Vascular pharmacology of methylene blue in vitro and in vivo: a comparison with NG-nitro-L-arginine and diphenyleneiodonium.

1. The vascular effects of the soluble guanylyl cyclase inhibitor, methylene blue as well as the nitric oxide (NO) synthase inhibitors, NG-nitro-L-arginine (L-NOARG) and diphenyleneiodonium (DPI) were studied in rat isolated aortic rings and conscious, unrestrained rats. 2. Acetylcholine (ACh) and sodium nitroprusside (SNP) caused concentration-dependent relaxation of preconstricted aortic rings. Both methylene blue (1 x 10(-5) M) and L-NOARG (3 x 10(-5) M) abolished ACh-induced relaxation; however, methylene blue but not L-NOARG shifted the concentration-response curve of SNP to the right. 3. In conscious rats, i.v. infusion of methylene blue (1.1 x 10(-5) mol kg-1 min-1), at a concentration which reduced the aortic tissue level of cyclic GMP by 50%, did not significantly alter mean arterial pressure (MAP) and heart rate (HR). In contrast, i.v. bolus injection of L-NOARG (1.5 x 10(-4) mol kg-1) markedly increased MAP and decreased HR. 4. Both ACh and SNP dose-dependently decreased MAP in conscious rats. Methylene blue did not alter the magnitude or duration of ACh- or SNP-induced depressor responses. L-NOARG, on the other hand, significantly though incompletely, reduced the magnitude and duration of the depressor response to ACh but not SNP. The depressor response to ACh or SNP was not altered by pretreatment with indomethacin (1.4 x 10(-5) mol kg-1) or capsaicin (3.3 x 10(-4) mol kg-1). 5. NG-nitro-L-arginine methyl ester (L-NAME) also caused dose-dependent increases in MAP in conscious rats. Both methylene blue and DPI (1 x 10-5 mol kg-1) selectively shifted the dose-pressor response curve of L-NAME to the right.6. These results suggest that: (1) the inhibition of endogenous NO biosynthesis does not necessarily lead to pressor response in vivo, (2) L-NOARG may not produce pressor response solely via the inhibition of endogenous endothelial NO biosynthesis, and (3) the depressor responses to ACh and SNP may not involve the release of NO or prostanoids or afferent nerve transmitters.