Intramitochondrial and extramitochondrial free calcium ion concentrations of suspensions of heart mitochondria with very low, plausibly physiological, contents of total calcium

Abstract

The 2-oxoglutarate dehydrogenase of intact rat heart mitochondria is activated by Ca²⁺, with 50% activation at approximately 0.5 nmol of total Ca/mg of mitochondrial protein, in the presence of P_i and Mg²⁺. Mitochondrial Ca contents in excess of 2 nmol/mg of protein result in 100% activation of the enzyme. Investigation of Ca²⁺ release from the mitochondria using the metallochromic indicator Arsenazo III defines aS _0.5 of 5.4±0.4 nmol of Ca/mg of protein, when the endogenous Ca content of the mitochondria is progressively depleted with EGTA, prior to the initiation of the release process being studied. The subsequent determination of matrix free Ca²⁺ concentration by the “null-point” technique has allowed expression of these results in terms of free concentration rather than Ca content, with an activity coefficient of approximately 0.001 for matrix Ca²⁺. From the above, Ca²⁺ efflux from heart mitochondria is not saturated at the mitochondrial Ca contents or Ca²⁺ concentrations which give effective regulation of dehydrogenase activity. A consequence is that heart mitochondria do not buffer the pCa of the extramitochondrial medium at these Ca contents (2+ in normal heart.