Differential effects of thyrotropin-releasing hormone, vasoactive intestinal peptide, phorbol ester, and depolarization in GH4C1 rat pituitary cells.

The sequence of PRL and GH release from GH4C1 cells was studied in perifusion and static culture systems. The secretory pattern elicited by TRH differed from those caused by depolarizing concentrations of KCl (Ca2+-initiated secretion), vasoactive intestinal peptide (VIP), 8-bromo-cAMP, and forskolin (cAMP-mediated secretion), and 12-O-tetradecanoylphorbol-13-acetate (TPA) (protein kinase C activation). TRH, K+, VIP, and TPA all caused secretion within 1 min in the perifusion system but the peak response to TRH and depolarization occurred earlier than the peak responses to TPA and VIP. PRL and GH release in response to a pulsatile application of TRH (0.4-min pulse every 5 min for 25 min) did not decline with a low dose, indicating that acute desensitization does not occur, but did decrease with a high concentration. When cells in the perifusion system were subjected to continuous stimulation, TRH caused a biphasic response with a 2- to 3-min period of high secretion followed by a second phase in which GH and PRL secretion were 60-70% the rates in the first phase. KCl caused predominantly first-phase secretion, and TPA caused a biphasic secretory pattern with a delay in its peak of action. VIP caused a modest but prolonged response whether administered in a pulsatile or sustained manner. When GH-cells were exposed to 100 nM TRH for 2 days, [3H] [N3-methyl-His2]TRH binding was decreased (down-regulation), intracellular PRL was increased (170% of control), and intracellular GH was decreased (65% of control). In these down-regulated cells, baseline PRL and GH secretion were changed in proportion to the relative intracellular hormone content. The responsiveness to TRH, KCl, and TPA during the initial 10-min period (first phase) was reduced; however, the responsiveness to these substances in the subsequent 50-min period (second phase) was unchanged. The ED50 for TRH stimulation of hormone release was increased 2- to 4-fold in down-regulated cells, but the dose-response curves for other secretagogues were not shifted. These data suggest that the initial burst of hormone release caused by TRH is mediated by Ca2+, and that prolonged exposure to TRH causes homologous desensitization.