Phosphate-induced efflux of adenine nucleotides from heart mitochondria

Adenine nucleotide efflux from isolated rat heart mitochondria was studied. Inorganic phosphate induced efflux of adenine nucleotides from the mitochondria. This efflux was inhibited by carboxyatractyloside and atractyloside. The rate of efflux showed saturation kinetics with respect to extramitochondrial phosphate (K(m), 9.5 mM). Lowering the pH from 7.4 to 6.8 had little or no effect on the rate of efflux. Deenergizing the mitochondria enhanced carboxyatractyloside-insensitive efflux, but it did not affect carboxyatractyloside-sensitive efflux. Extramitochondrial ATP (200 μM) or AMP (200 μM) prevented efflux when the phosphate concentration was 10 mM. AMP (200 μM) did not inhibit efflux when the phosphate concentration was 40 mM. Atractyloside inhibited efflux noncompetitively with respect to inorganic phosphate. Mersalyl (10 nmol/mg protein) did not inhibit efflux. Phenyl-succinate (20 mM) totally inhibited phosphate-induced efflux. The results of this study indicate that under conditions found in the ischemic heart cell (low ATP, high phosphate), adenine nucleotides may be lost from the mitochondria via the adenine nucleotide translocase. Phosphate does not induce this efflux by interacting with the translocase or the phosphate-hydroxyl carrier. The site of action of phosphate may be the dicarboxylate carrier.