Comparison of two soluble guanylyl cyclase inhibitors, methylene blue and ODQ, on sodium nitroprusside-induced relaxation in guinea-pig trachea

1 To clarify further the role of cyclic GMP in mediating the relaxant response in guinea-pig trachea induced by sodium nitroprusside (SNP), the effects of soluble guanylyl cyclase inhibitors, methylene blue and 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ) on SNP-induced muscle relaxation and cyclic GMP accumulation were determined. 2 SNP (0.3-100 mu M) evoked a concentration-dependent relaxation of guinea-pig isolated tracheas precontracted with 0.3 mu M carbachol. Preincubation of the preparations with methylene blue (10, 30 and 100 mu M) resulted in a slight but concentration-dependent prevention of the relaxant response to SNP. In contrast, the relaxation to SNP was extensively prevented by 3 mu M ODQ and almost abolished by 10 mu M ODQ. 3 SNP (30 mu M) induced a significant elevation of cyclic GMP accumulation (from 1.34+/-0.14 to 5.39+/-0.28 pmol mg(-1) protein, n=5; P<0.001), which was partially attenuated by 100 mu M methylene blue (3.11+/-0.51 pmol mg(-1) protein, n=5; P<0.05), whereas completely abolished by 10 mu M ODQ (1.31+/-0.28 pmol mg(-1) protein, n=5; P<0.001). 4 Methylene blue, but not ODQ and N-omega-nitro-L-arginine methyl ester (L-NAME), caused a concentration-dependent contraction in the tracheal preparation. The tension produced by 100 mu M methylene blue was 41.8+/-4.3% (0.3 mu M carbachol as 100%; n=12). Moreover, the non-selective muscarinic receptor antagonist atropine and the M-3-selective antagonist 4-diphenylacetoxy-N-methylpiperidine methiodine greatly inhibited the contractile effect evoked by methylene blue (100 mu M). 5 In conclusion, this study provides substantial evidence that SNP-induced muscle relaxation in guineapig trachea is completely via a cyclic GMP-dependent mechanism. Furthermore, ODQ, but not methylene blue, will likely become an important tool in differentiating between cyclic GMP-dependent and -independent effects of nitric oxide.