Classification of postjunctional beta adrenoceptors mediating relaxation of canine bronchi
Janssen, L.J.; Daniel, E.E.
Journal of Pharmacology and Experimental Therapeutics 256(2): 670-676
1991
ISSN/ISBN: 0022-3565 PMID: 1671600 Document Number: 374581
Adrenoceptors mediating relaxant responses to exogenously added or endogenously released catecholamines in isolated canine bronchi (3rd-6th order) were characterized using selective beta receptor agonists and antagonists. Norepinephrine (3 .times. 107 to 3 .times. 10-5 M) or isoproterenol (3 .times. 10-8 to 10-6 M) fully relaxed tissues precontracted with 3 .times. 10-7 M carbachol (Cch). Salbutamol (Sal) also relaxed Cch-precontracted tissues, but this relaxant effect was extremely sensitive to the concentration of precontracting agent used: the maximal effect of Sal was 100, 79 and 28% reversal of tone in tissues precontracted with 2 .times. 10-8, 10-7 and 3 .times. 10-7 M Cch. The effects of isoproterenol were antagonized by propranolol. Norepinephrine relaxations were antagonized by the beta-1-selective antagonist lCl 89,406 (pA2 = 7.70) and the beta-2-selective antagoniist lCl 118,551 (pA2 = 6.33). Sal-relaxations were antagonized by lCl 118,551 (pA2 = 8.91). Field stimulation in tissues precontracted with McNeil A343 (M1-selective muscarinic agonist) produced transient relaxations which were antagonzied by lCl 89,406 but not lCl 118,551 (both 10-9 to 10-7 M). Thus, exogenous and endogenous catecholamines relax canine bronchial smooth muscle by activating both beta-1 and beta-2 adrenoceptors, although the latter seem to play a significant role only when low concentrations of Cch were used.