Modulation of ion transport across rat distal colon by cysteine

Abstract

The aim of this study was to identify the actions of stimulation of endogenous production of H 2S by cysteine, the substrate for the two H 2S-producing enzymes, cystathionine-β-synthase and cystathionine-γ-lyase, on ion transport across rat distal colon. Changes in short-circuit current (Isc) induced by cysteine were measured in Ussing chambers. Free cysteine caused a concentration-dependent, transient fall in Isc, which was sensitive to amino-oxyacetate and β-cyano-L-alanine, i.e., inhibitors of H 2S-producing enzymes. In contrast, Na cysteinate evoked a biphasic change in Isc, i.e., an initial fall followed by a secondary increase, which was also reduced by these enzyme inhibitors. All responses were dependent on the presence of Cl - and inhibited by bumetanide, suggesting that free cysteine induces an inhibition of transcellular Cl - secretion, whereas Na cysteinate - after a transient inhibitory phase - activates anion secretion. The assumed reason for this discrepancy is a fall in the cytosolic pH induced by free cysteine, but not by Na cysteinate, as observed in isolated colonic crypts loaded with the pH-sensitive dye, BCECF Intracellular acidification is known to inhibit epithelial K + channels. Indeed, after preinhibition of baso-lateral K + channels with tetrapentylammonium or Ba 2+, the negative Isc induced by free cysteine was reduced significantly. In consequence, stimulation of endogenous H 2S production by Na cysteinate causes, after a short inhibitory response, a delayed activation of anion secretion, which is missing in the case of free cysteine, probably due to the cytosolic acidification. In contrast, diallyl trisulfide, which is intracellularly converted to H 2S, only evoked a monophasic increase in Isc without the initial fall observed with Na cysteinate. Consequently, time course and amount of produced H 2S seem to strongly influence the functional response of the colonic epithelium evoked by this gasotransmitter. © 2012 Pouokam and Diener.