Regulation of ileal electrolyte transport by K+ channel activators

L-Cysteine enhances nutrient absorption via a cystathionine-β-synthase-derived H2 S pathway in rodent jejunum

Hydrogen sulphide (H2 S) is generated endogenously from L-cysteine (L-Cys) by the enzymes cystathionine-β-synthase (CBS) and cystathionine-γ-lyase (CSE). In addition, L-Cys is commonly used as a precursor in the food and pharmaceutical industries. The aim of the present study is to determine whether L-Cys regulates intestinal nutrient transport. To that end, the presence of CBS and CSE in the jejunum epithelium was assessed by immunohistochemistry, Western blotting and the methylene blue assay. In addition, in vivo L-Cys (100 mg/kg, administered immediately after the glucose load) significantly increased blood glucose levels 30 min after the oral administration of glucose to mice. This effect of L-Cys was completely blocked by amino-oxyacetic acid (AOA; 50 mg/kg; administered at the same time as L-Cys) an inhibitor of CBS. Measurements of the short-circuit current (Isc) in the rat jejunum epithelium revealed that L-Cys (1 mmol/L; 6 min before the administration of L-alanine) enhances Na(+)-coupled L-alanine or glucose transport, and that this effect is inhibited by AOA (1 mmol/L;10 min before the administration of L-Cys), but not D,L-propargylglycine (PAG;1 mmol/L; 10 min before the administration of L-Cys), a CSE inhibitor. Notably, L-Cys-evoked enhancement of nutrient transport was alleviated by glibenclamide (Gli;0.1 mmol/L; 10 min before the administration of L-Cys), a K(+) channel blocker. Together, the data indicate that L-Cys enhances jejunal nutrient transport, suggesting a new approach to future treatment of nutrition-related maladies, including a range of serious health consequences linked to undernutrition.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

Berberine inhibits ion transport in human colonic epithelia

The effects of berberine on ion transport in both human colonic mucosal epithelia and an intestinal epithelial cell line (T84) were examined. Berberine (concentration range 0-500 microM) reduced both basal and stimulated ion transport responses in human colonic mucosae in a manner which was non-specific for Ca2+ -or cAMP-mediated signals. Similarly, in cultured intestinal epithelial monolayers, berberine inhibited Ca2+ -and cAMP-mediated responses indicating an inhibitory activity directly at the level of the epithelium rather than an indirect effect through other mucosal element(s). Berberine did not alter the rate of generation of cAMP by adenylyl cyclase or the activity of protein kinase A, the effector enzyme of the cAMP pathway. Berberine inhibited carbachol-stimulated 86Rb+ efflux from T84 monolayers. Berberine also inhibited K+ conductance in apically-permeabilised re-sected mucosae. These results indicate i) that berberine exerts an anti-secretory action directly upon epithelial cells and ii) the mechanism of action may be at the level of blockade of K+ channels.

Regulation of electrolyte transport by nitric oxide in the mouse cecum

The effect and role of nitric oxide (NO) in the regulation of ion transport in the mouse cecum were investigated. L-arginine, used to increase NO production, increased short-circuit current (Isc), a measure of ion transport, in a concentration-dependent manner with a maximal increase of 193.8+/-65.5 microA/cm2. This increase was not changed in Cl-- or HCO3--free buffers, but was significantly decreased in Na+-free buffer. Using immunohistochemistry, the constitutive form of nitric oxide synthase was found not to be different in the inflamed cecum. The inducible form of the enzyme, however, which was absent in the cecum of normal mice, was present in high levels in the cecum of the colitic mouse. These results suggest that NO causes an increase in Na+ absorption. The increased levels of inducible NO synthase in the inflamed cecum suggest a role for NO in the pathophysiology of inflammatory bowel disease.

Involvement of K+ channel modulation in the proabsorptive effect of nitric oxide in the rat jejunum in vivo

The role of K+ channels in the mediation of the nitric oxide(NO)-induced proabsorptive effect in intestinal fluid transport was investigated in a functional study, using a model of ligated jejunal loops of anaesthetized rats in vivo. The K+ channel opener cromakalim and the K+ channel blocker glibenclamide were administered under basal conditions as well as under conditions, when fluid secretion was influenced by N omega-nitro-L-arginine methyl ester (L-NAME), prostaglandin E2, Escherichia coli heat stable enterotoxin a (E. coli STa) or L-arginine. Intravenous infusion of cromakalim (63.5 micrograms/kg per min) significantly enhanced net fluid absorption compared to controls, totally abolished net fluid secretion induced by L-NAME (0.55 mg/kg per min), reversed net fluid secretion induced by intraluminal instillation of E. coli STa (10 units/ml) to absorption, but did not influence fluid secretion elicited by close i.a. infusion of prostaglandin E2 (79 ng/min). Close i.a. infusion of glibenclamide (0.16 mg/kg per min) reversed net fluid absorption to net secretion, blocked the absorptive effect of L-arginine (8.88 mg/kg per min) and reduced the proabsorptive effect of cromakalim. The secretory effect of L-NAME was not further enhanced by glibenclamide. These results suggest that modulation of basolateral K+ channels by NO is involved in the mediation of its proabsorptive effect, since opening and closure of K+ channels mimicked, respectively counteracted, the action of NO-donors and inhibitors of NO-synthesis on intestinal fluid transport. The role of prostaglandins in the proabsorptive effect of NO remains to be elucidated. These results furthermore support the role of K+ channel openers as potential new antidiarrheal drugs.

K+ channel openers delay intestinal transit and have antidiarrheal activity

The effect of pinacidil and cromakalim, two KATP channel openers, on intestinal transit and castor oil-induced diarrhea was studied in mice. Both drugs, administered orally, dose dependently inhibited the intestinal propulsion of charcoal, and castor oil-induced diarrhea, comparing favorably with morphine. These results may suggest a new approach for the symptomatic treatment of diarrhea.