Possible role of intracellular Ca2+ in the toxicity of phenformin.

Selective use of various mitochondrial Ca2+ transport inhibitors indicated that significant Ca2+ redistribution may occur during the isolation of mitochondria. Exposure of guinea-pig liver mitochondria to phenformin (beta-phenethylbiguanide) during the isolation procedure resulted in decreased mitochondrial Ca2+. Novel isolation conditions were developed to determine liver mitochondrial calcium content considered to reflect that in vivo. Administration of phenformin to rats and guinea-pigs resulted in decreased mitochondrial Ca2+. Decreased liver mitochondrial Ca2+ correlated inversely with raised blood lactate concentrations in the guinea-pig; 2-oxoglutarate, but not succinate oxidation, was inhibited in these mitochondrial preparations. A mechanism of action for phenformin-associated lactic-acidosis, attributable to impaired mitochondrial function arising from inactivation of Ca2+-sensitive, NAD+-dependent mitochondrial dehydrogenases (e.g. 2-oxoglutarate dehydrogenase) due to alteration in mitochondrial calcium content, is proposed.