MECHANISMS OF RELAXATIONS OF BOVINE ISOLATED BRONCHIOLES BY THE NITRIC-OXIDE DONOR, GEA-3175

1 The present study was designed to investigate the effects and mechan isms of relaxation induced by the nitric oxide (NO) donor, GEA 3175 (a 3-aryl-substituted oxatriazole derivative) on bovine bronchioles (eff ective lumen diameter 200-800 mu m) suspended in microvascular myograp hs for isometric tension recording. 2 In segments of bovine bronchiole s contracted to 5-hydroxytryptamine, GEA 3175 (10(-8)-10(-4) M) induce d concentration-dependent reproducible relaxations. These relaxations were slow in onset compared to other NO-donors such as 3-morpholinosyd onimine-hydrochloride (SIN-1) and S-nitroso-N-acetylpenicillamine (SNA P). 3 In 5-hydroxytryptamine-contracted preparations the order of rela xant potency (pD(2)) was: salbutamol (7.80)> GEA 3175 (6.18)> SIN-1 (4 .90)> SNAP (3.55). In segments contracted to acetylcholine, the relaxa nt responses were reduced and GEA 3175 relaxed the bronchioles with pD (2)=4.41+/-0.12 and relaxations of 66 +/- 10% (n = 4), while SNAP and salbutamol caused relaxations of 19 +/- 6% (n = 4) and 27 +/- 6% (n = 8) at the highest concentration used, respectively. 4 Oxyhaemoglobin ( 10(-5) M), the scavenger of nitric oxide, caused rightward shifts of t he concentration-relaxation curves to GEA 3175 and NO. 1H-[1,2,4]oxadi azolo[4,3,-a]quinoxalin-1-one(ODQ, 3 x 10(-6) M) and LY 83583 (10(-6) M), the inhibitors of soluble guanylate cyclase, also reduced the rela xations induced by GEA 3175 and nitric oxide. However, ODQ did not aff ect salbutamol-evoked relaxation in the bovine small bronchioles. 5 GE A 3175-induced relaxations were reduced in potassium-rich (60 mmol l(- 1) K+) solution. Glibenclamide (10(-6) M) markedly inhibited the relax ations induced by the opener of ATP-sensitive K+ channels, levcromakal im (3 x 10(-8)-10(-5) M), but it did not modify the relaxations induce d by GEA 3175 or salbutamol. Apamin (5 x 10(-7) M), a blocker of the s mall Ca2+-activated K+-channels did not affect the relaxations to GEA 3175. In contrast, blockers of large Ca2+-activated K+-channels, chary bdotoxin (3 x 10(-8)-10(-7) M) and iberiotoxin (10(-8) M), did inhibit the relaxations to GEA 3175. The combination of apamin and charybdoto xin did not induce an additional inhibitory effect on the relaxations to GEA 3175 compared to charybdotoxin alone. 6 In preparations where a concentration-response curve to GEA 3175 or NO was first obtained in the presence of LY 83583, incubation with charybdotoxin (10(-7) M) did produce an additional inhibitory effect of the relaxations. However, in the presence of ODQ (3 x 10(-6) M), iberiotoxin (10(-8) M) did not produce additional reduction of the NO- or GEA 3175-induced relaxation s. 7 The present results suggest that the slow-releasing NO-donor GEA 3175 is more potent than the traditional NO donors in inducing relaxat ions of bovine bronchioles. GEA 3175, as for exogenously added NO, eli cits relaxations through a cyclic GMP-dependent mechanism followed by opening of large conductance Ca2+-activated K+-channels.