Stimulation of pyruvate dehydrogenase activity in adipocytes by oxytocin: Evidence for mediation of the insulin-like effect by endogenous hydrogen peroxide independent of glucose transport

Oxytocin, a nonapeptide posterior pituitary hormone, which is known to increase glucose oxidation in fat cells like insulin, is shown here to stimulate pyruvate dehydrogenase activity in these cells. The process appears to involve the activation of preexisting molecules since there was no change in the total enzyme content after full activation. The effect of oxytocin, as well as of insulin, appears to be mediated by endogenous H₂O₂ formation, as evident from (i) the enhanced ¹⁴C]formate oxidation and its greater inhibition by 3-amino-1,2,4-triazole in the hormone-treated cells than in the control. This is a measure of the catalase:H₂O₂ complex, and the dose dependence of this response is found to be identical with that of glucose oxidation via the hexose monophosphate shunt pathway and of pyruvate dehydrogenase activity; and (ii) treatment of the cells with low concentration of exogenous H₂O₂ causes the activation of pyruvate dehydrogenase to the extent which is comparable with the effect of the hormones. The ED₅₀ of oxytocin was 7 × 10^?9m, whereas the ED₅₀ of insulin was 5 × 10^?11m. The reduced, inactive (SH) derivatives of the hormones had the same dose-response relationship, but considerably lower effect (10 to 20% of the native molecules of the hormones), indicating the significant role of the disulfide bridge(s) in eliciting these metabolic responses. The stimulation of PDH by oxytocin or insulin is found to be essentially independent of medium glucose which, however, can sustain the response apparently by recycling the intracellular oxidation-reduction state. However, unlike insulin, oxytocin fails to stimulate the rapid uptake of 3-O-³H]methyl-d-glucose in these cells. The data illustrate that the major metabolic actions of insulin, viz., glucose utilization and lipogenesis, are shared by another heterologous polypeptide hormone, e.g., oxytocin, through a common effector, H₂O₂. It is suggested that (i) oxytocin may play a limited surrogate role for insulin in these cells; and (ii) H₂O₂ production may be the general basis of oxytocin's action.