Mechanisms of TSH release: studies with forskolin, phorbol ester and A23187

We have investigated the interactions of intracellular messenger systems which may regulate thyrotrophin (TSH) release in primary cultures of rat anterior pituitary cells. Calcium ionophore A23187 was used to raise intracellular free calcium, the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA) to activate protein kinase C, and forskolin to activate adenylate cyclase. Each of these agents stimulated TSH secretion in a dose-dependent manner, but the effect of forskolin was delayed for at least 6 h. The combined effects of A23187 and TPA on TSH secretion were simply additive, but forskolin synergistically enhanced the effect of A23187 and 55 mM potassium, but not that of TPA. Co-incubation of cells with 1 microM A23187 caused parallel upward shift of the TPA dose-response curve, and the further addition of 10 microM forskolin led to further upward shift. These results suggest that calcium mobilization and adenylate cyclase activation may interact synergistically to modulate TSH secretion. Raised cellular cyclic AMP content may amplify further the TSH release stimulated by the combination of protein kinase C activation and raised intracellular free calcium.