Energy Dissipation by Calcium Recycling and the Efficiency of Calcium Transport in Rat-Liver Mitochondria

Abstract

The recycling of endogenous calcium across the inner membrane of incubated mitochondria consists of a passive efflux of calcium from the matrix which is compensated by an active re-uptake of this ion from the medium. Upon addition of ruthenium red which inhibits only the re-uptake, mitochondrial calcium accumulated in the medium at a rate of 3.5 nmol × min−1× mg protein−1. The ATP balance of this experiment, measured with the help of [2-14C]pyruvate as a substrate, revealed that ruthenium red inhibited about 20% of the energy dissipation of the mitochondria. It is concluded that the calcium recycling is one of the processes related to state 4 respiration of the mitochondria. Ca/O ratios for the calcium recycling were calculated from the rates of calcium release into the medium and the concomitant decrease in the respiratory rates observed after addition of ruthenium red. When the calcium transport system was progressively inhibited with increasing concentrations of ruthenium red, the intramitochondrial ATP/ADP ratios and the rates of calcium release gradually increased up to a maximal value, whereas the Ca/O ratios concomitantly decreased. Moreover, in the presence of barely uncoupling concentrations of dinitrophenol the Ca/O ratios were larger than in the control, whereas in the presence of oligomycin they were smaller. These results indicate that the efficiency of mitochondrial calcium transport decreases when the mitochondria approach a state of a maximal energy charge.