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

We investigated the pharmacological actions of hydrogen sulfide (H(2)S) using sodium hydrosulfide (NaHS) and sodium sulfide (Na(2)S) 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 H(2)S 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 Na(2)S 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 H(2)S, prostanoid and nitric oxide production. In the concentration range, 10 nM to 100 microM, NaHS produced a concentration-dependent relaxation of carbachol-induced tone reaching a maximum of inhibition of 28% at 30 microM. The cyclooxygenase inhibitor, flurbiprofen (1 microM), enhanced relaxations induced by both NaHS and Na(2)S yielding IC(50) values of 7 microM and 70 microM, respectively. With exception of l-NAME (300 muM) inhibitors of cystathionine gamma-lyase, propargylglycine, (PAG) (1 mM) and beta-cyanoalanine, (BCA) (1 mM) and inhibitors of cystathionine beta-synthase, aminooxyacetic acid (AOA) (30 microM) and hydroxylamine (HOA) (30 microM) caused significant (P < 0.001) rightward shifts in the concentration-response curves to NaHS. An activator of cystathionine beta-synthase, SAM (100 microM), enhanced relaxations elicited by low concentrations of NaHS but attenuated responses caused by the higher concentrations of this H(2)S donor. The inhibitor of K(ATP) channel, glibenclamide (100 and 300 microM), blocked relaxations induced by NaHS. We conclude that the observed inhibitory action of NaHS and Na(2)S in isolated porcine irides is dependent on endogenous production of prostanoids and the biosynthesis of H(2)S by cystathionine gamma-lyase and cystathionine beta-synthase. Furthermore, relaxation induced by H(2)S is mediated, at least in part, by K(ATP) channels. Nitric oxide is not involved in the relaxation induced by this gas in the isolated porcine irides.