Reversal of mitochondrial Na Ca exchange during metabolic inhibition in rat cardiomyocytes

During hypoxia of isolated cardiomyocytes, Ca2+ entry into mitochondria may occur via the Na/Ca exchanger, the normal efflux pathway, and not the Ca-uniporter, the normal influx route. If this is the case, then depletion of myocyte Na+ should inhibit Ca2+ uptake, and collapse of the mitochondrial membrane potential (Delta psi(m)) would inhibit the uniporter, To test these hypotheses, isolated rat myocytes were exposed to metabolic inhibition, to mimic hypoxia, and [Ca2+](m) and [Ca2+](e) determined by selective loading of indo-1 into these compartments. Delta psi(m) was determined using rhodamine 123, Following metabolic inhibition, [Ca2+](m) was significantly lower in Na-depleted cells than controls (P<0.001), [Ca2+](c) was approximately the same in both groups, and mitochondria depolarised completely. Thus Na-depletion inhibited mitochondrial Ca2+ uptake, suggesting that Ca2+ entry occurred via Na/Ca exchange, and the collapse of Delta psi(m) during metabolic inhibition is consistent with inactivity of the Ca-uniporter. (C) 1999 Federation of European Biochemical Societies.