Mitochondrial Ca2+ influx and efflux rates in guinea pig cardiac mitochondria:Low and high affinity effects of cyclosporine A

Ca2+ plays a central role in energy supply and demand matching in cardiomyocytes by transmitting changes in excitation-contraction coupling to mitochondrial oxidative phosphorylation. Matrix Ca2+ is controlled primarily by the mitochondrial Ca2+ uniporter and the mitochondrial Na+/Ca2+ exchanger, influencing NADH production through Ca2+-sensitive dehydrogenases in the Krebs cycle. In addition to the well-accepted role of the Ca2+-triggered mitochondrial permeability transition pore in cell death, it has been proposed that the permeability transition pore might also contribute to physiological mitochondrial Ca2+ release. Here we selectively measure Ca2+ influx rate through the mitochondrial Ca2+ uniporter and Ca2+ efflux rates through Na+-dependent and Na+-independent pathways in isolated guinea pig heart mitochondria in the presence or absence of inhibitors of mitochondrial Na+/Ca2+ exchanger (CGP 37157) or the permeability transition pore (cyclosporine A). cyclosporine A suppressed the negative bioenergetic consequences (Delta Psi(m) loss, Ca2+ release, NADH oxidation, swelling) of high extramitochondrial Ca2+ additions, allowing mitochondria to tolerate total mitochondrial Ca2+ loads of >400 nmol/mg protein. For Ca2+ pulses up to 15 mu M, Na+-independent Ca2+ efflux through the permeability transition pore accounted for similar to 5% of the total Ca2+ efflux rate compared to that mediated by the mitochondrial Na+/Ca2+ exchanger (in 5 mM Na+). Unexpectedly, we also observed that cyclosporine A inhibited mitochondrial Na+/Ca2+ exchanger-mediated Ca2+ efflux at higher concentrations (IC50 = 2 mu M) than those required to inhibit the permeability transition pore, with a maximal inhibition of similar to 40% at 10 mu M cyclosporine A, while having no effect on the mitochondrial Ca2+ uniporter. The results suggest a possible alternative mechanism by which cyclosporine A could affect mitochondrial load in cardiomyocytes, potentially explaining the paradoxical toxic effects of cyclosporine A at high concentrations. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection. (C) 2011 Elsevier B.V. All rights reserved.