Mitochondrial calcium transporting pathways during hypoxia and reoxygenation in single rat cardiomyocytes

Mitochondrial [Ca2+] ([Ca2+]m) rises in parallel with cytosolic [Ca2+] ([Ca2+]c) following ATP-depletion rigor contracture induced by hypoxia in isolated cardiomyocytes. We investigated the pathways involved in the hypoxia induced changes in [Ca2+]m by using known inhibitors of mitochondrial Ca2+ transport, namely ruthenium red, an inhibitor of the Ca2+ uniporter (the normal influx route) and clonazepam, an inhibitor of Na+/Ca2+ exchange, (the normal efflux route).

[Ca2+]m was determined from indo-1/am loaded rat myocytes where the cytosolic fluorescence signal had been quenched by superfusion with Mn2+. [Ca2+]c was measured by loading myocytes with indo-1 pentapotassium salt during the isolation procedure. Cells were placed in a specially developed chamber for induction of hypoxia and reoxygenated 40 min after rigor development.

50% of control cells hypercontracted upon reoxygenation; this correlated with a [Ca2+]m or [Ca2+]c higher than approximately 350 nM at the end of rigor. Clonazepam completely abolished the rigor-induced rise in [Ca2+]m but not [Ca2+]c. On reoxygenation [Ca2+]m increased over the first 5 min and remained elevated whereas [Ca2+]c fell. In the presence of ruthenium red a dramatic increase in [Ca2+]m occurred 5-10 min after rigor development (the indo-1 fluorescence signal was saturated); [Ca2+]c also increased but to a lesser extent. On reoxygenation, [Ca2+]m fell rapidly even though cells hypercontracted and [Ca2+]c remained elevated.

During hypoxia following rigor development Ca2+ uptake into mitochondria occurs largely via the Na+/Ca2+ exchanger rather than the Ca2+ uniporter whereas on reoxygenation the transporters resume their normal directionality.