A dual mechanism of action of ocytocin in rat epididymal fat cells.

Ocytocin (oxytocin) at concentrations below lo-' M is shown to possess a wide range of insulin-like activities in rat epididymal fat cells, although the effects are small when compared with those of insulin.Newly described activities include stimulation of glucose incorporation into glycogen and of intracellular Hz02 production, as well as inhibition of hormone-stimulated lipolysis.These low concentration range effects are thought to be mediated by the high affinity ocytocin receptors of adipocytes.Considering that ocytocin and insulin are known to act via separate receptors, the likeness of the spectrum of activities of the two hormones and the similarity in order of potency of ocytocin and insulin effects on various metabolic processes, together with some evidence for competition between ocytocin and insulin for stimulation of glucose C-1 oxidation, suggests that the two hormones share a common post-receptor mechanism for the effects described above.On the other hand, ocytocin at concentrations greater than M is now shown to activate the glucose transport system while simultaneously inhibiting cellular glycogen deposition, the latter likely through activation of glycogenolysis.Further, its inhibition of hormone-stimulated lipolysis is diminished, and ocytocin per se becomes lipolytic.Notably, stimulation of the glucose transport system by ocytocin and insulin is always additive, suggesting different mechanisms of actions for the two hormones.These high concentration range effects are thought to be mediated by the low affinity ocytocin receptor.Although ocytocin, like vasopressin, can activate adenylate cyclase in some tissues, it fails to do so in adipocytes.Further, ocytocin activates glycogenolysis in the liver without involvement of cyclic AMP.It i s proposed that the low affinity ocytocin receptor mediates the hormone's effects in adipocytes in a like manner to its effects in the liver, i.e. probably involving cytosolic Ca2+ elevation.This dual mechanism of action of ocytocin in adipocytes explains certain discrepancies between our results and data reported by others.