ROLE OF CATION GRADIENTS IN HYPERCONTRACTURE OF MYOCYTES DURING SIMULATED ISCHEMIA AND REPERFUSION

We examined the relationship between transsarcolemmal cation gradients and hypercontracture of cardiac myocytes in ischemia and reperfusion using adult rat ventricular myocytes superfused with buffer mimicking normal or ischemic extracellular fluid. Contractile performance of ele ctrically stimulated cells was recorded by an optical video system sim ultaneously with measurements of intracellular Ca2+ concentration ([Ca 2+]i) Using fura-2 or intracellular pH (pH(i)) using 2',7'-bis(2-carbo xyethyl)-5(6)-carboxyfluorescein. While cells were exposed to simulate d ischemia buffer, the transsarcolemmal H+ gradient was abolished, [Ca 2+]i transient stopped, and twitch contraction of myocytes ceased. Upo n reperfusion with normal buffer, H+ gradient was quickly restored, Ca 2+ transients restarted with transient increase in systolic Ca2+, and twitch contraction restarted with development of hypercontracture, whi ch continued after [Ca2+]i returned to preischemic level even in the p resence of near-normal concentrations of high-energy phosphates. When the transsarcolemmal proton, Na+, and Ca2+ gradients were altered so t hat Na+ entry via Na+-H+ exchange and Ca2+ entry via Ca2+-Na+ exchange were made less favorable, the transient systolic overshoot of Ca2+ at reperfusion and development of hypercontracture was largely avoided. These results suggest that Na+ and then Ca2+ entry via the Na+-H+ and Na+-Ca2+ exchangers, respectively, probably contribute to the increase in [Ca2+]i and hypercontracture of myocytes at time of reperfusion in this model.