Inhibition of renin secretion by clonidine after alpha-adrenoceptor blockade in anesthetized dogs

Dogs anaesthetised with pentobarbital were used to study the effects of the alpha-adrenoceptor antagonists phentolamine and piperoxan on the clonidine-induced suppression of plasma renin activity (PRA). Given alone, clonidine (30 microgram/kg, i.v.) produced an initial rise in mean arterial pressure (MAP) which was followed by a hypotensive response. These changes in blood pressure were accompanied by decreased in PRA and heart rate (HR). Pretreatment with phentolamine, 1 mg/kg i.v. or 3.3 microgram/kg/min infused into the third cerebral ventricle, or piperoxan, 20 microgram/kg stat. + 5 microgram/kg/min infused either i.v. or intraventricularly, did not modify the clonidine-induced falls in PRA, MAP or HR. All pretreatment regimes, with the exception of intraventricular phentolamine, virtually abolished the initial pressor response to clonidine. These results demonstrate that the renin-lowering action of clonidine in the dog is not inhibited by two classical alpha-adrenoceptor antagonists.