Cardiovascular properties of medroxalol, a new antihypertensive drug.

Medroxalol is a new antihypertensive agent that is presently undergoing clinical trial. Its cardiovascular properties were studied using spontaneously hypertensive rats (SHR), anesthetized dogs, and isolated tissues. Medroxalol produced a long-lasting fall in blood pressure when given by the oral route to SHR. It was more potent than phentolamine in antihypertensive effectiveness. Given intravenously to dogs, medroxalol reduced the blood pressure and heart rate of doses that did not greatly reduce cardiac output. The hypotensive effect of medroxalol was reduced but not abolished following alpha- and beta-adrenergic-receptor blockade. Medroxalol inhibited heart rate and blood pressure responses to isoproterenol and phenylephrine in dogs. In vitro medroxalol resembled a competitive antagonist at alpha-adrenergic receptors in rabbit aortic strips (pA2 6.09) and beta-adrenergic receptors in guinea pig atria (pA2 7.73). It was 0.02 as potent as phentolamine at alpha-receptors and 0.09 as potent as propranolol at beta-receptors. It was concluded that the principal action of medroxalol was to produce a fall in blood pressure by decreasing peripheral vascular resistance more than cardiac output. Adrenergic alpha- and beta-receptor blockade alone does more than cardiac output. Adrenergic alpha- and beta-receptor blockade alone does not satisfactorily explain the hypotension. A contribution by an active vasodilatory component appears likely.