Inhibitory action of hydrogen sulfide on muscarinic receptor-induced contraction of isolated porcine irides

We investigated the pharmacological actions of hydrogen sulfide (H2S) using sodium hydrosulfide (NaHS) and sodium sulfide (Na2S) as donors on isolated porcine irides in the presence of tone induced by muscarinic receptor stimulation. Furthermore, we also investigated the mechanism of action of H2S in this smooth muscle. Isolated porcine iris muscle strips were set up in organ baths and prepared for measurement of longitudinal isometric tension. The relaxant action of NaHS or Na2S on carbachol-induced tone was studied in the absence and presence of a K+-channel inhibitor and inhibitors/activators of enzymes of the biosynthetic pathways for H2S, prostanoid and nitric oxide production. In the concentration range, 10 nM to 100 μM, NaHS produced a concentration-dependent relaxation of carbachol-induced tone reaching a maximum of inhibition of 28% at 30 μM. The cyclooxygenase inhibitor, flurbiprofen (1 μM), enhanced relaxations induced by both NaHS and Na2S yielding IC50 values of 7 μM and 70 μM, respectively. With exception of l-NAME (300 μM) inhibitors of cystathionine γ-lyase, propargylglycine, (PAG) (1 mM) and β-cyanoalanine, (BCA) (1 mM) and inhibitors of cystathionine β-synthase, aminooxyacetic acid (AOA) (30 μM) and hydroxylamine (HOA) (30 μM) caused significant (P < 0.001) rightward shifts in the concentration–response curves to NaHS. An activator of cystathionine β-synthase, SAM (100 μM), enhanced relaxations elicited by low concentrations of NaHS but attenuated responses caused by the higher concentrations of this H2S donor. The inhibitor of KATP channel, glibenclamide (100 and 300 μM), blocked relaxations induced by NaHS. We conclude that the observed inhibitory action of NaHS and Na2S in isolated porcine irides is dependent on endogenous production of prostanoids and the biosynthesis of H2S by cystathionine γ-lyase and cystathionine β-synthase. Furthermore, relaxation induced by H2S is mediated, at least in part, by KATP channels. Nitric oxide is not involved in the relaxation induced by this gas in the isolated porcine irides.