Calcitonin Inhibits Basal and Thyrotropin-Releasing Hormone-Induced Release of Prolactin from Anterior Pituitary Cells: Evidence for a Selective Action Exerted Proximal To Secretagogue-Induced Increases in Cytosolic Ca2+*

Abstract

Salmon calcitonin (sCT)-like peptide is present in the central nervous system and pituitary gland of the rat, and this peptide inhibits basal and TRH-stimulated PRL release from cultured rat anterior pituitary (AP) cells. The present studies were designed to examine further the inhibitory actions of sCT on basal and TRH-stimulated PRL release and investigated 1) the temporal dynamics of the responses, 2) the effects of sCT on PRL release induced by other secretogogues, and particularly those acting via elevations of cytosolic Ca2+, and 3) the selectivity of sCT action on basal and stimulated AP hormone release. The inhibition of basal PRL release by sCT (0.1-10 nM) was dose-dependent and was characterized by a rapid onset with a gradual recovery to normal rates of release after the period of sCT inhibition. The inhibitory effect of sCT on basal PRL release was reversed by treatment with either the Ca2+ ionophore A23187 or with the phorbol ester, phorbol myristate acetate (PMA). sCT infusion did not affect the basal release of GH, TSH, FSH, or LH by perifused AP cells. When administered in short pulses, TRH, at concentrations from 1-100 nM, elicited a dose-dependent increase in PRL release. When coadministered with short 10 nM TRH, sCT (1-100 nM) inhibited TRH-induced PRL release in a dose-dependent manner, with a maximal inhibition of 78% at a concentration of 10 nM, and an ED50 concentration of approximately 3 nM. During longer (30 min) pulses of TRH (100 nM), PRL release increased sharply over 4-fold within 2 min, followed within 12 min by a rapid decline to a level 1.5-2-fold higher than basal, and this level was maintained for the remainder of the stimulation period. sCT pretreatment inhibited the overall PRL response to TRH. In contrast to its inhibition of TRH-induced PRL release, sCT failed to prevent the stimulation of PRL release by either ionophore A23187, PMA, vasoactive intestinal peptide, or forskolin. In addition, sCT failed to block TRH-induced TSH release or GnRH-induced LH release.(ABSTRACT TRUNCATED AT 400 WORDS)