Mechanism of loss of adenine nucleotides from mitochondria during myocardial ischemia.

We tested the hypothesis that loss of mitochondrial adenine nucleotides during myocardial ischemia is induced by the accumulation of inorganic phosphate (Pi) and a decrease in cytosolic ATP. In the isolated perfused rat heart, loss of mitochondrial adenine nucleotides (ATP + ADP + AMP) was preceded by the rise in tissue Pi and the loss of tissue ATP. After 30 min ischemia, the average rate of loss of mitochondrial adenine nucleotides was c. 1.5% of the initial pool/min. In isolated heart mitochondria, there are two pathways for adenine nucleotide release: a 'fast', phosphate-dependent pathway, which is inhibited by atractyloside; and a 'slow', phosphate-independent pathway, which is insensitive to atractyloside. Decreasing the pH from 7.4 to 6.5 significantly decreased the rate of release by the phosphate-dependent pathway (but not the phosphate-independent pathway). Analysis of release rates indicated that HPO4-2 is responsible for the phosphate-induced release; Vmax = 53.8% of the pool/per minute, Km = 7.5 mM. In vitro, extramitochondrial ATP inhibited adenine nucleotide release in the presence of Pi such that the rate of release was inversely proportional to the extramitochondrial [ATP]; extrapolation to zero ATP indicated a release rate of 2 to 3% of the pool/per minute, which is approximately equal to the rate of the 'slow' phosphate-independent pathway. Moreover, increasing the Pi concentration did not increase the rate of adenine nucleotide release in the presence of extramitochondrial ATP. Accumulation of mitochondrial adenine nucleotides was observed when the mitochondria were incubated in the presence of 4 mM or greater ATP. The results suggest that the rise in intracellular Pi during myocardial ischemia does not induce the loss of adenine nucleotides from the mitochondrial compartment, but rather that degradation of cytosolic ATP results in a slowing of ATP influx such that the rate of efflux (phosphate-independent) exceeds the rate of influx.