Biosynthesis and degradation of prolactin in the rat anterior pituitary gland. Time course of incorporation of label in vitro and evidence for rapid degradation

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Abstract
Biosynthesis of prolactin [PRL] was studied in anterior pituitary glands from female rats, incubated in vitro. In this system [3H]L was incorporated into pituitary proteins, including somatotropin (growth hormone, GH) and PRL. The rate of uptake of label into PRL (and to a lesser extent into total protein) slowed considerably during the first 2 h of incubation, although the rate of uptake into GH was constant for 8 h. The most probable explanation for this apparent decrease in the rate of PRL synthesis is degradation of PRL in the gland. Degradation of this hormone was also demonstrated by incubating prelabeled pituitaries in unlabeled medium and following the content of labeled PRL, and by studying the hormonal content of pituitary glands (by radioimmunoassay) before and after incubation. Degradation of PRL appears to be much more rapid than that of GH, and may represent a physiological mechanism whereby over-accumulation of PRL is prevented when secretion of the hormone has been rapidly switched off.