Atypical β-adrenoceptors of rat thoracic aorta

Abstract

The possible existence of atypical β-adrenoceptors in vascular smooth muscle of rat isolated thoracic aorta was investigated. Isoprenaline (10 −8–10 −4 M) produced concentration-dependent relaxation of phenylephrine (10 −5 M) precontracted rings of endothelium-denuded rat aorta in vitro. Isoprenaline-induced relaxation was resistant to blockade by atenolol (10 −6 M). But, propranolol (2 × 10 −7 M) caused a non-competitive inhibition and SR 59230A (6.6 × 10 −6 M), a β 3-adrenoceptor selective antagonist, failed to produce additional antagonism in presence of propranolol. BRL 37344 (10 −8–10 −4 M), a β 3-selective agonist, did not relax ring segments precontracted with phenylephrine (10 −5 M) in the absence of endothelium. The non-conventional partial agonist (–)-cyanopindolol (5 × 10 −6–10 −4 M) induced a marked relaxation in phenylephrine (10 −5 M) precontracted aortic rings without endothelium. This vasodilation was resistant to blockade by propranolol (2 × 10 −7 M) and SR 59230A (10 −5 M). Salbutamol (10 −8–10 −4 M) produced concentration-dependent relaxation in isolated endothelium-denuded aortic rings precontracted with phenylephrine (10 −5 M). Propranolol (2 × 10 −7 M), but not atenolol (10 −6 M), inhibited this relaxant response. It is concluded that in endothelium-denuded thoracic aorta, salbutamol acts through β 2-adrenoceptors whereas isoprenaline seems to activate both β 2-adrenoceptors and an atypical β-adrenergic receptor. This atypical β-adrenoceptor is distinct from putative β 3-adrenoceptor and maybe resembles the reported fourth cardiac β-adrenoceptor.