Noradrenaline-Induced Relaxation of Urinary Bladder Smooth Muscle Is Primarily Triggered through the β 3 -Adrenoceptor in Rats

β-Adrenoceptors are subclassified into 3 subtypes (β -β ). Among these, β -adrenoceptors are present in various types of smooth muscle and are believed to play a role in relaxation responses of these muscles. β -Adrenoceptors are also present in urinary bladder smooth muscle (UBSM), although their expression varies depending on the animal species. To date, there has been little information available about the endogenous ligand that stimulates β -adrenoceptors to produce relaxation responses in UBSM. In this study, to determine whether noradrenaline is a ligand of UBSM β -adrenoceptors, noradrenaline-induced relaxation was analyzed pharmacologically using rat UBSM. We also assessed whether noradrenaline metabolites were ligands in UBSM. In isolated rat urinary bladder tissues, mRNAs for β -, β -, and β -adrenoceptors were detected using RT-PCR. In UBSM preparations contracted with methacholine (3 × 10 M), noradrenaline-induced relaxation was not inhibited by the following antagonists: atenolol (10 M; selective β -adrenoceptor antagonist), ICI-118,551 (3 × 10 M; selective β -adrenoceptor antagonist), propranolol (10 M; non-selective β-adrenoceptor antagonist), and bupranolol (10 M; non-selective β-adrenoceptor antagonist). In the presence of propranolol (10 M), noradrenaline-induced relaxation was competitively inhibited by bupranolol (3 × 10 -3 × 10 M) or SR59230A (10 -10 M; selective β -adrenoceptor antagonist), with their pA values calculated to be 6.64 and 7.27, respectively. None of the six noradrenaline metabolites produced significant relaxation of methacholine-contracted UBSM. These findings suggest that noradrenaline, but not its metabolites, is a ligand for β -adrenoceptors to produce relaxation responses of UBSM in rats.