Atypical Adrenoceptor-Mediated Relaxation of Canine Pulmonary Artery through a cAMP-Dependent Pathway

Abstract

We studied the existence of β3-adrenoceptors in canine pulmonary artery smooth muscle under isometric conditionsin vitro.A rank order potency of vascular relaxation was isoproterenol > salbutamol > selective β3-adrenoceptor agonists, CL 316243 and BRL 37344. A marked desensitization to salbutamol occurred by pretreatment with salbutamol but not with CL 316243. When β1-adrenoceptors were blocked, the relaxant responses to salbutamol were competitively antagonized by the β2-adrenoceptor antagonist ICI 118551, whereas the response to CL 316243 was not. Cyanopindolol, a non-selective β-adrenoceptor antagonist, antagonized CL 316243-induced relaxation in a competitive manner with apA2of 6.10, and this value was lower than that when salbutamol was used as an agonist (6.69). Intracellular cAMP levels were increased by CL 316243, an effect that was not altered by ICI 118551. Therefore, β3-adrenoceptors may be present and functioning in canine pulmonary artery.