Vasodilating antihypertensive drug-induced aldosterone release--a study of endogenous angiotensin-mediated aldosterone release in the rat

The vasodilatory drugs, minoxidil and hydralazine, induce renin release in the rat, man and the dog. Previous reports suggest that the rat adrenal cortex was insensitive to angiotensin stimulation. As a result these studies were designed to obtain evidence for or against the hypothesis that the control of aldosterone release in the rat is unique among mammalian species. Minoxidil and hydralazine induced a time-related increase in both serum renin activity and serum aldosterone. Minoxidil caused a dose-related, proportional increase in serum renin and aldosterone. This response was blocked by prior bilateral nephrectomy but was not affected by hypophysectomy. A competitive angiotensin antagonist, saralasin (1-Sar-8-Ala angiotensin II), impaired minoxidil-induced aldosterone release in a dose-related manner while potentiating minoxidil-induced renin release. Pretreatment with propranolol, a beta adrenergic blocking drug, impaired minoxidil-induced renin and aldosterone release. Only small changes in serum corticosterone occurred after minoxidil or hydralazine administration. These results indicate that minoxidil-induced aldosterone release was mediated by the endogenous angiotensin II formed from renin release. They also support the unanesthetized rat as an appropriate animal model for study of the renin-angiotensin-aldosterone axis and its modification by drugs.

Control plasma renin activity and changes in sympathetic tone as determinants of minoxidil-induced increase in plasma renin activity

A study was made of the possible mechanism(s) underlying minoxidil-induced increase in plasma renin activity (PRA). 10 patients with essential hypertension were treated with minoxidil and subsequently with a combination of minoxidil plus propranolol. Minoxidil lowered mean arterial pressure 31.6 plus or minus 3.3 mm Hg, mean plus or minus SEM. There was an associated increase in both PRA, 6.26 plus or minus 2.43 NG/ML/H, and heart rate, 21.4 plus or minus 2.7 beats/min. The changes in PRA and heart rate were positively correlated, r, 0.79. Addition of propranolol reduced mean arterial pressure by a further 10.1 plus or minus 1.5 mm Hg and returned heart rate to control levels. Propranolol reduced PRA significantly but not to control levels. Control PRA positively correlated with PRA on minoxidil, r, 0.97, and with PRA on minoxidil plus propranolol, r, 0.98. We conclude that control PRA is a major determinant of change in PRA with minoxidil. Minoxidil increased PRA by at least two mechanisms: (a) an adrenergic mechanism closely related to change in heart rate and blocked by propranolol, and (b) a mechanism(s) not sensitive to propranolol and possibly related to decrease in renal perfusion pressure.

Altered renin release and propranolol potentiation of vasodilatory drug hypotension

Vasodilating antihypertensive drugs induce hypotension with reflex tachycardia, renin release, and fluid and electrolyte retention. Propranolol can impair this renin release. The studies described here were designed to determine the hemodynamic role of vasodilatory drug-induced renin release and inhibition thereof by propranolol in two animals models, the unanesthetized, normotensive and the unanesthetized, genetically hypertensive rat. In studies with normotensive rats, propranolol impaired renin release and tachycardia resulting from hydralazine and minoxidil and potentiated their hypotensive action. Two additional interventions against the renin-angiotensin system were used in evaluating the mechanism of this potentiation. One was removal of the renin source by nephrectomy, and the second was blockade of angiotensin's vasoconstrictor action using a selective angiotensin antagonist, saralasin (1-Sar-8-Ala-angiotensin II (previously known as P113)).. Both interventions potentiated vasocilatory drug hypotension, as did propranolol, but did not prevent reflex tachycardia. When combined with saralasin propranolol did not add to protentiation by this peptide. A similar pattern of blood pressure decrement and potentiation was seen in genetically hypertensive rats when propranolol or saralasin treatment preceded hydralazine. Propranolol was demonstrated to block hydralazine-induced increases in serum renin activity in genetically hypertensive rats. We conclude that hypotensive potentiation of vasocilating drugs by propranolol in these animal models is mediated to a large extent by impairment of renin release. Persistence of hypotensive tachycardia after nephrectomy and after saralasin in normotensive rats suggests the irrelevance of angiotensin's central nervous system stimulation to this cardiac effect. Clinical studies are underway to quantify the potential importance of this beneficial drug interaction in man.

Stimulation of vascular smooth muscle sodium, potassium--adenosinetriphosphatase by vasodilators

A ouabain-sensitive, Mg 2+ -dependent, Na + , K + -stimulated adenosinetriphosphatase (ATPase) isolated from canine mesenteric arteries was activated by the following vasodilators: hydralazine, diazoxide, PGE 1 , PGE 2 , PGA 2 , and minoxidil. Epinephrine, norepinephrine, and isoproterenol also stimulated the ATPase, but PGF 2α was ineffective. Since these vasodilators activate the adenylate cyclase of vascular smooth muscle, the effects of cyclic adenosine monophosphate (AMP) and theophylline on the Na + , K + -ATPase were studied; both substances caused a concentration-dependent increase in enzymatic activity. Propranolol blocked the catecholamine-induced stimulation of Na + , K + -ATPase, and polyphloretin phosphate antagonized the effects of the prostaglandins. It is concluded that vasodilatation in response to these substances is associated with the stimulation of the Na + , K + -ATPase of vascular smooth muscle probably mediated through cyclic AMP.

Adrenergic component of renin release induced by vasodilating antihypertensive drugs in the rat

The capacity of the vasodilating drugs, minoxidil and hydralazine, for inducing renin release was characterized in rats according to chronology and dose response. Propranolol inhibition of this renin release was also characterized and related to serum levels of propranolol. Minoxidil and hydralazine (1.0 mg/kg) induced sevenfold elevations of serum renin activity. Treatment with propranolol, resulting in plasma propranolol concentrations as low as 50 ng/ml, impaired vasodilatory drug-induced renin release. The propranolol inhibition suggests a beta-adrenergic component to this type of renin release and the potential for a clinically efficacious drug interaction.