Abstract
Rat heart mitochondria respiring on succinate in the presence of Ru red (to inhibit uptake on the Ca2+ uniporter) released Ca2+ on the Ca/Na antiporter until a steady state was reached. Addition of the ionophore A23187 (which catalyzes Ca2+/2H+ exchange) did not perturb this steady state. Thermodynamic analysis showed that if a Ca2+/nNa+ exchange with any value of n was at equilibrium, addition of A23187 would cause an obvious change in extramitochondrial free [Ca2+]. The endogenous Ca/Na antiporter of mitochondria catalyses electroneutral Ca2+/2Na+ exchange.