Role of mitochondrial re-energization and Ca2+ influx in reperfusion injury of metabolically inhibited cardiac myocytes.

OBJECTIVE: We used isolated myocytes to investigate the role of mitochondrial re-energization and Ca2+ influx during reperfusion on hypercontracture, loss of Ca2+ homeostasis and contractile function.

METHODS: Isolated adult rat ventricular myocytes were exposed to metabolic inhibition (NaCN and iodoacetate) and reperfusion injury was assessed from hypercontracture, loss of Ca2+ homeostasis ([Ca2+]i measured with fura-2) and failure of contraction in response to electrical stimulation. Mitochondrial membrane potential was followed using the potentiometric dye tetramethylrhodamine ethyl ester.

RESULTS: Metabolic inhibition led to contractile failure and rigor accompanied by a sustained increase in [Ca2+]i. Reperfusion after 10 min metabolic inhibition led to an abrupt repolarization of the mitochondrial membrane potential (after 25.5+/-1.2 s), a transient fall in [Ca(2+]i followed by an abrupt hypercontracture (37.1+/-1.8 s) in 84% of myocytes. Ca2+ homeostasis (diastolic [Ca2+]i < 250 nM) recovered in only 23.3+/-5.1% of cells and contractions recovered in 15.3+/-2.2%. Oligomycin abolished the hypercontracture on reperfusion, but mitochondrial repolarization was unaffected. Preventing Ca2+ influx during reperfusion with [Ca2+]i-free Tyrode or with an inhibitor of Na(+)/Ca2+ exchange did not prevent the hypercontracture, but increased the percentage of cells recovering Ca2+ homeostasis and contractile function. The presence of 0.5 microM cyclosporin A did not prevent hypercontracture but increased the percentage of cells recovering Ca2+ homeostasis to 56.2+/-3.6% and contractile function to 52+/-4.3%.

CONCLUSIONS: Reperfusion-induced hypercontracture, and loss of Ca2+ homeostasis and contractile function are initiated following mitochondrial re-energization. The hypercontracture requires the production of oxidative ATP but not Ca2+ influx during reperfusion. Loss of Ca2+ homeostasis and contractile function are linked to Ca2+ influx during reperfusion, probably via opening of mitochondrial permeability transition pores.