DES-ACETYLATED VARIANTS OF ALPHA-MELANOCYTE-STIMULATING HORMONE AND BETA-ENDORPHIN CAN ANTAGONIZE THE MAMMOTROPE-RECRUITING ACTIVITY OF THEIR ACETYLATED FORMS

We have previously reported that hypophysial neurointermediate lobe pe ptides, di-acetylated alpha-melanocyte-stimulating hormone (di-ac-alph a-MSH) and N-acetylated beta-endorphin (N-ac-beta-END), can acutely in crease the relative number of prolactin-secreting cells in anterior pi tuitary cell cultures from ovariectomized rats. Inasmuch as the des-ac etylated forms of these peptides (des-ac-alpha-MSH and beta-END) were not effective in this regard, we concluded that acetylation was an abs olute requirement for manifestation of the recruitment response. The a im of the present study was to determine whether these des-acetylated variants could antagonize the mammotrope-recruiting activity of their acetylated congeners. Treatment of anterior pituitary cell cultures wi th di-ac-alpha-MSH and N-ac-beta-END increased the relative amount of prolactin secretors above control values. Interestingly, des-acetylate d variants of alpha-MSH and beta-END blocked the mammotrope-recruitmen t activity of their respective acetylated forms. In addition, beta-END antagonized the mammotrope-recruitment activity of di-ac-alpha-MSH wh ile des-ac-alpha-MSH did not attenuate the stimulatory effect of N-ac- beta-END. Given that mammotropes maintained in vivo are exposed to all these peptides, it is possible that these acetylated and non-acetylat ed congeners may act in an opposing manner to regulate dynamic prolact in release.