title: Adenosine receptor-mediated relaxation of rabbit airway smooth muscle: a role for nitric oxide.
abstract: In this study, we investigated the relaxant effect of adenosine receptor agonists on KCl-precontracted airway smooth muscle from rabbits and characterized the type of receptor involved in bronchorelaxation in the presence and absence of epithelium. We further defined the role of epithelium-derived relaxing factor, i.e., nitric oxide (NO), on these responses. In both epithelium-intact and -denuded tertiary airway rings from rabbits, the adenosine receptor agonists 2-[p-(2-carboxyethyl)]phenylethylamino-5-N'-ethylcarboxamidoadenos ine (CGS-21680), 5'-(N-ethyl-carboxamido)adenosine (NECA), 2-chloroadenosine (CAD), and (-)-N6-(2-phenylisopropyl)adenosine (R-PIA) relaxed airway smooth muscle with a potency order of CGS-21680 > NECA > CAD > R-PIA. A 98.5, 89.7, 73.2, and 64.7% relaxation was observed at 10(-5) M by CGS-21680, NECA, CAD, and R-PIA in the epithelium-intact bronchial rings, respectively. The 50% maximum effective concentration (EC50; x 10(-7) M) values for CGS-21680, NECA, CAD, and R-PIA were 2, 4, 9, and 80, respectively. Denuded rings, however, showed much less relaxant responses to various adenosine agonists compared with epithelium-intact rings. The adenosine receptor antagonist 8-(sulfophenyl)theophylline significantly attenuated the relaxant responses to all the agonists in the epithelium-intact and -denuded rings. The epithelium-dependent relaxant effect of the agonists in airway rings was inhibited by NG-monomethyl-L-arginine (L-NMMA; 30 microM). The EC50 (x 10(-6) M) values for CGS-21680, NECA, CAD, and R-PIA in the presence of inhibitor were 5.5, 8, 30, and 200, respectively. The L-NMMA produced an insignificant inhibitory effect in the epithelium-denuded rings. L-Arginine but not D-arginine (100 microM) reversed the inhibitory effect of L-NMMA on adenosine agonist-induced relaxation. In primary epithelial cells in culture, CGS-21680 (10(-5) M) induced a fourfold increase in NO production over the control. The CGS-21680-induced NO production in epithelial cells was significantly inhibited by NG-nitro-L-arginine methyl ester (L-NAME). Moreover, L-arginine reversed the inhibitory effect of L-NAME in the epithelial cells. The data suggest that adenosine relaxes rabbit airway smooth muscle through an A2 adenosine receptor and the epithelium serves as a source of NO.
authors.affiliation: Department of Pharmacology, East Carolina University, Greenville, North Carolina 27858, USA.
authors.forename: S
authors.initials: S
authors.lastname: Ali
authors.forename: W J
authors.initials: WJ
authors.lastname: Metzger
authors.forename: H A
authors.initials: HA
authors.lastname: Olanrewaju
authors.forename: S J
authors.initials: SJ
authors.lastname: Mustafa
mesh_terms: D015762:2-Chloroadenosine / Q000494:pharmacology; D000241:Adenosine / Q000031:analogs & derivatives* / Q000494:pharmacology*; D019830:Adenosine-5'-(N-ethylcarboxamide); D000818:Animals; D001120:Arginine / Q000494:pharmacology; D001980:Bronchi / Q000187:drug effects / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D004848:Epithelium / Q000502:physiology; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology*; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology*; D019331:NG-Nitroarginine Methyl Ester / Q000494:pharmacology*; D009569:Nitric Oxide / Q000502:physiology*; D010627:Phenethylamines / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D058906:Purinergic P1 Receptor Agonists; D011817:Rabbits; D018047:Receptors, Purinergic P1 / Q000502:physiology*; D019323:omega-N-Methylarginine / Q000494:pharmacology*
chemical_list: D010627:Phenethylamines; D058906:Purinergic P1 Receptor Agonists; D018047:Receptors, Purinergic P1; C061282:2-(4-(2-carboxyethyl)phenethylamino)-5'-N-ethylcarboxamidoadenosine; D015762:2-Chloroadenosine; C047235:N-(1-methyl-2-phenylethyl)adenosine; D019323:omega-N-Methylarginine; D009569:Nitric Oxide; D019830:Adenosine-5'-(N-ethylcarboxamide); D011189:Potassium Chloride; D001120:Arginine; D000241:Adenosine; D019331:NG-Nitroarginine Methyl Ester