Modulation of Ca2+ Efflux and Rebounding Ca2+ Transport in Rat Liver Mitochondria

The paper analyzes the factors affecting the H+-K+ exchange catalyzed by rat liver mitochondria depleted of endogenous Mg2+ by treatment with the ionophore A23187. The exchange has been monitored as the rate of K+ efflux following addition of A23187 in low-K+ media. (1) The H+-K+ exchange is abolished by uncouplers and respiratory inhibitors. The inhibition is not related to the depression of ΔpH, whereas a dependence is found on the magnitude of the transmembrane electrical potential, Δψ. Maximal rate of K+ efflux is observed at 180–190 mV, whereas K+ efflux is inhibited below 140–150 mV. (2) Activation of H+-K+ exchange leads to depression of ΔpH but not of Δψ. Respiration is only slightly stimulated by the onset of H+-K+ exchange in the absence of valinomycin. These findings indicate that the exchange is electroneutral, and that the Δψ control presumably involves conformational changes of the carrier. (3) Incubation in hypotonic media at pH 7.4 or in isotonic media at alkaline pH results in a marked activation of the rate of H+-K+ exchange, while leaving unaffected the level of Mg2+ depletion. This type of activation results in partial ‘uncoupling’ from the Δψ control, suggesting that membrane stretching and alkaline pH induce conformational changes on the exchange carrier equivalent to those induced by high Δψ. (4) The available evidence suggests that the activity of the H+-K+ exchanger is modulated by the electrical field across the inner mitochondrial membrane.