β1-Adrenoceptors in Rat Anterior Pituitary May Be Constitutively Active. Inverse Agonism of CGP 20712A on Basal 3′,5′-Cyclic Adenosine 5′-Monophosphate Levels

Abstract

Catecholamines directly stimulate GH, ACTH, and prolactin secretion from rat anterior pituitary through the beta(2)-adrenoceptor (AR). We recently showed that gonadotrophs express the beta(1)-AR and that glucocorticoids drastically increase its mRNA expression level. The present investigation explores whether beta(1)-ARs are functionally coupled to adenylate cyclase. In anterior pituitary cell aggregates, the highly selective beta(1)-AR antagonists CGP 20712A and ICI 89,406-8a attenuated isoproterenol-stimulated cAMP accumulation, but no agonist action of norepinephrine could be detected. Remarkably, CGP 20712A inhibited basal cAMP levels by its own for at least 50%, an action that tended to be more effective in dexamethasone-supplemented medium. The latter effect was abolished by the beta-AR antagonist carvedilol, but not by other beta-AR antagonists. Pretreatment with pertussis toxin abolished the action of CGP 20712A on basal cAMP. CGP 20712A also attenuated isoproterenol-induced cAMP accumulation in the gonadotroph cell lines alphaT3-1 and LbetaT2, but not in the somatotroph precursor cell line GHFT and the folliculo-stellate cell line TtT/GF. However, in LbetaT2 cells CGP 20712A did not inhibit basal cAMP levels by its own. The present data suggest that beta(1)-AR in the anterior pituitary is positively coupled to adenylyl cyclase but is constitutively active in a pertussis toxin-sensitive manner. CGP 20712A may act as an inverse agonist with approximately 50% negative intrinsic activity, suggesting that the beta(1)-AR significantly contributes to basal adenylate cyclase activity in the pituitary.