On the Nature of Pi-Induced, Mg2+-Prevented Ca2+ Release in Rat Liver Mitochondria

Since 1964 it has been known that inorganic phosphate (Pi) releases Ca2+ from liver mitochondria. The Ca2+ release is irreversible and accompanied by indefinite stimulation of respiration. Although these phenomena have been attributed to increased Ca2+ cycling, and therefore to stimulation by Pi of a specific Ca2+ efflux pathway, direct evidence for such a specific pathway and for real Ca2+ cycling has never been provided. Here, we aim to clarify the nature of Pi-induced, Mg2+-prevented Ca2+ release and of the related respiratory stimulation. We show that, in the absence of Mg2+ and BSA, phosphate addition to Ca2+-preloaded mitochondria causes a sharp decrease of deltapH. Since deltapsi tends to decline as well, the H+ electrochemical gradient (deltamuH+) decreases dramatically. The nature of Ca2+ release induced by Pi in the absence of Mg2+ appears therefore to be due to reversal of the uniport carrier, consequent to decrease of deltapsi rather than to activation of an independent efflux pathway. Accordingly, the indefinite stimulation of respiration is not due to increased Ca2+ cycling but to increased H+ recycling across the inner membrane.