The relative role of various Ca2+ transport systems in the regulation of Ca2+ cytosolic free Ca2+ concentration was evaluated using rabbit renal proximal tubules. Intracellular compartmentation was evaluated through Ca2+ releases induced by carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), A23187, and ruthenium red (RR) alone and in combination. In a Ca2+-free solution after 1 h of incubation, FCCP released 43 +/- 4%, A23187 54 +/- 3%, and RR 29 +/- 5% of total cellular Ca2+; in addition, RR inhibited the rate of FCCP-induced release, confirming its mitochondrial origin. In 1 mM Ca2+, the releases were 57 +/- 9%, 70 +/- 5%, and 34 +/- 10%, respectively. Therefore, the mitochondrial Ca2+ content is 20-60 nmol/mg of mitochondrial protein, sufficiently large to effectively buffer cell Ca2+. To evaluate the role of the plasma membrane Na:Ca exchanger, 10(-4) M ouabain was added and caused a slight decline in total cell Ca2+ content and no change in ionized Ca2+ measured by the null-point method, suggesting that the plasmalemmal Na+:Ca2+ exchanger does not play an important role in Ca2+ extrusion. Cytosolic free Ca2+ increased when 100 mM sodium was replaced with equimolar choline or tetramethylammonium. However, tetramethylammonium replacement released 55% of the mitochondrial Ca2+ content by increasing mitochondrial Ca2+ efflux without affecting the Ca2+ influx pathway. These results suggest that Na+ replacements in this tissue increase ionized Ca2+ by increasing mitochondrial Ca2+ efflux and not by inhibition of Na+:Ca2+ exchange at the plasma membrane.