Cardiac inotropic responses to calcium and forskolin are not altered by prolonged isoproterenol infusion

Effects of prolonged isoproterenol infusion upon the density of cardiac calcium channels, calcium-mediated contractile responses, and the ability of forskolin to enhance tension development and cyclic AMP accumulation were studied in ventricular muscle preparations from Sprague-Dawley rats. Isoproterenol infusion (400 μg/kg per h s.c., 4 days) significantly decreased calcium channel density (B max) in cardiac microsomal membranes as quantified by a 32% decrease in specific [ 3H]nitrendipine binding sites; binding affinity (K D) was unchanged. A 57% decrease of β-adrenoceptors confirmed homologous down regulation. To examine functional effects of decreased [ 3H]nitrendipine binding sites, responses to calcium, BAY K8644 and nifedipine were determined in isolated right ventricular strips. Significant decreases in basal developed tension were observed in muscles from isoproterenal-infused rats. However, concentration-dependent increases in contractility in response to CaCl 2 or BAY K8644 were comparable, and the negative inotropic effect of nifedipine was unchanged. Whereas isoproterenol infusion was associated with significantly decreased basal cardiac cyclic AMP concentrations, exposure of ventricular strips from either vehicle- or isoproterenol-infused rats to 10 μM forskolin resulted in comparable increases in cyclic AMP and in developed tension. Cumulative, submaximal concentrations of forskolin also produced similar increases in contraclity with maximum responses in ventricular strips from vehicle-infused animals attained at 4.4 μM forskolin. Higher concentrations resulted in automaticity. By contrast, ventricle from isoproterenol-infused animals responded to 14.4 μM forskoklin with maximal increases in force of contraction.