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

BACKGROUND AND AIMS: Hydrogen sulfide (H(2)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 Ca(2+) concentration ([Ca(2+)](i)) and force were simultaneously monitored in fura-2-loaded strips with front-surface fluorometry. The contribution of endogenous H(2)S to LES contractility was investigated by examining the effects of inhibitors of H(2)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 [Ca(2+)](i) and inversely correlated with the extracellular Na(+) concentration ([Na(+)](o)) (25-137.4 mM), indicating involvement of an Na(+)/Ca(2+) exchanger. The reduction in the basal [Ca(2+)](i) level by AOA was significantly augmented in the antral smooth muscle sheets of Na(+)/Ca(2+) exchanger transgenic mice compared with wild-type mice. CONCLUSIONS: Endogenous H(2)S regulates the LES myogenic tone by maintaining the basal [Ca(2+)](i) via Na(+)/Ca(2+) exchanger. H(2)S-generating enzymes may be a potential therapeutic target for esophageal motility disorders, such as achalasia.