Donors of H₂S may be beneficial in treating cardiovascular diseases where the plasma levels of H₂S are decreased. Therefore, we investigated the mechanisms involved in relaxation of small arteries induced by GYY4137 [(4-methoxyphenyl)-morpholin-4-yl-sulfanylidene-sulfido-λ5-phosphane;morpholin-4-ium], which is considered a slow-releasing H₂S donor. Sulfides were measured by use of 5,5'-dithiobis-(2-nitro benzoic acid), and small rat mesenteric arteries with internal diameters of 200-250 µm were mounted in microvascular myographs for isometric tension recordings. GYY4137 produced similar low levels of sulfides in the absence and the presence of arteries. In U46619-contracted small mesenteric arteries, GYY4137 (10^-6-10^-3 M) induced concentration-dependent relaxations, while a synthetic, sulfur-free, GYY4137 did not change the vascular tone. L-cysteine (10^-6-10^-3 M) induced only small relaxations reaching 24 ± 6% at 10^-3 M. Premixing L-cysteine (10^-3 M) with Na₂S and GYY4137 decreased Na₂S relaxation and abolished GYY4137 relaxation, an effect prevented by an nitric oxide (NO) synthase inhibitor, L-NAME (N^ω-nitro-L-arginine methyl ester). In arteries without endothelium or in the presence of L-NAME, relaxation curves for GYY4137 were rightward shifted. High extracellular K⁺ concentrations decreased Na₂S and abolished GYY4137 relaxation suggesting potassium channel-independent mechanisms are also involved Na₂S relaxation while potassium channel activation is pivotal for GYY4137 relaxation in small arteries. Blockers of large-conductance calcium-activated (BK_Ca) and voltage-gated type 7 (K_V7) potassium channels also inhibited GYY4137 relaxations. The present findings suggest that L-cysteine by reaction with Na₂S and GYY4137 and formation of sulfides, inhibits relaxations by these compounds. The low rate of release of H₂S species from GYY4137 is reflected by the different sensitivity of these relaxations towards high K⁺ concentration and potassium channel blockers compared with Na₂S. The perspective is that the rate of release of sulfides plays an important for the effects of H₂S salt vs. donors in small arteries, and hence for a beneficial effect of GYY4137 for treatment of cardiovascular disease.

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