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We have previously reported that hypophysial neurointermediate lobe peptides, di-acetylated -melanocyte-stimulating hormone (di-ac--MSH) and N-acetylated β-endorphin (N-ac-β-END), can acutely increase the relative number of prolactin-secreting cells in anterior pituitary cell cultures from ovariectomized rats. Inasmuch as the des-acetylated forms of these peptides (des-ac--MSH and β-END) were not effective in this regard, we concluded that acetylation was an absolute requirement for manifestation of the recruitment response. The aim 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 with di-ac--MSH and N-ac-β-END increased the relative amount of prolactin secretors above control values. Interestingly, des-acetylated variants of -MSH and β-END blocked the mammotrope-recruitment activity of their respective acetylated forms. In addition, β-END antagonized the mammotrope-recruitment activity of di-ac--MSH while des-ac--MSH did not attenuate the stimulatory effect of N-ac-β-END. Given that mammotropes maintained in vivo are exposed to all these peptides, it is possible that these acetylated and non-acetylated congeners may act in an opposing manner to regulate dynamic prolactin release.

Journal of Endocrinology (1993) 139, 295–300