Effect of enalaprilic acid on cardiac contractile response to beta-adrenergic stimulation

Studies in two-kidney--one clip hypertensive rats have demonstrated that long-term treatment with enalapril induced regression of cardiac hypertrophy, but the cardiac contractile response to beta-adrenergic stimulation remained depressed. In the present study, we evaluate the contractile response to beta-adrenergic stimulation of isolated papillary muscle in normal rats with isoproterenol (10(-11) M to 10(-4) M) in the presence of enalaprilic acid (10(-6) M or 10(-4) M) or enalaprilic acid (10(-4) M) and angiotensin II (10(-6) M). Myocardial contractility was characterized by maximal developed tension and maximal rate of rise of tension (+T), and the relaxant effect of isoproterenol by the ratio of (+T), and the maximal velocity of relaxation (-T)(+T/-T ratio). The rest tension (g/mm2) and the cross-sectional area (mm2) were similar in all the muscles studied. Enalaprilic acid (either 10(-6) M or 10(-4) M) in the bath did not induce any change in contractile and relaxation parameters. The increment in +T and -T (expressed as percentage) in response to cumulative doses of isoproterenol (10(-11) M to 10(-4) M) was significantly depressed in the presence of enalaprilic acid (10(-4) M) when compared with control hearts in which only vehicle was added before isoproterenol (p less than 0.05). The addition of angiotensin II after enalaprilic acid (10(-4) M) did not normalize the response in +T and -T. Enalaprilic acid diminishes the contractile response of the papillary muscle to beta-adrenergic stimulation. The inhibition of the local angiotensin II does not seem to be involved in this result.