CA2-HEART MITOCHONDRIA( STIMULATES BOTH THE RESPIRATORY AND PHOSPHORYLATION SUBSYSTEMS IN RAT)

Stimulation of mitochondrial respiration by physiological concentratio ns of Ca2+ was studied to determine which components of oxidative phos phorylation are affected by Ca2+. The kinetic dependence of the respir atory chain, phosphorylation subsystem and proton leak on the mitochon drial membrane potential in isolated rat heart mitochondria respiring on 2-oxoglutarate or succinate was measured at two different concentra tions of external free Ca2+. The results show that proton leak is not directly affected by Ca2+, but that both the respiratory and phosphory lation systems can be directly stimulated by Ca2+ depending on conditi ons. Although Ca2+ directly stimulates the phosphorylation system, thi s has relatively little effect on respiration rate with 2-oxoglutarate in States 3 and 4 because the subsystem has little control over respi ration. However, in intermediate states, the phosphorylation system ha s greater control and Ca2+ stimulation of this system contributes subs tantially to the stimulation of respiration and phosphorylation. In th e case of succinate oxidation neither the respiratory subsystem nor th e phosphorylation system is stimulated by Ca2+.