Evidence that phorbol ester-activated pathways are not directly involved in the action of insulin in rat adipocytes

The effects of 12-phorbol 13-myristate acetate (PMA) on glucose transport, glucose metabolism and lipolysis in rat adipocytes were examined. Alone, PMA (100 ng/ml) stimulated 2-deoxyglucose transport, glucose oxidation and lipogenesis by an amount corresponding to about 30-50% of the maximal insulin effect. The effect of PMA on the insulin-stimulated processes was additive at all insulin concentrations. Adenosine deaminase-stimulated lipolysis was enhanced by about 50% by PMA (100 ng/ml). Thus, in contrast to insulin, PMA had a lipolytic effect. The antilipolytic effect of insulin was unaffected by the presence of PMA, both ED50 and maximal inhibition of insulin were unchanged. In conclusion, it is suggested that phorbol esters may activate pathways other than those activated by insulin in rat adipocytes.