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

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Abstract
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 μm) evoked a concentration‐dependent relaxation of guinea‐pig isolated tracheas precontracted with 0.3 μm carbachol. Preincubation of the preparations with methylene blue (10, 30 and 100 μm) 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 μm ODQ and almost abolished by 10 μm ODQ. 3 SNP (30 μm) 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 μm methylene blue (3.11±0.51 pmol mg−1 protein, n = 5; P < 0.05), whereas completely abolished by 10 μm ODQ (1.31±0.28 pmol mg−1 protein, n = 5; P < 0.001). 4 Methylene blue, but not ODQ and Nω‐nitro‐l‐arginine methyl ester (l‐NAME), caused a concentration‐dependent contraction in the tracheal preparation. The tension produced by 100 μm methylene blue was 41.8±4.3% (0.3 μm carbachol as 100%; n=12). Moreover, the non‐selective muscarinic receptor antagonist atropine and the M3‐selective antagonist 4‐diphenylacetoxy‐N‐methylpiperidine methiodine greatly inhibited the contractile effect evoked by methylene blue (100 μm). 5 In conclusion, this study provides substantial evidence that SNP‐induced muscle relaxation in guinea‐pig 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. British Journal of Pharmacology (1998) 125, 1158–1163; doi:10.1038/sj.bjp.0702181