Comparison of a New Renin Inhibitor and Enalaprilat in Renal Hypertensive Dogs

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1 Comparison of a New Renin Inhibitor and Enalaprilat in Renal Hypertensive Dogs SHALER G. SMITH III, ANDREA A. SEYMOUR, ELAINE K. MAZACK, JOSHUA BOGER, AND EDWARD H. BLAINE SUMMARY The hypotensive efficacy of a potent new renin inhibitor (iva-isovaleryl-l-histidyl-lprolyl-l-phenylalanyl-l-hlstidyl-achpa-l-leucyl-l-phenylalanyl amide) containing (S,S)--amino--cyclohexyl--hydroxy pentanoic acid (ACHPA) was compared with the converting enzyme inhibitor (enalaprilat) (MK-22) in conscious one-kidney dogs before and after tightening a renal artery clamp. Dose-response curves to 0.00 to 0.1 mg/kg/min i.v. infusions of the ACHPA-containing renin inhibitory peptide or enalaprilat ( mg/kg i.v. bolus) were obtained in one-kidney dogs before and days and days after renal artery constriction. The ACHPA-containing renin inhibitory peptide and enalaprilat maximally decreased blood pressure by 10 ± 2 and 9 ± 2 mm Hg before constriction and by ± 2 and ± mm Hg in dogs treated days after renal artery constriction, respectively. Glomerular filtration rate and effective renal plasma flow were unaltered or slightly improved. In sharp contrast, both compounds elicited significant, dose-related decreases in blood pressure ( 26 ± and 20 ± mm Hg, respectively), glomerular filtration rate ( 21 ± and 2 ± ml/min), and renal plasma flow (- ± and - 8 ± 1 ml/min) in dogs examined days after renal artery constriction. These data demonstrate that ACHPA-containing renin inhibitory peptide and enalaprilat are equally effective antihypertensive agents in dogs with renin-dependent renovascular hypertension and lend credence to the contention that the renin-angiotensin system supports renal function in hypertensive states in which renin levels are elevated. (Hypertension 9: 106, 1987) KEY WORDS blood pressure renal function renin-angiotensin system IN experimental renovascular hypertension produced by partial constriction of the remaining renal artery in unilaterally nephrectomized animals, mean arterial pressure (MAP) rises in concert with plasma renin activity (PRA). 1 ' The PRA in these animals remains elevated for approximately 1 week and then returns to normal levels, even though the hypertension is sustained.' 2 Until recently, examinations of the involvement of the renin-angiotensin system in the pathogenesis of experimental renovascular From the Departments of Pharmacology and Medicinal Chemistry, Merck Institute for Therapeutic Research, West Point, Pennsylvania. Present address for Dr. Seymour The Squibb Institute for Medical Research, P.O. Box 000, Princeton, NJ Present address for Dr. Blaine: Dept. of Pharmacology, Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, MO 60. Address for reprints: Shaler G. Smith III, Merck Institute for Therapeutic Research, W26-208, West Point, PA Received January 27, 1986; accepted August 20, hypertension primarily have employed competitive angiotensin II antagonists or angiotensin I (ANG I) converting enzyme (ACE) inhibitors to interfere with the pressor actions or formation of angiotensin II. " 9 Administration of angiotensin II antagonists or ACE inhibitors lowers blood pressure in all of the animal models in which the elevated pressure is accompanied by hyperreninemia. 2 ' 6^> Although the renin-angiotensin system undoubtedly participates in the pathogenesis of experimental renovascular hypertension, the exact role played by angiotensin II has been difficult to elucidate. The angiotensin II analogues, frequently used as competitive inhibitors of the endogenous hormone, possess both antagonistic and agonistic activity in vitro, and thus the effect of complete angiotensin II blockade cannot be appreciated fully. 6 ' 10 Additionally, the ACE inhibitors may increase the plasma levels of the vasodilative kinins, since converting enzyme (kininase II) also degrades kinins." 1 Definition of the exact contribution of the renin-angiotensin system therefore requires a selective blockade. 10

2 COMPARISON OF A RENIN INHIBITOR AND ENALAPRILAT/Sm/r/i et al. Recently, several groups have reported that some renin substrate analogues show a high degree of specificity as competitive renin inhibitors in vitro and are effective hypotensive agents in rats in which blood pressure is supported by an infusion of exogenous renin, 1 ' H in sodium-deficient dogs, " 16 in normotensive and hypertensive sodium-deficient monkeys, 17 and in sodium-deficient humans. 18 One such peptide contains the unusual amino acid statine, a feature that contributes to its efficacy as a renin inhibitor. 1 Replacement of statine with (S,S)--amino--cyclohexyl--hydroxy pentanoic acid (ACHPA) has led to a new series of peptides possessing a more potent activity profile. Afa-Isovaleryl-L-histidyl-L-prolyl-L-phenylalanyl-L- histidyl-(s,s)-[-amino--cyclohexyl--hydroxy]- pentanoyl-l-leucyl-l-phenylalanyl amide is a potent new ACHPA-containing renin inhibitory peptide (ACRIP) that has been shown to specifically inhibit rat, rabbit, dog, and human plasma renins in vitro (IC 0 = 21 nm, 0. nm, 1.9 nm, and 0.17 nm, respectively). 19 The present study compared the cardiovascular and renal effects of ACRIP with those of enalaprilat (MK-22), an ACE inhibitor, in conscious dogs. To evaluate ACRIP in renin-dependent and renin-independent hypertensive states, the compound was infused into conscious one-kidney dogs on three occasions: 1) before renal artery constriction (RAC), 2) during the initial phase of experimental renovascular hypertension when PRA was elevated, and ) during chronic experimental hypertension after PRA had returned to normal levels. The hypotensive activity of ACRIP was comparable to that of the ACE inhibitor, enalaprilat {N[(S) -carboxy--phenylpropyl]-l-ala- L-Pro) at all stages of the experiment and was most evident during the early phase of experimental renovascular hypertension. Materials and Methods Conditioned female mongrel dogs (Haycock Farms, Quakertown, PA, USA) weighing 10 to 2 kg and maintained on a normal sodium diet (60-6 meq/day) were unilaterially nephrectomized under sterile conditions using halothane/no/o 2 anesthesia. Chronic indwelling iliac arterial and venous Tygon catheters were implanted, and the free ends were exteriorized between the shoulder blades. The dogs were allowed to recover for several days, during which time they were trained to stand calmly in nylon mesh support slings (Alice King Chatham, Los Angeles, CA, USA). Two to weeks later, the renal artery of the remaining kidney was exposed through a flank incision made under sterile operating conditions. An adjustable clamp was placed around the artery, and an electromagnetic flow probe (Carolina Medical Instruments, King, NC, USA) temporarily was positioned distal to the clamp to measure renal blood flow. Once flow had stabilized, the clamp was tightened until the flow was reduced by to 60%, a procedure previously reported to produce benign renal hypertension in one-kidney dogs within 2 hours. The flow probe was removed, and the animal was allowed to recover. Each animal was treated with several predetermined doses of either ACRIP or enalaprilat on three occasions: 1) when the dog had recovered from the unilateral nephrectomy, 2) days after RAC, and ) days after RAC. For each dose-response determination, the dog was placed in a support sling and a sterile Foley catheter was inserted into the bladder for the collection of timed urine samples. At least 1 hour before beginning each experiment, a priming injection containing creatinine (0 mg/kg) andp-aminohippurate (PAH), 8 mg/kg, in 0.9% sterile saline was administered as a bolus into the indwelling venous catheter and followed by a continuous 1 ml/min sustaining infusion of creatinine and PAH (1.0 and 0. mg/kg/min, respectively). After the 1 hour equilibration period, the bladder was emptied andrinsedwith 2 ml of sterile distilled water, and the experiment was begun. Once two 0-minute control urine collections were obtained, four doses of the test compound were administered at 1 hour intervals, during which two 0-minute clearance periods were observed at each dose. ACRIP was added to the creatinine-pah solution and, because of its relatively short half-life, was infused continuously for an hour at rates of 0.00, 0.01, 0.0, and 0.1 mg/kg/min. Enalaprilat was dissolved in 0.9% saline and injected intravenously as a bolus for cumulative doses of 0.00, 0.01, 0.0, and 0.1 mg/kg. Each dog was randomly assigned one of the test agents and received only that compound during the -week period. In each experiment, blood pressure was monitored continuously by a pressure transducer (Micron Instruments, Los Angeles, CA, USA) connected to the indwelling iliac arterial catheter and placed at heart level. Heart rate (HR) was derived electronically from the pressure pulses. These parameters were recorded simultaneously on punched paper tape (data obtained at 1-minute intervals) by a data logger (Digitec; United Systems, Dayton, OH, USA) and displayed on a Gould 2600 strip chart recorder (Cleveland, OH, USA). The minute-to-minute HR and MAP data were averaged later for each 0-minute period, and these mean values were used in all subsequent analyses. Arterial blood samples for measurements of hematocrit and plasma electrolyte, creatinine, and PAH concentrations were drawn at the midpoint of each clearance period and placed in heparinized tubes. The volume of urine collected throughout each 0-minute interval was determined gravimetrically. Finally, an arterial blood sample for PRA analysis was collected in a chilled test tube containing disodium EDTA at the end of each period. Urine and plasma samples were analyzed for sodium, potassium, and chloride content using ion-selective electrodes (C800 System, Technicon Instruments, Tarrytown, NY, USA), and urinary electrolyte excretions were calculated by standard formulas. Creatinine and PAH concentrations were determined using a colorimetric assay adapted to an autoanalyzer (Technicon Instruments). Effective renal plasma flow (ERPF) and glomerular filtration rate

3 12 HYPERTENSION VOL 9, No 2, FEBRUARY 1987 (GFR) were estimated by the renal clearances of PAH and creatinine, respectively. PRA was measured by radioimmunoassay (Clinical Assays, Cambridge, MA, USA) of samples generated at ph 6.0 for 1 hour. Upper and lower limits of detection were 6 and 0.22 ng angiotensin I/ml/hr, respectively. All procedures were performed in accordance with our institutional guidelines for animal care and use. The smaller group size in the post-rac groups, compared with the large pre-rac numbers, reflects the fact that several dogs failed to become hypertensive following RAC, and others died shortly after the procedure. Furthermore, malignant hypertension developed in two of the five dogs in the ACRJP-treated group during the second week of the study. The individual response profiles observed in the remaining three ACRTP-treated dogs were, however, representative of the overall group responses at the previous time points (normotensive and days post-rac). Statistical differences between treatment means were ascertained by analysis of variance for repeated measures. Significant deviations from controls were detected by Dunnett's t test (p < 0.0). Student's / test (p < 0.00) was used to compare differences between ACRIP and enalaprilat treatments at specific time points. Results ACRIP produced small but statistically significant falls in MAP (Figure 1) at several doses in normotensive dogs without affecting HR (Table 1). ACRIP treatment did not alter either ERPF (see Table 1) or GFR (Figure 2) in normotensive dogs. Administration of ACRIP resulted in a significant, dose-related natriuresis and a less consistent diuresis in normotensive dogs (see Table 1). The initial dose of ACRIP reduced arterial PRA (Table 1) to levels approaching the lower limits of our assay sensitivity. Therefore, any additional decreases would not have been detected. Three days after RAC, when arterial pressure was increased by an average of 2 ± 2 mm Hg, increasing doses of ACRIP progressively lowered MAP until a maximum decrease of 26 ± mm Hg was reached during the infusion of 0.1 mg/kg/min. HR rose several times during ACRIP infusion (see Table 1), but the response was not consistent nor dose-related. In this early phase of experimental hypertension, ACRIP produced significant reductions in GFR (see Figure 2) and small, nonsignificant decreases in ERPF (see Table 1). Urine flow and natriuresis (see Table 1) were significantly depressed by ACRIP, as were excretion rates of potassium (from 0 ± 7 t o l O ± 2 /ieq/min) and of chloride (from ± to 10 ± /ieq/min). Again, all doses of ACRIP significantly reduced PRA from 2. ± 0.67 ng angiotensin I/ml/hr to undetectable levels (see Table 1). In dogs with chronic renovascular hypertension ( days post-rac), ACRIP administration diminished PRA to nearly undetectable levels (see Table 1) and decreased potassium excretion from 21 ± to 10 ± 2 AiEq/min at the highest dose. There were no statistical- WO Time (min) Tlme(min) FIGURE 1. MAP in dogs treated with ACHPA-containing renin inhibitory peptide (ACRIP; A) or enalaprilat (MK-22; B) at various stages in the development of one-kidney, one clip hypertension. The values represent the mean ± SEM for normotensive dogs (*), dogs days after renal artery constriction (RAC; ), dogs days after RAC (A), and dogs with malignant hypertension (o). Single (p<0.0) and double (p<0.01) asterisks indicate that the value is significantly different from the corresponding predrug value. The number of animals in each treatment group is shown in parentheses. ly significant changes in the other measured variables. During the second week of the study, two dogs exhibited signs of malignant hypertension, such as severely elevated blood pressure (182 ± 1 mm Hg) and poor renal function (GFR = 18 ± ; ERPF = 77 ± 6), and one of the dogs exhibited hyperreninemia (.6 ng angiotensin I/ml/hr). When challenged with ACRIP, MAP fell dramatically in these dogs (see Figure 1A). The hypotensive and HR responses to enalaprilat in normotensive dogs (Table 2; see Figure IB) were nearly identical to those produced by ACRIP. The ERPF (see Table 2) and GFR (see Figure 2) tended to improve following treatment with enalaprilat but, as with ACRIP, a clear dose-response relationship could not be defined. However, enalaprilat did elicit a doserelated saluresis and diuresis (see Table 2). PRA was significantly elevated by all doses of enalaprilat. During the early phase of experimental renovascular hypertension, enalaprilat significantly decreased MAP at all but the lowest dose (see Figure IB). At no time was MAP lower than that measured in the ACRTPtreated dogs. The changes in HR (see Table 2) were not significant as a group, even though tachycardia was observed in several dogs at the higher dose of enalaprilat. The significant reductions in ERPF (see Table 2) and GFR (see Figure 2) produced by administration of the ACE inhibitor were similar to those induced by

4 COMPARISON OF A RENIN INHIBITOR AND ENALAPRILAT/Smif/i et al. 1 ACRIP infusion. Although enalaprilat tended to attenuate urine flow and electrolyte excretion in these animals, only the fall in potassium excretion (from 28 ± 7 to 8 ± 2 /i.eq/min) was significant. As expected, ACE inhibition produced dose-related increases in PRA (see Table 2). The highest dose of enalaprilat elicited a small but significant drop in MAP in dogs with established experimental renovascular hypertension (see Figure IB). PRA was elevated by all doses of the compound in these animals, but no other significant changes were observed (see Table 2 and Figure 2). The depressor responses to ACRIP and to enalaprilat in normotensive dogs and in each phase of experimental hypertension are summarized in Figure. At no time during the course of the study were the maximal hypotensive responses produced by ACRIP significantly different.from those obtained with enalaprilat. A VI «- = " -DO E 0 E X) B VI 0 E 1 f 2 - " in 1o o _tn (SI t 0 O0O r 0 0O0 OOI 0D OJ 1 I 1 K» Time (nun) frvkq MK-22 Q01 O l Discussion Most of the investigations seeking to establish the role of the renin-angiotensin system in the genesis of hypertension in various experimental models have been fraught with technical difficulties. The intrinsic agonistic activity associated with the angiotensin II analogues rendered these compounds less than ideal tools for examining the renin-angiotensin system. Likewise, the involvement of converting enzyme in at least two different biochemical pathways has led to the generation of contradictory results when ACE inhibitors have been used to establish cause and effect relationships. The recent advent of peptides that interfere directly with the enzyme renin has provided investigators with a precise antagonist with which to experimentally inhibit the renin-angiotensin system. These potent and highly specific peptides, which should be devoid of any actions outside of the renin-angiotensin system, have been shown to be effective hypotensive agents in animal models in which PRA was elevated. 1 " 17 The present study compared the effects of the most potent of these new specific renin inhibitors, ACRIP, with those of the ACE inhibitor enalaprilat in an effort to define precisely the role of the reninangiotensin system in canine experimental renovascular hypertension. In the current study, moderate RAC in unilaterally nephrectomized dogs produced a sustained hypertension accompanied by an elevated PRA during the first week (see Figure 1 and Tables 1 and 2). During the second week, PRA values declined while MAP remained elevated. Administration of the ACE inhibitor enalaprilat significantly reduced MAP during the early phase of experimental hypertension but yielded only modest falls in chronic hypertensive and normotensive dogs (see Figure 1). Similar results have been obtained by others, 2 ' *"* who have suggested that the hypotensive efficacy of the ACE inhibitors is related to the preexisting level of PRA in the subject; an explanation that assumes that the hemodynamic changes associated with ACE inhibition resulted solely from reduced formation of the pressor hormone angiotensin II. Addi- Y\LtOO Time (mm) FIGURE 2. Glomerular filtration rate (GFR) in dogs treated with ACHPA-containing renin inhibitory peptide (ACRIP; A) or enalaprilat (MK-22; B) at various stages in the development of one-kidney, one clip hypertension. The values represent the mean ± SEM for normotensive dogs ( ) and dogs days ( ) and days (A) after renal artery constriction. Single (p<0.0) and double (p<o.oi) indicate that the value is significantly different from the corresponding predrug value. The number of animals in each treatment group is shown in parentheses. o 0 x E <] Normotensive - T1 (ID in* L-NS ' 0ays Post-RAC T 1 1() () i A) () 1 -NS- ' ' MOoys Post-RAC \\ Vs. \\ 1 L-NS ^ \ 1 r~] MK-22 > /SCRIP FIGURE. The maximal observed changes in MAP in dogs treated with either ACHPA-containing renin inhibitory peptide (ACRIP) or enalaprilat (MK-22) at various stages in the development of one-kidney, one clip hypertension. The values represent the mean ± SEM of the maximal changes at the indicated stage of renovascular hypertension. The number of animals per group is indicated in parentheses. NS not statistically significant (at p<0.0); RAC = renal artery constriction.

5 1 HYPERTENSION VOL 9, No 2, FEBRUARY 1987 TABLE 1. Effects of Administration of ACHPA-Containing Renin Inhibitory Peptide on Selected Parameters at Various Stages in the Development of One-Kidney, One Clip Hypertension Variable HR (beats/min) ERPF (ml/min) PRA (ng ANG I/ml/hr) UV (ml/min) Days post- RAC n Control 98± 81± 86± 19±17 80 ±20 9±2 1.00± ± ± ± ±0.0 ACRIP, 0.00 mg/kg/min 60 min 90 min 99± 101 ±6 87± 8 ± 87 ±2 86± 161 ± 17 ±16 7 ±19 67 ± 99±2 1±0 0.0±0.02t O.O±O.O2t 0.0±0.0t 0.10±0.06t O.O±O.Ot 0.7± ± ± ±10 20 ±* ± ±0.02t 0.67±0.1* * 0.8±0.10 ACRIP, 0.01mg/kg/min 0 min 10 min 102 ±6 10 ±6 90±6 9 ±7* 8±7 9±1 ±1 8 ±2 9 ±17 70±21 1±29 101±2 O.O±O.Ot 0.08±0.0t O.O±O.Ot 0.0± ±0.06t 0.0±0. 2±1* 0.0±0.02t 0.0±0.02t 0±0t 0.±0. 0.±0.08* 0.2 ±0.02 U N,V OxEq/min) 28±8 2 ±19* 62±1t 7±7 10±t 8±t ll±t 9 ±8 6± 7 ±2 8 ±2 Values are means ± SEM. RAC renal artery constriction; ACRIP = ACHPA-containing renin inhibitory peptide; HR = heart rate; ERPF = effective renal plasma flow; ANG I = angiotensin I; UV = urine flow; U N,V = urinary sodium excretion. *p < 0.0, tp < 0.01, comparedwith control values. TABLE 2. Effects of Administration of Enalaprilat on Selected Parameters at Various Stages in the Development of One-Kidney, One Clip Hypertension Days Enalaprilat, 0.00 mg/kg Enalaprilat, 0.01 mg/kg DOSt- Variable RAC n Control 60 min 90 min 0 min 10 min HR (beats/min) ERPF (ml/min) PRA (ng ANG I/ml/hr) UV (ml/min) U N,V (jieq/min) 9± 80± 8± ±17 81± 1.26 ± ± ±0.0 0.± ± ±9 1±7 9±21 Values are means ± SEM. See Table 1 for key to abbreviations. *p < 0.0, tp < 0.01, compared with control values. 92± 88±8 8± 7 ±9* 91± 90± 1.9 ± ± ±.9 0.9± ±0.06 8± 18± 9 ±20 91 ± 80±6 87± 1±9 87±9 8±6 1.± ± ± ± ±0.0 ±8 22 ± 61 ± 9O± ± ±10 70±19 8 ±10 2.1± ±.2t.9± ±0.0* 0.19± ±9 8± 69 ±2 96±6 8± 9 ± 8± 9 + 8±6 2.00± ±.69t ±0.0* 0.72 ± ±0.0 ±9* ±19 tional factors may be involved, since Blaine et al. u could not demonstrate a direct relationship between suppression of PRA by a statine-containing renin inhibitor and the fall in blood pressure in sodium-deficient dogs. Alternatively, several investigators have proposed additional mechanisms of action ' 16 for the ACE inhibitors. One group reported that the hypotensive response achieved following ACE inhibition with captopril in patients with low and normal renin levels was attenuated by pretreatment with the kallikrein in-

6 COMPARISON OF A RENIN INHIBITOR AND ENALAPRILAT/SmM et al. 1 TABLE 1. (continued) ACRIP, 0.0mg/kg/min 180 min 102 ±6 89± 8±6 16±8 8 ±19 9 ±22 0.0±0.0t 0.0±0.0t o±ot ±0.09t 0. ±0.08 6±1t 7±2t 6 ±2 210 min 10 ±6 91±6 9 ± ±18 60±2 96 ±2 0.02±0.01t 0.0±0.0t 0.07±0.0t 0.1 ± ±0.0t 0.1 ± ±t 9±t ±7 TABLE 2. (continued) ACRIP, 0.1 mg/kg/min 20 min 270 min 10 ± 102 ± 99±1t 91 ± 9± 9± 209 ±6* ±29 6 ±21 1 ±1 ± ±0.01t o±ot 0.06±0.0t 0. ± ±0.29* ±t 1±9t 6±9 0.02±0.02t 0.07±0.07f o±ot 0.6±0.08 O.O±O.1* 0.7± ±1t ll±t 8 ±2 inhibitors in vivo. Leckie et al. 20 and Wood et al. 21 were unable to demonstrate additional hypotensive effects when converting enzyme inhibitors were administered to animals receiving infusions of a renin inhibitor. These data contrast with those of Oldham et al. 16 and Blaine et al. 22 who have reported additional, albeit small, reductions in arterial pressure attributable to ACE inhibition under similar circumstances. The reasons for the discrepancy remain unclear. In the present study, short-term administrations of the ACE inhibitor enalaprilat and ACRIP, a novel and potent renin inhibitor, had equally hypotensive effects in normotensive and hypertensive one-kidney dogs (see Figure ). Additionally, both of these compounds were most active in dogs days post-rac, a time when MAP is thought to be supported largely by the renin-angiotensin system. - 7 Some concern has been raised regarding the effects of surgery on renin release in the one-kidney hypertensive dog model and how this might affect the observed parameters after operation. Although our study did not include a sham-operated control group, others 2-2 have shown that surgically induced renin release returns to presurgical levels within 2 hours. In addition, Gutmann et al. 2 showed that reducing the renal blood flow to the remaining kidney in a onekidney, one clip conscious dog 9 days postoperatively elicited renin release within minutes. Since the PRA patterns observed in the present study follow the patterns routinely seen in one-kidney, one clip renal hypertensive dogs, it seems unlikely that our findings days post-rac result from surgical trauma. Enalaprilat, 0.0 mg/kg 180 min 210 min 102 ±6 96± 107 ±2 107 ±22 9± 88± 7±1 1 ± 70 ±20 71 ± ±0.96* 2.61 ±0.02* ±2.07t 2.10±2.78t 6.7±.78 6.± ±0.0t 0.7±0.0t 0.± ± ± ± t 67± lit 7±1 17± 6±21 77 ±26 Enalaprilat, 0.1 mg/kg 20 min 00 min 108 ±8* ±20 0±22 91 ±1 91± 8 ±1* +* 0 ±10* 6 ±19 8± 91 ±10.0±1.19t lot 7.9 ± ±0.06t 0.± ±1t 6±1 6 ± t 2.78±2.78t.7± t ±18t 7±1 72±21 hibitor aprotinin. These data supported the contention that potentiation of the kallikrein-kinin system may contribute to the hemodynamic actions of captopril. In an effort to resolve this issue, several investigators have directly compared the antihypertensive activity of the ACE inhibitors with that of the renin Interestingly, renal function also appeared dependent upon the renin-angiotensin system days post- RAC, since treatment with either enalaprilat or ACRIP diminished each measure of renal hemodynamics and excretory function (see Figure 2 on p. 1 and Tables 1 and 2, opposite). These findings were consistent with the suggestion of Hall and co-workers that angiotensin II-induced vasoconstriction of the renal efferent arterioles was necessary for autoregulation of GFR in ischemic kidneys. These authors showed that angiotensin II levels increased during reduction in renal blood flow in untreated dogs, leading to enhanced efferent arteriolar tone and maintenance of glomerular perfusion pressure. However, when the vasoconstricting actions of the renin-angiotensin system were inhibited by [Sar 1, Ile 8 ]angiotensin II, a competitive antagonist, the renal efferent arterioles dilated, and filtration pressure was reduced. Additionally, ACE inhibitors diminished renal function in patients with a single stenotic kidney and in patients with bilateral renal artery stenoses. 28 " 2 Withdrawal of the inhibitor restored renal function in those patients. In the present study, administration of enalaprilat or ACRIP compromised renal function in the one-kidney renovascular hypertensive dogs only when PRA was elevated days after RAC. Renal function had returned to normal by the time these animals were reexamined days after RAC. These data are consistent with the hypothesis that angiotensin II is essential for autoregulation in ischemic kidneys.

7 16 HYPERTENSION VOL 9, No 2, FEBRUARY 1987 In conclusion, a potent new renin inhibitor, ACRIP, and the ACE inhibitor enalaprilat produced quantitatively similar effects on MAP and renal function in conscious normotensive dogs and in dogs in two stages of experimental renovascular hypertension. Both compounds were most efficacious in dogs with acute renovascular hypertension associated with an elevated PRA and were modestly depressor in normotensive dogs and dogs with chronic renovascular hypertension. Each agent compromised renal function in animals in which renin levels were elevated, demonstrating that the renin-angiotensin system is a major component of renal autoregulation in hypertensive, as well as normotensive, animals. 26 ' 27 By no measure was enalaprilat more effective than ACRIP, indicating that suppression of the renin-angiotensin system was the primary, if not the sole, antihypertensive effect during short-term treatment with an ACE inhibitor in this model. Acknowledgments We thank Mrs. Joan Brooke for her assistance in preparation of this manuscript and Dr. Charles Sweet for his scientific advice. References 1. Bianchi G, Tenconi LT, Lucca R. Effect in the conscious dog of constriction of the renal artery to a sole remaining kidney on haemodynamics, sodium balance, body fluid volume, plasma renin concentration and pressor responsiveness to angiotensin. Clin Sci 1970;8: Davis JO. The pathogenesis of chronicrenovascularhypertension. Clin Res 1977;0:9-. Bianchi G, Baldoli E, Lucca R, Barbin P. Pathogenesis of arterial hypertension after the constriction of the renal artery leaving the opposite kidney intact both in the anesthetized and in the conscious dog. Clin Sci 1972;2: Bumpus FM, Sen S, Smeby RR, Sweet C, Ferrario CM, Khosla MC. Use of angiotensin II antagonists in experimental hypertension. Circ Res 197;2(suppl I): 108. Ayers CR, Vaughn ED Jr, Yancy MR, Bing KT, Johnson CC, Morton C. Effect of l-sarcosine-8-alanine angiotensin II and converting enzyme inhibitor onreninreleasein dogs with acute renovascular hypertension. Circ Res 197;(suppl I):27 6. Sweet CS, Ferrario CM, Kosoglov A, Bumpus FM. Cardiovascular evaluation of [ 1 -sarcosine-8-isoleucine] angiotensin II and its effects on blood pressure of conscious renal hypertensive dogs. In: Wesson LG, Fanelli GM Jr, eds. Recent advances in renal physiology and pharmacology. Baltimore: University Park Press, 197: Watkins BE, Davis JO, Freeman RH, Stephens GA, DeForrest JM. Effects of the oral converting enzyme inhibitor (SQ 22) onrenovascularhypertension in the dog. Proc Soc Exp Biol Med 1978; 17: Watkins BE, Davis JO, Freeman RH, DeForrest JM, Stephans GA. Continuous angiotensin II blockade throughout the acute phase of one-kidney hypertension in the dog. Circ Res 1978:2: Sweet CS, Gross DM, Arbegast PT, et al. Antihypertensive activity of N-[(S)-l-{ethoxycarbonyl)--phenylpropyl]-L-Ala- L-Pro (MK-21), an orally active converting enzyme inhibitor. J Pharmacol Exp Ther 1981;216: Anderson GH Jr, Streeten DHP, Dalakos TG. Pressor response to l-sar-8-ala angiotensin II (saralasin) in hypertensive subjects. Circ Res 1977;0:2-20. Engel SL, Schaeffer TR, Gold BI, Rubin B. Inhibition of pressor effects of angiotensin I and augmentation of depressor effects of bradykinin by synthetic peptides. Proc Soc Exp Biol Med 1972; 0:20-2. Overlack A, Stumpe KO, Kuhnert M, et al. Evidence for participation of kinins in the antihypertensive effect of converting enzyme inhibitor. Klin Wochenschr 1981;9: Boger J, Lohr NS, Ulm EH, et al. Novel renin inhibitors containing the amino acid statine. Nature I98;0:81-8. Blaine EH, Schorn TW, Boger J. Statine-containing renin inhibitor: dissociation of blood pressure lowering and renin inhibitor in sodium-deficient dogs. Hypertension 198;6(suppl I):I Szelke M, Leckie BJ, Tree M, et al. H-77: a potent new renin inhibitor, in vitro and in vivo studies. Hypertension 1982; (suppl U):II-9-II Oldham AA, Amstein MJA, Major JS, Clough DP. In vivo comparison of the renin inhibitor H77 with the angiotensinconverting enzyme inhibitor captopril. J Cardiol Pharmacol 198;6: Burton J, Cody RJ Jr, Herd JA, Haber E. Specific inhibition of renin by an angiotensinogen analog: studies in sodium depletion and renin-dependent hypertension. Proc Natl Acad Sci USA 1980;77: Zusman R, Christensen D, Burton J, Nussberger J, Dodds A, Haber E. Hemodynamic effects of a competitive renin inhibitory peptide in man: evidence for multiple mechanisms of action [Abstract]. Clin Res 198;1:8A 19. Boger J, Payne LS, Perlow DS, et al. Renin inhibitors: syntheses of sub-nanomolar, competitive, transition-state analog inhibitors containing a novel analog of statine. J Med Chem 198; O 20. Leckie B, Szelke M, Hallett A, et al. Peptide inhibitors of renin. Clin Exp Hypertens [A] 198;(7,8): Wood JM, Forgiarini P, Hofbauer KG. Inhibition of renin and converting enzyme in conscious volume depleted marmosets. J Hypertens 198;l(suppl 2): Blaine EH, Nelson BJ, Seymour AA, et al. Comparison of renin and converting enzyme inhibition in sodium-deficient dogs. Hypertension 198;7(suppl I):I-66-I McKenzie JK, Ryan JW, Lee MR. Effect of laparotomy on plasma renin activity in the rabbit. Nature 1967;21:2-2. Robertson D, Michelakis AM. Effect of anesthesia and surgery on plasmareninactivity in man. J Clin Endocrinol Metab 1972; : Gutmann FD, Tagawa H, Haber E, Barger AC. Renal arterial pressure, renin secretion and blood pressure control in trained dogs. Am J Physiol 197;22: Hall JE, Guyton AC, Trippodo NC, Lohmeier TE, McCaa RE, Cowley AW Jr. Intrarenal control of electrolyte excretion by angiotensin II. Am J Physiol 1977;22:F8-F 27. Hall JE, Guyton AC, Jackson TE, Coleman TG, Lohmeier TE, Trippodo NC. Control of glomerular filtration rate by reninangiotensin system. Am J Physiol 1977;2:F66-F Hricik DE, Browning PJ, Kopelman R, Goomo WE, Madias NE, Dzau VJ. Captopril-induced functional renal insufficiency in patients with bilateral renal-artery stenoses or renal-artery stenosis in a solitary kidney. N Engl J Med 198;08: Curtis JJ, Luke RG, Whelchel JD, Diethelm AG, Jones P, Dustan HP. Inhibition of angiotensin-converting enzyme in renal-transplant recipients with hypertension. N Engl J Med 198;08: Blythe WB. Captopril and renal autoregulation. N Engl J Med 198;O8:9O Mason JC, Hilton PJ. Reversible renal failure due to captopril in a patient with transplant artery stenosis: Case report. Hypertension 198;: Bender W.LaFranceN, Walker WG. Mechanism of deterioration in renal function in patients with renovascular hypertension treated with enalapril. Hypertension 198;6(suppl I): I997