Regulation of Ca 2+ transport in brain mitochondria. II. The mechanism of the adenine nucleotides enhancement of Ca 2+ uptake and retention

Abstract

ADP greatly enhances the rate of Ca 2+ uptake and retention in Ca 2+ loaded mitochondria. Atractyloside, a specific inhibitor of the ADP/ATP translocator, completely inhibits the ADP effect, while bongkrekate, another specific inhibitor of the translocator enhances the effect of ADP. These results indicate that locking the ADP/ATP translocator in the M-state is sufficient to produce the ADP effect. Cyclosporin A, a specific inhibitor of the Ca 2+-induced membrane permeabilization does not substitute for ADP, indicating that ADP directly affect the rate of electrogenic Ca 2+ uptake. The effect of the translocator conformation on the rate of electrogenic Ca 2+ uptake is independent of the concentration of P i and is not caused by changes in membrane potential. However, locking the carrier in the M-state appears to increase the negative surface charge on the matrix face of the inner membrane. This may lead to an enhanced rate of Ca 2+ dissociation from the electrogenic carrier at the matrix surface. The rate of Na +-independent Ca 2+ efflux is only slightly inhibited by locking the carrier in the M-state, presumably due to the same mechanism. In the presence of ADP, P i inhibits the Na +-independent efflux. In the presence of physiological concentrations of spermine, P i and Mg 2+, the rate of Ca 2+ uptake, Ca 2+ retention and Ca 2+ set points depend sharply on ADP concentration at the physiological range of ADP. Thus, changes of cytosolic ADP concentration may lead to change in the rate of Ca 2+ uptake by mitochondria and thus modulate the excitation-relaxation cycles of cytoplasmic free calcium.