NA-DEPENDENT CA2+ EFFLUX MECHANISM OF HEART-MITOCHONDRIA IS NOT A PASSIVE CA2+()2NA+ EXCHANGER/

Net Ca2+ flux across the inner membrane of respiring heart mitochondri a was evaluated under conditions in which virtually all Ca2+ movement can be attributed to the Na+/Ca2+ antiport. If this antiport promotes a passive electroneutral exchange of Ca2+ for 2Na(+), the Ca2+ gradien t should be equal to the square of the Na+ gradient at equilibrium. Be cause the mitochondrial Na+/H+ antiport equilibrates the Na+ and H+ gr adients, the Ca2+ gradient should also equal the square of the H+ grad ient. In a series of > 20 determinations at different matrix [Ca2+], d ifferent Delta pH, and varying membrane potential, it was found that C a2+ is transported out of the mitochondrion against gradients from 15- to 100-fold greater than the value predicted for passive electroneutr al exchange. It is concluded that the observed gradients are too large to be sustained by passive Ca2+/2Na(+) exchange. The observed gradien ts are compatible with an electrogenic Ca2+/3Na(+) exchange. Alternati vely another source of energy is available to support these gradients.