Role of hydrogen sulfide production in inhibitory action of L-cysteine on isolated porcine irides.

Purpose: To investigate the direct pharmacological actions of L-cysteine, a substrate for the production of H2S, on isolated porcine irides in the presence of tone induced by muscarinic receptor stimulation. Furthermore, we examined the underlying mechanism of action of L-cysteine in this smooth muscle. Methods: Isolated porcine iris muscle strips were set up in organ baths containing oxygenated Krebs buffer solution at 37 degrees C. Longitudinal isometric tension was recorded via a grass FT03 Force-Displacement Transducer and analyzed using the PolyView computer software. The relaxant action of L-cysteine on carbachol-induced tone was studied in the absence and presence of inhibitors of enzymes of the biosynthetic pathways for H2S, and prostanoids. In addition, we also examined the effect of ATP-sensitive K+ (K-ATP) channel antagonist, glibenclamide on relaxations induced by L-cysteine. Results: L-cysteine (30 nM(-1) mM) evoked concentration-dependent relaxations of carbachol-induced tone in isolated porcine irides, reaching a maximum inhibition of 43% at 1 mM. This response was enhanced significantly (P < 0.001) in the presence of the COX inhibitor, flurbiprofen (3 mu M). Additionally, in the presence of flurbiprofen, the H2S donors, NaHS and Na2S, mimicked the relaxations produced by L-cysteine, yielding IC50 values of 5.8 mu M and 180 mu M, espectively. Both the inhibitor of cystathionine beta-synthase, AOA (30 mu M) and the K-ATP channel antagonist, glibenclamide (100 mu M) caused significant (P < 0.001) rightward shifts in the concentration-response curves to L-cysteine and attenuated the maximum inhibitory response. Conversely, the inhibitor of cystathionine gamma-lyase, PAG (1 mM) blocked only relaxations caused by high concentrations of L-cysteine (> 100 mu M). Conclusions: The inhibitory action of L-cysteine in isolated porcine irides is dependent on the endogenous production of H2S by cystathionine.-lyase and cystathionine beta-synthase. Furthermore, prostanoids and K-ATP channels are involved in the inhibitory action of L-cysteine in this tissue.