Effects of calcium on respiration and ATP content of isolated, leaky, heart muscle cells

Abstract

Calcium influx secondary to increased sarcolemmal permeability has been implicated in the irreversibility of cell injury associated with myocardial infarction or excessive doses of β-adrenergic agents such as isoproterenol. Cell death has been attributed, at least in part, to Ca uncoupling of oxidative phosphorylation. We have, therefore, studied the effect of Ca on State 3 respiration and steady state ATP levels in a preparation of spontaneously beating myocardial fragments (MF) which are highly permeable to solutes added to the medium. The response of MF respiration to Ca was examined between pH 6.0 and pH 7.4, using three separate buffer systems: phosphate (Pi), PIPES, and Bis-Tris propane. When Ca was added to give concentrations anticipated in cells in vivo with leaky sarcolemmas, State 3 respiration was markedly inhibited. Regardless of the buffer used, inhibition became progressively greater with reduction of pH. Only at the highest pH levels employed did we observe respiratory responses which could be interpreted as Ca-induced uncoupling of oxidative phosphorylation. Steady state ATP levels declined in the presence of Ca at a rate similar to that observed in the absence of substrate. Thus Ca effectively prevented phosphorylation, while inhibiting rather than stimulating oxygen uptake. It is concluded that Ca influx could be a determinant of cell death in a provisional zone of myocardial necrosis, but since such tissue is generally acidotic, the effect would occur through respiratory inhibition rather than uncoupling of oxidative phosphorylation. This inhibition of respiration would spare available oxygen for adjacent cells which might thus be able to survive.