Intracellular free calcium and mitochondrial membrane potential in ischemia/reperfusion and preconditioning.

Moderation of calcium perturbations has been implicated in ischemic preconditioning. As mitochondria possess an effective Ca(2+)transporting system driven by the mitochondrial membrane potential, experiments were performed to study time-averaged intracellular free calcium and the mitochondrial membrane potential during preconditioning and ischemia-reperfusion. Isolated rat hearts were subjected to 5 min of preconditioning, a 9-min intervening reperfusion and 21 min of ischemia with subsequent reperfusion. The hearts were preloaded with the Ca(2+)indicator Fura-2 or the mitochondrial membrane potential probe safranine. A method was devised for correction for NADH autofluorescence in time-averaged Ca(2+)probing with Fura-2. The pH dependence of the apparent dissociation constant of the Ca(2+)complex of Fura-2 was determined. Intracellular free Ca(2+)increased during the 5-min ischemia, and this was reversed upon reperfusion. During protracted ischemia a continual Ca(2+)rise was observed when the fluorescence data were corrected for changes in pH. An initial sharp Fura-2 fluorescence spike upon final reperfusion was caused by a pH-dependent change in the dissociation constant of the Ca(2+)complex of Fura-2. In preconditioned hearts the free Ca(2+)was somewhat lower during reperfusion, but a major effect of preconditioning was observed during the prolonged ischemia. The decrease in mitochondrial membrane potential during prolonged ischemia was faster in the preconditioned heart with no difference during the final reperfusion. The effect of preconditioning on cell survival was reflected in a decrease in the post-ischemic washout of creatine kinase. The moderation of the ischemic and post-ischemic intracellular Ca(2+)increase, and the acceleration of the ischemic mitochondrial membrane potential decrease by ischemic preconditioning is in accord with the notion of the involvement of mitochondrial ATP sensitive K(+)channels in preconditioning. In studies on ischemia it is absolutely necessary to correct for the pH-sensitivity of the apparent dissociation constant of the calcium complex of Fura-2 to obtain reliable data for intracellular free calcium.