Stable changes to calcium fluxes in mitochondria isolated from rat livers perfused with α-adrenergic agonists and with glucagon

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Abstract
Mitochondria isolated from rat liver after a short-term perfusion with the α-adrenergic agonist phenylephrine or with glucagon exhibited enhanced rates of uptake of Ca2+ and prolonged retention of Ca2+ in the presence of 4mm-Pi. The effect of Ca2+ retention was apparent after perfusion with phenylephrine for only 1min and was maximal after 7min of treatment. The changes induced by glucagon, although similar, were less rapid. Adrenaline caused similar changes to phenylephrine and its effects were blocked by the α-adrenergic antagonist phenoxybenzamine, but not by the β-antagonist propranolol. The Ca2+ content of the isolated mitochondria decreased by 30% 1min after the onset of perfusion with phenylephrine; by 6min it had begun to return to the original value which was reached at 10min. A similar loss in calcium content was induced by glucagon but the changes were not as great and occurred more slowly. Mitochondria from phenylephrine-treated livers exhibited decreased rates of Ca2+ efflux induced by addition of 2mm-EGTA, a 50% increase in the contents of ADP and total adenine nucleotides, a small increase in the transmembrane pH gradient, and a reduced rate of oxaloacetate-induced NADPH oxidation. This study thus shows that stimulation of liver by α-adrenergic agonists, like that by glucagon, induces within minutes a stable modification of mitochondria leading to alterations in the Ca2+-translocation cycle (increased Ca2+ uptake and retention) and alterations in mitochondrial energy-linked reactions.

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