Correlation between effects of hydralazine on force and on the adenyl cyclase system of ventricular myocardium in dogs and cats

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Abstract
Hydralazine is a potent arteriolar dilator, which increases cardiac output in patients with heart failure. Previous studies suggested that these beneficial effects might be due in part to a positive inotropic effect. The present study further investigated the effect of hydralazine on myocardial contractility and adenyl cyclase activity. In isolated cat papillary muscles, bath concentrations of hydralazine up to 10−4 mol·litre−1 did not alter force development, whereas 10−3 mol·litre−1 hydralazine increased isometric force by 31%. This effect was blocked by 10−6 mol·litre−1 propranolol and was absent after catecholamine depletion produced by previous reserpine treatment. In canine ventricular myocardium hydralazine in all concentrations used (10−7 to 1(T−3 mol·litre−1) increased control adenyl cyclase activity. This increase was statistically significant in 10−6 to 10−3 mol·litre−1concentrations, reaching a maximum of 69.5% at 10−4 mol·litre−1. In cat ventricular myocardium 10−6 to 10−3 mol·litre−1 hydralazine increased the cyclic AMP production, although to a lesser magnitude than that in canine tissue. Hydralazine 10−5 mol·litre−1 produced a 37.8% increase, and the maximum effect of 45.2% occurred at 10−3 mol·litre−1. The positive effects of hydralazine were completely abolished by the addition of propranolol in dogs as well as in cats. Thus the adenyl cyclase stimulation induced by hydralazine is mediated through the beta receptor. Although there is quantitative dissociation concerning the effective dose on force compared with the effective dose on the adenylate cyclase system, the overall data produce the evidence for an adrenergic mechanism that might be responsible for inotropic stimulation.

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