OBJECTIVE The aim of this study was to identify the actions of H(2)S on ion transport across rat distal colon. METHODS Changes in short-circuit current (Isc) induced by the H(2)S-donor, NaHS, were measured in Ussing chambers. Cytosolic Ca(2+) concentration was evaluated using fura-2. RESULTS NaHS concentration-dependently induced a change in Isc, that was only partially inhibited by the neurotoxin, tetrodotoxin. Lower concentrations (< or =10(-3) mol.L(-1)) of NaHS induced a monophasic increase in Isc, whereas higher concentrations induced an additional, secondary fall of Isc, before a third phase when Isc rose again. Blockers of H(2)S-producing enzymes (expression demonstrated immunohistochemically) decreased basal Isc, suggesting that endogenous production of H(2)S contributes to spontaneous anion secretion. The positive Isc phases induced by NaHS were due to Cl(-) secretion as shown by anion substitution and transport inhibitor experiments, whereas the transient negative Isc induced by higher concentrations of the H(2)S-donor was inhibited by mucosal tetrapentylammonium suggesting a transient K(+) secretion. When applied from the serosal side, glibenclamide, an inhibitor of ATP-sensitive K(+) channels, and tetrapentylammonium, a blocker of Ca(2+)-dependent K(+) channels, suppressed NaHS-induced Cl(-) secretion suggesting different types of K(+) channels are stimulated by the H(2)S-donor. NaHS-induced increase in cytosolic Ca(2+) concentration was confirmed in isolated, fura-2-loaded colonic crypts. This response was not dependent on extracellular Ca(2+), but was inhibited by blockers of intracellular Ca(2+) channels present on Ca(2+) storage organelles. CONCLUSIONS H(2)S induces colonic ion secretion by stimulation of apical as well as basolateral epithelial K(+) channels.