Cultured cells from adult rat anterior pituitaries or intermediate lobes were treated with the proteinase inhibitor tripeptide aldehydes BOC-DPhe-Pro-Arg-H (Boc-fPRH) and DPhe-Pro-Arg-H (fPRH), ovine corticotropin-releasing factor (oCRF), and bromocriptine. One millimolar fPRH stimulated basal, and slightly enhanced oCRF-induced ACTH release by melanotrophs in short-term experiments. The basal release of alpha-MSH was also stimulated by the drug. In long-term experiments, fPRH elevated markedly both the release and the intracellular level of ACTH; BOC-fPRH caused an increased alpha-MSH release. Tritiated fPRH had no preference for POMC-producing cells and BOC-fPRH or fPRH were harmless to the cell morphology. In anterior pituitary cell cultures, fPRH diminished slightly basal and oCRF-induced ACTH release. Bromocriptine was ineffective on corticotrophs, however, in melanotrophs it inhibited ACTH release markedly with or without fPRH in the medium. The dissimilar responsiveness of the corticotrophs and melanotrophs to the peptide aldehydes may be interpreted in terms of their differing membrane receptors or intracellular mechanism of stimulus-secretion coupling.
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