Renal kallikrein-kinin system and the depressor effect of angiotensin converting enzyme inhibitors MK 421, SA 446, and captopril in rats

Responses in urinary kallikrein and kinin excretion and systolic blood pressure to MK 421, SA 446 or captopril were studied in normotensive rats fed on a regular or a low sodium diet to assess the role of renal kallikrein-kinin system in their hypotensive effect. MK 421, SA 446 or captopril were infused at a rate of 6 mg/kg/day by osmotic minipump implanted intraperitoneally for up to 6 days. The magnitude of fall in systolic blood pressure was greater on a low sodium diet when compared to on a regular diet, whereas the pattern of the fall was similar on both diets. The magnitude of falls in plasma angiotensin II and aldosterone concentration induced by MK 421, SA 446 and captopril was not significantly different between both regular and low sodium diets. Urinary kallikrein and kinin excretion and sodium excretion were increased during infusion of MK 421, SA 446 or captopril on a low sodium diet, however any significant changes were not found in each of them on a regular diet. The present results suggest that on a low sodium diet the augmented hypotensive response to angiotensin converting enzyme inhibitors in the rats might be due to the enhanced renal kallikrein-kinin system in addition to suppressed renin-angiotensin system.

Two inhibitors of angiotensin-converting enzyme, enalapril and captopril, increase salt appetite of rats

Male rats were orally administered an inhibitor of angiotensin-converting enzyme (ACE), N-[(S)-1-(ethoxycarbonyl)-3-phenylpropyl]-1-ala-1-pro maleate (enalapril, MK-0421) at dosage levels of 10, 30, and 90 mg/kg X d. After 2-6 wk of dosing, the rats receiving 30 and 90 mg/kg X d produced large numbers of seminal plugs and had lacerated penises due to licking in an attempt to recover urine. Providing 0.9% saline as the source of drinking water prevented this behavior and subsequent lesions. There were no adverse effects on reproductive performance. A subsequent study showed that enalapril at 5 mg/kg X d po and captopril (another ACE inhibitor) at 25 mg/kg X d po increased NaCl intake in rats. Our results with captopril confirm those of Fregly (1980) and Evered and Robinson (1983) and show that both converting-enzyme inhibitors (enalapril and captopril) increase salt appetite in rats.

Effect of captopril, MK-421 and MK-422 on fibrinolysis determined by 3 "in vitro" assays

The effect of captopril, MK-421 and MK-422 on fibrinolysis was investigated in vitro with 3 assay methods. Incorporation of captopril, but not MK-421 and MK-422, into fibrin clots resulted in a reduction of fibrinolysis by purified human plasmin. The concentrations of captopril required to demonstrate this effect far exceeded the therapeutic doses. Pre-mixing plasmin with captopril did not lead to a decreased lysis of fibrin plates. Furthermore, none of these agents affected the lysis of 125I-fibrin plates by a mixture of human serum and streptokinase. Captopril and MK-421, and to a much lesser extent MK-422, inhibited the amidolysis of a fluorogenic synthetic substrate by human plasmin; the inhibition was ameliorated by increasing the substrate concentration. Apparently, the inhibition of fibrinolysis by high doses of captopril, and of amidolysis by captopril and MK-421, appears to lie in their effects on the substrate (fibrin and synthetic substrate) rather than on the enzyme (plasmin).

Enalapril improves systemic and renal hemodynamics and allows regression of left ventricular mass in essential hypertension

Enalapril, a new angiotensin-converting enzyme inhibitor, is an effective antihypertensive agent for both renovascular and essential hypertension. It is structurally different from captopril in that it does not possess a sulfhydryl group. The systemic and renal hemodynamic, biochemical and cardiac adaptive changes induced by enalapril were studied in 8 patients with essential hypertension before and after 12 weeks of therapy. Mean arterial pressure decreased from 110 to 90 mm Hg (p less than 0.01), and this was mediated through a decrease in total peripheral resistance from 42 +/- 3 to 32 +/- 3 U (p less than 0.01). Cardiac index and heart rate did not change. Renal plasma flow was increased in 6 of 8 patients and renal vascular resistance decreased from 123 +/- 6 to 91 +/- 7 U (p less than 0.001). Left ventricular mass index decreased from a mean of 166 +/- 29 to 117 +/- 8 g/m2 (p less than 0.05) without impaired myocardial contractility. Thus, enalapril lowers arterial pressure by reducing total peripheral resistance without reflexive cardiac effects. It also has favorable hemodynamic effects on the kidney. This is the first report of regression of LV mass with this agent in man.

Mechanism for hypotensive action of angiotensin converting enzyme inhibitors

The mechanism(s) for the hypotensive effect of Angiotensin Converting Enzyme (ACE) inhibitors remains elusive. This is because of the multiplicity of the biological actions of angiotensin, the dual role of ACE and the ability of the inhibitors to induce the enzyme. After a single dose of enalapril (MK421), a new ACE inhibitor, in patients with essential hypertension a close linear relationship between the plasma level of enalaprilic acid (MK422) and the degree of ACE inhibition could be demonstrated. Furthermore the degree of ACE inhibition was linearly related to the hormonal changes and to the fall in blood pressure. After chronic administration of enalapril the plasma levels of MK422 were found to be dose dependent. As in the acute study there was also a linear relationship between the plasma level of MK422 and the degree of ACE inhibition. However, the plasma enalaprilic acid level-ACE inhibition dose response curve after chronic administration was shifted to the right, compared to the dose response curve after acute administration suggesting that ACE had been induced during chronic administration of enalapril in humans. There were direct linear relationships between both the degree of ACE inhibition the plasma and enalaprilic acid (MK422) level to the fall in mean arterial pressure. These results suggest that regardless of the final mechanism for the hypotensive action of ACE inhibitors it is a consequence of their inhibition of the enzyme.

The chemistry of enalapril

The design origins of the potent non-mercapto angiotensin converting enzyme inhibitors enalaprilat and its mono ethyl ester enalapril are described. Lactam analogues of enalaprilat have provided some insight into the conformation of this inhibitor when it is bound to converting enzyme. X-ray crystallographic studies of a related enzyme/inhibitor complex offer an explanation for the high potency and specificity of these and related inhibitors.

Tolerance and safety of enalapril

Enalapril is the result of a targeted research programme to develop a non-mercapto converting enzyme inhibitor with a long duration of action and an improved safety profile for use in the therapy of hypertension and congestive heart failure. Over 3500 patients world-wide have received enalapril or enalaprilat. Long-term experience at present includes over 2500 patients. While enalapril and captopril produce similar efficacy, enalapril is better tolerated and appears not to be associated with occurrence of captopril-type side-effects, particularly the skin rash, taste loss, leukopenia and proteinuria. Enalapril and other converting enzyme inhibitors may be associated with renal insufficiency when given to patients with bilateral renovascular hypertension.

ACE inhibition and brachial artery haemodynamics in hypertension

Brachial artery haemodynamics, including arterial diameter, blood flow velocity, blood flow and compliance of the brachial artery may be evaluated non-invasively in man, using pulsed Doppler methods. In patients with sustained untreated essential hypertension, brachial artery diameter is increased, blood flow is normal and arterial compliance is reduced independently of the level of blood pressure. Converting enzyme inhibition may reverse the large artery abnormalities, causing an increase in arterial diameter, blood flow and arterial compliance both in acute and long-term investigations. These changes in large arteries may contribute towards improved cardiovascular morbidity and less mortality in treated hypertensive patients.

Left ventricular changes after chronic therapy with enalapril maleate in moderate to severe hypertensive patients

A randomized double-blind trial was carried out to determine the relationship of the changes in blood pressure and heart rate with changes in echocardiographic left ventricular indices in moderate to severe hypertensive patients with established left ventricular hypertrophy who were being treated chronically with enalapril or hydrochlorothiazide plus propranolol for 26 weeks. After a 2-week period on placebo, drug dosages in the two groups were adjusted to individual needs until blood pressure was normalized (diastolic less than 90 mmHg). Patients in Group I received 10 to 40 mg enalapril/day; those in Group II received 50 mg hydrochlorothiazide plus 80 to 240 mg propranolol/day. Echocardiographic measurements were made at the end of the placebo and 26-week active treatment periods. Significant correlations were observed between the changes in four pairs of variables in each group. In the 8 patients receiving enalapril, there were negative correlations between interventricular septal thickness and supine systolic blood pressure, erect and supine heart rates, and a positive correlation between relative wall thickness and erect diastolic blood pressure. In the 7 patients on hydrochlorothiazide plus propranolol, there were negative correlations between relative wall thickness and erect and supine heart rate, and positive correlations between left ventricular mass and erect diastolic blood pressure, and the percentage change in internal diameter of the left ventricle and supine systolic blood pressure. Possible explanations for and implications of these regional changes are discussed.

Enalapril in heart failure

Serum MK-422 and plasma angiotensin converting enzyme activity were measured during the introduction of enalapril therapy in eight patients with heart failure. In a second study of 16 patients, we recorded exercise tolerance, clinical status and haemodynamics before and after 12 weeks of placebo or enalapril treatment. Increasing doses of enalapril gave step-wise increments in serum MK-422. Plasma converting enzyme activity remained low for at least 24 h after each dose of enalapril (5, 10 and 20 mg). Compared to placebo patients (n = 8), those receiving enalapril (n = 8) tended to improve their exercise performance and clinical status, and showed a fall in right heart pressures after 12 weeks of treatment.

Effect of converting enzyme inhibition by enalapril on sodium homeostasis in the rat

The effect of oral treatment with the converting enzyme inhibitor enalapril on sodium homeostasis was investigated in the rat. Treatment by enalapril prior to and during a 6 day period following abrupt suppression of dietary Na+ was associated with a sodium wasting state (urinary Na+ always exceeded intake during the observation period) and blunting by 90% of the aldosterone response to Na+ restriction In rats on chronic low Na+ intake, enalapril produced a slight, transient natriuresis together with a marked increase in drinking volume. In Na+ replete rats, enalapril had no influence on sodium balance. Converting enzyme inhibition markedly impaired the systemic and renal response to Na restriction and enalapril had no natriuretic effect in the Na+ replete state.

An overview of the clinical pharmacology of enalapril

Enalapril maleate is a prodrug which when administered orally is hydrolysed to release the active converting enzyme inhibitor enalaprilat. Enalapril maleate is 60% absorbed and 40% bioavailable as enalaprilat. Both compounds undergo renal excretion without further metabolism. The functional half-life for accumulation of enalaprilat is 11 h, and this is increased in the presence of a reduction in renal function. Inhibition of converting enzyme inhibition is associated with reductions in plasma angiotensin II and plasma aldosterone, and with increases in plasma renin activity and plasma angiotensin I. Acute and chronic effects have been reviewed. When given with hydrochlorothiazide, enalapril attenuates the secondary aldosteronism and ameliorates the hypokalaemia from diuretics. Both acutely and chronically in patients with essential hypertension, enalapril reduced blood pressure with a rather flat dose-response curve. No evidence of a triphasic response such as seen with captopril has been demonstrated with enalapril, and blood pressure returns smoothly to pretreatment levels when the drug is abruptly discontinued. Once- or twice-daily dosing gives similar results. The antihypertensive effects of enalapril are potentiated by hydrochlorothiazide. Haemodynamically, blood pressure reduction is associated with a reduced peripheral vascular resistance and an increase in cardiac output and stroke volume with little change in heart rate. Renal vascular resistance decreases, and renal blood flow may increase without an increase in glomerular filtration in patients with normal renal function. In patients with essential hypertension and glomerular filtration rates below 80 ml/min/m2, both renal blood flow and glomerular filtration rates may increase.

Effect of enalapril (MK-421), an orally active angiotensin I converting enzyme inhibitor, on blood pressure, active and inactive plasma renin, urinary prostaglandin E2, and kallikrein excretion in conscious rats

The angiotensin I converting enzyme (ACE) inhibitor enalapril (MK-421), at a dose of 1 mg/kg or more by gavage twice daily, effectively inhibited the pressor response to angiotensin I for more than 12 h and less than 24 h. Plasma renin activity (PRA) did not change after 2 or 4 days of treatment at 1 mg/kg twice daily despite effective ACE inhibition, whereas it rose significantly at 10 mg/kg twice daily. Blood pressure fell significantly and heart rate increased in rats treated with 10 mg/kg of enalapril twice daily, a response which was abolished by concomitant angiotensin II infusion. However, infusion of angiotensin II did not prevent the rise in plasma renin. Enalapril treatment did not change urinary immunoreactive prostaglandin E2 (PGE2) excretion and indomethacin did not modify plasma renin activity of enalapril-treated rats. Propranolol significantly reduced the rise in plasma renin in rats receiving enalapril. None of these findings could be explained by changes in the ratio of active and inactive renin. Water diuresis, without natriuresis and with a decrease in potassium urinary excretion, occurred with the higher dose of enalapril. Enalapril did not potentiate the elevation of PRA in two-kidney one-clip Goldblatt hypertensive rats. In conclusion, enalapril produced renin secretion, which was in part beta-adrenergically mediated.(ABSTRACT TRUNCATED AT 250 WORDS)

A possible role for the brain renin-angiotensin system in the regulation of LH secretion

To investigate the possibility that angiotensin II (ANG II) is involved in the regulation of luteinizing hormone (LH) secretion, ANG II was injected intraventricular ANG II caused an increase in plasma LH concentrations in 10 min, whereas intravenous ANG II in the same dose had no significant effect. Intraventricular administration of the ANG II antagonist, saralasin, during the afternoon of proestrus induced a significant decrease in the number of rats ovulating and abolished the ovulatory surge in LH secretion seen in saline-injected control animals. Intravenous saralasin in two different doses failed to inhibit ovulation and produced only a small decrease in the LH surge. In animals treated with intraventricular saralasin, the increase in plasma LH concentration produced by intravenous injection of 50 ng of LH-releasing hormone (LHRH) was normal, indicating no alteration in the sensitivity of the gonadotrops to LHRH. Intraventricular administration of the converting-enzyme inhibitor, enalapril diacid, inhibited ovulation and the ovulatory surge, whereas systemically administered enalapril diacid had no effect on LH secretion. The data suggest that ANG II generated in the brain may play a significant role in the regulation of LH secretion on the day of proestrus.

Acute effects of MK421, a new angiotensin converting enzyme inhibitor, in man

Acute effects of MK421 were examined in 11 male normal volunteers. Oral administration of 5 mg of MK421 did not induce any changes in blood pressure or pulse rate. Plasma renin activity increased significantly from 7.3 +/- 2.9 ng/ml to 22.2 +/- 7.1 (p less than 0.05), whereas plasma aldosterone concentration did not change. Blood kinin, or urinary excretion of kallikrein and of prostaglandin E did not change. However, urinary excretion of sodium increased significantly from 44.0 +/- 6.8 mEq/4 hr to 56.5 +/- 6.5 (p less than 0.02) following the administration of MK421. The present results show that an increased renin release induced by MK421 is independent of change in blood pressure, and also suggest that MK421 may have a direct action on the renal tubules.

Enalapril in the treatment of hypertension with renal artery stenosis

The converting enzyme inhibitor enalapril, in single daily doses of 10-40 mg, was given to 20 hypertensive patients with renal artery stenosis. The blood pressure fall six hours after the first dose of enalapril was significantly related to the pretreatment plasma concentrations of active renin and angiotensin II and to the concurrent fall in angiotensin II. Blood pressure fell further with continued treatment; the long term fall was not significantly related to pretreatment plasma renin or angiotensin II concentrations. At three months, 24 hours after the last dose of enalapril, blood pressure, plasma angiotensin II, and converting enzyme activity remained low and active renin and angiotensin I high; six hours after dosing, angiotensin II had, however, fallen further. The rise in active renin during long term treatment was proportionally greater than the rise in angiotensin I; this probably reflects the fall in renin substrate that occurs with converting enzyme inhibition. Enalapril alone caused reduction in exchangeable sodium, with distinct increases in serum potassium, creatinine, and urea. Enalapril was well tolerated and controlled hypertension effectively long term; only two of the 20 patients required concomitant diuretic treatment.

Postsynaptic alpha adrenoceptors and attenuation of vascular sympathetic responses in SHRs by captopril and enalapril

The effects of a short-term oral treatment with captopril or enalapril, two converting enzyme inhibitors (CEIs) administered in equipotent antihypertensive doses, on the systemic vasopressor and on the renal, mesenteric and hind-limb vascular responses to cirazoline and UK 14304, respectively alpha1- and alpha2- adrenoceptor specific agonists, were investigated in the adult pithed SHR. Cirazoline and UK 14304 induced vasoconstrictor responses in the three investigated territories, demonstrating the location and a functional role at these levels of the two sub-types of alpha-adrenoceptors. Both captopril and enalapril reduced, to the same extent, the systemic pressor and the regional vasoconstrictor responses, especially in the renal and mesenteric territories, elicited by cirazoline and UK 14304, demonstrating that both subtypes of alpha-adrenoceptors are involved in the CEIs-induced attentuation of sympathetic responses.

Cardiac and hormonal effects of enalapril in hypertension

Systolic time measurements, echocardiography, and bicycle exercise testing with cardiac output determinations (CO2 rebreathing) were used to evaluate cardiac performance in 16 male hypertensives at the end of a 4-wk placebo period and after 12 wk of treatment with increasing doses (maximum = 40 mg/day) of enalapril maleate (N = 11) and of placebo (N = 5). The effect of exercise on plasma renin activity (PRA) and plasma norepinephrine (NE) concentration was also measured. Mean arterial pressure was reduced by 10 mm Hg or more in all but one subject who received enalapril. In both the enalapril- and placebo-treated subjects, the preejection period/left ventricular ejection time ratio and fractional shortening of the left ventricle at rest and cardiac output and stroke volume during moderate exercise did not change during the study. Enalapril induced a compensatory rise in PRA (N = 10). Compared to plasma NE concentration, 1124 +/- 380 pg/ml (mean +/- SD), during exercise at the end of the initial placebo period, there was attenuation of the rise of plasma NE concentration, 851 +/- 290, at the same load of exercise during enalapril therapy. Unchanged cardiac performance despite effective long-term lowering of blood pressure with enalapril may relate to inhibition of angiotensin II-mediated facilitation of NE release from peripheral nerve endings.

Enalapril: a new angiotensin-converting enzyme inhibitor in chronic heart failure: acute and chronic hemodynamic evaluations

Hemodynamic effects of the new oral angiotensin-converting enzyme inhibitor, enalapril, were evaluated acutely in 15 patients with chronic heart failure and in 7 patients after 4 weeks of maintenance therapy. Initial hemodynamic effects were characterized by a significant increase in cardiac index (from 2.1 +/- 0.7 to 2.6 +/- 0.7 liters/min per m2) and a decrease in pulmonary capillary wedge pressure (from 30 +/- 6 to 24 +/- 7 mm Hg), right atrial pressure (from 14 +/- 5 to 11 +/- 4 mm Hg), mean arterial pressure (from 96 +/- 16 to 80 +/- 17 mm Hg) and systemic vascular resistance (from 1,820 +/- 480 to 1,200 +/- 410 dynes . s . cm-5) without any significant change in heart rate, pulmonary artery pressure and pulmonary vascular resistance. During maintenance therapy, the dose of diuretic drugs had to be increased because of systemic venous hypertension. Repeat hemodynamic study showed that after chronic therapy, cardiac index (2.1 +/- 0.7 vs. 3.0 +/- 0.08 liters/min per m2) and stroke volume index (24 +/- 10 vs. 36 +/- 7 ml/m2) remained elevated and pulmonary capillary wedge pressure was lower than control (30 +/- 6 vs. 16 +/- 6 mm Hg), indicating sustained improvement in left ventricular performance. Plasma renin activity increased and plasma norepinephrine levels decreased after enalapril therapy and these humoral changes persisted during maintenance therapy. All patients receiving chronic therapy had symptomatic improvement. Significant hypotension, which occurred in five patients at the initiation of therapy, appears to be the major side effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolated liver granulomas of murine Schistosoma mansoni contain components of the angiotensin system

Angiotensin I (AI) and angiotensin II/III (AII/III) were detected by radioimmunoassay in homogenates of isolated liver granulomas from mice infected for 8 wk with Schistosoma mansoni. Angiotensin I converting enzyme (ACE) activity, which could be completely inhibited by captopril, a specific ACE inhibitor, was also present as determined by radioassay. Spontaneous angiotensin I-generating activity was detected in homogenates that received supplemental angiotensinogen (protein renin substrate). This activity was partly inhibited by pepstatin, an acid protease inhibitor, indicating the presence of angiotensinogenase(s). Trypsinization of homogenates resulted in some AI generation, which suggests that homogenates had AI precursor. Treatment of infected mice with MK421, another specific ACE inhibitor, decreased granuloma ACE activity and AII content and size. AII, and to a lesser extent AIII, inhibited mouse peritoneal macrophage migration in an in vitro assay. These data support the contention that components of the angiotensin system are in the granuloma and may serve a function in regulation of the inflammation.

Immunoreactive bradykinin and [des-Arg9]-bradykinin in low-renin essential hypertension--before and after treatment with enalapril (MK 421)

Bradykinin (BK) and [des-Arg9]-bradykinin (-9BK) concentrations in blood and urine samples from 18 normotensive subjects and 23 patients with low-renin essential hypertension were determined by radioimmunoassay. BK and -9BK levels in venous blood from normotensive subjects were 67.1 +/- 60.8 pg/ml and 204.1 +/- 44.5 (mean +/- S.D.), respectively, and levels in urine from normotensive subjects were 5.3 +/- 5.3 ng/ml and 1.6 +/- 1.2, respectively. The blood and urinary levels of BK and -9BK in low-renin essential hypertensives were not significantly different from those of normotensives and did not change when the hypertensives were treated with the new orally active angiotensin I-converting enzyme (ACE) inhibitor, enalapril (MK421). It has been proposed that BK levels do not change with ACE inhibition because under these conditions BK might be metabolized to -9BK by kininase I. Since -9BK levels did not increase with MK421 treatment, this possibility can be excluded. The absence of elevations in blood and urine BK and -9BK after administration of MK421 does not support an involvement of kinins in the mechanism of antihypertensive action of MK421 in these patients. On the basis of the data, it is not possible to exclude such an involvement, however, because local changes in kinin concentrations could occur that are not reflected by changes in circulating or urinary kinin levels.

Time course and effect of sodium intake on vascular and hormonal responses to enalapril (MK 421) in normal subjects

Enalapril (MK 421) is a long-acting angiotensin converting-enzyme inhibitor which specifically lacks the sulfhydryl group of captopril. We measured arterial pressure and hormonal responses that occurred in 22 normotensive subjects treated with increasing doses of enalapril (2.5, 5, 10, and 20 mg) once daily, who were on either low (10 mEq) or high (200 mEq) sodium diets. In sodium-restricted subjects (n = 12), mean diastolic blood pressure declined from control levels in a dose-related fashion during the first 2 h. The mean 24-h diastolic blood pressure values were significantly lower than control (p less than 0.01) at all doses, with the greatest declines occurring with the 10 and 20 mg doses. Accompanying the decline in blood pressure was a significant increase in plasma renin activity (PRA) and reduction in plasma angiotensin II (AII) and aldosterone levels. As with blood pressure, the greatest changes were observed with the 5, 10, and 20 mg doses. However, only the 20 mg dose of enalapril produced prolonged and significant PRA increments (p less than 0.01) and AII decrements (p less than 0.05) of at least 24 h in duration. In sodium-replete subjects (n = 10), a similar pattern of diastolic blood pressure responses was observed; however, the effect of the drug was not so profound. No significant decrease in AII occurred at any dose of enalapril; however, PRA increased modestly but significantly (p less than 0.01) after administration of 5, 10, and 20 mg enalapril. Plasma aldosterone declined modestly but significantly, 2 (p less than 0.05) and 4 (p less than 0.01) h after the 10 mg dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Enalapril (MK-421), a new angiotensin-converting enzyme inhibitor. Acute and chronic effects in heart failure

Enalapril (MK-421) is a new oral angiotensin-converting enzyme inhibitor which was administered to eight patients with chronic congestive heart failure. Four hours after enalapril administration (10 to 20 mg), mean arterial pressure fell from 95.6 +/- 11.4 (SD) to 84.8 +/- 17.6 mm Hg (p less than 0.05), systemic vascular resistance fell from 18.5 +/- 3.0 to 15.8 +/- 4.1 units (p less than 0.02), while pulmonary artery wedge pressure changed insignificantly from 17.5 +/- 9.2 to 14.5 +/- 10.2 mm Hg and cardiac index rose insignificantly from 2.63 +/- 0.46 to 2.82 +/- 0.75 L/min/m2. These hemodynamic effects persisted during one month of enalapril administration. Baseline plasma renin activity of 0.76 +/- 1.07 ng/ml/hr rose to 3.23 +/- 2.87 ng/ml/hr (p less than 0.05) after one month of enalapril administration. During the month of enalapril administration, maximal exercise duration rose from 465.1 +/- 233.0 to 572.3 +/- 233.7 seconds (p less than 0.05), and maximal oxygen uptake increased from 12.3 +/- 2.7 to 16 +/- 11.2 ml/min/kg (p less than 0.05). No major side effect occurred. These sustained effects may be clinically beneficial, and enalapril deserves further evaluation in the long-term treatment of patients with chronic congestive heart failure.

Effects of angiotensin I converting enzyme inhibitors on the renal excretory function, hemodynamics and renin release in isolated perfused rat kidney

Direct renal effects of angiotensin I converting enzyme inhibitors (CEIs), captopril, SA446 and MK421, were examined in isolated rat kidneys perfused with a renin-substrate-free medium. Among three CEIs, only captopril induced a significant natriuresis, whereas SA446 and MK421 did not. UKV, renal vascular resistance and creatinine clearance were not affected by any of these CEIs. Renin release from perfused rat kidneys were not influenced by CEIs under the present experimental conditions. These results suggest that among three different types of CEIs, only captopril possesses natriuretic action in the isolated perfused rat kidney and that this action may be independent of its inhibitory action on angiotensin converting enzyme. It is also suggested that these three CEIs themselves do not have a direct effect on the renal vascular bed.

Inhibitors of the angiotensin I converting enzyme as antihypertensive drugs

Inhibitors of the angiotensin I converting enzyme (captopril, enalapril) offer a new principle in the drug treatment of hypertension and congestive heart failure. The present survey deals with the mode of action of the converting enzyme inhibitors, including possible interference with the renin-angiotensin systems in the kidney, the vascular wall and the brain, with the kallikrein--bradykinin system and with the sympathetic nervous system, at both pre- and postjunctional sites. Furthermore, the haemodynamic pattern as well as the therapeutic applications are discussed, including the most important side-effects, contra-indications, interactions and clinical pharmacokinetic properties.

Cardiac regression and blood pressure control in the Dahl rat treated with either enalapril maleate (MK 421, an angiotensin converting enzyme inhibitor) or hydrochlorothiazide

Enalapril maleate (MK 421), and hydrochlorothiazide were used to evaluate the control of hypertension and reversal of myocardial hypertrophy in Dahl sensitive (DS) and Dahl resistant (DR) rats given either a high (8% NaCl) or a low salt (0.4% NaCl) diet. Groups of six-week-old male DS and DR rats were treated with enalapril (15-100 mg/kg/day) in drinking water for eight weeks. Additional comparable groups of DS and DR were also treated with hydrochlorothiazide (60-400 mg/kg/day). Systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate (HR) and heart weight/body weight (Hwt/Bwt) ratio were determined. We observed significant reduction in Hwt/Bwt ratio (P less than 0.001) along with control of SBP and DBP in the DS given a high salt diet treated with either enalapril or hydrochlorothiazide. However, in the DR given a high salt diet, cardiac regression (Hwt/Bwt ratio, P less than 0.05), SBP and DBP (P less than 0.01) reduction were seen only with enalapril. Similarly, cardiac regression (Hwt/Bwt ratio, P less than 0.05) was observed along with reduction of SBP and DBP (P less than 0.001) in the DS given a low salt diet and DR given enalapril. These data indicate that enalapril reduced SBP and DBP in association with cardiac regression in hypertensive and normotensive rats. In contrast, hydrochlorothiazide only reduced SBP, DBP and caused cardiac reversal (Hwt/Bwt ratio) in DS placed on a high salt diet.

Endogenous angiotensin II as a determinant of sodium-modulated changes in tissue responsiveness to angiotensin II in normal man

Dietary sodium restriction reduces vascular smooth muscle, particularly renovascular, responsiveness to infused angiotensin II (AII), while the responsiveness of the adrenal and the AII-renin short feedback loop to AII is enhanced. To determine whether circulating AII mediates these changes in responsiveness, we studied 11 sodium-restricted and 9 sodium-replete normal subjects before and after 75 h of converting enzyme inhibitor pretreatment with MK421. All subjects received infusions of paraaminohippurate (PAH) to assess renal plasma flow during graded AII infusion (0.3-10 ng/kg X min) before and after MK421 administration. Plasma aldosterone, cortisol, PRA, AII, sodium, potassium, and PAH clearance were measured at the onset and end of each AII dose. In sodium-restricted subjects, preinfusion AII and aldosterone levels were significantly reduced, (P less than 0.001) to the range found in sodium-replete subjects, after 75 h of MK421 administration, whereas blood pressure and PAH responses to infused AII were significantly enhanced (P less than 0.01). Blood pressure and PAH responses to infused AII in sodium-replete subjects were not significantly modified by MK421 treatment, confirming that the drug effect was specific. In contrast, the plasma aldosterone increment and PRA decrement after AII infusion were similar before and after MK421 on both diets. Thus, sodium-modulated changes in PAH and blood pressure responsiveness to infused AII depend on circulating AII levels. However, circulating AII does not mediate sodium modulation of adrenal or PRA short-feedback loop responsiveness to infused AII. Two different mechanisms determine sodium modulation of tissue responsiveness to AII; in one, circulating AII via a receptor mechanism is the mediator, and in the other, some other factor(s) also linked to sodium intake must be responsible.

Successful management of severe congestive cardiac failure with enalapril

The new angiotensin-converting enzyme inhibitor enalapril (MK-421) was administered to a patient with severe congestive cardiac failure. Dramatic and sustained symptomatic and hemodynamic improvement is reported. The possible clinical significance of this agent's favorable profile is briefly discussed.

Increased renal plasma flow in long-term enalapril treatment of hypertension

The renal effects of long-term antihypertensive treatment with enalapril were evaluated in 34 subjects (age, 53 yr; range, 27 to 65) with mild, uncomplicated hypertension. After receiving placebo for 4 wk, subjects were randomly assigned to groups receiving incremental doses of enalapril (10, 20, or 40 mg/day for 4 wk each) in a single morning dose or two divided doses, or of placebo. One subject who received enalapril developed acute renal failure by the end of the study. There was no evidence of glomerular or tubular damage in the other subjects; as measured by 24-hr urinary protein excretion, urinary activity of N-acetyl-beta-D-glucosaminidase, and uric acid clearance. During treatment with enalapril, renal plasma flow (measured with 131I-iodohippurate sodium) and glomerular filtration rate increased by 12.1% and 6.8%. Changes in renal plasma flow correlated inversely with age and final mean arterial pressure and correlated positively with initial plasma renin activity of subjects. Except for an occasional idiosyncratic adverse reaction, enalapril is a safe and effective antihypertensive drug with the unique ability to increase renal function despite a fall in renal perfusion pressure.

Humoral and renal effects of MK-421 (enalapril) in hypertensive subjects

To assess the effect of MK-421 (enalapril) we treated six hospitalized hypertensive patients receiving constant sodium intake with incremental doses of this new angiotensin-converting enzyme blocking drug. After a few days of placebo treatment, MK-421 was given in single daily doses, starting with 1.25 mg and increasing until blood pressure was adequately controlled. On the lowest dose, converting enzyme activity was reduced by 50%, but angiotensin II and blood pressure did not change significantly. There were, however, significant increases in noradrenaline, renin, and aldosterone. With higher doses there was a more pronounced reduction in converting enzyme activity, while angiotensin II, aldosterone, and blood pressure all fell significantly. Renin levels rose, but noradrenaline and adrenaline were reduced. Orthostatic hypotension was not observed. With continued treatment, renal vasodilatation and enhanced natriuresis occurred together with a 1.2 kg decrement in body weight. Concurrently plasma volume rose, but renal blood flow remained unchanged. The data indicate that MK-421 effectively lowers blood pressure, and it does so by converting enzyme inhibition; sodium loss and a decrease in sympathetic activity are associated features. Since plasma volume increased despite enhanced natriuresis, the drug may act both at the arteriolar and at the venular level.

Enalapril in low-renin essential hypertension

The antihypertensive efficacy of N-[(S)-1-(ethoxycarbonyl)-3-phenyl-propyl]-L-alanyl-L-proline (enalapril maleate) was evaluated in a randomized, double-blind trial in 23 patients with mild low-renin essential hypertension ranging in age from 32 to 70 yr (20 were black and 3 were white). All underwent a 4-wk washout-placebo phase and were then assigned to a dosing schedule of either 10 mg enalapril once daily, 5 mg enalapril twice daily, or placebo twice daily for 12 wk. Conditional on diastolic pressure, the dose was increased at 4-wk intervals to a maximum of 40 mg daily or until control was achieved or the end of the study reached. At the end of the 12-wk titration phase, there was a follow-up period during which measurements were made after discontinuation of the medication. Mean supine diastolic pressure decreased from baseline (98.5 +/- 2.6 mm Hg) during the titration phase (86.3 +/- 4.6 mm Hg) in the group taking enalapril once daily. In three of the eight patients in the once-daily group and five of eight in the twice-daily group, supine diastolic pressures fell below 90 mm Hg. Neither supine nor standing systolic pressure nor standing diastolic pressure decreased significantly from pretreatment levels during enalapril once or twice daily. Heart rates measured after 5 min supine rest were not altered by enalapril. Enalapril induced inhibition of converting enzyme activity at all dose levels and with both dosing schedules. No adverse effect attributable to enalapril occurred during the study. The data indicate that once-daily enalapril is safe and effective treatment for mild low-renin essential hypertension.

The role of angiotensin II in the development of hypertension and in the maintenance of glomerular filtration rate during 48 hours of renal artery stenosis in conscious dogs

The responses to 48 h of renal artery stenosis were compared in uninephrectomized, chronically-instrumented dogs with or without inhibition of angiotensin II (AII) formation by enalapril. Mean arterial pressure rose by an average of 29.9 mmHg (s.e.m. 3.5) in untreated dogs and by 14.5 mmHg (s.e.m. 2.8) in enalapril-treated dogs over the two days of stenosis. Renal artery stenosis reduced glomerular filtration rate (GFR) by 49% (s.e.m. 9) in untreated dogs and by 86% (s.e.m. 8) in enalapril-treated dogs. Compared to untreated dogs, enalapril-treated dogs also had lower renal artery pressure distal to the stenosis, drank less water and had larger rises in plasma K+ following renal artery stenosis. There were no differences in renal blood flow or urinary Na+ excretion in the two groups of dogs. Thus blockade of AII production did not prevent hypertension occurring in response to renal artery stenosis, but the rise in blood pressure was only about half that which occurred in normal dogs and GFR was much more severely reduced.

Blood pressure maintenance in awake dehydrated rats: renin, vasopressin, and sympathetic activity

The role of vasopressin, the renin system, and sympathetic activity in sustaining blood pressure in the dehydrated state was investigated in normotensive nonanesthetized male Wistar rats. After 48-h dehydration, plasma arginine vasopressin was 14.0 +/- 1.7 pg/ml and plasma norepinephrine 0.46 +/- 0.05 ng/ml. In another group of rats in which the angiotensin converting enzyme inhibitor (MK 421, 5 mg po twice daily) was administered throughout the dehydration period, blood pressure was reduced by more than 20% (P less than 0.001), and both plasma arginine vasopressin and norepinephrine were higher at 23.4 +/- 3.9 pg/ml (P less than 0.01) and 0.83 +/- 0.07 ng/ml (P less than 0.01), respectively. Taken together, in rats with or without converting enzyme blockade, there was an inverse correlation between mean blood pressure and plasma arginine vasopressin (r = 0.67, P less than 0.01) as well as plasma norepinephrine (r = 0.82, P less than 0.01) levels. The acute administration of a specific vasopressin pressor inhibitor (dPVDAVP) reduced mean blood pressure in the rats with a blocked renin system by 16.9 mmHg (P less than 0.001). In rats without converting enzyme inhibition, the induced fall was only 6.4 mmHg. These results indicate that following 48-h dehydration the renin angiotensin system interacts with the vasopressin secretory mechanism to sustain blood pressure, with renin playing a predominant role. They further suggest that, following blockade of the renin system, activation of the sympathetic nervous system probably also contributes to blood pressure maintenance.

Haemodynamic, hormonal, and electrolyte effects of enalapril in heart failure

Enalapril, the new converting enzyme inhibitor, was administered to eight patients with heart failure (NYHA Functional Class II to IV) during standardised and intensive haemodynamic, hormone, and electrolyte monitoring. The first dose (5 mg) of enalapril induced a fall in plasma angiotensin II and noradrenaline levels, and prolonged decrements in systemic vascular resistance, arterial pressure, heart rate, and right heart pressures. Maximum haemodynamic effects were evident four to eight hours after the first dose, with return to baseline by 24 hours. Plasma angiotensin II levels, however, were still suppressed at 24 hours. The magnitude of haemodynamic response was related closely to baseline (pre-enalapril) activity of the renin-angiotensin system and the sympathetic system. Enalapril treatment over three days induced a positive cumulative balance of sodium and potassium, and a small increase in plasma potassium. Urine aldosterone excretion decreased in a stepwise fashion. Continued enalapril administration for four to eight weeks resulted in improved clinical status (NYHA Functional Class) and exercise tolerance in patients who initially were most severely incapacitated, but little change was observed in healthier subjects. We conclude that in heart failure, enalapril is a long acting converting enzyme inhibitor with clear cut beneficial haemodynamic effects in the short term. Long term controlled studies of enalapril in heart failure are warranted.

Angiotensin-converting enzyme inhibitory activity of SCH 31846, a new non-sulfhydryl inhibitor

SCH 31846, 1-(N-[1(S)-(ethoxycarbonyl)-3-phenylpropyl]-(S)-alanyl)-cis, syn-octahydro-(H-indole-2-S)-carboxylic acid; CI-907; PD 109, 763-2, is a new non-sulfhydryl-containing, angiotensin-converting enzyme (ACE) inhibitor. The present investigation describes its ACE inhibitory properties and compares them to those of MK 421. The diacid of SCH 31846 inhibited rabbit pulmonary ACE with an IC50 of 2.2 nM (MK 421 diacid 2.5 nM). The drug behaved as a competitive and specific inhibitor in vitro. SCH 31846 and its diacid effectively inhibited pressor actions of intravenous injection of angiotensin I (AI) in anesthetized rats. ID50 values were 27 and 11 micrograms/kg for SCH 31846 and SCH 31846 diacid, respectively (MK 421 and MK 421 diacid 57 and 15 micrograms/kg, respectively). Oral administration of SCH 31846 (0.03-1 mg/kg) inhibited pressor actions of AI in conscious rats with a duration of over 16 h at 0.3 and 1 mg/kg. SCH 31846 was 2.2 times as potent as MK 421 in this regard. The diacid of SCH 31846 was considerably less potent than the ester, implying poor oral absorption of the former. Effective ACE inhibition, as judged by attenuation of pressor actions of AI, was noted in dogs after both intravenous and oral administrations of SCH 31846. Onset of action was more rapid than that of MK 421. Intravenous administration of SCH 31846 inhibited the renal vascular actions of intrarenal injection of AI, indicating effective blockade of the renal enzyme. Intracerebroventricular administration of SCH 31846 diacid blocked pressor responses to intracerebroventricular AI, whereas oral administration of SCH 31846 (10 mg/kg) did not, implying that SCH 31846 inhibits brain ACE but does not gain access to the cerebral enzyme when administered orally. These data indicate that SCH 31846 is a potent and specific non-sulfhydryl ACE inhibitor. As such, it should be useful in the treatment of hypertension and heart failure.

Reversal of diuretic-induced secondary hyperaldosteronism and hypokalemia by enalapril (MK-421): a new angiotensin-converting enzyme inhibitor

The study reported here prospectively evaluated the prevention of diuretic-induced secondary hyperaldosteronism and hypokalemia by a converting enzyme inhibitor, enalapril (MK 421). Eighteen normal subjects were randomized into three groups: (1) a HCTZ group (hydrochlorothiazide (HCTZ) 50 mg/day); (2) a MK-421 group (MK-421 10 mg/day); and (3) a HCTZ + MK-421 group [HCTZ 50 mg/day plus MK-421 10 mg/day]. Following a five-day control and a 28-day treatment period, the HCTZ group demonstrated an attenuated but persistent secondary hyperaldosteronism and hypokalemia, the MK-421 group manifested a gradual decline in aldosterone secretion, and the HCTZ + MK-421 group had a delayed but effective correction of secondary hyperaldosteronism and hypokalemia at 28 days but not before. In conclusion, MK-421 reversed diuretic-induced secondary hyperaldosteronism and hypokalemia after 28 days of hydrochlorothiazide therapy. Therefore, converting enzyme inhibitors, such as enalapril, provide useful adjunctive therapy in diuretic-treated patients, but potassium supplementation may be required before the start of four weeks of combined therapy.

Antihypertensive and metabolic effects of a new converting enzyme inhibitor, enalapril

Thirty-one men with mild, uncomplicated essential hypertension were studied for 18 wk in a double-blind, placebo-controlled, randomized clinical trial. Those whose supine diastolic pressure was 90 to 104 mm Hg after 4 wk of placebo were randomly assigned to three groups. Group I (11 subjects) initially received one 10-mg enalapril capsule in the morning and one placebo capsule in the evening. Group II (10 subjects) received one 5-mg enalapril capsule twice daily. Group III (10 subjects) received one placebo capsule twice daily. Drug dosages were doubled and then quadrupled in all groups at wk 8 and 12. Metabolic, ophthalmologic, and audiometric studies were done on all subjects at wk 2, 4, 8, 12, and 16. Enalapril lowered diastolic pressure in the supine and upright positions in single and divided doses. Its antihypertensive effect was dose dependent, and it was greater in white patients than in black patients. The drug was well tolerated by all subjects and did not cause clinical or metabolic complications. It is concluded that enalapril is effective in lowering the arterial pressure in single and divided daily doses; its effect is dose dependent and is greater on the diastolic arterial pressure than on the systolic pressure; and it is well tolerated.

The effect of enalapril on serum prolactin

Twelve healthy male volunteers received single daily oral doses of enalapril (EN) 10 mg for 8 consecutive days. Serum samples for prolactin (PRL) assay were taken on day 0 (baseline) at 12.00 h and 16.00 h, and on days 1 and 8 at 08.00 h (pre-drug), 12.00 h and 16.00 h. The 08.00 h values on day 1 served as the pre-drug baseline. There were no significant changes from baseline in serum PRL levels on days 1 and 8. All mean serum PRL levels on day 8 were significantly (P less than 0.01) lower than the upper limit of the normal range (2-15 ng/ml) found for healthy males in this laboratory. It is concluded that therapeutic doses of EN (10 mg/day, p.o.) for 8 consecutive days do not raise mean serum PRL levels above the normal range in healthy male volunteers.

Effects of captopril and enalapril on regional vascular resistance and reactivity in spontaneously hypertensive rats

The present study compares the effects of short-term treatments with captopril and enalapril, administered in equipotent antihypertensive doses, on the regional vascular resistances and on the regional vascular responsiveness to vasopressor agents of adult spontaneously hypertensive rats (SHRs). Three groups of animals were treated by gavage with captopril (100 mg/kg), enalapril (25 mg/kg), or distilled water for 8 days. Arterial blood pressure (BP), heart rate (HR), plasma renin concentration (PRC), and plasma converting-enzyme activity (CEA) were measured. Cardiac index (CI), total peripheral resistance (PR), and organ flow distribution were determined using microspheres. Renal and mesenteric vascular responsiveness to vasopressor agents was evaluated by continuous measurement of renal and mesenteric blood flows with miniaturized pulsed Doppler flow probes. Data showed that in the anesthetized SHR the two drugs induced similar reductions in BP, PR, and HR, without affecting CI. They simultaneously produced a strong converting-enzyme inhibition as evidenced by the suppression of angiotensin I effects accompanied by a potentiation of angiotensin II responses, a reduction in CEA, and an increase in PRC. Organ flows were similarly and homogeneously increased, especially in the kidneys, in both treated groups. Norepinephrine (NE) vasoconstrictor responses were abolished in the mesenteric vascular bed by both drugs, but in the renal, NE responses although completely abolished by captopril were only partially reduced by enalapril. It thus appears that diminished vascular responsiveness to NE, especially in the case of captopril, is probably involved along with converting-enzyme inhibition in the antihypertensive action of converting enzyme inhibitors (CEI), the mechanism of the difference between captopril and enalapril remaining still speculative.

Comparative antihypertensive effects of enalapril maleate and hydrochlorothiazide, alone and in combination

Enalapril maleate is an investigational oral prodrug whose hydrolyzed diacid metabolite is a potent angiotensin-converting enzyme inhibitor. Fourteen patients with mild to moderate hypertension were evaluated after receiving placebo, and two weeks of treatment with each of the following: enalapril maleate (20 mg b.i.d.), hydrochlorothiazide (25 mg b.i.d.), and the two in combination. In comparison to placebo, the magnitudes of the blood pressure reduction after enalapril and hydrochlorothiazide alone were comparable. The reduction in blood pressure following enalapril was evident throughout the 12-hour dosing interval. The combination of enalapril and hydrochlorothiazide resulted in a marked further reduction in blood pressure that was greater than that predicted from the responses to the individual drugs (P less than 0.05). Biochemical parameters confirmed inhibition of angiotensin-converting enzyme during enalapril treatment; serum angiotensin-converting enzyme activity proved an excellent monitor of compliance. Enalapril was generally well tolerated. Adverse effects included symptomatic hypotension in three patients when enalapril was first added to hydrochlorothiazide and hyperesthesia of the oral mucosa without a loss of taste in one patient on enalapril. Enalapril maleate alone and especially in combination with hydrochlorothiazide appears to be an effective, well-tolerated converting enzyme inhibitor with at least a 12-hour duration of action.

Effects of a new angiotensin converting enzyme inhibitor, enalapril, in acute and chronic left ventricular failure in dogs

Enalapril (MK-421) is a new angiotensin converting enzyme inhibitor which effectively lowers elevated blood pressure and might also be useful in heart failure. Enalapril was infused into six awake dogs 2 hours after left circumflex coronary artery embolization (acute failure group) and into six other awake dogs two to six months after coronary embolization (chronic failure group). In the acute failure group 2 hours after embolization, increased left ventricular end-diastolic pressure and reduced cardiac output remained unchanged during enalapril infusion. In the chronic failure group, increased left ventricular end-diastolic pressure also remained unchanged during enalapril infusion, but cardiac output, which had fallen to 131.8 +/- 11.9 (S.D.) from 165.8 +/- 17.9 ml/min/kg (P less than 0.01) by two to six months in this group rose during enalapril infusion to 154.5 +/- 27.7 ml/min/kg (P less than 0.05). Heart rate and blood pressure were not changed during enalapril in either group, but stroke volume rose (26.0 +/- 5.9 to 29.2 +/- 6.9 ml, P less than 0.01) and systemic vascular resistance fell (58.5 +/- 10.3 to 39.3 +/- 4.3 units, P less than 0.01) during enalapril only in the chronic failure group. Plasma renin activity after embolization was slightly but not significantly higher in the acute failure group. Thus, enalapril appears to be an arterial vasodilator in dogs with chronic but not acute left ventricular failure.