ADENOSINE RECEPTOR-MEDIATED RELAXATION OF RABBIT AIRWAY SMOOTH-MUSCLE- A ROLE FOR NITRIC-OXIDE

In this study, we investigated the relaxant effect of adenosine recept or agonists on KCl-precontracted airway smooth muscle from rabbits and characterized the type of receptor involved in bronchorelaxation in t he 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 ring s from rabbits, the adenosine receptor agonists )]phenylethylamino-5-N -6-ethylcarboxamidoadenosine (CGS-21680), 5'-(N-ethyl-carboxamido)aden osine (NECA), 2-chloroadenosine (CAD), and (-)-N-6-(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% relaxat ion was observed at 10(-5) M by CGS-21680, NECA, CAD, and R-PIA in the epithelium-intact bronchial rings, respectively. The 50% maximum effe ctive 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 co mpared with epithelium-intact rings. The adenosine receptor antagonist 8-(sulfophenyl)theophylline significantly attenuated the relaxant res ponses to all the agonists in the epithelium-intact and -denuded rings . The epithelium-dependent relaxant effect of the agonists in airway r ings was inhibited by N-6-monomethyl-L-arginine (L-NMMA; 30 mu M). The EC50 (X 10(-6) M) values for CGS-21680, NECA, CAD, and R-PIA in the p resence of inhibitor were 5.5, 8, 30, and 200, respectively. The L-NMM A produced an insignificant inhibitory effect in the epithelium-denude d rings. L-Arginine but not D-arginine (100 mu M) reversed the inhibit ory effect of L-NMMA on adenosine agonist-induced relaxation. In prima ry epithelial cells in culture, CGS-21680 (10(-5) M) induced a fourfol d increase in NO production over the control. The CGS-21680-induced NO production in epithelial cells was significantly inhibited by N-6-nit ro-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 A(2) ad enosine receptor and the epithelium serves as a source of NO.