Studies on the Possible Involvement of Prostaglandins in Insulin Generation of Pyruvate Dehydrogenase Activator

Insulin-exposed liver particulate fraction supernatants from control rats stimulated mitochondrial pyruvate dehydrogenase (PDH) activity by 26%, while the stimulation by similar preparations from indomethacin-injected rats (5 mg/kg twice daily, i.p., for 2 days) was 4%. In vitro addition of indomethacin to the particulate fraction during insulin exposure also inhibited stimulation of PDH by insulin. This inhibitory effect of indomethacin was completely overcome by the in vitro addition of prostaglandin E₂ (PGE₂) to the liver particulate incubation mixture. Intact adipocytes showed a similar (62%) decrease in insulin activation of PDH in the presence of indomethacin. In a cell-free adipocyte system (co-incubation of mitochondria and plasma membrane), indomethacin addition resulted in 90% decrease in insulin-stimulated PDH response. PGE₂ addition completely reversed this inhibition. In contrast to its effects on PDH activation, indomethacin had no effect on insulin-stimulated glucose oxidation. In vitro incubation of fat cells with dexamethasone (1 μLM) also resulted in decreased insulin activation of PDH. Inclusion of arachidonic acid during dexamethasone exposure of fat cells resulted in partial restoration of the insulin effect on PDH in fat cells and in cell-free preparations. However, addition of PGE₂ during insulin exposure of plasma membranes from dexamethasonetreated preparations showed no significant restoration of the insulin effect on PDH. These studies suggest that: (1) PG metabolism is involved in insulin's generation of the second messenger, and (2) the mechanism of dexamethasone-induced inhibition of insulin effect on PDH is a complex phenomenon involving the synthesis and action of eicosanoids.