On the involvement of cyclic AMP and extracellular Ca in the regulation of hormone release from rat pituitary tumour (GH) cells in culture

Thyroliberin (TRH), dibutyryl cyclic AMP (db-cAMP), and 3-isobutyl-l-methylxanthine (MIX) had a stimulatory effect on prolactin (PRL) and growth hormone (GH) release from GH 3 cells. Half-maximal and maximal effects were observed for TRH at 2.5 nM and 10 nM; for db-cAMP at 0.6 mM and 5 mM, respectively. MIX (0.1 mM-1 mM) induced a dose-dependent accumulation of cellular cyclic AMP, while the hormone release was already maximally stimulated at 0.1 mM MIX. The maximal effects on hormone release of TRH and db-cAMP, but not of TRH and MIX, were additive. The Ca channel blockers Co (5 mM) and verapamil (100 μM) and the Ca chelator EGTA (4 mM) abolished the stimulatory effect of TRH (1 μM) on hormone release. Co and verapamil, but not EGTA, inhibited the stimulatory effect of db-cAMP (5 mM) on hormone release. The inhibitory effects of Co and verapamil on GH release were counteracted by the combination of TRH and db-cAMP. For PRL release Co, but not verapamil, was able to inhibit the combined action of TRH and db-cAMP. Co, verapamil, and EGTA eliminated the stimulatory effect of MIX (1 mM) on PRL release while only Co and EGTA affected the GH release. Hormone release in the presence of MIX plus verapamil or EGTA, but not Co, was increased by TRH. The calmodulin antagonist trifluoperazine (TFP) at 30 μM inhibited basal hormone release and hormone release stimulated by TRH (1 μM), db-cAMP (5 mM), and MIX (1 mM). The Ca ionophore A23187 (5 μM) had a stimulatory effect on basal hormone release which was abolished by 30 μM TFP. © 1987 Plenum Publishing Corporation.