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 ⁴⁵Ca, subjected to ⁴⁵Ca washout in the presence of different cytosolic energy levels, then stimulated with 10 mM caffeine to release residual sarcoplasmic reticular ⁴⁵Ca. When ryanodine (1 μM) was applied to open Ca²⁺ channels and thereby released ⁴⁵Ca from the sarcoplasmic reticulum during washout, caffeine-stimulated ⁴⁵Ca release was decreased 96%, demonstrating that virtually the entire caffeine-sensitive ⁴⁵Ca 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 ⁴⁵Ca pool size was observed. Thus, caffeine-induced ⁴⁵Ca 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.