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To examine the cellular mechanisms of contractile dysfunction in postinfarction heart failure, we studied the effects of beta-adrenergic receptor stimulation on contractile function and Ca2+i handling of noninfarcted papillary muscles from sham-operated (n = 17) and infarcted (n = 17) rats. Ca2+i transients measured with the bioluminescent protein aequorin and parameters of isometric contraction were recorded during graded isoproterenol stimulation. Developed tension and peak rate of tension rise were depressed (p less than 0.05) in muscles from infarcted rats at physiological and maximally stimulating [Ca2+]oS. The time to peak tension was prolonged in the muscles from the infarcted rats, corresponding with prolongation of the time to peak Ca2+i. In muscles from sham-operated rats, isoproterenol increased both the amplitude of the Ca2+i transient and the peak rate of tension rise. In contrast, the inotropic response to isoproterenol was severely blunted in the muscles from infarcted rats despite a large increase in the amplitude of the Ca2+i transient. Isoproterenol abbreviated the time course of the isometric twitch and the Ca2+i transient in both groups. These findings suggest that postinfarction heart failure may be related in part to decreased force-generating capacity of the myofilaments. Treatment with captopril for 5 weeks, beginning 1 week after infarction (n = 14), resulted in reduction of left ventricular filling pressures and partial normalization of myocardial contractility and Ca2+i handling. In addition, compared with muscles from untreated infarcted rats, muscles from the captopril-treated rats exhibited improved contractile responses to increasing [Ca2+]o or isoproterenol. The inotropic response to isoproterenol in muscles from all three groups of rats had a significant negative correlation (r = -0.64, p less than 0.0001) with left ventricular end-diastolic pressure measured in vivo. Thus, the defect in excitation-contraction coupling in rats with postinfarction heart failure may be partially normalized by chronic load reduction with an angiotensin converting enzyme inhibitor.

Captopril enhances intracellular calcium handling and beta-adrenergic responsiveness of myocardium from rats with postinfarction failure.