ENERGETIC SUPPORT OF POST-ISCHEMIC CARDIAC PERFORMANCE

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Contractile function is depressed for prolonged periods in reversibly injured, viable myocardium following relatively brief coronary occlusions. Although recognized as an important clinical problem, the pathophysiological mechanisms of this myocardial 'stunning' are controversial. We have developed and characterized an isolated working guinea-pig heart model of stunning. In these hearts, the energy-yielding substrate pyruvate produces marked, parallel enhancements of ventricular performance and myocytic energy level (cytosolic ATP phosphorylation potential and Gibbs free energy of ATP hydrolysis). The overall goal of this research is to delineate the subcellular mechanisms responsible for these energy-linked functional improvements. Myocardial function is controlled in large measure by the Ca2+ pumping activity of the sarcoplasmic reticulum Ca2+ ATPase. Our preliminary findings indicate that pyruvate-energization enhances sarcoplasmic reticulum Ca2+_ transport in non-ischemic heart. Project one will test the hypothesis that cytosolic energization stimulates Ca2+ uptake by sarcoplasmic reticulum in post-ischemic 'stunned' myocardium. Cytosolic energy level in 'stunned' guinea-pig hearts will be varied by altering substrate composition of perfusion media or by beta-adrenergic stimulation with isoproterenol, and Ca2+ uptake and Ca2+ ATPase activity will be quantitated in sarcoplasmic reticulum isolated from stop-frozen hearts. Project two will test the hypothesis that substrate-enhancement of sarcoplasmic reticular function stems from increased phospholamban phosphorylation, a well-characterized mechanism for beta-adrenergic stimulation of cardiac inotropism. To effect radiolabelling of cardiac phosphoproteins, intramyocytic high-energy phosphate pools will be labelled by perfusion with [32P]inorganic phosphate; during tracer-free washout, cardiac inotropism will be increased by pyruvate-energization or by beta-adrenergic stimulation with isoproterenol. Proteins in isolated sarcoplasmic reticulum will be separated by electrophoresis, and 32P incorporation detected and quantitated by autoradiography and scintillation counting. This investigation will delineate the bioenergetic mechanisms for the observed highly significant relationship between contractility and cytosolic energy level in normoxic and especially 'stunned' myocardium in the absence of adrenergic stimulation. Of special clinical interest, pyruvate energization may be effective in reversing postischemic impairment of sarcoplasmic reticulum Ca2+ transport. Since pyruvate, unlike catecholamines, increases cytosolic energy level, this investigation may indicate that pyruvate could be a valuable cardioprotective intervention in clinical situations of energy-depleted heart.