Characteristics of two basolateral potassium channel populations in human colonic crypts

Potassium Derangements: A Pathophysiological Review, Diagnostic Approach, and Clinical Management

Potassium is an essential cation critical in fluid and electrolyte balance, acid–base regulation, and neuromuscular functions. The normal serum potassium is kept within a narrow range of 3.5–5.2 meq/L while the intracellular concentration is approximately 140–150 meq/L. The total body potassium is about 45–55 mmol/kg; thus, a 70 kg male has an estimated ~136 g and 60 kg female has ~117 g of potassium. In total, 98% of the total body potassium is intracellular. Skeletal muscle contains ~80% of body potassium stores. The ratio of intracellular to extracellular potassium concentration (Ki/Ke) maintained by Na+/K+ ATPase determines the resting membrane potential. Disturbances of potassium homeostasis lead to hypo- and hyperkalemia, which if severe, can be life-threatening. Prompt diagnosis and management of these problems are important.

The LRRC family of BK channel regulatory subunits: potential roles in health and disease

Large conductance K⁺ channels, termed BK channels, regulate a variety of cellular and physiological functions. Although universally activated by changes in voltage or [Ca²⁺ ]_i , the threshold for BK channel activation varies among loci of expression, often arising from cell-specific regulatory subunits including a family of leucine rich repeat-containing (LRRC) γ subunits (LRRC26, LRRC52, LRRC55 and LRRC38). The 'founding' member of this family, LRRC26, was originally identified as a tumour suppressor in various cancers. An LRRC26 knockout (KO) mouse model recently revealed that LRRC26 is also highly expressed in secretory epithelial cells and partners with BK channels in the salivary gland and colonic goblet cells to promote sustained K⁺ fluxes likely essential for normal secretory function. To accomplish this, LRRC26 negatively shifts the range of BK channel activation such that channels contribute to K⁺ flux near typical epithelial cell resting conditions. In colon, the absence of LRRC26 increases vulnerability to colitis. LRRC26-containing BK channels are also likely important regulators of epithelial function in other loci, including airways, female reproductive tract and mammary gland. Based on an LRRC52 KO mouse model, LRRC52 regulation of large conductance K⁺ channels plays a role both in sperm function and in cochlear inner hair cells. Although our understanding of LRRC-containing BK channels remains rudimentary, KO mouse models may help define other organs in which LRRC-containing channels support normal function. A key topic for future work concerns identification of endogenous mechanisms, whether post-translational or via gene regulation, that may impact LRRC-dependent pathologies.

Calmodulin Antagonist W-7 Enhances Intermediate Conductance Ca2+- Sensitive Basolateral Potassium Channel (IKCa) Activity in Human Colonic Crypts

Intermediate conductance potassium (IK_Ca) channels are exquisitively Ca²⁺ sensitive, intracellular Ca²⁺ regulating channel activity by complexing with calmodulin (CaM), which is bound to the cytosolic carboxyl tail. Although CaM antagonists might be expected to decrease IK_Ca channel activity, the effect of W-7 in human T lymphocytes are conflicting. We therefore evaluated the effect of W-7 on basolateral IK_Ca channels in human colonic crypt cells. Intact crypts obtained from normal human colonic biopsies by Ca²⁺ chelation were used for patch clamp studies of basolateral IK_Ca channels in the cell-attached configuration. IK_Ca channel activity was studied when the bath Ca²⁺ concentration was changed from 1.2 mmol/L to 100 μmol/L and back to 1.2 mmol/L, as well as from 100 μmol/L to 1.2 mmol/L and back to 100 μmol/L, both in the absence and presence of 25 μmol/L W-7. Decreasing bath Ca²⁺ from 1.2 mmol/L to 100 μmol/L decreased IK_Ca channel activity reversibly in the absence of W-7, whereas there was a uniformly high level of channel activity at both bath Ca²⁺ concentrations in the presence of W-7. In separate experiments, increasing bath Ca²⁺ from 100 μmol/L to 1.2 mmol/L increased IK_Ca channel activity reversibly in the absence of W-7, whereas there was again a uniformly high level of channel activity at both bath Ca²⁺ concentrations in the presence of W-7. We, therefore, propose that W-7 has a specific stimulatory effect on basolateral IK_Ca channel activity, despite its ability to inhibit Ca²⁺/CaM-mediated, IK_Ca channel-dependent Cl⁻ secretion in human colonic epithelial cells.

Goblet cell LRRC26 regulates BK channel activation and protects against colitis in mice

Goblet cells (GCs) are specialized cells of the intestinal epithelium contributing critically to mucosal homeostasis. One of the functions of GCs is to produce and secrete MUC2, the mucin that forms the scaffold of the intestinal mucus layer coating the epithelium and separates the luminal pathogens and commensal microbiota from the host tissues. Although a variety of ion channels and transporters are thought to impact on MUC2 secretion, the specific cellular mechanisms that regulate GC function remain incompletely understood. Previously, we demonstrated that leucine-rich repeat-containing protein 26 (LRRC26), a known regulatory subunit of the Ca²⁺-and voltage-activated K⁺ channel (BK channel), localizes specifically to secretory cells within the intestinal tract. Here, utilizing a mouse model in which MUC2 is fluorescently tagged, thereby allowing visualization of single GCs in intact colonic crypts, we show that murine colonic GCs have functional LRRC26-associated BK channels. In the absence of LRRC26, BK channels are present in GCs, but are not activated at physiological conditions. In contrast, all tested MUC2^- cells completely lacked BK channels. Moreover, LRRC26-associated BK channels underlie the BK channel contribution to the resting transepithelial current across mouse distal colonic mucosa. Genetic ablation of either LRRC26 or BK pore-forming α-subunit in mice results in a dramatically enhanced susceptibility to colitis induced by dextran sodium sulfate. These results demonstrate that normal potassium flux through LRRC26-associated BK channels in GCs has protective effects against colitis in mice.

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.

Colonic Potassium Absorption and Secretion in Health and Disease

The colon has large capacities for K⁺ absorption and K⁺ secretion, but its role in maintaining K⁺ homeostasis is often overlooked. For many years, passive diffusion and/or solvent drag were thought to be the primary mechanisms for K⁺ absorption in human and animal colon. However, it is now clear that apical H⁺ ,K⁺ -ATPase, in coordination with basolateral K⁺ -Cl^- cotransport and/or K⁺ and Cl^- channels operating in parallel, mediate electroneutral K⁺ absorption in animal colon. We now know that K⁺ absorption in rat colon reflects ouabain-sensitive and ouabain-insensitive apical H⁺ ,K⁺ -ATPase activities. Ouabain-insensitive and ouabain-sensitive H⁺ ,K⁺ -ATPases are localized in surface and crypt cells, respectively. Colonic H⁺ ,K⁺ -ATPase consists of α- (HKC_α ) and β- (HKC_β ) subunits which, when coexpressed, exhibit ouabain-insensitive H⁺ ,K⁺ -ATPase activity in HEK293 cells, while HKC_α coexpressed with the gastric β-subunit exhibits ouabain-sensitive H⁺ ,K⁺ -ATPase activity in Xenopus oocytes. Aldosterone enhances apical H⁺ ,K⁺ -ATPase activity, HKC_α specific mRNA and protein expression, and K⁺ absorption. Active K⁺ secretion, on the other hand, is mediated by apical K⁺ channels operating in a coordinated way with the basolateral Na⁺ -K⁺ -2Cl^- cotransporter. Both Ca²⁺ -activated intermediate conductance K⁺ (IK) and large conductance K⁺ (BK) channels are located in the apical membrane of colonic epithelia. IK channel-mediated K⁺ efflux provides the driving force for Cl^- secretion, while BK channels mediate active (e.g., cAMP-activated) K⁺ secretion. BK channel expression and activity are increased in patients with end-stage renal disease and ulcerative colitis. This review summarizes the role of apical H⁺ ,K⁺ -ATPase in K⁺ absorption, and apical BK channel function in K⁺ secretion in health and disease. © 2018 American Physiological Society. Compr Physiol 8:1513-1536, 2018.

Small conductance Ca2+-activated K+ channels in the plasma membrane, mitochondria and the ER: Pharmacology and implications in neuronal diseases

Ca²⁺-activated K⁺ (K_Ca) channels regulate after-hyperpolarization in many types of neurons in the central and peripheral nervous system. Small conductance Ca²⁺-activated K⁺ (K_Ca2/SK) channels, a subfamily of K_Ca channels, are widely expressed in the nervous system, and in the cardiovascular system. Voltage-independent SK channels are activated by alterations in intracellular Ca²⁺ ([Ca²⁺]_i) which facilitates the opening of these channels through binding of Ca²⁺ to calmodulin that is constitutively bound to the SK2 C-terminus. In neurons, SK channels regulate synaptic plasticity and [Ca²⁺]_i homeostasis, and a number of recent studies elaborated on the emerging neuroprotective potential of SK channel activation in conditions of excitotoxicity and cerebral ischemia, as well as endoplasmic reticulum (ER) stress and oxidative cell death. Recently, SK channels were discovered in the inner mitochondrial membrane and in the membrane of the endoplasmic reticulum which sheds new light on the underlying molecular mechanisms and pathways involved in SK channel-mediated protective effects. In this review, we will discuss the protective properties of pharmacological SK channel modulation with particular emphasis on intracellularly located SK channels as potential therapeutic targets in paradigms of neuronal dysfunction.

Segmental differences in upregulated apical potassium channels in mammalian colon during potassium adaptation

Rat proximal and distal colon are net K⁺ secretory and net K⁺ absorptive epithelia, respectively. Chronic dietary K⁺ loading increases net K⁺ secretion in the proximal colon and transforms net K⁺ absorption to net K⁺ secretion in the distal colon, but changes in apical K⁺ channel expression are unclear. We evaluated expression/activity of apical K⁺ (BK) channels in surface colonocytes in proximal and distal colon of control and K⁺-loaded animals using patch-clamp recording, immunohistochemistry, and Western blot analyses. In controls, BK channels were more abundant in surface colonocytes from K⁺ secretory proximal colon (39% of patches) than in those from K⁺-absorptive distal colon (12% of patches). Immunostaining demonstrated more pronounced BK channel α-subunit protein expression in surface cells and cells in the upper 25% of crypts in proximal colon, compared with distal colon. Dietary K⁺ loading had no clear-cut effects on the abundance, immunolocalization, or expression of BK channels in proximal colon. By contrast, in distal colon, K⁺ loading 1) increased BK channel abundance in patches from 12 to 41%; 2) increased density of immunostaining in surface cells, which extended along the upper 50% of crypts; and 3) increased expression of BK channel α-subunit protein when assessed by Western blotting (P < 0.001). Thus apical BK channels are normally more abundant in K⁺ secretory proximal colon than in K⁺ absorptive distal colon, and apical BK channel expression in distal (but not proximal) colon is greatly stimulated as part of the enhanced K⁺ secretory response to dietary K⁺ loading.

Evidence that two distinct crypt cell types secrete chloride and potassium in human colon

BACKGROUND

Human colon may secrete substantial amounts of water secondary to chloride (Cl(-)) and/or potassium (K(+)) secretion in a variety of diarrhoeal diseases. Ion secretion occurs via Cl(-) and K(+) channels, which are generally assumed to be co-located in the colonocyte apical membrane, although their exact cellular sites remain unclear.

OBJECTIVE

 To investigate the location of apical Cl(-) (CFTR) and apical K(+) (large conductance; BK) channels within human colonic epithelium.

DESIGN

Whole-cell patch clamp recordings were obtained from intact human colonic crypts. Specific blockers of K(+) channels and CFTR identified different types of K(+) channel and CFTR under resting conditions and after stimulating intracellular cAMP with forskolin. The BK channel β3-subunit was localised by immunostaining.

RESULTS

Two types of crypt cells were identified. One (73% of cells) had whole-cell currents dominated by intermediate conductance (IK) K(+) channels under resting conditions, which developed large CFTR-mediated currents in response to increasing intracellular cAMP. The other (27% of cells) had resting currents dominated by BK channels inhibited by the BK channel blocker penitrem A, but insensitive to both forskolin and the IK channel blocker clotrimazole. Immunostaining showed co-localisation of the BK channel β3-subunit and the goblet cell marker, MUC2.

CONCLUSIONS

In human colon, Cl(-) secretion originates from the dominant population of colonocytes expressing apical CFTR, whereas K(+) secretion is derived from a smaller population of goblet cells expressing apical BK channels. These findings provide new insights into the pathophysiology of secretory diarrhoea and should be taken into account during the development of anti-diarrhoeal drugs.

Progress in understanding the relationship between diarrhea and intestinal ion transport

Diarrhea is a major cause of morbidity and mortality in the world. There are millions of people dying of diarrhea, and most of them are children. Diarrhea can be divided into acute diarrhea and chronic diarrhea based on the length of the course, and into infectious diarrhea and noninfectious diarrhea according to the etiology. Diarrhea is an imbalance in absorption and secretion of water and electrolytes in the intestine, which involves abnormal ion transport. This paper reviews recent advances in understanding the causes of diarrhea, the relationship between intestinal ion transport and diarrhea, and ion transport in different kinds of diarrhea, with an aim to providing a reference and some new ideas on the comprehensive understanding of the pathogenesis, pathophysiology and treatment of diarrhea.

Characteristics of Kcnn4 channels in the apical membranes of an intestinal epithelial cell line

Intermediate-conductance K(+) (Kcnn4) channels in the apical and basolateral membranes of epithelial cells play important roles in agonist-induced fluid secretion in intestine and colon. Basolateral Kcnn4 channels have been well characterized in situ using patch-clamp methods, but the investigation of Kcnn4 channels in apical membranes in situ has been hampered by a layer of mucus that prevents seal formation. In the present study, we used patch-clamp methods to characterize Kcnn4 channels in the apical membrane of IEC-18 cells, a cell line derived from rat small intestine. A monolayer of IEC-18 cells grown on a permeable support is devoid of mucus, and tight junctions enable selective access to the apical membrane. In inside-out patches, Ca(2+)-dependent K(+) channels observed with iberiotoxin (a Kcnma1/large-conductance, Ca(2+)-activated K(+) channel blocker) and apamin (a Kcnn1-3/small-conductance, Ca(2+)-activated K(+) channel blocker) present in the pipette solution exhibited a single-channel conductance of 31 pS with inward rectification. The currents were reversibly blocked by TRAM-34 (a Kcnn4 blocker) with an IC(50) of 8.7 ± 2.0 μM. The channels were not observed when charybdotoxin, a peptide inhibitor of Kcnn4 channels, was added to the pipette solution. TRAM-34 was less potent in inhibiting Kcnn4 channels in patches from apical membranes than in patches from basolateral membranes, which was consistent with a preferential expression of Kcnn4c and Kcnn4b isoforms in apical and basolateral membranes, respectively. The expression of both isoforms in IEC-18 cells was confirmed by RT-PCR and Western blot analyses. This is the first characterization of Kcnn4 channels in the apical membrane of intestinal epithelial cells.

Small conductance potassium channels drive ATP-activated chloride secretion in the oviduct

The molecular mechanisms controlling fluid secretion within the oviduct have yet to be determined. As in other epithelia, both secretory and absorptive pathways are likely to work in tandem to drive appropriate ionic movement to support fluid movement across the oviduct epithelium. This study explored the role of potassium channels in basolateral extracellular ATP (ATP(e))-stimulated ion transport in bovine oviduct epithelium using the Ussing chamber short-circuit current (I(SC)) technique. Basal I(SC) in bovine oviduct epithelium comprises both chloride secretion and sodium absorption and was inhibited by treatment with basolateral K(+) channel inhibitors tetrapentlyammonium chloride (TPeA) or BaCl(2). Similarly, ATP-stimulated chloride secretion was significantly attenuated by pretreatment with BaCl(2,) tetraethylammonium (TEA), tolbutamide, and TPeA. Basolateral K(+) current, isolated using nystatin-perforation technique, was rapidly activated by ATP(e), and pretreatment of monolayers with thapsigargin or TPeA abolished this ATP-stimulated K(+) current. To further investigate the type of K(+) channel involved in the ATP response in the bovine oviduct, a number of specific Ca(2+)-activated K(+) channel inhibitors were tested on the ATP-induced ΔI(SC) in intact monolayers. Charbydotoxin, (high conductance and intermediate conductance inhibitor), or paxilline, (high conductance inhibitor) did not significantly alter the ATP(e) response. However, pretreatment with the small conductance inhibitor apamin resulted in a 60% reduction in the response to ATP(e). The presence of small conductance family member KCNN3 was confirmed by RT-PCR and immunohistochemistry. Measurements of intracellular calcium using Fura-2 spectrofluorescence imaging revealed the ability of ATP(e) to increase intracellular calcium in a phospholipase C-inositol 1,4,5-trisphosphate pathway-sensitive manner. In conclusion, these results provide strong evidence that purinergic activation of a calcium-dependent, apamin-sensitive potassium conductance is essential to promote chloride secretion and thus fluid formation in the oviduct.

Basolateral potassium (IKCa) channel inhibition prevents increased colonic permeability induced by chemical hypoxia

Major liver resection is associated with impaired intestinal perfusion and intestinal ischemia, resulting in decreased mucosal integrity, increased bacterial translocation, and an increased risk of postoperative sepsis. However, the mechanism by which ischemia impairs intestinal mucosal integrity is unclear. We therefore evaluated the role of Ca(2+)-sensitive, intermediate-conductance (IK(Ca)) basolateral potassium channels in enhanced intestinal permeability secondary to chemical hypoxia. The effects of chemical hypoxia induced by 100 μM dinitrophenol (DNP) and 5 mM deoxyglucose (DG) on basolateral IK(Ca) channel activity and whole cell conductance in intact human colonic crypts, and paracellular permeability (G(S)) in isolated colonic sheets, were determined by patch-clamp recording and transepithelial electrical measurements, respectively. DNP and DG rapidly stimulated IK(Ca) channels in cell-attached basolateral membrane patches and elicited a twofold increase (P = 0.004) in whole cell conductance in amphotericin B-permeabilized membrane patches, changes that were inhibited by the specific IK(Ca) channel blockers TRAM-34 (100 nM) and clotrimazole (CLT; 10 μM). In colonic sheets apically permeabilized with nystatin, DNP elicited a twofold increase (P = 0.005) in G(S), which was largely inhibited by the serosal addition of 50 μM CLT. We conclude that, in intestinal epithelia, chemical hypoxia increases G(S) through a mechanism involving basolateral IK(Ca) channel activation. Basolateral IK(Ca) channel inhibition may prevent or limit increased intestinal permeability during liver surgery.

Apical potassium (BK) channels and enhanced potassium secretion in human colon

The human colon has the capacity to secrete potassium (K(+)) ions and enhanced K(+) secretion is a feature of a variety of diarrhoeal diseases. Recent work points to K(+) secretion in human colon being mediated by high conductance (BK) K(+) channels located in the apical membrane of colonic epithelial cells. The aim of this review is to highlight the importance of these channels in maintaining K(+) homoeostasis in health and disease.

Regulation of colonic apical potassium (BK) channels by cAMP and somatostatin

High-conductance apical K+ (BK) channels are present in surface colonocytes of mammalian (including human) colon. Their location makes them well fitted to contribute to the excessive intestinal K(+) losses often associated with infective diarrhea. Since many channel proteins are regulated by phosphorylation, we evaluated the roles of protein kinase A (PKA) and phosphatases in the modulation of apical BK channel activity in surface colonocytes from rat distal colon using patch-clamp techniques, having first increased channel abundance by chronic dietary K+ enrichment. We found that PKA activation using 50 micromol/l forskolin and 5 mmol/l 3-isobutyl-1-methylxanthine stimulated BK channels in cell-attached patches and the catalytic subunit of PKA (200 U/ml) had a similar effect in excised inside-out patches. The antidiarrheal peptide somatostatin (SOM; 2 micromol/l) had a G protein-dependent inhibitory effect on BK channels in cell-attached patches, which was unaffected by pretreatment with 10 micromol/l okadaic acid (an inhibitor of protein phosphatase type 1 and type 2A) but completely prevented by pretreatment with 100 micromol/l Na+ orthovanadate and 10 micromol/l BpV (inhibitors of phosphoprotein tyrosine phosphatase). SOM also inhibited apical BK channels in surface colonocytes in human distal colon. We conclude that cAMP-dependent PKA activates apical BK channels and may enhance colonic K+ losses in some cases of secretory diarrhea. SOM inhibits apical BK channels through a phosphoprotein tyrosine phosphatase-dependent mechanism, which could form the basis of new antidiarrheal strategies.

The cystic fibrosis transmembrane conductance regulator and chloride-dependent ion fluxes of ovine vocal fold epithelium

PURPOSE

Ion-driven transepithelial water fluxes participate in maintaining superficial vocal fold hydration, which is necessary for normal voice production. The authors hypothesized that Cl(-) channels are present in vocal fold epithelial cells and that transepithelial Cl(-) fluxes can be manipulated pharmacologically.

METHOD

Immunohistochemical assays were used to identify cystic fibrosis transmembrane regulator Cl(-) channels in ovine vocal fold mucosae (n = 2). Electrophysiological responses of vocal fold mucosae (n = 80) to Cl(-) channel inhibitors and secretagogues were evaluated in an ovine model using a randomized controlled experimental design.

RESULTS

Cystic fibrosis transmembrane regulator channels were localized to the plasma membranes of epithelial cells. The Cl(-) transport inhibitor, diphenylamine-2-carboxylate, elicited a 30% decrease in mean short-circuit current (I(sc); n = 10). The secretagogue, isobutylmethylxanthine, yielded a 31.7% increase in mean I(sc) (n = 10). Another secretagogue, uridine triphosphate, elicited a 48.8% immediate and 17.3% sustained increase in mean I(sc) (n = 10). No sustained increases occurred following application of secretagogues to mucosae bathed in a low Cl(-) environment (n = 10), suggesting that responses were Cl(-) dependent.

CONCLUSIONS

The authors provide structural and functional evidence for the presence of a transepithelial pathway for Cl(-) fluxes. Pharmacological manipulation of this pathway may offer a mechanism for maintaining superficial vocal fold hydration.

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.

Very mild disease phenotype of congenic CftrTgH(neoim)Hgu cystic fibrosis mice

BACKGROUND

A major boost to cystic fibrosis disease research was given by the generation of various mouse models using gene targeting in embryonal stem cells. Moreover, the introduction of the same mutation on different inbred strains generating congenic strains facilitated the search for modifier genes. From the original CftrTgH(neoim)Hgu mouse model with a divergent genetic background (129/Sv, C57BL/6, HsdOla:MF1) two inbred mutant mouse strains CF/1-CftrTgH(neoim)Hgu and CF/3-CftrTgH(neoim)Hgu had been generated using strict brother x sister mating. CF/1-CftrTgH(neoim)Hgu and CF/3-CftrTgH(neoim)Hgu mice were fertile and showed normal growth and lifespan. In this work the CftrTgH(neoim)Hgu insertional mutation was backcrossed from CF/3-CftrTgH(neoim)Hgu onto the inbred backgrounds C57BL/6J and DBA/2J generating congenic animals in order to clarify the differential impact of the Cftr mutation and the genetic background on the disease phenotype of the cystic fibrosis mutant mice. Clinical and electrophysiological features of the two congenic strains were compared with those of CF/1-CftrTgH(neoim)Hgu and CF/3-CftrTgH(neoim)Hgu and wild type controls.

RESULTS

Under the standardized housing conditions of the animal facility, the four mouse strains CF/1-CftrTgH(neoim)Hgu, CF/3-CftrTgH(neoim)Hgu, D2.129P2(CF/3)-CftrTgH(neoim)Hgu and B6.129P2(CF/3)-CftrTgH(neoim)Hgu exhibited normal life expectancy. Growth of congenic cystic fibrosis mice was comparable with that of wild type controls. All mice but D2.129P2(CF/3)-CftrTgH(neoim)Hgu females were fertile. Short circuit current measurements revealed characteristic response profiles of the HsdOla:MF1, DBA/2J and C57BL/6J backgrounds in nose, ileum and colon. All cystic fibrosis mouse lines showed the disease-typical hyperresponsiveness to amiloride in the respiratory epithelium. The mean chloride secretory responses to carbachol or forskolin were 15-100% of those of the cognate wild type control animals.

CONCLUSION

The amelioration of the clinical features and of the basic defect that had emerged during the generation of CF/3-CftrTgH(neoim)Hgu mice was retained in the congenic mice indicating that the Cftr linkage group or other loci shared between the inbred strains contain(s) the major modifier(s) of attenuation of cystic fibrosis symptoms.

Role of protein kinase C in aldosterone-induced non-genomic inhibition of basolateral potassium channels in human colonic crypts

Aldosterone produces rapid, non-genomic, inhibition of basolateral intermediate conductance K(+) (IK(Ca)) channels in human colonic crypt cells but the intracellular second messengers involved are unclear. We therefore evaluated the role of protein kinase C (PKC) in aldosterone's non-genomic inhibitory effect on basolateral IK(Ca) channels in crypt cells from normal human sigmoid colon. Patch clamp studies revealed that in cell-attached patches, IK(Ca) channel activity decreased progressively to 38+/-8% (P<0.001) of the basal value 10 min after the addition of 1 nmol/L aldosterone, and decreased further to 23+/-6% (P<0.02) of the basal value 5 min after increasing the aldosterone concentration to 10 nmol/L. Pre-incubation of crypts with 1 micromol/L chelerythrine chloride or 1 micromol/L Gö 6976 (PKC inhibitors) prevented the inhibitory effect of aldosterone. Conversely, channel activity decreased to 60+/-9% (P<0.02) of the basal value 10 min after the addition of 500 nmol/L PMA (a PKC activator), whereas 4alpha-PMA (an inactive ester) had no effect. When aldosterone (10 nmol/L) and PMA were added together, IK(Ca) channel activity was inhibited to the same extent as with aldosterone alone. These results indicate that aldosterone's non-genomic inhibitory effect on the macroscopic basolateral K(+) conductance in human colonic crypts reflects PKC-mediated inhibition of IK(Ca) channels.

Altered cryptal expression of luminal potassium (BK) channels in ulcerative colitis

Decreased sodium (Na(+)), chloride (Cl(-)), and water absorption, and increased potassium (K(+)) secretion, contribute to the pathogenesis of diarrhoea in ulcerative colitis. The cellular abnormalities underlying decreased Na(+) and Cl(-) absorption are becoming clearer, but the mechanism of increased K(+) secretion is unknown. Human colon is normally a K(+) secretory epithelium, making it likely that K(+) channels are expressed in the luminal (apical) membrane. Based on the assumption that these K(+) channels resembled the high conductance luminal K(+) (BK) channels previously identified in rat colon, we used molecular and patch clamp recording techniques to evaluate BK channel expression in normal and inflamed human colon, and the distribution and characteristics of these channels in normal colon. In normal colon, BK channel alpha-subunit protein was immunolocalized to surface cells and upper crypt cells. By contrast, in ulcerative colitis, although BK channel alpha-subunit protein expression was unchanged in surface cells, it extended along the entire crypt irrespective of whether the disease was active or quiescent. BK channel alpha-subunit protein and mRNA expression (evaluated by western blotting and real-time PCR, respectively) were similar in the normal ascending and sigmoid colon. Of the four possible beta-subunits (beta(1-4)), the beta(1)- and beta(3)-subunits were dominant. Voltage-dependent, barium-inhibitable, luminal K(+) channels with a unitary conductance of 214 pS were identified at low abundance in the luminal membrane of surface cells around the openings of sigmoid colonic crypts. We conclude that increased faecal K(+) losses in ulcerative colitis, and possibly other diseases associated with altered colonic K(+) transport, may reflect wider expression of luminal BK channels along the crypt axis.

Role of Ca2+-activated K+ channels in duodenal mucosal ion transport and bicarbonate secretion

Stimulation of muscarinic receptors in the duodenal mucosa raises cytosolic free Ca(2+) concentration ([Ca(2+)](cyt)), thereby regulating duodenal epithelial ion transport. However, little is known about the downstream molecular targets that account for this Ca(2+)-mediated biological action. Ca(2+)-activated K(+) (K(Ca)) channels are candidates, but the expression and function of duodenal K(Ca) channels are poorly understood. Therefore, we determined whether K(Ca) channels are expressed in the duodenal mucosa and investigated their involvement in Ca(2+)-mediated duodenal epithelial ion transport. Two selective blockers of intermediate-conductance Ca(2+)-activated K(+) (IK(Ca)) channels, clotrimazole (30 muM) and 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34; 10 muM), significantly inhibited carbachol (CCh)-induced duodenal short-circuit current (I(sc)) and duodenal mucosal bicarbonate secretion (DMBS) in mice but did not affect responses to forskolin and heat-stable enterotoxin of Escherichia coli. Tetraethylammonium, 4-aminopyridine, and BaCl(2) failed to inhibit CCh-induced I(sc) and DMBS. A-23187 (10 muM), a Ca(2+) ionophore, and 1-ethyl-2-benzimidazolinone (1-EBIO; 1 mM), a selective opener of K(Ca) channels, increased both I(sc) and DMBS. The effect of 1-EBIO was more pronounced with serosal than mucosal addition. Again, both clotrimazole and TRAM-34 significantly reduced A23187- or 1-EBIO-induced I(sc) and DMBS. Moreover, clotrimazole (20 mg/kg ip) significantly attenuated acid-stimulated DMBS of mice in vivo. Finally, the molecular identity of IK(Ca) channels was verified as KCNN4 (SK4) in freshly isolated murine duodenal mucosae by RT-PCR and Western blotting. Together, our results suggest that the IK(Ca) channel is one of the downstream molecular targets for [Ca(2+)](cyt) to mediate duodenal epithelial ion transport.

Differential expression of small-conductance Ca2+-activated K+ channels SK1, SK2, and SK3 in mouse atrial and ventricular myocytes

Small-conductance Ca2+-activated K+ channels (SK channels, KCa channels) have been reported in excitable cells, where they aid in integrating changes in intracellular Ca2+ with membrane potential. We recently reported for the first time the functional existence of SK2 (KCa2.2) channels in human and mouse cardiac myocytes. Here, we report cloning of SK1 (KCa2.1) and SK3 (KCa2.3) channels from mouse atria and ventricles using RT-PCR. Full-length transcripts and their variants were detected for both SK1 and SK3 channels. Variants of mouse SK1 channel (mSK1) differ mainly in the COOH-terminal structure, affecting a portion of the sixth transmembrane segment (S6) and the calmodulin binding domain (CaMBD). Mouse SK3 channel (mSK3) differs not only in the number of polyglutamine repeats in the NH2 terminus but also in the intervening sequences between the polyglutamine repeats. Full-length cardiac mSK1 and mSK3 show 99 and 91% nucleotide identity with those of mouse colon SK1 and SK3, respectively. Quantification of SK1, SK2, and SK3 transcripts between atria and ventricles was performed using real-time quantitative RT-PCR from single, isolated cardiomyocytes. SK1 transcript was found to be more abundant in atria compared with ventricles, similar to the previously reported finding for SK2 channel. In contrast, SK3 showed similar levels of expression in atria and ventricles. Together, our data are the first to indicate the presence of the three different isoforms of SK channels in heart and the differential expression of SK1 and SK2 in mouse atria and ventricles. Because of the marked differential expression of SK channel isoforms in heart, specific ligands for Ca2+-activated K+ currents may offer a unique therapeutic opportunity to modify atrial cells without interfering with ventricular myocytes.

Pathways for K+ Efflux in Isolated Surface and Crypt Colonic Cells. Activation by Calcium

K+ -conductive pathways were evaluated in isolated surface and crypt colonic cells, by measuring (86)Rb efflux. In crypt cells, basal K+ efflux (rate constant: 0.24 +/- 0.044 min(-1), span: 24 +/- 1.3%) was inhibited by 30 mM TEA and 5 mM Ba2+ in an additive way, suggesting the existence of two different conductive pathways. Basal efflux was insensitive to apamin, iberiotoxin, charybdotoxin and clotrimazole. Ionomycin (5 microM) stimulated K+ efflux, increasing the rate constant to 0.65 +/- 0.007 min(-1) and the span to 83 +/- 3.2%. Ionomycin-induced K+ efflux was inhibited by clotrimazole (IC(50) of 25 +/- 0.4 microM) and charybdotoxin (IC(50) of 65 +/- 5.0 nM) and was insensitive to TEA, Ba2+, apamin and iberiotoxin, suggesting that this conductive pathway is related to the Ca2+-activated intermediate-conductance K+ channels (IK(ca)). Absence of extracellular Ca2+ did neither affect basal nor ionomycin-induced K+ efflux. However, intracellular Ca2+ depletion totally inhibited the ionomycin-induced K+ efflux, indicating that the activation of these K+ channels mainly depends on intracellular calcium liberation. K+ efflux was stimulated by intracellular Ca(2+) with an EC(50) of 1.1 +/- 0.04 microM. In surface cells, K+ efflux (rate constant: 0.17 +/- 0.027 min(-1); span: 25 +/- 3.4%) was insensitive to TEA and Ba2+. However, ionomycin induced K+ efflux with characteristics identical to that observed in crypt cells. In conclusion, both surface and crypt cells present IK(Ca) channels but only crypt cells have TEA- and Ba2+-sensitive conductive pathways, which would determine their participation in colonic K+ secretion.

Pathogenesis of diarrhea in ulcerative colitis: new views on an old problem

BACKGROUND

Whereas water movement into the intestinal lumen occurs secondary to Cl secretion in secretory diarrheal diseases, defects in key transport processes lead to profound decreases in colonic Na, Cl, and water absorption in ulcerative colitis.

STUDIES AND RESULTS

Recent studies indicate reduced expression/activity of apical Na channels and basolateral Na, K-ATPase, leading to loss of electrogenic Na absorption in the distal colon and rectum. There is also likely to be a decrease in electroneutral NaCl cotransport, which is present throughout the colon. Preliminary work on basolateral K channel abundance and activity in colonic epithelial cells suggests that whole-cell K conductance is decreased in ulcerative colitis, leading to epithelial cell depolarization, and further limitation of Na absorption. In addition, there is a marked reduction in colonic epithelial resistance, which reflects a decrease in the integrity of intercellular tight junctions and the presence of apoptotic foci.

CONCLUSIONS

Impaired Na and Cl transport, combined with enhanced epithelial "leakiness," results in a profound decrease in the capacity of the inflamed colon to absorb salt and water. Transport abnormalities in ulcerative colitis may at least partly reflect the effects of proinflammatory cytokines, raising the possibility of novel approaches to the restoration of colonic absorptive capacity in this disease.

Enhanced large intestinal potassium permeability in end-stage renal disease

The capacity of the colon for potassium (K+) secretion increases in end-stage renal disease (ESRD), to the extent that it makes a substantial contribution to K+ homeostasis. This colonic K+ adaptive response may reflect enhanced active K+ secretion, and be associated with an increase in apical membrane K+ permeability. In this study, this hypothesis was tested in patients with normal renal function or ESRD, by evaluating the effect of barium ions (a K+ channel inhibitor) on rectal K+ secretion using a rectal dialysis technique, and the expression of high conductance (BK) K+ channel protein in colonic mucosa by immunohistochemistry. Under basal conditions, rectal K+ secretion was almost threefold greater (p < 0.02) in ESRD patients (n = 8) than in patients with normal renal function (n = 10). Intraluminal barium (5 mmol/l) decreased K+ secretion in the ESRD patients by 45% (p < 0.05), but had no effect on K+ transport in patients with normal renal function. Immunostaining using a specific antibody to the BK channel alpha-subunit revealed greater (p < 0.001) levels of BK channel protein expression in surface colonocytes and crypt cells in ESRD patients (n = 9) than in patients with normal renal function (n = 9), in whom low levels of expression were mainly restricted to surface colonocytes. In conclusion, these results suggest that enhanced colonic K+ secretion in ESRD involves an increase in the apical K+ permeability of the large intestinal epithelium, which most likely reflects increased expression of apical BK channels.

Hypotonic stress activates an intermediate conductance K+ channel in human colonic crypt cells

AIM

To investigate the effect of hypotonic stress on human colonic crypts cells in terms of ion channel activity and intracellular Ca2+ concentration.

METHODS

Single crypts were isolated from biopsies taken during colonoscopy. The patch clamp technique was used (in the cell-attached mode) to observe the activity of ion channels during hypotonic stress. Calcium measurements were made using the fluophores Fluo 3 or 4.

RESULTS

The intermediate conductance (29 pS), Ca2+ -sensitive, K+ channel (also known as KCNN4) previously described (Sandle et al. 1994) was seen in 54 of 149 patches (36%) when the crypts were bathed in normal extracellular solution (290 mOsm kg(-1)). Forty-one patches could be used for further analysis. Activation of one or several 29 pS channels was seen in 15 of 41 patches (39%) after 30 s to 4 min of exposure to hypotonic solution (160 mOsm kg(-1)). The open probability increased from 0.0043 in control solution to 0.44 at 5 min of hypotonic stress. When the crypts were exposed to hypotonic solution, an increase in intracellular Ca2+ could be seen. The increase in intracellular Ca2+ emanates mainly from intracellular stores.

CONCLUSION

The 29 pS K+ channel takes part in volume regulation in human colonic crypt cells. The activation of this channel is mediated through an increase in intracellular Ca2+.

The CLCA gene locus as a modulator of the gastrointestinal basic defect in cystic fibrosis

To determine whether the CLCA gene family of calcium-activated chloride channels is a modulator of the basic defect of cystic fibrosis (CF), an association study was performed with polymorphic microsatellite markers covering a 40-Mbp region spanning the CLCA gene locus on human chromosome 1p in CF patients displaying CF transmembrane conductance regulator (CFTR)-independent residual chloride conductance in gastrointestinal epithelia. Statistically significant association of the electrophysiological phenotype with the allele distribution of markers 5' of and within the CLCA locus was observed. Transmission disequilibrium and the significance of the association decreased within the locus from hCLCA2 towards hCLCA4. Expression of hCLCA1 and hCLCA4 in human rectal mucosa was proven by microarray analysis. The CLCA gene region was identified to encode mediators of DIDS-sensitive anion conductance in the human gastrointestinal tract that modulate the CF basic defect.

Potassium channels in epithelial transport

Epithelial cells in the kidney, gastrointestinal tract and exocrine glands are engaged in vectorial transport of salt and nutrients. In these tissues, K(+) channels play an important role for the stabilization of membrane voltage and maintenance of the driving force for electrogenic transport. Luminal K(+) channels represent an exit pathway for the excretion of K(+) in secreted fluid, urine and faeces, thereby effecting body K(+) homeostasis. Indeed, the expression and function of several luminal K(+) channels is modulated by hormones regulating water, Na(+), and K(+) metabolism. In addition to net transport of K(+) in the serosal (or apical) direction, K(+) channels can be coupled functionally to K(+)-transporting ATPases such as the basolateral Na(+)/K(+) ATPase or the luminal H(+)/K(+) ATPase. These ATPases export Na(+) or H(+) and take up K(+), which is then recycled via K(+) channels. This review gives a short overview on the molecular identity of epithelial K(+) channels and summarizes the different mechanisms of K(+) channel function during transport in epithelial cells.

Active K+ secretion through multiple KCa-type channels and regulation by IKCa channels in rat proximal colon

Colonic K+ secretion stimulated by cholinergic agents requires activation of muscarinic receptors and the release of intracellular Ca2+. However, the precise mechanisms by which this rise in Ca2+ leads to K+ efflux across the apical membrane are poorly understood. In the present study, Northern blot analysis of rat proximal colon revealed the presence of transcripts encoding rSK2 [small conductance (SK)], rSK4 [intermediate conductance (IK)], and rSlo [large conductance (BK)] Ca2+-activated K+ channels. In dietary K+-depleted animals, only rSK4 mRNA was reduced in the colon. On the basis of this observation, a cDNA encoding the K+ channel rSK4 was cloned from a rat colonic cDNA library. Transfection of this cDNA into Chinese hamster ovary (CHO) cells led to the expression of Ca2+-activated K+ channels that were blocked by the IK channel inhibitor clotrimazole (CLT). Confocal immunofluorescence confirmed the presence of IK channels in proximal colonic crypts, and Western blotting demonstrated that IK protein sorted to both the apical and basolateral surfaces of colonic epithelia. In addition, transcellular active K+ secretion was studied on epithelial strips of rat proximal colon using unidirectional 86Rb+ fluxes. The addition of thapsigargin or carbachol to the serosal surface enhanced net 86Rb+ secretion. The mucosal addition of CLT completely inhibited carbachol-induced net 86Rb+ secretion. In contrast, only partial inhibition was observed with the BK and SK channel inhibitors, iberiotoxin and apamin, respectively. Finally, in parallel with the reduction in SK4 message observed in animals deprived of dietary K+, carbachol-induced 86Rb+ secretion was abolished in dietary K+-depleted animals. These results suggest that the rSK4 channel mediates K+ secretion induced by muscarinic agonists in the rat proximal colon and that transcription of the rSK4 channel is downregulated to prevent K+ loss during dietary K+ depletion.

Non-genomic regulation of intermediate conductance potassium channels by aldosterone in human colonic crypt cells

BACKGROUND

Aldosterone has a rapid, non-genomic, inhibitory effect on macroscopic basolateral K(+) conductance in the human colon, reducing its capacity for Cl(-) secretion. The molecular identity of the K(+) channels constituting this aldosterone inhibitable K(+) conductance is unclear.

AIM

To characterise the K(+) channel inhibited by aldosterone present in the basolateral membrane of human colonic crypt cells.

METHODS

Crypts were isolated from biopsies of healthy sigmoid colon obtained during colonoscopy. The effect of aldosterone on basolateral K(+) channels, and the possible involvement of Na(+):H(+) exchange, were studied by patch clamp techniques. Total RNA from isolated crypts was subjected to reverse transcriptase-polymerase chain reaction (RT-PCR) using primers specific to intermediate conductance K(+) channels (KCNN4) previously identified in other human tissues.

RESULTS

In cell attached patches, 1 nmol/l aldosterone significantly decreased the activity of intermediate conductance (27 pS) K(+) channels by 31%, 53%, and 54% after 1, 5 and 10, minutes, respectively. Increasing aldosterone concentration to 10 nmol/l produced a further 56% decrease in channel activity after five minutes. Aldosterone 1-10 nmol/l had no effect on channel activity in the presence of 20 micro mol/l ethylisopropylamiloride, an inhibitor of Na(+):H(+) exchange. RT-PCR identified KCNN4 mRNA, which is likely to encode the 27 pS K(+) channel inhibited by aldosterone.

CONCLUSION

Intermediate conductance K(+) channels (KCNN4) present in the basolateral membranes of human colonic crypt cells are a target for the non-genomic inhibitory effect of aldosterone, which involves stimulation of Na(+):H(+) exchange, thereby reducing the capacity of the colon for Cl(-) secretion.

Localization of Ca2+ -activated big-conductance K+ channels in rabbit distal colon

Big-conductance Ca(2+)-activated K(+) channels (BK channels) may play an important role in the regulation of epithelial salt and water transport, but little is known about the expression level and the precise localization of BK channels in epithelia. The aim of the present study was to quantify and localize the BK channels in the distal colon epithelium by iberiotoxin (IbTX) binding using the radiolabeled iberiotoxin analogue (125)I-IbTX-D19Y/Y36F, by autoradiography and by immunohistochemical studies. The results showed that the surface cells, responsible for Na(+) absorption, contained a high number of BK channels, whereas the abundance of the channels in the Cl(-)-secreting crypt cells was very low or absent. Surprisingly, the (125)I-IbTX-D19Y/Y36F binding and immunohistochemical studies showed expression of BK channels in the apical as well as in the basolateral membranes of the surface cells. In conclusion, the significant and distinct expression of BK channels in epithelia, combined with their strict regulation, indicate that these channels may play an important role in the overall regulation of salt and water transport.

Preliminary pharmacological characterisation of an interleukin-13-enhanced calcium-activated chloride conductance in the human airway epithelium

Interleukin (IL)-13 (10 ng/ml for 48 h) treatment of human bronchial epithelial cells induced a hypersecretory ion transport phenotype. Ussing chamber experiments demonstrated that this phenotypic change was characterised by an almost complete inhibition of the amiloride-sensitive short circuit current (ISC) and the appearance of an enhanced calcium-activated chloride conductance (CaCC). The peak increases in ISC (anion secretion) in response to UTP and ionomycin were increased by >8 fold and >13 fold respectively following IL-13 treatment. Changes in intra-cellular Ca(2+) levels following agonist exposure were not different between control and IL-13 treatments. The sensitivity of this IL-13-enhanced CaCC to several chloride channel-blocking molecules was determined following permeabilisation of the basolateral membrane and the establishment of a basolateral to apical chloride gradient. Under these conditions changes in ISC were regulated exclusively by the apical membrane and the current stimulated by ionomycin was sensitive to the chloride channel blockers diisothocyanatostilbene-2,2'-disulfonic acid (DIDS), dinitrostilben-2,2'-disulfonic acid (DNDS) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) but was insensitive to tamoxifen. An understanding of the pharmacological profile of this conductance will ultimately aid in the determination of its molecular identity and function in the human airway epithelium.

The low conductance K(+) channel in human colonic crypt cells has a voltage-dependent permeability not affected by Mg(++)

The low conductance K(+) channel found in human colonocytes was investigated using the patch-clamp technique. The channel is Ca(++)-dependent and is blocked by Ba(++) (5 mM) with a decrease in open probability from 0.42 to 0.19. At -40 mV the slope conductance was 29 pS (using intracellular solution in the pipette). In inside-out patches, inward rectification was seen both with KCl (pipette)/NaCl (bath) solutions as well as KCl/KCl solutions. The rectification could not be affected by omitting Mg(++) from the pipette or the bath solution, neither by exposing the patches to the polyamine spermine (1 mM). Using the Goldman-Hodgkin-Katz equation we show that the permeability decreased in a linear fashion from approximately 5.2 x 10(-14) cm(3)/s to 1.8 x 10(-14) cm(3)/s (-100 to +100 mV), both with and without Mg(++) in the solutions. There was no significant difference in the nominal values of permeability. This property of the K(+) channel may facilitate the hyperpolarization needed to sustain a chloride secretion.

Secretory modulation of basolateral membrane inwardly rectified K(+) channel in guinea pig distal colonic crypts

Cell-attached recordings revealed K(+) channel activity in basolateral membranes of guinea pig distal colonic crypts. Inwardly rectified currents were apparent with a pipette solution containing 140 mM K(+). Single-channel conductance (gamma) was 9 pS at the resting membrane potential. Another inward rectifier with gamma of 19 pS was observed occasionally. At a holding potential of -80 mV, gamma was 21 and 41 pS, respectively. Identity as K(+) channels was confirmed after patch excision by changing the bath ion composition. From reversal potentials, relative permeability of Na(+) over K(+) (P(Na)/P(K)) was 0.02 +/- 0.02, with P(Rb)/P(K) = 1.1 and P(Cl)/P(K) < 0.03. Spontaneous open probability (P(o)) of the 9-pS inward rectifier ((gp)K(ir)) was voltage independent in cell-attached patches. Both a low (P(o) = 0.09 +/- 0.01) and a moderate (P(o) = 0.41 +/- 0.01) activity mode were observed. Excision moved (gp)K(ir) to the medium activity mode; P(o) of (gp)K(ir) was independent of bath Ca(2+) activity and bath acidification. Addition of Cl(-) and K(+) secretagogues altered P(o) of (gp)K(ir). Forskolin or carbachol (10 microM) activated the small-conductance (gp)K(ir) in quiescent patches and increased P(o) in low-activity patches. K(+) secretagogues, either epinephrine (5 microM) or prostaglandin E(2) (100 nM), decreased P(o) of (gp)K(ir) in active patches. This (gp)K(ir) may be involved in electrogenic secretion of Cl(minus sign) and K(+) across the colonic epithelium, which requires a large basolateral membrane K(+) conductance during maximal Cl(-) secretion and, presumably, a lower K(+) conductance during primary electrogenic K(+) secretion.

Basolateral potassium channels of rabbit colon epithelium: role in sodium absorption and chloride secretion

In order to assess the role of different classes of K(+) channels in recirculation of K(+) across the basolateral membrane of rabbit distal colon epithelium, the effects of various K(+) channel inhibitors were tested on the activity of single K(+) channels from the basolateral membrane, on macroscopic basolateral K(+) conductance, and on the rate of Na(+) absorption and Cl(-) secretion. In single-channel measurements using the lipid bilayer reconstitution system, high-conductance (236 pS), Ca(2+)-activated K(+) (BK(Ca)) channels were most frequently detected; the second most abundant channel was a low-conductance K(+) channel (31 pS) that exhibited channel rundown. In addition to Ba(2+) and charybdotoxin (ChTX), the BK(Ca) channels were inhibited by quinidine, verapamil and tetraethylammonium (TEA), the latter only when present on the side of the channel from which K(+) flow originates. Macroscopic basolateral K(+) conductance, determined in amphotericin-permeabilised epithelia, was also markedly reduced by quinidine and verapamil, TEA inhibited only from the lumen side, and serosal ChTX was without effect. The chromanol 293B and the sulphonylurea tolbutamide did not affect BK(Ca) channels and had no or only a small inhibitory effect on macroscopic basolateral K(+) conductance. Transepithelial Na(+) absorption was partly inhibited by Ba(2+), quinidine and verapamil, suggesting that BK(Ca) channels are involved in basolateral recirculation of K(+) during Na(+) absorption in rabbit colon. The BK(Ca) channel inhibitors TEA and ChTX did not reduce Na(+) absorption, probably because TEA does not enter intact cells and ChTX is 'knocked off' its extracellular binding site by K(+) outflow from the cell interior. Transepithelial Cl(-) secretion was inhibited completely by Ba(2+) and 293B, partly by quinidine but not by the other K(+) channel blockers, indicating that the small (<3 pS) K(V)LQT1 channels are responsible for basolateral K(+) exit during Cl(-) secretion. Hence different types of K(+) channels mediate basolateral K(+) exit during transepithelial Na(+) and Cl(-) transport.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

Neuropeptide Y inhibits the protein kinase C-stimulated Cl(-) secretion in the human colonic cell line HT29cl.19A cell line via multiple sites

Neuropeptide Y is known to exert inhibitory effects on ion secretion in the intestine by reducing the activity of adenylyl cyclase. In the human intestinal epithelial cell line HT29cl.19A, it has been previously shown that neuropeptide Y inhibits the electrophysiological phenomena related to Cl(-) secretion, when induced by elevation of cAMP via forskolin. Moreover, the secretion induced via elevation of intracellular calcium levels via muscarinic activation can be inhibited by neuropeptide Y. Part of these inhibitions appeared to be due to lowered calcium activity in the epithelial cells, thereby reducing the basolateral K(+) conductance. The phorbol ester 4-phorbol-12,13-dibutyrate (PDB) can induce secretion in this cell line via activation of protein kinase C as well; however, the effect of neuropeptide Y on this pathway has not yet been studied. In the present experiments, it is shown that neuropeptide Y inhibits the PDB-induced secretion at two sides: one located in the apical membrane and another in the basolateral membrane. It is shown that the latter effect can, at least partially, be explained via a direct effect of neuropeptide Y on the K(+) conductance. This was concluded from the observation that neuropeptide Y could also reduce basolateral K(+) conductance when intracellular calcium was dramatically increased by ionomycin. The observed inhibitory effects suggest that neuropeptide Y is a very powerful antisecretory peptide in human intestinal epithelial cells.

Chloride secretion by the intestinal epithelium: molecular basis and regulatory aspects

Chloride secretion is the major determinant of mucosal hydration throughout the gastrointestinal tract, and chloride transport is also pivotal in the regulation of fluid secretion by organs that drain into the intestine. Moreover, there are pathological consequences if chloride secretion is either reduced or increased such as in cystic fibrosis and secretory diarrhea, respectively. With the molecular cloning of many of the proteins and regulatory factors that make up the chloride secretory mechanism, there have been significant advances in our understanding of this process at the cellular level. Similarly, emerging data have clarified the intercellular relationships that govern the extent of chloride secretion. The goal of our article is to review this area of investigation, with an emphasis on recent developments and their implications for the physiology and pathophysiology of chloride transport.

Role of K(V)LQT1 in cyclic adenosine monophosphate-mediated Cl(-) secretion in human airway epithelia

Ion transport defects underlying cystic fibrosis (CF) lung disease are characterized by impaired cyclic adenosine monophosphate (cAMP)-dependent Cl(-) conductance. Activation of Cl(-) secretion in airways depends on simultaneous activation of luminal Cl(-) channels and basolateral K(+) channels. We determined the role of basolateral K(+) conductance in cAMP- dependent Cl(-) secretion in native human airway epithelium obtained from non-CF and CF patients. CF tissues showed typical alterations of short-circuit currents with enhanced amiloride-sensitive Na(+) conductance and defective cAMP-mediated Cl(-) conductance. In non-CF tissues, Cl(-) secretion was significantly inhibited by the chromanol 293B (10 micromol/liter), a specific inhibitor of K(V)LQT1 K(+) channels. Inhibition was increased after cAMP-dependent stimulation. Similar effects were obtained with Ba(2+) (5 mmol/liter). In patch-clamp experiments with a human bronchial epithelial cell line, stimulation with forskolin (10 micromol/liter) simultaneously activated Cl(-) and K(+) conductance. The K(+) conductance was reversibly inhibited by Ba(2+) and 293B. Analysis of reverse-transcribed messenger RNA from non-CF and CF airways showed expression of human K(V)LQT1. We conclude that the K(+) channel K(V)LQT1 is important in maintaining cAMP-dependent Cl(-) secretion in human airways. Activation of K(V)LQT1 in CF airways in parallel with stimulation of residual CF transmembrane conductance regulator Cl(-) channel activity or alternative Cl(-) channels could help to circumvent the secretory defect.

Residual chloride secretion in intestinal tissue of deltaF508 homozygous twins and siblings with cystic fibrosis. The European CF Twin and Sibling Study Consortium

BACKGROUND & AIMS

Cholinergic stimulation of chloride secretion is impaired in the intestines of patients with cystic fibrosis (CF). However, intestinal chloride secretion has been observed in patients with mild CF mutations. The aim of this study was to investigate residual Cl(-) secretion in the intestine of DeltaF508 homozygous CF patients, and examine the contribution of cystic fibrosis transmembrane conductance regulator (CFTR) and alternative Cl(-) conductances. Twins and siblings with identical CFTR genotypes were investigated to determine the impact of factors other than CFTR on chloride secretion.

METHODS

Chloride secretion in rectal tissue was investigated by applying Ca(2+) and adenosine 3',5'-cyclic monophosphate (cAMP)-linked agonists before and after the inhibition of alternative Cl(-) conductances with 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid (DIDS).

RESULTS

cAMP-mediated Cl(-) secretion was observed in 73% of patients, and 20% showed DIDS-sensitive Ca(2+)-activated Cl(-) secretion. This DIDS-sensitive alternative chloride conductance was seen only in CF patients who also responded to cAMP agonists. Chloride secretion was more concordant within monozygous twins than within dizygous pairs.

CONCLUSIONS

These results suggest the presence of CFTR-mediated Cl(-) secretion in a subgroup of patients, implying that a portion of deltaF508 CFTR can be processed in vivo and function as a chloride channel in the apical membrane of intestinal cells. Moreover, a considerable number of deltaF508 homozygous patients express chloride conductances other than CFTR in their intestinal epithelia.

Mobilization of intracellular Ca(2+) by thromboxane A(2) does not affect Ca(2+)-activated K(+) channels in rat colonic crypt cells

The effects of 9,11-epithio-11,12-methano-thromboxane A(2) (STA(2)), a stable thromboxane A(2) analogue, and carbachol on colonic Ca(2+)-activated K(+) channels were studied. In indo-1-loaded single cells in isolated rat colonic crypts, both STA(2) (0.1 microM) and carbachol (10 microM) transiently increased intracellular free Ca(2+) concentration ([Ca(2+)](i)) by 136 and 155 nm, respectively. In whole-cell current-clamp experiments of the colonic crypt cells with Cl(-)-free solutions, carbachol (10 microM) hyperpolarized the cell by 19.7 mV, while STA(2) (0.1 microM) did not affect the membrane potential. In the isolated colonic mucosa that was permeabilized mucosally by a monovalent ionophore nystatin in the presence of a serosally directed K(+) gradient, carbachol (10 microM) transiently elicited K(+) current, but STA(2) (0.1 microM) did not. These results indicate that STA(2) elevates [Ca(2+)](i) in rat colonic crypt cells but does not activate basolateral Ca(2+)-activated K(+) channels.

Multiple action sites of flufenamate on ion transport across the rat distal colon

The antisecretory effects of flufenamate in the rat distal colon were investigated with the Ussing-chamber and the patch-clamp method as well as by measurements of the intracellular Ca(2+) concentration using fura-2-loaded isolated crypts. Flufenamate (5.10(-4) mol l(-1)) suppressed the short-circuit current (Isc) induced by carbachol (5.10(-5) mol l(-1)), forskolin (5.10(-6) mol l(-1)) and the Isc induced by the membrane-permeable analogue of cyclic AMP, CPT - cyclic AMP (10(-4) mol l(-1)). Indomethacin (10(-6) - 10(-4) mol l(-1)) did not mimic the effect of flufenamate, indicating that the antisecretory effect of flufenamate is not related to the inhibition of the cyclo-oxygenase. When the basolateral membrane was depolarized by a high K(+) concentration and a Cl(-) current was induced by a mucosally directed Cl(-) gradient, the forskolin-stimulated Cl(-) current was blocked by flufenamate, indicating an inhibition of the cyclic AMP-stimulated apical Cl(-) conductance. When the apical membrane was permeabilized by the ionophore, nystatin, flufenamate decreased the basolateral K(+) conductance and inhibited the Na(+) - K(+)-ATPase. Patch-clamp experiments revealed a variable effect of flufenamate on membrane currents. In seven out of 11 crypt cells the drug induced an increase of the K(+) current, whereas in the remaining four cells an inhibition was observed. Experiments with fura-2-loaded isolated crypts indicated that flufenamate increased the basal as well as the carbachol-stimulated intracellular Ca(2+) concentration. These results demonstrate that flufenamate possesses multiple action sites in the rat colon: The apical Cl(-) conductance, basolateral K(+) conductances and the Na(+) - K(+)-ATPase.

Somatostatin peptides inhibit basolateral potassium channels in human colonic crypts

Somatostatin is a powerful inhibitor of intestinal Cl(-) secretion. We used patch-clamp recording techniques to investigate the effects of somatostatin on low-conductance (23-pS) K(+) channels in the basolateral membrane of human colonic crypts, which are an important component of the Cl(-) secretory process. Somatostatin (2 microM) elicited a >80% decrease in "spontaneous" K(+) channel activity in cell-attached patches in nonstimulated crypts (50% inhibition = approximately 8 min), which was voltage-independent and was prevented by pretreating crypts for 18 h with pertussis toxin (200 ng/ml), implicating a G protein-dependent mechanism. In crypts stimulated with 100-200 microM dibutyryl cAMP, 2 microM somatostatin and its synthetic analog octreotide (2 microM) both produced similar degrees of K(+) channel inhibition to that seen in nonstimulated crypts, which was also present under low-Cl(-) (5 mM) conditions. In addition, 2 microM somatostatin abolished the increase in K(+) channel activity stimulated by 2 microM thapsigargin but had no effect on the thapsigargin-stimulated rise in intracellular Ca(2+). These results indicate that somatostatin peptides inhibit 23-pS basolateral K(+) channels in human colonic crypt cells via a G protein-dependent mechanism, which may result in loss of the channel's inherent Ca(2+) sensitivity.

Anthrax toxin entry into polarized epithelial cells

We examined the entry of anthrax edema toxin (EdTx) into polarized human T84 epithelial cells using cyclic AMP-regulated Cl- secretion as an index of toxin entry. EdTx is a binary A/B toxin which self assembles at the cell surface from anthrax edema factor and protective antigen (PA). PA binds to cell surface receptors and delivers EF, an adenylate cyclase, to the cytosol. EdTx elicited a strong Cl- secretory response when it was applied to the basolateral surface of T84 cells but no response when it was applied to the apical surface. PA alone had no effect when it was applied to either surface. T84 cells exposed basolaterally bound at least 30-fold-more PA than did T84 cells exposed apically, indicating that the PA receptor is largely or completely restricted to the basolateral membrane of these cells. The PA receptor did not fractionate with detergent-insoluble caveola-like membranes as cholera toxin receptors do. These findings have implications regarding the nature of the PA receptor and confirm the view that EdTx and CT coopt fundamentally different subcellular systems to enter the cell and cause disease.

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.