[Relaxation of canine pulmonary arteries caused by stimulation of atypical beta-adrenergic receptors].

To examine possible contributions of beta(3)-adrenoceptors to catecholamine-induced pulmonary vasodilation, we studied isolated canine pulmonary arterial segments under isometric conditions. Addition of beta-adrenoceptor agonists produced a concentration-dependent relaxation of tissues precontracted with 50 mM KCl; the rank order of potency was isoproterenol (ISO, 1) > salbutamol (SAL, 0.97) > selective beta(3)-adrenoceptor agonists CL 316243 (CL, 0.87) and BRL 37344 (BRL, 0.86). Relaxant responses to SAL were competitively antagonized by the beta(2)-adrenoceptor antagonist ICI 118551 and the pA2 value was 6.67 +/- 0.21 (mean +/- SE), whereas the response to CL was weekly antagonized only by a high concentration of ICI 118551 (10(-5) M) and the apparent pA2 value was 5.24 when alpha-and beta(1)-adrenergic receptors were blocked. By contrast, the atypical beta-adrenoceptor antagonist cyanopindolol antagonized CL-induced relaxation in a competitive manner; the pA2 value was 6.71 +/- 0.12, which was lower than that with salbutamol (p < 0.05). Intracellular cyclic AMP levels were increased in a contraction-dependent manner by CL. These results suggest that beta(3)-adrenoceptors may exist in canine pulmonary arterial smooth muscle and that stimulation of this atypical receptor causes vasodilation through a cyclic AMP-dependent pathway.