Effect of blockade of angiotensin-I converting enzyme on the blood pressure of renal hypertensive rabbits

Effects of captopril (SQ 14, 225) on the renin-angiotensin-aldosterone system in normal rats

High doses of captopril (SQ 14, 225) (120-160 mg/kg/day) were administered orally to normal rats, and the effects on the renin-angiotensin-aldosterone system were observed. Plasma angiotensin converting enzyme (ACE) activity was elevated significantly on the 3rd, 7th and 30th days of captopril administration. ACE activity in the lung and the kidney was significantly decreased on the 1st day then gradually increased, becoming significantly higher than that of controls by the 30th day. Plasma renin activity (PRA) was significantly elevated on the 1st day and remained at a high level until the 30th day. Renal renin content was found to be significantly lower on the 1st and 3rd days. Plasma aldosterone concentration was not affected by captopril treatment, whereas serum potassium concentration was found to be significantly lower on the 1st, 3rd and 30th days. It is suggested that besides its inhibitory action on ACE, captopril has a direct or indirect stimulating action on ACE production as well as on renin release.

Hypotension induced by inhibition of angiotensin-converting enzyme in pentobarbital-anesthetized dogs

The mechanism of the hypotensive response produced by inhibition of the angiotensin converting enzyme was studied in pentobarbital anesthetized dogs. A recently developed potent inhibitor of the converting enzyme, SQ 14,225 (D-3-mercapto-2-methyl propanoyl-L-proline), administered i.v. to intact dogs resulted in a rapid marked decrease in blood pressure. In nephrectomized dogs, SQ 14,225 retained significant hypotensive activity, although the absolute magnitude of the decreases in blood pressure were less than had been observed in dogs with intact kidneys. SQ 14,225 also lowered blood pressure when administered to intact dogs in which angiotensin II receptors had been blocked with the receptor antagonist Sar1,Ala8-angiotensin II. This apparent ability of SQ 14,225 to decrease blood pressure in the absence of a functional renin angiotensin system was shared by a structurally dissimilar, nonapeptide, angiotensin converting enzyme inhibitor, SQ 20,881 (Glu-Trp-Pro-Arg-Pro-Gln-Ile-Pro-Pro). SQ 20,881 also produced significant decreases in blood pressure in nephrectomized dogs. These findings indicate that the angiotensin converting enzyme inhibitors, SQ 14,225 and SQ 20,881 may lower blood pressure in anesthetized normotensive dogs via a mechanism unrelated to either the renin angiotensin system or the renal kinin system.

Inhibition of angiotensin converting enzyme by SQ 14,225 in conscious rabbits

The cardiovascular pharmacology of SQ 14,225 (D-3-mercapto-2-methylpropanoyl-L-proline), a new orally effective inhibitor of angiotensin-coverting enzyme (ACE) was investigated in conscious normotensive rabbits. Intravenous administration of SQ 14,225 (3.1-310.0 microgram/kg) resulted in a dose related inhibition of the pressor responses to 310 ng/kg, i.v. of angiotensin I (AI) without diminishing the pressor responses to 100 ng/kg, i.v. of angiotensin II (AII). In fact, the responsiveness of the rabbits to AII was significantly enhanced by higher doses of SQ 14,225. This enhancement of the pressor effects of AII was found to be related to the inhibition of ACE and the resulting decrease in the levels of endogenous AII. In addition, SQ 14,225 (1.0 mg/kg, i.v.) markedly potentiated the magnitude and duration of the vasodepressor responses elicited by bradykinin (1.0 microgram/kg. i.v.). At a dose of 1.0 mg/kg. i.v., SQ 14,225 had no effect on the vasodepressor effects of intravenously administered isoproterenol (0.4 microgram/kg), acetylcholine (1.0 microgram/kg) or prostaglandin E2 (3.0 microgram/kg, i.v.). The pressor responses to norepinephrine (3.0 microgram/kg, i.v.) were similarly unaffected by SQ 14,225 (1.0 mg/kg, i.v.). In normal rabbits SW 14,225 (1.0 mg/kg, i.v.) caused a small but significant decrease in arterial pressure; it had no such effect in anephric rabbits. The observation of this study indicate that SQ, 14,225 is a specific inhibitor of ACE in conscious rabbits.

Management of the patient with renovascular hypertension

Renal artery stenosis, either fibromuscular or atheromatous, is probably the most common cause of secondary hypertension in man. Both of these diseases are active, ongoing processes that may be ameliorated but not cured by medical or surgical treatment. The clinical history and examination of the patient with hypertension may help differentiate renovascular hypertension from essential hypertension. The presence of a systolic-diastolic or continuous bruit is often an indicator of severe renal artery stenosis. Systemic hypertension is the physiologic consequence of significant renal artery stenosis. Knowledge of the basic concepts of the renin-angiotensin-aldosterone system, as has evolved from experimental models of renovascular hypertension, forms the basis for understanding the process of evaluation and treatment of such patients. The treatment of choice for the patient with severe hypertension and a functionally significant renovascular lesion is surgical--both in terms of successful treatment of hypertension and improved long-term prognosis. Diligent periodic reevaluation of these patients as well as those with less severe hypertension who are receiving medical treatment enables the physician to select the proper management that offers optimal control of patient blood pressure and avoids target-organ damage to the kidneys, central nervous system, or cardiovascular system.

The effect of indomethacin blockade of prostaglandin synthesis on blood pressure of normal rabbits and rabbits with renovascular hypertension

Indomethacin inhibits the synthesis of prostaglandin and the release of renin. These effects were studied in normal rabbits and rabbits with two-kidney Goldblatt hypertension (2KGH) and one-kidney Goldblatt hypertension (1KGH) by giving daily intravenous injections of indomethacin (3mg/kg after two initial doses of 9 mg/kg), and in appropriate control rabbits given diluent phosphate buffer without indomethacin. In normal rabbits, indomethacin significantly decreased immunoreactive plasma prostaglandin E-like substance (IPGE) and plasma renin activity (PRA). Indomethacin did not change plasma creatinine (PCr) or mean blood pressure but it decreased renal blood flow (RBF) and glomerular filtration rate (GFR). In 2KGH rabbits, responses depended on the level of renal function and, to a lesser extent, on the level of PRA. In six of10 2KGH rabbits in which hypertension developed without significant changes in PRA, IPGE, PCr, RBF, and GFR, indomethacin produced changes similar to those seen in normals. In the other four rabbits, development of 2KGH was accompanied by increased PRA, increased IPGE, and decreased RBF and GFR, and indomethacin produced renal failure, oliguria, malignant hypertension, and death within 5 days. In 1KGH rabbits, indomethacin decreased IPGE, PRA, and renal function but increased mean blood pressure. These observations suggest that prostaglandins exert a protective effect on renal function in renovascular hypertension.

Conversion of angiotensin I to angiotensin II

The angiotensin I converting enzyme has two important functions: it inactivates bradykinin and converts angiotensin I to angiotensin II. Inhibition of the enzyme blocks the renin-angiotensin system and decreases systemic blood pressure if the pressure is maintained or increased by renin. The enzyme occurs in a variety of tissues and cell forms. The vascular endothelial cells of the lung and of peripheral blood vessels, and the epithelial cells of the kidney tubules are major sources of the enzyme. In addition to inactivating hypotensive peptides and activating a hypertensive one in the systemic circulation, the enzyme may affect organ functions by hydrolyzing peptides that are formed and released locally.

Control of renin release: a review of experimental evidence and clinical implications

Present knowledge of the mechanisms regulating release of renin is reviewed with particular emphasis on neural factors. Evidence is given for a direct effect of renal innervation on beta adrenergic receptors in juxtaglomerular cells, and for the involvement of reflex release of renin in conditions such as tilting and acute salt depletion. Participation of neural and nonneural mechanisms of control is also shown to occur in other conditions, such as aortic constriction and hemorrhage. The view is held that neural sympathetic factors might explain some of the renin disturbances found in essential hypertension. First, in patients with high renin hypertension part of the hypertension is renin-dependent, and these pressor levels of renin seem to be neurally induced since they can commonly be suppressed by beta adrenoreceptor blocking agents. Second, the hypothesis is presented that patients with low renin hypertension, at least those who have no volume disturbance, have a blunted sympathetic control of renin release. Therefore a sufficiently precise test of sympathetic activity, and possibly of body fluid volumes, should be associated with renin profiles for a better understanding of the pathophysiology of arterial hypertension and as a better guide to therapeutic management. Indeed, most of the available antihypertensive drugs act on sympathetic activity, body fluid volume or renin, and this multifaceted profile would provide more rational guidelines for treatment.

Role of prostaglandins in the reversal of one-kidney hypertension in the rabbit

Hypertensive rabbits with a clip on the renal artery of their solitary remaining kidney show an abrupt decrease in blood pressure after the arterial constriction is released. Although the mechanism underlying this phenomenon remains controversial, some experimental evidence suggests that it could be humorally mediated. The involvement of prostaglandins was investigated by examining the effect of the release of the arterial constriction on blood pressure, renal blood flow, glomerular filtration rate, and urinary output in five conscious single-kidney hypertensive rabbits in which prostaglandin synthesis was blocked with indomethacin (priming intravenous injection of 9 mg/kg followed by a constant infusion of 1 mg/kg hour-1). The results were compared with those obtained in another group of five single-kidney hypertensive rabbits submitted to the same protocol but not treated with indomethacin. The blockade of prostaglandin synthesis with indomethacin prevented the increments in renal blood flow and glomerular filtration rate seen in the control rabbits after unclipping a significantly retarded the appearance of diuresis and the fall in blood pressure. Despite these observations, the results do not indicate a major participation of prostaglandins in the reversal of single-kidney hypertension, because the decrease in blood pressure 9 hours after the removal of the arterial constriction was similar in both groups.