study, blood pressure was measured continuously 24 h a day, whereas in the study by McCaa et al. blood pressure was measured only l-2 h each day. Previous studies have indicated that blood pressure may fluctuate markedly from hour to hour in dogs, so that l-2 h of blood pressure recording may not provide an accurate assessment of the daily blood pressure under certain experimental conditions (6). Also, in the present study SQ 14225 was infused continuously 24 h a day so that a relatively constant blood level of the converting enzyme inhibitor was achieved. In the study by McCaa et al., SQ 14225 was administered orally only 2 times daily, and it is possible that blood levels fluctuated markedly during the day. When SQ 14225, or SQ 14225 plus aldosterone were infused continuously and mean arterial pressure was measured continuously in the present study, we found no evidence that the blood pressure or renal effects of SQ 14225 in sodium-deficient dogs were mediated by changes in plasma aldosterone concentration. Obviously our findings do not necessarily imply that changes in aldosterone secretion are unimportant in regulating blood pressure and renal excretion during sodium deprivation, or that the renin-angiotensin system is unimportant in regulating aldosterone secretion during sodium deprivation. In fact, during chronic blockade of AI1 formation, plasma aldosterone concentration decreased by approximately 36-40s. But, since plasma aldosterone l

remained greatly elevated above normal sodium-replete levels even after 8 days of angiotensin converting enzyme inhibition, it is clear that other mechanisms, besides increased formation of AII, are also involved in stimulating aldosterone secretion during sodium deprivation. It is also evident from the data of the present study that while the renin-angiotensin system is one of the principal compensatory mechanisms for regulating blood pressure and renal function during sodium deprivation, it does not mediate a major part of its effects through changes in aldosterone secretion. In addition to blocking AI1 formation and reducing plasma aldosterone concentration, inhibition of angiotensin converting enzyme has also been reported to increase plasma levels of bradykinin, a potentially important vasodilator (29). McCaa et al. (29) suggested that the increased blood levels of bradykinin observed during oral administration of SQ 14225 could mediate part of the increased urinary sodium excretion caused by converting enzyme inhibition. It is possible, therefore, that part of the effect of SQ 14225in reducing blood pressure, causing renal vasodilation, and increasing urinary sodium excretion is due to increased plasma levels of bradykinin. Although infusion of pharmacological amounts of bradykinin causes renal vasodilation and increased urinary excretion (1, 31), there has been no direct evidence that endogenous levels of bradykinin are sufficient to cause marked effects on blood pressure and renal function chronically. In fact, Jaeger et al. (24) found no evidence in sodium-depleted normal or spontaneously hypertensive rats that the angiotensin converting enzyme inhibitor, teprotide, exerts its effects on blood pressure via a mechanism independent of AI1 blockade. In an attempt to determine whether the effects of the converting enzyme inhibitor on blood pressure and renal function were due to inhibition of AI1 formation or to the vasodilator effect of bradykinin potentiation during SQ 14225 infusion, we infused AII along with the converting enzyme inhibitor at a rate of 10 ngakg-’ emin-’ for 8 days in four dogs, and at a rate of 20 ng . kg-’ min-’ for 5 days in four additional dogs. The lower rate of AI1 infusion almost completely reversed the effects of SQ 14225 on blood pressure, plasma aldosterone, and renal function. Blood pressure and plasma aldosterone concentration were only 10 and ll%, respectively, below the control levels of sodium-deficient dogs. Infusion of AI1 at a rate of 20 ng kg-‘. min-’ increased blood pressure and plasma aldosterone concentration to slightly above control, and also restored renal function toward control levels. Infusion of 10 ng .kg-’ .min-’ of AI1 was calculated to increase plasma concentration of AI1 to approximately 100-150 pmol/liter (4, 30), a value well within the range of AI1 levels reported for dogs or man during sodium deficiency or renal artery constriction (4, 30). However, the observation that the blood pressure, and renal and aldosterone effects of SQ 14225 are reversed by infusion of physiological amounts of AI1 does not constitute direct evidence that SQ 14225 acts solely by blockade of AI1 formation. Further insight can be gained by comparing the blood pressure and aldosterone responsesto converting enzyme inhibition and AI1 infusion. If endogenously produced l

l

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F430 to reduce bradykinin is acting as a potent vasodilator blood pressure after converting enzyme inhibition, then it seems likely that complete restoration of arterial pressure to the control level bY infusion of AI1 would require a greater amount of AI1 tha .n would be required to restore plasma aldosterone concentration to the control level, assuming that increased plasma bradykinin does not simultaneously decrease aldosterone secretion. To our knowledge there is no evidence that increased plasma levels of bradykinin decrease aldosterone secretion; in fact, infusion of bradykinin has been reported to cause either an increase or no change in aldosterone secretion (3). Since proportional increases in plasma aldosterone concentration and blood pressure were observed after infusion of 10 or 20 ng . kg-’ emin-’ of AI1 along with SQ 14225, the data from the present study are not consistent with the hypothesis that the blood pressure and renal effects of SQ 14225 are due to a major extent to bradykinin potentiation. There are several other potentially more important mechanisms besides changes in aldosterone secretion or increased plasma levels of bradykinin to link infusion of SQ 14225 and blood pressure reductions in sodium-deficient dogs. One possibility, which would account for the rapid reduction in blood pressure observed during SQ 14225 infusion, is that converting enzyme inhibition prevented the vasoconstrictor effect of AI1 on peripheral arterioles. However, when arterial pressure is chronically reduced by a fall in total peripheral resistance without a simultaneous decrease in renal vascular resistance, sodium excretion usually decreases, extracellular fluid volume increases, and there is at least a partial restoration of blood pressure toward the normal level (5, 15). In the present study, the hypotension observed during infusion of SQ 14225 was followed by an increased sodium excretion, rather than additional sodium retention, and a further slow decline in arterial pressure. These observations suggest that other mechanisms in addition to dilation of peripheral arterioles are involved in causing chronic hypotension during SQ 14225 infusion in sodiumdeficient dogs. Another mechanism by which SQ 14225 infusion could help to sustain a reduction in arterial pressure is by blocking the antinatriuretic effec t of AII. In recent shortterm studies from our laboratories (17, 20, 28) and by other investigators (26), intrarenal infusion of AI1 antagonist or the angiotensin converting enzyme inhibitor teprotide caused a marked increase in renal blood flow as well as urinary sodium excretion but did not change plasma aldosterone concentration. The data indicate that endogenously produced AI1 has an important antinatriuretic action mediated either through changes in renal hemodynamics or renal tubular reabsorption, or both. However, prior to the present study, it was not clear whether the effects of AI1 blockade on renal hemodynamits and sodium excretion could be maintained chronically, or if additional compensatory mechanisms would be activated to restore the normal arterial pressure-urinary output relationship. Data from the present study indicate that renal vasodilation and increased capability of the kidneys to excrete sodium are sustained during chronic blockade of AI1 formation. Furthermore, the

HALL,

GUYTON,

SMITH,

AND

COLEMAN

direct effects of AI1 on renal function and chronic blood pressure regulation during sodium deprivation apnear to I be much more important, quantitatively, than the effects of AI1 mediated by changes in aldosterone secretion. The intrarenal mechanisms by which blockade of AI1 formation tends to increase urinary sodium excretion are not entirely clear. Transiently, before blood pressure decreases markedly, infusion of AI1 antagonist or converting enzyme inhibitor greatly increases renal excretion (20, 28). However, as blood pressure decreased in the present study, renal excretion of sodium must also have decreased somewhat, so that after several days of converting enzyme inhibition, urinary sodium excretion was only slightly elevated above control values for sodiumdeficient dogs. This small increase in sodium excretion, however, may represent a large increase in renal excretory capability, since elevated rates of sodium excretion were accomplished despite large decreases in arterial pressure, which normally would tend to greatly depress sodium excretion (15, 19, 21). The enhanced capability to excrete sodium after SQ 14225 was not due to an increase in filtered sodium load, since both GFR and plasma sodium concentration were substantialIy reduced during chronic converting enzyme inhibition. Accordingly, during long-term blockade of AI1 formation in sodium-deficient dogs, there is a marked decrease in absolute renal tubular sodium reabsorption that is not cau .sed by a reduction in plasma aldosterone because aldosterone infusion does not reconcentration, store GFR or absolute tubular sodium reabsorption to control levels. Whether this reduction in sodium reabsorption caused by converting enzyme inhibition occurs secondarily to a reduced GFR and increased effective renal plasma flow, or occurs as a direct effect of AI1 blockade on the renal tubule cannot be determined from the results of the present study. A reduction in GFR alone would tend to decrease sodium reabsorption if the tubules are capable of automatically adjusting their reabsorption rate to changes in filtered load (glomerulotubular balance) (14). However, this adjustment of the tubules to changes in GFR would not cause sodium excretion to increase above control levels. A reduction in filtration fraction, due to a decreased GFR and an increased renal plasma flow, could also decrease sodium reabsorption by decreasing peritubular capillary oncotic pressure and perhaps also by increasing peritubular capillary hydrostatic pressure (11). Finally, the enhanced capability to excrete sodium after SQ 14225 may be due in part to blockade of a direct effect of AI1 on the renal tubules; in fact, several recent short-term studies suggest that AI1 does directly increase renal tubular sodium reabsorption (12, 25). Whatever the exact intrarenal mechanisms responsible for the tendency to increase sodium excretion after SQ 14225, the data from the present study indicate that this effect is not due to a reduction in plasma aldosterone concentration or to an increase in filtered sodium load. Another important finding of the present study is that during chronic infusion of SQ 14225, regulation of effective renal plasma flow and GFR were markedly dissociated; although GFR decreased as blood pressure was reduced by converting enzyme inhibition, effective renal

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CHRONIC

BLOCKADE

OF

ANGIOTENSIN

F431

II FORMATION

plasma flow actually increased by approximately 20%. Therefore, the filtration fraction decreased by 2529% during chronic converting enzyme inhibition. In previous short-term studies using anesthetized dogs, we postulated that uncoupling of renal blood flow and GFR autoregulation in dogs with a nonfunctional renin-angiotensin system was due to blockade of the vasoconstrictor effect of AI1 on efferent arterioles. Calculations of efferent arteriolar resistance, based on indirect methods we have previously described (19,21), indicate that during chronic infusion of SQ 14225 in the present study efferent arteriolar resistance was reduced by approximately 3548%. Infusion of exogenous angiotensin II at a rate of 10 or 20 ng* kg-’ emin-’ completely restored the filtration fraction and calculated efferent arteriolar resistance, but did not completely restore GFR to control levels, possibly because these rates of AI1 infusion reduced effective renal plasma flow to levels that were actually slightly below the control levels. In summary, data from the present study indicate that the renin-angiotensin system plays a major role in main-

taining blood pressure and GFR during chronic sodium deprivation, while decreasing renal plasma flow and helping to conserve sodium. An important component of the long-term control of GFR and sodium excretion by the renin-angiotensin system may involve an efferent arteriolar vasoconstrictor mechanism. The results of this study also suggest that the chronic effects of angiotensin converting enzyme inhibition on blood pressure, renal hemodynamics, and sodium excretion during sodium deprivation are not primarily mediated by changes in aldosterone secretion.

We thank Dr. Zola P. Horovitz of the Squibb Institute, Princeton, NJ for the generous supply of SQ 14225 used in these studies, and Mr. Douglas N. Packer for excellent technical assistance. This work was supported by National Institutes of Health Grants HL 23502 and HL 11678 and by a grant from the Mississippi Heart Association. J. E. Hall is the recipient of National Institutes of Health Research Career Development Award K04 HL 00502. Received

26 February

1979; accepted

in final form

3 August

1979.

REFERENCES 1. BARRACLOUGH, M. A., AND I. H. MILLS. Effect of bradykinin on renal function. Clin. Sci. 28: 69, 1965. 2. BENGIS, R. G., T. G. COLEMAN, D. B. YOUNG, AND R. E. MCCAA. Long-term blockade of angiotensin formation in various normotensive and hypertensive rat models using converting enzyme inhibitor @Q-14225). Circ. Res. 43, Suppl. I: 145-153, 1978. 3. BLAIR-WEST, J. R., J. P. COGHLAN, D. A. DENTON, J. R. GODING, J. A. MUNRO, R. E. PETERSON, AND M. WINTOUR. Humoral stimulation of adrenal cortical secretion. J. Clin. Inuest. 41: 1606-1627, 1962. 4. CARAVAGGI, A. M., G. BIANCHI, J. J. BROWN, A. F. LEVER, J. J. MORTON, J. D. POWELL-JACKSON, J. I. S. ROBERTSON, AND P. F. SEMPLE. Blood pressure and plasma angiotensin II concentration after renal artery constriction and angiotensin infusion in the dog: [5-isoleucine] angiotensin II and its breakdown fragments in the dog blood. Circ. Res. 38: 315-321, 1976. 5. COLEMAN, T. G., A. W. COWLEY, JR., AND A. C. GUYTON. Experimental hypertension and the long-term control of arterial pressure. In: International Review of Science. Cardiovascular Physiology, edited by A. C. Guyton and C. E. Jones. Baltimore: Univ. Park Press, 1975, vol. 1, p. 259-297. 6. COWLEY, A. W., JR., J. F. LIARD, AND A. C. GUYTON. Role of the baroreceptor reflex in daily control of arterial blood pressure and other variables in the dog. Circ. Res. 32: 544-546, 1975. 7. DAVIS, J. 0. Regulation of aldosterone secretion. In: Handbook of PhysioZogy. Endocrinology, edited by H. Blaschko, G. Sayers, and A. D. Smith. Washington, DC: Am. Physiol. Sot., 1975, vol. VI, p. 77-106. 8. DECLUE, J. W., A. C. GUYTON, A. W. COWLEY, JR., T. G. COLEMAN, R. A. NORMAN, AND R. E. MCCAA. Subpressor angiotensin infusion, renal sodium handling and salt-induced hypertension. Circ. Res. 43: 503-512, 1978. 9. DICKINSON, C. J., AND R. Yu. Mechanism involved in the progressive pressor response to very sm!ll quantities of angiotensin. Circ. Res. 21, Suppl. II: 157-165, 1967. 10. DUNNETT, C. W. A multiple comparison procedure for comparing several treatments with a control. J. Am. Stat. Assoc. 50: 10961121,1955. 11. EARLEY, L. E., AND R. W. SCHRIER. Intrarenal control of sodium excretion by hemodynamic and physical factors. In: Handbook of Physiology. Renal Physiology, edited by J. Orloff and R. W. Berliner. Washington, DC: Am. Physiol. Sot., 1973, p. 721-762. 12. FREEDLANDER, A. B., AND T. L. GOODFRIEND. Angiotensin receptors and sodium transport in renal tubules (Abstract). Federation Proc. 36: 481, 1977. 13. FREEMAN, R. H., J. 0. DAVIS, AND W. S. SPIELMAN. Renin-angiotensin system and aldosterone secretion during aortic constriction

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

in the rat. Am. J. Physiol. 232: F434-F437,1977 or Am. J. Physiol.: RenaZ FZuid Electrolyte PhysioZ. 1: F434-F437, 1977. GERTZ, K. H., AND J. W. BOYLAN. Glomerulo-tubular balance. In: Handbook of Physiology. Renal Physiology, edited by J. Orloff and R. W. Berliner. Washington, DC: Am. Physiol. Sot., 1973, p. 763-790. GUYTON, A. C., A. W. COWLEY, JR., D. B. YOUNG, T. G. COLEMAN, J. E. HALL, AND J. W. DECLUE. Integration and control of circulatory function. In: International Review of Physiology. Cardiovascular Physiology II, edited by A. C. Guyton and A. W. Cowley, Jr. Baltimore: Univ. Park Press, 1976, vol. 9, p. 341-385. HABER, E. T., T. KOERNER, L. B. PAGE, B. KLIMAN, AND A. PURNODE. Application of radioimmunoassay of angiotensin I to the physiologic measurements of plasma renin activity in normal human subjects. J. CZin. EndocrinoZ. 29: 1349-1355, 1969. HALL, J. E., T. G. COLEMAN, A. C. GUYTON, J. W. BALFE, AND H. C. SALGADO. Intrarenal role of angiotensin II and [Des-Asp’langiotensin II. Am. J. Physiol. 236: F252-F259, 1979 or Am. J. PhysioZ.: Renal Fluid EZectroZyte Physiol. 5: F252-F259, 1979. HALL, J. E., A. C. GUYTON, AND B. M. FARR. A single-injection method for measuring glomerular filtration rate. Am. J. Physiol. 232: F72-F76, 1977 or Am. J. Physiol.: Renal Fluid Electrolyte Physiol. 1: F72-F76, 1977. HALL, J. E., A. C. GUYTON, AND A. W. COWLEY, JR. Dissociation of renal blood flow and filtration rate autoregulation by renin depletion. Am. J. Physiol. 232: F215-F221, 1977 or Am. J. Physiol.: Renal Fluid Electrolyte Physiol. 1: F215-F221, 1977. HALL, J. E., A. C. GUYTON, N. C. TRIPPODO, T. E. LOHMEIER, R. E. MCCAA, AND A. W. COWLEY, JR. Intrarenal control of electrolyte excretion by angiotensin II. Am. J. Physiol. 232: F538-F544, 1977 or Am. J. Physiol.: Renal FZuid Electrolyte Physiol. 1: F538-F544, 1977. HALL, J. E., A. C. GUYTON, T. E. JACKSON, T. G. COLEMAN, T. E. LOHMEIER, AND N. C. TRIPPODO. Control of glomerular filtration rate by renin-angiotensin system. Am. J. Physiol. 233: F366-F372, 1977 or Am. J. Physiol.: Renal Fluid Electrolyte Physiol. 2: F366F372, 1977. HALL, J. E., A. C. GUYTON, H. C. SALGADO, R. E. MCCAA, AND J. W. BALFE. Renal hemodynamics in acute and chronic angiotensin hypertension. Am. J. Physiol. 235: F174-F179, 1978 or Am. J. Physiol.: Renal FZuid Electrolyte Physiol. 4: F174-F179, 1978. HARRIS, P. J., AND J. A. YOUNG. Dose dependent stimulation and inhibition of proximal tubular sodium reabsorption by angiotensin II in the rat kidney. Pfluegers Arch. 367: 295-297, 1977. JAEGER, P., R. K. FERGUSON, H. R. BRUNNER, E. J. KIRCHERTZ, AND H. GAVRAS. Mechanism of blood pressure reduction by teprotide (SQ-20881) in rats. Kidney Int. 13: 289-296, 1978.

Downloaded from www.physiology.org/journal/ajprenal by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 21, 2019.

F432 25. JOHNSON, M. D., AND R. L. MALVIN. Stimulation of renal sodium reabsorption by angiotensin II. Am. J. Physiol. 232: F298-F306, 1977 or Am. J. Physiol.: RenaZ Fluid ElectroZyte Physiol. 1: F298F306, 1977. 26. KIMBROUGH, H. M., JR., D. VAUGHAN, JR., R. M. CAREY, AND C. R. AYERS. Effect of intrarenal angiotensin II blockade on renal function in conscious dogs. Circ. Res. 40: 174-178, 1977. 27. LARAGH, J. H., AND J. E. SEALEY. The renin-angiotensin-aldosterone hormonal system and regulation of sodium, potassium, and blood pressure homeostasis. In: Handbook of Physiology. RenaZ Physiology, edited by J. Orloff and R. W. Berliner. Washington, DC: Am. Physiol. Sot., 1973, p. 831-908. 28. LOHMEIER, T. E.,bA. W. COWLEY, JR., N. C. TRIPPODO, J. E. HALL, AND A. C. GUYTON. Effects of endogenous angiotensin II on renal sodium excretion and renal hemodynamics. Am. J. PhysioZ. 233:

HALL,

GUYTON,

SMITH,

AND

COLEMAN

F388-F395,1977 or Am. J. Physiol.: RenaZ FZuid Electrolyte PhysioZ. 2: F388-F395, 1977. 29. MCCAA, R. E., J. E. HALL, AND C. S. MCCAA. The effects of angiotensin I-converting enzyme inhibitors on arterial blood pressure and urinary sodium excretion: role of the renal renin-angiotensin and kallikrein-kinin systems. Circ. Res. 43, Suppl. I: 132-139, 1978. 30. SEMPLE, P. F., A. S. BOYD, P. M. DAWES, AND J. J. MORTON. Angiotensin II and its heptapeptide (2-8)) hexapeptide (3-8)) and pentapeptide (4-8) metabolites in arterial and venous blood of man. Circ. Res. 39: 671-678, 1976. 31. STEIN, J. R., R. C. CONGBALAY, D. L. KARSH, R. W. OSGOOD, AND T. E. FERRIS. The effect of bradykinin on proximal tubular sodium reabsorption in the dog: evidence for functional nephron heterogeneity. J. CZin. Invest.: 51: 1709-1721, 1972.

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