ATP-sensitive K(+) channels in rat colonic epithelium

Hydrogen sulfide induces Ca2+ influx in the principal cells of rat cortical collecting ducts

Hydrogen sulfide (H2S) acts as a gas-signaling agent in various tissues. Although it has been reported that endogenous enzymes that generate H2S are expressed abundantly in the kidney, few reports examine cellular responses to H2S in renal tubular epithelial cells. In this study, we investigated the effects of NaHS, an H2S donor, and l-cysteine, a substrate for H2S production, on the principal cells of rat cortical collecting ducts (CCDs). NaHS increased the intracellular Ca2+ concentration ([Ca2+]i) in the principal cells. The removal of extracellular Ca2+ largely attenuated the [Ca2+]i response. The TRPV4 channel blocker significantly inhibited the effect of NaHS. Extracellular administration of l-cysteine also elicited a rise in [Ca2+]i. Prior treatment of CCDs with AOAA, an inhibitor of H2S production enzyme, l-cysteine-induced [Ca2+]i response was significantly reduced. These results suggest that not only exogenous H2S but also endogenously produced H2S triggers the extracellular influx pathway of Ca2+ in the principal cells of rat CCDs.

Proteinase-activated receptors regulate intestinal functions in a segment-dependent manner in rats

Proteinases released e.g. during inflammatory or allergic responses affect gastrointestinal functions via proteinase-activated receptors such as PAR1 and PAR2. As the gastrointestinal tract exerts pronounced gradients along its longitudinal axis, the present study focuses on the effect of PAR1 and PAR2 agonists on electrogenic ion transport (measured as short-circuit current; I_sc), tissue conductance (G_t) and contractility of the longitudinal muscle layer of rats. In Ussing chamber experiments, the PAR1 agonist TFLLR-NH₂, which mimics the tethered ligand liberated after cleavage of the receptor, evoked only a modest increase in I_sc (<0.5 μEq·h^-1·cm^-2) in small intestine, but a strong increase (3-4 μEq·h^-1·cm^-2) in colon. Pretreatment with tetrodotoxin reduced the response of the colonic segments to the level of the small intestine. Thrombin, the natural activator of PAR1, was much less effective suggesting biased activation by this peptidase. A similar gradient along the longitudinal axis of the intestine was observed with trypsin, the endogenous activator of PAR2. Divergent actions of PAR1 activation by enzymatic cleavage or a mimetic peptide were also observed when recording isometric contractions of longitudinal muscle. For example, in the jejunum TFLLR-NH₂ concentration-dependently induced a contractile response, whereas thrombin showed only inconsistent effects. The PAR2 activator AC264613 induced a concentration-dependent decrease in muscle tone combined with an inhibition of phasic spontaneous contractions. PCR experiments and immunohistochemical stainings confirmed the expression of PAR1 and PAR2. The data implies that PAR1 and PAR2 functions vary depending on the intestinal segment.

The role of HCO3- in propionate-induced anion secretion across rat caecal epithelium

Propionate, a metabolite from the microbial fermentation of carbohydrates, evokes a release of epithelial acetylcholine in rat caecum resulting in an increase of short-circuit current (I_sc) in Ussing chamber experiments. The present experiments were performed in order to characterize the ionic mechanisms underlying this response which has been thought to be due to Cl⁻ secretion. As there are regional differences within the caecal epithelium, the experiments were conducted at oral and aboral rat corpus caeci. In both caecal segments, the propionate-induced I_sc (I_Prop) was inhibited by > 85%, when the experiments were performed either in nominally Cl⁻- or nominally HCO₃⁻-free buffer. In the case of Cl⁻, the dependency was restricted to the presence of Cl⁻ in the serosal bath. Bumetanide, a blocker of the Na⁺-K⁺-2Cl⁻-cotransporter, only numerically reduced I_Prop suggesting that a large part of this current must be carried by an ion other than Cl⁻. In the aboral caecum, I_Prop was significantly inhibited by mucosally administered stilbene derivatives (SITS, DIDS, DNDS), which block anion exchangers. Serosal Na⁺-free buffer reduced I_Prop significantly in the oral (and numerically also in aboral) corpus caeci. RT-PCR experiments revealed the expression of several forms of Na⁺-dependent HCO₃⁻-cotransporters in caecum, which might underlie the observed Na⁺ dependency. These results suggest that propionate sensing in caecum is coupled to HCO₃^– secretion, which functionally would stabilize luminal pH when the microbial fermentation leads to an increase in the concentration of short-chain fatty acids in the caecal lumen.

Potassium channels in intestinal epithelial cells and their pharmacological modulation: a systematic review

Several potassium channels (KCs) have been described throughout the gastrointestinal tract. Notwithstanding, their contribution to both physiologic and pathophysiologic conditions, as inflammatory bowel disease (IBD), remains underexplored. Therefore, we aim to systematically review, for the first time, the evidence on the characteristics and modulation of KCs in intestinal epithelial cells (IECs). PubMed, Scopus, and Web of Science were searched to identify studies focusing on KCs and their modulation in IECs. The included studies were assessed using a reporting inclusiveness checklist. From the 745 identified records, 73 met the inclusion criteria; their reporting inclusiveness was moderate-high. Some studies described the physiological role of KCs, while others explored their importance in pathological settings. Globally, in IBD animal models, apical K_Ca1.1 channels, responsible for luminal secretion, were upregulated. In human colonocytes, basolateral K_Ca3.1 channels were downregulated. The pharmacological inhibition of K_2P and K_v influenced intestinal barrier function, promoting inflammation. Evidence suggests a strong association between KCs expression and secretory mechanisms in human and animal IECs. Further research is warranted to explore the usefulness of KC pharmacological modulation as a therapeutic target.

ATP-dependent potassium channels contribute to motor regulation of esophageal striated muscle in rats

The aim of the present study was to clarify roles of ATP-dependent potassium channels (K_ATP channels) in motility of the striated muscle portion in the esophagus. An isolated segment of the rat esophagus was placed in an organ bath and mechanical responses were recorded using a force transducer. Electrical stimulation of the vagus nerve evoked contractile response of striated muscle in the esophageal segment. Application of glibenclamide, an antagonist of K_ATP channels, increased amplitude of vagally mediated twitch contractions of the rat esophagus. On the other hand, minoxidil, an agonist of K_ATP channels, decreased amplitude of twitch contractions. RT-PCR revealed the expression of subunits of K_ATP channels in esophageal tissue. In addition, immunopositivity for subunits of K_ATP channels was observed in the striated muscle cells of the esophageal muscle layer. These findings indicate that K_ATP channels contribute to motor regulation of striated muscle in the rat esophagus.

Potassium Channelopathies and Gastrointestinal Ulceration

Potassium channels and transporters maintain potassium homeostasis and play significant roles in several different biological actions via potassium ion regulation. In previous decades, the key revelations that potassium channels and transporters are involved in the production of gastric acid and the regulation of secretion in the stomach have been recognized. Drugs used to treat peptic ulceration are often potassium transporter inhibitors. It has also been reported that potassium channels are involved in ulcerative colitis. Direct toxicity to the intestines from nonsteroidal anti-inflammatory drugs has been associated with altered potassium channel activities. Several reports have indicated that the long-term use of the antianginal drug Nicorandil, an adenosine triphosphate-sensitive potassium channel opener, increases the chances of ulceration and perforation from the oral to anal regions throughout the gastrointestinal (GI) tract. Several of these drug features provide further insights into the role of potassium channels in the occurrence of ulceration in the GI tract. The purpose of this review is to investigate whether potassium channelopathies are involved in the mechanisms responsible for ulceration that occurs throughout the GI tract.

Hypoxia/Reoxygenation Effects on Ion Transport across Rat Colonic Epithelium

Ischemia causes severe damage in the gastrointestinal tract. Therefore, it is interesting to study how the barrier and transport functions of intestinal epithelium change under hypoxia and subsequent reoxygenation. For this purpose we simulated hypoxia and reoxygenation on mucosa-submucosa preparations from rat distal colon in Ussing chambers and on isolated crypts. Hypoxia (N₂ gassing for 15 min) induced a triphasic change in short-circuit current (I_sc): a transient decrease, an increase and finally a long-lasting fall below the initial baseline. During the subsequent reoxygenation phase, I_sc slightly rose to values above the initial baseline. Tissue conductance (G_t) showed a biphasic increase during both the hypoxia and the reoxygenation phases. Omission of Cl⁻ or preincubation of the tissue with transport inhibitors revealed that the observed changes in I_sc represented changes in Cl⁻ secretion. The radical scavenger trolox C reduced the I_sc response during hypoxia, but failed to prevent the rise of I_sc during reoxygenation. All changes in I_sc were Ca²⁺-dependent. Fura-2 experiments at loaded isolated colonic crypts revealed a slow increase of the cytosolic Ca²⁺ concentration during hypoxia and the reoxygenation phase, mainly caused by an influx of extracellular Ca²⁺. Surprisingly, no changes could be detected in the fluorescence of the superoxide anion-sensitive dye mitosox or the thiol-sensitive dye thiol tracker, suggesting a relative high capacity of the colonic epithelium (with its low O₂ partial pressure even under physiological conditions) to deal with enhanced radical production during hypoxia/reoxygenation.

Altered expression of ATP-sensitive K(+) channels in Hirschsprung's disease

PURPOSE

Hirschsprung's disease-associated enterocolitis (HAEC) is the most common cause of morbidity and mortality in Hirschsprung's disease (HSCR). Altered intestinal epithelial barrier function has been suggested to play a role in the causation of HAEC. In rodent experimental models of colitis, a decreased expression of K(ATP) channels (Subunits: Kir6.1/6.2 and SUR1/2) is reported. We designed this study to determine if K(ATP) channels exist within the human colon and to investigate the expression of different subunits in Hirschsprung's disease.

METHODS

We investigated Kir6.1, Kir6.2, SUR1, and SUR2 expression in ganglionic and aganglionic bowel of HD patients (n=5) and controls (n=5). Western blotting and confocal immunofluorescence were performed.

MAIN RESULTS

Western blot analysis revealed that Kir6.1, Kir6.2, SUR1, and SUR2 are strongly expressed in the normal human colon. Kir6.1, Kir6.2, SUR1, and SUR2 expression was significantly decreased in the aganglionic bowel compared to ganglionic bowel and controls. Kir6.1 and SUR1 expression were also significantly decreased in the ganglionic bowel of HSCR patients compared to controls.

CONCLUSION

We demonstrate for the first time the existence of K(ATP) channels in the human colon. The decreased K(ATP) channel expression in HSCR specimens suggests that an altered K(ATP) expression may interfere with intestinal epithelium barrier function and predispose to HAEC.

Role of epithelial ion transports in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic inflammatory disorder with a complex pathogenesis. Diarrhea is a highly prevalent and often debilitating symptom of IBD patients that results, at least in part, from an intestinal hydroelectrolytic imbalance. Evidence suggests that reduced electrolyte absorption is more relevant than increased secretion to this disequilibrium. This systematic review analyses and integrates the current evidence on the roles of epithelial Na(+)-K(+)-ATPase (NKA), Na(+)/H(+) exchangers (NHEs), epithelial Na(+) channels (ENaC), and K(+) channels (KC) in IBD-associated diarrhea. NKA is the key driving force of the transepithelial ionic transport and its activity is decreased in IBD. In addition, the downregulation of apical NHE and ENaC and the upregulation of apical large-conductance KC all contribute to the IBD-associated diarrhea by lowering sodium absorption and/or increasing potassium secretion.

Epithelial Electrolyte Transport Physiology and the Gasotransmitter Hydrogen Sulfide

Hydrogen sulfide (H2S) is a well-known environmental chemical threat with an unpleasant smell of rotten eggs. Aside from the established toxic effects of high-dose H2S, research over the past decade revealed that cells endogenously produce small amounts of H2S with physiological functions. H2S has therefore been classified as a "gasotransmitter." A major challenge for cells and tissues is the maintenance of low physiological concentrations of H2S in order to prevent potential toxicity. Epithelia of the respiratory and gastrointestinal tract are especially faced with this problem, since these barriers are predominantly exposed to exogenous H2S from environmental sources or sulfur-metabolising microbiota. In this paper, we review the cellular mechanisms by which epithelial cells maintain physiological, endogenous H2S concentrations. Furthermore, we suggest a concept by which epithelia use their electrolyte and liquid transport machinery as defence mechanisms in order to eliminate exogenous sources for potentially harmful H2S concentrations.

Regulation of Intestinal Glucose Absorption by Ion Channels and Transporters

The absorption of glucose is electrogenic in the small intestinal epithelium. The major route for the transport of dietary glucose from intestinal lumen into enterocytes is the Na⁺/glucose cotransporter (SGLT1), although glucose transporter type 2 (GLUT2) may also play a role. The membrane potential of small intestinal epithelial cells (IEC) is important to regulate the activity of SGLT1. The maintenance of membrane potential mainly depends on the activities of cation channels and transporters. While the importance of SGLT1 in glucose absorption has been systemically studied in detail, little is currently known about the regulation of SGLT1 activity by cation channels and transporters. A growing line of evidence suggests that cytosolic calcium ([Ca²⁺]_cyt) can regulate the absorption of glucose by adjusting GLUT2 and SGLT1. Moreover, the absorption of glucose and homeostasis of Ca²⁺ in IEC are regulated by cation channels and transporters, such as Ca²⁺ channels, K⁺ channels, Na⁺/Ca²⁺ exchangers, and Na⁺/H⁺ exchangers. In this review, we consider the involvement of these cation channels and transporters in the regulation of glucose uptake in the small intestine. Modulation of them may be a potential strategy for the management of obesity and diabetes.

Acute exercises induce disorders of the gastrointestinal integrity in a murine model

PURPOSE

Many endurance athletes complain about gastrointestinal (GI) symptoms. It is assumed that exercise-induced shift of perfusion with consecutive hypoperfusion of the enteral vascular system leads to an increased GI permeability and tissue damage. Therefore, the aim of the study was to investigate permeability, apoptosis, electrogenic ion transport (Isc), and tissue conductance (Gt) of the small intestine in a murine exercise model.

METHODS

After spirometry, male Swiss CD-1 mice were subjected to an intensive treadmill exercise (80% VO2max). Sedentary mice served as controls. The small intestine was removed at several time intervals post-exercise. Apoptotic cells were determined by the TUNEL method, while fluorescein isothiocyanate dextran permeation indicated intestinal permeability. The Gt and Isc measurements were carried out in a modified Ussing chamber.

RESULTS

Apoptosis of epithelial cells increased continuously until 24 h post exercise (0.8 ± 0.42 versus 39.2 ± 26.0%; p < 0.05). Compared with the control group the permeability increased 2 h after exercise (0.47 ± 0.07 versus 0.67 ± 0.14 FU/min; p < 0.05). Isc measurements of the ileum were augmented after 24 h (3.33 ± 0.56 versus 5.77 ± 1.16 μEq/h/cm(2); p < 0.05). At this time the Gt increased as well (28.8 ± 3.37 versus 32.5 ± 2.59 mS/cm(2); p < 0.05).

CONCLUSION

In the murine exercise model there is evidence that after intense endurance exercise repair processes occur in small intestinal epithelial cells, which affect permeability, Gt, and Isc. The formation of lamellipodia to close the "leaky" tight junctions caused by apoptosis might be an underlying mechanism.