On the state of calcium ions in isolated rat liver mitochondria. I. Ion fluxes and volume changes upon Ca2+ uptake under various ionic conditions.

As to functional consequences of Ca2+ uptake in isolated rat liver mitochondria, we simultaneously measured 3H2O and [14C]sucrose spaces, monovalent cation distribution, membrane potential and delta pH across the inner membrane, and [32P]phosphate and 45Ca2+ content in parallel incubations of different ionic composition. Without added Ca2+ and phosphate, mitochondrial matrix volume, membrane potential, and delta pH depended on the concentration and permeability of monovalent cations. Despite large differences in membrane potential, maximal Ca2+ uptake was identical under all conditions. Ca2+ uptake never provoked a volume change from which an osmotic active state of mitochondrial Ca2+ could be concluded. If matrix volume shrunk this could be totally accounted for by the loss of alkali ions exchanging for calcium ions. Even phosphate taken up in conjunction with Ca2+ was osmotically silent. Volume increases here occurring if K+ was permeabilized, solely resulted from K+ uptake, though this condition may give rise to irreversible mitochondrial damage with Ca2+ and phosphate release. As mitochondrial Ca2+ is bound, an electro-chemical equilibrium across the membrane is impossible for this ion. This has to be considered in any model describing equilibria of Ca2+ with mitochondria, though present models neglect this state of mitochondrial Ca2+.