Modulation of Ca2+ efflux and rebounding Ca2+ transport in rat liver mitochondria

The independent pathway for Ca2+ efflux of rat liver mitochondria exhibits a sharp temperature and pH dependence. The Arrhenius plot displays a break at 18 degrees C, activation energy being about 117 kJ/mol below 18 degrees C and 59 kJ/mol above 18 degrees C. The pH profile is bell-shaped, with a broad optimum at pH 7.0. These properties of the efflux pathway, together with the membrane potential modulation recently described (Bernardi, P. and Azzone, G.F. (1983) Eur. J. Biochem. 134, 377-383), suggest an explanation for the phenomenon of rebounding Ca2+ transport. Addition of a Ca2+ pulse to respiring mitochondria causes (i) a phase of rapid Ca2+ uptake, leading to a decrease of extramitochondrial free Ca2+ to a lower level with respect to that maintained before Ca2+ addition, and (ii) a slower phase of net Ca2+ efflux, leading to restoration of the steady-state extramitochondrial free Ca2+ preceeding Ca2+ addition. Evidence is provided that the excess Ca2+ uptake is linked to transient inactivation of the efflux pathway due to membrane depolarization. Conversely, the efflux phase is linked to reactivation of the efflux pathway upon repolarization. The efflux component of the rebound cycle and the isolated efflux pathway exhibit similar dependence on temperature, pH and membrane potential.