Effect of α1- and β2- adrenoceptor agonists and antagonists on ACTH secretion in rats

Abstract

For studing which α adrenoceptor α1- or α2- receptor) is mainly responsible for ACTH secretion in rats, prazosin and yohimbine were used as selective ρ1- and ς2- adrenoceptor antagonists, respectively, and B-HT933 was used as a selective α2- adrenoceptor agonist. Drugs were injected i.p., except for prazosin which was injected s.c. Rats were decapitated after suitable periods and the trunk blood was collected. The amount of ACTH in the serum was determined by a bioassay. (1) ACTH secretion induced by phenylephrine was inhibited by prazosin (P<0.01), but it was not inhibited by yohimbine. (2) Norepinephrine-induced ACTH secretion was inhibited by both prazosin (P<0.01) and yohimbine (P<0.001). (3) Injection of clonidine (0.06-2.5 mg/kg) increased ACTH secretion dose-dependently. Prazosin inhibited ACTH secretion induced by the lower dose of clonidine (P<0.02) and more clearly blocked ACTH secretion induced by the higher dose of it (P<0.01). Yohimbine also blocked ACTH secretion induced by the lower dose of clonidine (P<0.01). However, it was ineffective in inhibiting ACTH secretion induced by the higher dose of clonidine. (4) B-HT933 induced dose-related ACTH secretion (2.5-20 mg/kg) which was inhibited by yohimbine (P<0.01), but was not inhibited by prazosin.