Synthesis of Growth Hormone, Prolactin, and Proopiomelanocortin by Ovine Fetal Anterior and Neurointermediate Lobes*

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Abstract
Previous studies have shown that plasma concentrations of ovine GH, PRL, and corticotropin show characteristic changes during gestation and postnatally. The mechanisms regulating these changes are unknown but could occur at the levels of gene transcription, mRNA processing, mRNA translation, processing of hormonal precursors, or release of hormones. We have studied the ontogeny of pituitary hormone synthesis by incubating explants of fetal anterior pituitary and neurointermediate lobe in medium containing [35S]methionine. Labeled proteins were displayed by 2D gel electrophoresis, and the spots representing GH, PRL, and proopiomelanocortin (POMC; the precursor to corticotropin) were quantitated. GH and PRL were produced only in the anterior pituitary, whereas POMC was synthesized in both the anterior and neurointermediate lobes. GH was the most abundant protein synthesized by the pituitary at 50 days gestation and represented a fairly constant proportion of anterior pituitary protein synthesis thereafter and into adult life. PRL was made in miniscule quantities at 50 days, but its production increased steadily during the latter two thirds of gestation and exceeded the production of GH in perinatal and adult anterior pituitaries. Synthesis of POMC remained a fairly constant, low percentage of total protein synthesis in all anterior pituitary samples. In contrast, synthesis of POMC in the neurointermediate lobe increased steadily during the latter two thirds of gestation and remained constant thereafter. These data indicate that pituitary hormone production in the ovine fetus precedes hormone release by several weeks. Furthermore, the changes in hormonal synthesis do not correlate with the established changes in fetal and perinatal plasma hormone concentrations. This indicates that the variations in fetal plasma GH, PRL, and ACTH are regulated posttranslationally, perhaps at the level of hormone release.