The Ba 2+ sensitivity of the Na +-induced Ca 2+ efflux in heart mitochondria: the site of inhibitory action

Abstract

The Na +-induced Ca 2+ release from rat heart mitochondria was measured in the presence of Ruthenium red. (1) Ba 2+ effectively inhibited the Na +-induced Ca 2+ release. At 10 mM Na + 50% inhibition was reached by 1.51 ± 0.48 (S.D., n = 8) μM Ba 2+ in the presence of 0.1 mg/ml albumin and by 0.87 ± 0.25 (S.D., n = 3) μM Ba 2+ without albumin. (2) In order to inhibit, it was not required that Ba 2+ ions enter the matrix. 140Ba 2+ was not accumulated in the mitochondrial matrix space; further, in contrast to liver mitochondria, Ba 2+ inhibition was immediate. (3) The Na +-induced Ca 2+ release was inhibited by Ba 2+ non-competitively, with respect of the extramitochondrial Na +. (4) The double inhibitor titration of the Na +-Ca 2+ exchanger with Ba 2+ in the presence and absence of extramitochondrial Ca 2+ revealed that the exchanger possesses a common binding site for extramitochondrial Ca 2+ and Ba 2+, presumably the regulatory binding site of the Na +-Ca 2+ exchanger, which was described by Hayat and Crompton (Biochem. J. 202 (1982) 509–518). All these observations indicate that Ba 2+ acts at the cytoplasmic surface of the inner mitochondrial membrane. The inhibitory properties of Ba 2+ on the Na +-dependent Ca 2+ release in heart mitochondria are basically different from those found on Na +-independent Ca 2+ release in liver mitochondria (Lukács, G.L. and Fonyó, A. (1985) Biochim. Biophys. Acta 809, 160–166).