Endogenous Hydrogen Sulfide Contributes to Tone Generation in Porcine Lower Esophageal Sphincter Via Na + /Ca 2+ Exchanger.

Background and Aims: Hydrogen sulfide (H2 S) is a major physiologic gastrotransmitter. Its role in the regulation of the lower esophageal sphincter (LES) function remains unknown. The present study addresses this question.

Methods: Isometric contraction was monitored in circular smooth muscle strips of porcine LES. Changes in cytosolic Ca2+  concentration ([Ca2+ ]i ) and force were simultaneously monitored in fura-2-loaded strips with front-surface fluorometry. The contribution of endogenous H2 S to LES contractility was investigated by examining the effects of inhibitors of H2 S-generating enzymes, including cystathionine-β-synthase, cystathionine-γ-lyase, and 3-mercaptopyruvate sulfurtransferase, on the LES function.

Results: Porcine LES strips myogenically maintained a tetrodotoxin-resistant basal tone. Application of AOA (cystathionine-β-synthase inhibitor) or L-aspartic acid (L-Asp; 3-mercaptopyruvate sulfurtransferase inhibitor) but not DL-PAG (cystathionine-γ-lyase inhibitor), decreased this basal tone. The relaxant effects of AOA and L-Asp were additive. Maximum relaxation was obtained by combination of 1 mM AOA and 3 mM L-Asp. Immunohistochemical analyses revealed that cystathionine-β-synthase and 3-mercaptopyruvate sulfurtransferase, but not cystathionine-γ-lyase, were expressed in porcine LES. AOA+L-Asp-induced relaxation was accompanied by a decrease in [Ca2+ ]i and inversely correlated with the extracellular Na+ concentration ([Na+ ]o ) (25-137.4 mM), indicating involvement of an Na+ /Ca2+ exchanger. The reduction in the basal [Ca2+ ]i level by AOA was significantly augmented in the antral smooth muscle sheets of Na+ /Ca2+ exchanger transgenic mice compared with wild-type mice.

Conclusions: Endogenous H2 S regulates the LES myogenic tone by maintaining the basal [Ca2+ ]i via Na+ /Ca2+ exchanger. H2 S-generating enzymes may be a potential therapeutic target for esophageal motility disorders, such as achalasia.