Ca++ Uptake by Fragments of Rat Liver Mitochondria and Its Dependence on Electron Transport*

It is well established that isolated intact mitochondria actively bind or accumulate various inorganic ions such as K+ and Na+ (l-9). It appears, however, that the intact mitochondrial structure may not be an absolute requirement for energy-linked active binding of inorganic ions, since Gamble (5) has demonstrated that phosphorylating digitonin fragments of rat liver mitochondria (10, 11) retained preexisting bound Kf in a process dependent upon respiration. Furthermore, when these fragments were incubated in a K+-containing medium, a respirationdependent exchange reaction occurred between endogenous and exogenous K+, without net uptake of K+. Recently we reported that isolated intact kidney mitochondria take up large quantities of Ca++ by a process dependent upon respiration and upon at least a portion of the mechanism of oxidative phosphorylation (12, 13). The data reported in this paper show that membrane fragments of rat liver mitochondria, prepared with digitonin (11) , also are able to accumulate Ca++ from the suspending medium; the stoichiometry and kinetics of this process are described. It is shown that the net uptake of Ca++ by mitochondrial membrane fragments requires electron transport in the presence of inorganic phosphate, and that Ca++ uptake is blocked by certain agents which uncouple oxidative phosphorylation. Some significant differences in the apparent cofactor requirements for the Ca++ uptake process in intact mitochondria and in digitonin fragments were observed. A preliminary report on some of these findings has been presented (12).