Insulin-like effects of ATP on adipocyte pyruvate dehydrogenase and phosphorylase

Extracellular ATP stimulated adipocyte pyruvate dehydrogenase in a time- and dose-dependent manner with an EC50 of 0.1 mM. The maximal effect was observed at 0.5 mM ATP after a 15-min incubation with a lag period of about 5 min. Depletion of intracellular Ca2+ with ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid reduced the effect of ATP by 50% and completely abolished the stimulatory effect of vasopressin on adipocyte pyruvate dehydrogenase but had no effect on the stimulation induced by insulin or adenosine. The effects of insulin and ATP on pyruvate dehydrogenase were glucose-dependent whereas the effect of adenosine was glucose-independent. Furthermore, ATP, like insulin, partially blocked the stimulatory effect of isoproterenol on phosphorylase. Adenosine, at a concentration of 1 mM, did not affect either basal or isoproterenol-stimulated phosphorylase activities. It is concluded that ATP activates adipocyte pyruvate dehydrogenase by at least two separate mechanisms: one is Ca2(+)-dependent and the other is Ca2(+)-independent. However, neither is the result of the formation of adenosine from ATP through hydrolysis.