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 Ca2+ channel blockers Co2+ (5 mM) and verapamil (100 μM) and the Ca2+ chelator EGTA (4 mM) abolished the stimulatory effect of TRH (1 μM) on hormone release. Co2+ and verapamil, but not EGTA, inhibited the stimulatory effect of db-cAMP (5 mM) on hormone release. The inhibitory effects of Co2+ and verapamil on GH release were counteracted by the combination of TRH and db-cAMP. For PRL release Co2+, but not verapamil, was able to inhibit the combined action of TRH and db-cAMP. Co2+, verapamil, and EGTA eliminated the stimulatory effect of MIX (1 mM) on PRL release while only Co2+ and EGTA affected the GH release. Hormone release in the presence of MIX plus verapamil or EGTA, but not Co2+, 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 Ca2+ ionophore A23187 (5 μM) had a stimulatory effect on basal hormone release which was abolished by 30 μM TFP.

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