ENERGETIC MODULATION OF CARDIAC INOTROPISM AND SARCOPLASMIC RETICULAR CA2+ UPTAKE

Myocardial contractile performance is a function of sarcoplasmic reticular Ca2+ uptake and release. Ca2+ handling is ATP-dependent and can account for up to 40% of total myocardial energy expenditure. We tested the hypothesis that the thermodynamics of the cytosolic adenylate system can modulate sarcoplasmic reticular Ca2+ handling and hence function in intact heart. Cellular energy level was experimentally manipulated by perfusing isolated working guinea-pig hearts with substrate-free medium or media fortified with lactate and/or pyruvate as the main energy substrate. Left ventricular contractile function was judged by stroke work and intraventricular dP/dt. Cytosolic energy level was indexed by measured creatine kinase reactants. Relative to 5 mM lactate, 5 mM pyruvate increased left ventricular stroke work, dP/dt(max), and dP/dt(min), while lowering left ventricular end-diastolic pressure at physiological left atrial and aortic pressures. Pyruvate also doubled cytosolic phosphorylation potentials and increased [ATP]/[ADP] ratio; this energetic enhancement distinguishes pyruvate from inotropic stimulation by catecholamines, which are known to decrease cytosolic energy level in perfused heart. Sarcoplasmic reticular Ca2+ handling was assessed in hearts prelabeled with Ca-45, subjected to Ca-45 washout in the presence of different cytosolic energy levels, then stimulated with 10 mM caffeine to release residual sarcoplasmic reticular Ca-45. When ryanodine (1 mu M) was applied to open Ca2+ channels and thereby released Ca-45 from the sarcoplasmic reticulum during washout, caffeine-stimulated Ca-45 release was decreased 96%, demonstrating that virtually the entire caffeine-sensitive Ca-45 pool was located in the sarcoplasmic reticulum. In detailed comparisons of pyruvate-energized vs. substrate-free deenergized hearts, an inverse relationship between cytosolic energy level and caffeine-mobilized Ca-45 pool size was observed. Thus, caffeine-induced Ca-45 release was decreased 60% by pyruvate energization and increased 2.5-fold by substrate-free deenergization. Taken together, these results support the hypothesis that enhancement of myocardial inotropism by energy-yielding substrate is mediated by increased sarcoplasmic reticular Ca2+ loading/release. Thus we propose that the known control of sarcoplasmic reticular Ca2+ turnover by the protein kinase/phospholamban system can be modulated by cytosolic energy level.