Modulation by Leu-enkephalin of peptide release from perifused neurointermediate pituitary. II. Inhibition of calcium-mediated secretion of alpha-MSH and beta-endorphin.

The present study examines the effect of opiates on alpha-MSH and beta-endorphin release from perifused neurointermediate rat pituitaries, as stimulated by various secretagogues for which Ca ions and/or cAMP serve as messengers. alpha-MSH release stimulated by high K+ concentrations (5-min pulses) and veratridine depolarization, which is closely dependent on Ca2+ fluxes, was abolished by both Leu-enkephalin and beta-endorphin. A dose-response relationship between inhibition of alpha-MSH secretion and the concentration of Leu-enkephalin, with ED50 approximately 10(-9) M, was observed. High K+-induced release of beta-endorphin was likewise blunted by Leu-enkephalin. The stimulatory effect of the Ca2+ ionophore A 23187 was inhibited in a similar way as was that of CRF, which requires both Ca2+ fluxes and cAMP formation. The antagonist naloxone not only reversed the action of opiates, but also enhanced spontaneous hormonal output. In contrast, the effects of l-isoproterenol and forskolin, for which cAMP serves as a primary messenger, were unaffected in the absence of extracellular Ca ions and, also, in the presence of Leu-enkephalin. We conclude that opioid peptides may exert a direct inhibitory influence on the release of both alpha-MSH and beta-endorphin and do so by interfering with the Ca2+ messenger system. In addition, these data also suggest the existence of an opiate-opiate negative feedback mechanism.