Ca2+ regulated K+ and non-selective cation channels in the basolateral membrane of rat colonic crypt base cells

Calcium-sensing receptor regulates intestinal dipeptide absorption via Ca2+ signaling and IKCa activation

Although absorption of di- and tripeptides into intestinal epithelial cells occurs via the peptide transporter 1 (PEPT1, also called solute carrier family 15 member 1 (SLC15A1)), the detailed regulatory mechanisms are not fully understood. We examined: (a) whether dipeptide absorption in villous enterocytes is associated with a rise in cytosolic Ca2+ ([Ca2+ ]cyt ), (b) whether the calcium sensing receptor (CaSR) is involved in dipeptide-elicited [Ca2+ ]cyt signaling, and (c) what potential consequences of [Ca2+ ]cyt signaling may enhance enterocyte dipeptide absorption. Dipeptide Gly-Sar and CaSR agonist spermine markedly raised [Ca2+ ]cyt in villous enterocytes, which was abolished by NPS-2143, a selective CaSR antagonist and U73122, an phospholipase C (PLC) inhibitor. Apical application of Gly-Sar induced a jejunal short-circuit current (Isc), which was reduced by NPS-2143. CaSR expression was identified in the lamina propria and on the basal enterocyte membrane of mouse jejunal mucosa in both WT and Slc15a1-/- animals, but Gly-Sar-induced [Ca2+ ]cyt signaling was significantly decreased in Slc15a1-/- villi. Clotrimazole and TRM-34, two selective blockers of the intermediate conductance Ca2+ -activated K+ channel (IKCa ), but not iberiotoxin, a selective blocker of the large-conductance K+ channel (BKCa ) and apamin, a selective blocker of the small-conductance K+ channel (SKCa ), significantly inhibited Gly-Sar-induced Isc in native tissues. We reveal a novel CaSR-PLC-Ca2+ -IKCa pathway in the regulation of small intestinal dipeptide absorption, which may be exploited as a target for future drug development in human nutritional disorders.

Tissue-specific ionomotive enzyme activity and K+ reabsorption reveal the rectum as an important ionoregulatory organ in larval Chironomus riparius exposed to varying salinity

A role for the rectum in the ionoregulatory homeostasis of larval Chironomus riparius was revealed by rearing animals in different saline environments and examining: (1) the spatial distribution and activity of keystone ionomotive enzymes Na(+)-K(+)-ATPase (NKA) and V-type H(+)-ATPase (VA) in the alimentary canal, and (2) rectal K(+) transport with the scanning ion-selective electrode technique (SIET). NKA and VA activity were measured in four distinct regions of the alimentary canal as follows: the combined foregut and anterior midgut, the posterior midgut, the Malpighian tubules and the hindgut. Both enzymes exhibited 10-20 times greater activity in the hindgut relative to all other areas. When larvae were reared in either ion-poor water (IPW) or freshwater (FW), no significant difference in hindgut enzyme activity was observed. However, in larvae reared in brackish water (BW), NKA and VA activity in the hindgut significantly decreased. Immunolocalization of NKA and VA in the hindgut revealed that the bulk of protein was located in the rectum. Therefore, K(+) transport across the rectum was examined using SIET. Measurement of K(+) flux along the rectum revealed a net K(+) reabsorption that was reduced fourfold in BW-reared larvae versus larvae reared in FW or IPW. Inhibition of NKA with ouabain, VA with bafilomycin and K(+) channels with charybdotoxin diminished rectal K(+) reabsorption in FW- and IPW-reared larvae, but not BW-reared larvae. Data suggest that the rectum of C. riparius plays an important role in allowing these larvae to cope with dilute as well as salinated environmental conditions.

Effects of carbon monoxide on ion transport across rat distal colon

The aim of the present study was to investigate whether carbon monoxide (CO) induces changes in ion transport across the distal colon of rats and to study the mechanisms involved. In Ussing chamber experiments, tricarbonyldichlororuthenium(II) dimer (CORM-2), a CO donor, evoked a concentration-dependent increase in short-circuit current (I(sc)). A maximal response was achieved at a concentration of 2.5·10(-4) mol/l. Repeated application of CORM-2 resulted in a pronounced desensitization of the tissue. Anion substitution experiments suggest that a secretion of Cl(-) and HCO(3)(-) underlie the CORM-2-induced current. Glibenclamide, a blocker of the apical cystic fibrosis transmembrane regulator channel, inhibited the I(sc) induced by the CO donor. Similarly, bumetanide, a blocker of the basolateral Na(+)-K(+)-2Cl(-) cotransporter, combined with 4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulfonic acid sodium salt, an inhibitor of the basolateral Cl(-)/HCO(3)(-) exchanger, inhibited the CORM-2-induced I(sc). Membrane permeabilization experiments indicated an activation of basolateral K(+) and apical Cl(-) channels by CORM-2. A partial inhibition by the neurotoxin, tetrodotoxin, suggests the involvement of secretomotor neurons in this response. In imaging experiments at fura-2-loaded colonic crypts, CORM-2 induced an increase of the cytosolic Ca(2+) concentration. This increase depended on the influx of extracellular Ca(2+), but not on the release of Ca(2+) from intracellular stores. Both enzymes for CO production, heme oxygenase I and II, are expressed in the colon as observed immunohistochemically and by RT-PCR. Consequently, endogenous CO might be a physiological modulator of colonic ion transport.

Mucosal potassium efflux mediated via Kcnn4 channels provides the driving force for electrogenic anion secretion in colon

Intermediate conductance K(+) (Kcnn4) channels are present in both mucosal and serosal membranes of colon. However, only serosal Kcnn4 channels have been shown to be essential for agonist-induced (cAMP and Ca(2+)) anion secretion. The present study sought to determine whether mucosal Kcnn4 channels also play a role in colonic anion secretion. Mucosal-to-serosal and serosal-to-mucosal unidirectional (86)Rb (K(+) surrogate) fluxes as well as short-circuit current (I(sc); a measure of anion secretion) were measured under voltage-clamp conditions in distal colon from rats fed either a standard or K(+)-free diet. 5,6-Dichloro-1-ethyl-1,3-dihydro-2H-benzimidazole-2-one (DC-EBIO) was used to activate Kcnn4 channels. Mucosal DC-EBIO both induced K(+) secretion and enhanced anion secretion in normal rat distal colon. The DC-EBIO-induced K(+) secretion was completely blocked by nonspecific (Ba(2+)) and Kcnn4-specific (TRAM-34) inhibitors, but was not blocked by the large-conductance K(+) (iberiotoxin), small-conductance K(+) (apamin), or KCNQ1 (chromanol 293B) specific blockers. Ba(2+) and TRAM-34 also inhibited DC-EBIO-enhanced anion secretion. The DC-EBIO-enhanced anion secretion was completely inhibited by the nonspecific anion channel blocker 5-nitro-2-(3-phenylpropyl-amino)benzoic acid, whereas it was only partially inhibited by CFTR [CFTR(inh)-172, glibenclamide]- and CaCC (niflumic acid)-specific Cl(-) channel blockers. In contrast, mucosal DC-EBIO-enhanced K(+) and anion secretion was not present in distal colon of dietary K-depleted rats, indicating absence of mucosal Kcnn4 channels. These observations indicate that mucosal Kcnn4 channels are capable of driving agonist-induced anion secretion mediated via CFTR and CaCC and likely contribute to stool K(+) losses that accompany diarrheal illnesses.

Cloning and identification of tissue-specific expression of KCNN4 splice variants in rat colon

KCNN4 channels that provide the driving force for cAMP- and Ca(2+)-induced anion secretion are present in both apical and basolateral membranes of the mammalian colon. However, only a single KCNN4 has been cloned. This study was initiated to identify whether both apical and basolateral KCNN4 channels are encoded by the same or different isoforms. Reverse transcriptase-PCR (RT-PCR), real-time quantitative-PCR (RT-QPCR), and immunofluorescence studies were used to clone and identify tissue-specific expression of KCNN4 isoforms. Three distinct KCNN4 cDNAs that are designated as KCNN4a, KCNN4b, and KCNN4c encoding 425, 424, and 395 amino acid proteins, respectively, were isolated from the rat colon. KCNN4a differs from KCNN4b at both the nucleotide and the amino acid level with distinct 628 bp at the 3'-untranslated region and an additional glutamine at position 415, respectively. KCNN4c differs from KCNN4b by lacking the second exon that encodes a 29 amino acid motif. KCNN4a and KCNN4b/c are identified as smooth muscle- and epithelial cell-specific transcripts, respectively. KCNN4b and KCNN4c transcripts likely encode basolateral (40 kDa) and apical (37 kDa) membrane proteins in the distal colon, respectively. KCNN4c, which lacks the S2 transmembrane segment, requires coexpression of a large conductance K(+) channel beta-subunit for plasma membrane expression. The KCNN4 channel blocker TRAM-34 inhibits KCNN4b- and KCNN4c-mediated (86)Rb (K(+) surrogate) efflux with an apparent inhibitory constant of 0.6 +/- 0.1 and 7.8 +/- 0.4 muM, respectively. We conclude that apical and basolateral KCNN4 K(+) channels that regulate K(+) and anion secretion are encoded by distinct isoforms in colonic epithelial cells.

Colonic potassium handling

Homeostatic control of plasma K+ is a necessary physiological function. The daily dietary K+ intake of approximately 100 mmol is excreted predominantly by the distal tubules of the kidney. About 10% of the ingested K+ is excreted via the intestine. K+ handling in both organs is specifically regulated by hormones and adapts readily to changes in dietary K+ intake, aldosterone and multiple local paracrine agonists. In chronic renal insufficiency, colonic K+ secretion is greatly enhanced and becomes an important accessory K+ excretory pathway. During severe diarrheal diseases of different causes, intestinal K+ losses caused by activated ion secretion may become life threatening. This topical review provides an update of the molecular mechanisms and the regulation of mammalian colonic K+ absorption and secretion. It is motivated by recent results, which have identified the K+ secretory ion channel in the apical membrane of distal colonic enterocytes. The directed focus therefore covers the role of the apical Ca2+ and cAMP-activated BK channel (KCa1.1) as the apparently only secretory K+ channel in the distal colon.

Loss of Ca-mediated ion transport during colitis correlates with reduced ion transport responses to a Ca-activated K channel opener

BACKGROUND AND PURPOSE

Epithelial surface hydration is critical for proper gut function. However, colonic tissues from individuals with inflammatory bowel disease or animals with colitis are hyporesponsive to Cl(-) secretagogues. The Cl(-) secretory responses to the muscarinic receptor agonist bethanechol are virtually absent in colons of mice with dextran sodium sulphate (DSS)-induced colitis. Our aim was to define the mechanism underlying this cholinergic hyporesponsiveness.

EXPERIMENTAL APPROACH

Colitis was induced by 4% DSS water, given orally. Epithelial ion transport was measured in Ussing chambers. Colonic crypts were isolated and processed for mRNA expression via RT-PCR and protein expression via immunoblotting and immunolocalization.

KEY RESULTS

Expression of muscarinic M(3) receptors in colonic epithelium was not decreased during colitis. Short-circuit current (I(SC)) responses to other Ca(2+)-dependent secretagogues (histamine, thapsigargin, cyclopiazonic acid and calcium ionophore) were either absent or severely attenuated in colonic tissue from DSS-treated mice. mRNA levels of several ion transport molecules (a Ca(2+)-regulated Cl(-) channel, the intermediate-conductance Ca(2+)-activated K(+) channel, the cystic fibrosis transmembrane conductance regulator, the Na(+)/K(+)-ATPase pump or the Na(+)/K(+)/2Cl(-) co-transporter) were not reduced in colonic crypts from DSS-treated mice. However, protein expression of Na(+)/K(+)-ATPase alpha1 subunits was decreased twofold during colitis. Activation of Ca(2+)-activated K(+) channels increased I(SC) significantly less in DSS colons compared with control, as did the protein kinase C activator, phorbol 12-myristate 13-acetate.

CONCLUSIONS AND IMPLICATIONS

Decreased Na(+)/K(+)-ATPase expression probably contributes to overall epithelial hyporesponsiveness during colitis, while dysfunctional K(+) channels may account, at least partially, for lack of epithelial secretory responses to Ca(2+)-mediated secretagogues.

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.

Sites of action of hydrogen peroxide on ion transport across rat distal colon

BACKGROUND AND PURPOSE

The aim of this study was the identification of the mechanism of oxidant-induced intestinal secretion.

EXPERIMENTAL APPROACH

The action of H2O2 on ion transport across rat distal colon was evaluated in Ussing chambers. Changes in cytosolic Ca2+ concentration were measured using fura-2.

KEY RESULTS

H2O2 concentration-dependently induced an increase in short-circuit current (Isc), which was due to a stimulation of Cl(-) secretion. The effect of H2O2 was dependent on the presence of serosal Ca2+. It was inhibited after emptying of intracellular Ca2+ stores by cyclopiazonic acid or blockade of ryanodine receptors by ruthenium red, whereas a blocker of inositol 1,4,5-trisphosphate receptors was less effective. Fura 2-experiments confirmed an increase in the cytosolic Ca2+ concentration in the presence of H2O2. Measurements of Cl- currents across the apical membrane at basolaterally depolarized epithelia revealed the activation of a glibenclamide-sensitive, SITS-resistant Cl- conductance by the oxidant. The activation of this conductance was inhibited after blockade of protein kinases with staurosporine. When the apical membrane was permeabilized with nystatin, two sites of action of H2O2 were identified at the basolateral membrane. The oxidant stimulated a basolateral tetrapentylammonium-sensitive K+ conductance and increased the current generated by the Na+-K+ pump. Pretreatment of the tissues with H2O2 reduced the action of subsequently administered Ca2+-, cAMP- and cGMP-dependent secretagogues demonstrating a long-term downregulation after the initial secretory response evoked by the oxidant.

CONCLUSIONS AND IMPLICATIONS

H2O2 affects colonic anion secretion by action sites at both the apical, as well as the basolateral membrane.

Characterization of inositol 1,4,5-trisphosphate (IP3) receptor subtypes at rat colonic epithelium

The aim of the present study was the characterization of the subtypes of inositol 1,4,5-trisphosphate receptors (IP3R) in rat colonic epithelium. A monoclonal antibody against IP3R1 did not stain the colonic epithelial cells. In contrast, IP3R2 and IP3R3 were found within the epithelium; however, with a distinct intracellular localization and differences in their distribution along the crypt axis. IP3R2 immunoreactivity was found within the nuclei of the epithelial cells. The signal was distributed all over the nucleus and not restricted to the nuclear envelope as demonstrated by counterstaining with lamin B1 and electron microscopical examination after immunogold labelling. In contrast, an antibody against IP3R3 stained the epithelial cells mostly in their apical half in accordance with the typical localization of IP3R in organelles such as the endoplasmic reticulum. In addition, there was a gradient from the surface region towards the crypt fundus, where the IP3R3 signal could not be detected. Despite the strong IP3R3-gradient, in saponin-permeabilized colonic crypts exogenously administered IP3 or adenophostin A evoked a similar depletion of mag-fura-2-loaded intracellular Ca2+ stores in crypt and surface cells suggesting a contribution of the nuclear IP3R2 to the Ca2+ release. This conclusion was confirmed by experiments with isolated nuclei from colonic epithelium, at which IP3 was able to induce changes in the Ca2+ concentration, which were inhibited by 2-aminoethoxy-diphenylborate (2-APB), a blocker of IP3 receptors. These results demonstrate that the colonic epithelial cells undergo changes in IP3R subtype expression during differentiation.

Role of cholinergic-activated KCa1.1 (BK), KCa3.1 (SK4) and KV7.1 (KCNQ1) channels in mouse colonic Cl- secretion

AIM

Colonic crypts are the site of Cl- secretion. Basolateral K+ channels provide the driving force for luminal cystic fibrosis transmembrane regulator-mediated Cl- exit. Relevant colonic epithelial K+ channels are the intermediate conductance Ca2+-activated K(Ca)3.1 (SK4) channel and the cAMP-activated K(V)7.1 (KCNQ1) channel. In addition, big conductance Ca2+-activated K(Ca)1.1 (BK) channels may play a role in Ca2+-activated Cl- secretion. Here we use K(Ca)1.1 and K(Ca)3.1 knock-out mice, and the K(V)7.1 channel inhibitor 293B (10 microm) to investigate the role of K(Ca)1.1, K(Ca)3.1 and K(V)7.1 channels in cholinergic-stimulated Cl- secretion.

METHODS

A Ussing chamber was used to quantify agonist-stimulated increases in short circuit current (Isc) in distal colon. Chloride secretion was activated by bl. forskolin (FSK, 2 microm) followed by bl. carbachol (CCH, 100 microm). Luminal Ba2+ (5 mm) was used to inhibit K(Ca)1.1 channels.

RESULTS

K(Ca)1.1 WT and KO mice displayed identical FSK and CCH-stimulated Isc changes, indicating that K(Ca)1.1 channels are not involved in FSK- and cholinergic-stimulated Cl- secretion. CCH-stimulated DeltaIsc was significantly reduced in K(Ca)3.1 KO mice, underscoring the known relevance of this channel in the activation of Cl- secretion by an intracellular Ca2+ increasing agonist. The residual CCH effect observed in K(Ca)3.1 KO mice suggests that yet another K+ channel is driving the CCH-stimulated Cl- secretion. In the presence of the specific K(V)7.1 channel blocker 293B, the residual CCH effect was abolished.

CONCLUSIONS

This demonstrates that both K(Ca)3.1 and K(V)7.1 channels are activated by cholinergic agonists and drive Cl- secretion. In contrast, K(Ca)1.1 channels are not involved in stimulated electrogenic Cl- secretion.

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.

Histamine-induced ion secretion across rat distal colon: involvement of histamine H1 and H2 receptors

The aim of the present study was to investigate the effect of histamine, a product of e.g. mast cells, on short-circuit current (I(sc)) across rat distal colon. Histamine concentration-dependently stimulated an increase in I(sc), which often was preceded by a transient negative current. Neither a release of neurotransmitters nor a release of prostaglandins contributed to the histamine response. The histamine-induced increase in I(sc) was blocked by the histamine H(1) antagonist, pyrilamine, but was resistant against the histamine H(2) antagonist, cimetidine. Conversely, the histamine H(1) agonist, TMPH (2-(3-trifluoromethylphenyl)histamine), exclusively evoked an increase in I(sc), whereas the histamine H(2) agonist, amthamine, evoked only a decrease in I(sc) suggesting that stimulation of different types of histamine receptors is responsible for the two phases of the response evoked by native histamine. Histamine induces the opening of glibenclamide-sensitive Cl(-) channels and of charybdotoxin-sensitive K(+) channels in the apical membrane as demonstrated by experiments at basolaterally depolarized epithelia. A further action site is the basolateral membrane, because histamine stimulates a charybdotoxin- and tetrapentylammonium-sensitive K(+) conductance in this membrane as observed in tissues, in which the apical membrane was permeabilized with an ionophore, nystatin. The increase in I(sc) evoked by histamine was blocked after depletion of intracellular Ca(2+) stores with cyclopiazonic acid and after blockade of inositol 1,4,5-trisphosphate (IP(3)) receptors, suggesting a release of stored Ca(2+). This was confirmed by the observation that the histamine H(1) agonist TMPH induced an increase in the fura-2 ratio signal of epithelial cells within isolated colonic crypts. Consequently, the mediator histamine seems to stimulate both histamine H(1) and H(2) receptors, from which the former seems to be prominently involved in the induction of epithelial chloride secretion.

Hypertonicity-induced cation channels

Whenever studied in a quantitative fashion, hypertonicity-induced cation channels (HICCs) are found to be the main mediators of regulatory volume increase. In most instances, these channels are either inhibited by amiloride (but insensitive to Gd3+ and flufenamate) or they are efficiently blocked by Gd3+ and flufenamate (but insensitive to amiloride). Of note, however, from two preparations so far a mixed type of pharmacology has also been reported. Whereas the ion selectivity of amiloride-sensitive HICCs has not been studied in much detail yet, amiloride-insensitive channels are either equally permeable to Na+, K+, Cs+ and Li+ but impermeable to N-methyl-D-glucamine (NMDG+) or they exhibit a permeability to Li+ and NMDG+ that amounts to some 50% when compared with that of Na+. Also in this respect, however, some peculiarities do exist. Concerning the actual molecular correlate, evidence was reported that HICCs may be related to the (amiloride-sensitive) epithelial Na+ channel and/or to transient receptor potential channels. Recent findings suggest that HICCs may contribute to cell proliferation, just as the K+ channels that are employed in regulatory volume decrease are mediators of the opposing process, i.e. apoptosis.

Distal Colonic K+ Secretion Occurs via BK Channels

K(+) secretion in the kidney and distal colon is a main determinant of K(+) homeostasis. This study investigated the identity of the relevant luminal secretory K(+) ion channel in distal colon. An Ussing chamber was used to measure ion transport in the recently generated BK channel-deficient (BK(-/-)) mice. BK(-/-) mice display a significant colonic epithelial phenotype with (1) lack of Ba(2+)-sensitive resting K(+) secretion, (2) absence of K(+) secretion stimulated by luminal P2Y(2) and P2Y(4) receptors, (3) absence of luminal Ca(2+) ionophore (A23187)-stimulated K(+) secretion, (4) reduced K(+) and increased Na(+) contents in feces, and (5) an increased colonic Na(+) absorption. In contrast, resting and uridine triphosphate (UTP)-stimulated K(+) secretion was not altered in mice that were deficient for the intermediate conductance Ca(2+)-activated K(+) channel SK4. BK channels localize to the luminal membrane of crypt, and reverse transcription-PCR results confirm the expression of the BK channel alpha-subunit in isolated distal colonic crypts. It is concluded that BK channels are the responsible K(+) channels for resting and stimulated Ca(2+)-activated K(+) secretion in mouse distal colon.

DCEBIO stimulates Cl- secretion in the mouse jejunum

We investigated the effects of 5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one(DCEBIO) on the Cl- secretory response of the mouse jejunum using the Ussing short-circuit current (Isc) technique. DCEBIO stimulated a concentration-dependent, sustained increase in Isc (EC50 41 +/- 1 microM). Pretreating tissues with 0.25 microM forskolin reduced the concentration-dependent increase in Isc by DCEBIO and increased the EC50 (53 +/- 5 microM). Bumetanide blocked (82 +/- 5%) the DCEBIO-stimulated Isc consistent with Cl- secretion. DCEBIO was a more potent stimulator of Cl- secretion than its parent molecule, 1-ethyl-2-benzimidazolinone. Glibenclamide or NPPB reduced the DCEBIO-stimulated Isc by >80% indicating the participation of CFTR in the DCEBIO-stimulated Isc response. Clotrimazole reduced DCEBIO-stimulated Isc by 67 +/- 15%, suggesting the participation of the intermediate conductance Ca2+-activated K+ channel (IKCa) in the DCEBIO-activated Isc response. In the presence of maximum forskolin (10 microM), the DCEBIO response was reduced and biphasic, reaching a peak response of the change in Isc of 43 +/- 5 microA/cm2 and then falling to a steady-state response of 17 +/- 10 microA/cm2 compared with DCEBIO control tissues (61 +/- 6 microA/cm2). The forskolin-stimulated Isc in the presence of DCEBIO was reduced compared with forskolin control tissues. Similar results were observed with DCEBIO and 8-BrcAMP where adenylate cyclase was bypassed. H89, a PKA inhibitor, reduced the DCEBIO-activated Isc, providing evidence that DCEBIO increased Cl- secretion via a cAMP/PKA-dependent manner. These data suggest that DCEBIO stimulates Cl- secretion of the mouse jejunum and that DCEBIO targets components of the Cl- secretory mechanism.

Class I antiarrhythmics inhibit Na+ absorption and Cl− secretion in rabbit descending colon epithelium

To clarify the mechanism of the diarrhea associated with the clinical use of antiarrhythmic drugs we assessed the effects of these agents on transepithelial Na+ absorption and Cl- secretion, on basolateral K+ conductance, and on the properties of single basolateral K+ channels of rabbit colon epithelium. Quinidine and propafenone, both at 10 microM, inhibited Na+ absorption by 27 and 38% respectively, compared with 50% with 5 mM Ba2+. The other tested class I antiarrhythmics disopyramide, mexiletine, lidocaine, and flecainide decreased Na+ current by 9-13%. Procainamide and the class III antiarrhythmics N-acetylprocainamide, sotalol, ibutilide, and amiodarone were no or were very weak inhibitors of Na+ absorption. Cl- secretion, stimulated with the adenosine analogue NECA (5'-N-ethylcarboxamide-adenosine), was reduced by 54% with quinidine and by 29% with propafenone compared with 100% with Ba2+. Mexiletine, lidocaine, and flecainide inhibited Cl- secretion by 10-23%, whereas the class III antiarrhythmics were no or were weak inhibitors. Those antiarrhythmics that inhibited Na+ and Cl- transport also reduced basolateral K+ conductance, determined in amphotericin B permeabilized epithelia. The activity of the high-conductance, Ca2+-activated, voltage-dependent K+ (BK(Ca)) channel, which is primarily responsible for basolateral K+ recycling during Na+ absorption, was inhibited by 10-30 microM quinidine or propafenone in the form of a rapidly dissociating block. Mexiletine and flecainide inhibited the single channel conductance at higher concentrations; disopyramide, lidocaine, and procainamide were ineffective. In conclusion, the present evidence suggests that the diarrhea caused by class I antiarrhythmic drugs such as quinidine and propafenone is a result of a reduction in basolateral K+ conductance and inhibition of BK(Ca) channels, thereby impeding transepithelial Na+ and water absorption.

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.

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+.

Regulatory volume increase after secretory volume decrease in colonic epithelial cells under muscarinic stimulation

To address the question of whether colonic secretory cells change their volume in response to carbachol (CCh) stimulation and, if so, the mechanisms involved therein, we used two-photon laser scanning microscopy to measure the volume of individual epithelial cells in the fundus region of crypts isolated from the guinea-pig distal colon. We also measured the volume of human colonic epithelial T84 cells using an electronic sizing technique. Both types of colonocytes responded to stimulation by CCh with shrinkage and then underwent a regulatory volume increase (RVI), even during continued stimulation by CCh. The secretory volume decrease (SVD) induced by CCh was antagonized by atropine, BAPTA loading and niflumic acid, a blocker of Ca(2+)-activated Cl(-) channels. An increase in the intracellular free [Ca(2+)] was observed with fura-2 during these volume responses to CCh. Removal of all Na(+) or K(+) or of most of the Cl(-) from the extracellular solution abolished the RVI, but not the preceding SVD. The RVI, but not the preceding SVD, was abolished by bumetanide, a blocker of the Na(+)-K(+)-2Cl(-) cotransporter. We conclude that guinea-pig crypt colonocytes and human T84 cells exhibit a cytosolic Ca(2+)-dependent SVD and undergo a subsequent RVI that is dependent on the operation of Na(+)-K(+)-2Cl(-) cotransporters.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

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.

Secretory activation of basolateral membrane Cl- channels in guinea pig distal colonic crypts

Cell-attached recordings revealed Cl(-) channel activity in basolateral membrane of guinea pig distal colonic crypts isolated from basement membrane. Outwardly rectified currents ((gp)Cl(or)) were apparent with a single-channel conductance (gamma) of 29 pS at resting membrane electrical potential; another outward rectifier with gamma of 24 pS was also observed ( approximately 25% of (gp)Cl(or)). At a holding potential of -80 mV gamma was 18 pS for both (gp)Cl(or) currents, and at +80 mV gamma was 67 and 40 pS, respectively. Identity as Cl(-) channels was confirmed in excised patches by changing bath ion composition. From reversal potentials, relative permeability of K(+) over Cl(-) (P(K)/P(Cl)) was 0.07 +/- 0.03, with relative permeability of Na(+) over Cl(-) (P(Na)/P(Cl)) = 0.08 +/- 0.04. A second type of Cl(-) channel was seen with linear current-voltage (I-V) relations ((gp)Cl(L)), having subtypes with gamma of 21, 13, and 8 pS. Epinephrine or forskolin increased the number of open (gp)Cl(or) and (gp)Cl(L). Open probabilities (P(o)) of (gp)Cl(or), (gp)Cl(L21), and (gp)Cl(L13) were voltage dependent in cell-attached patches, higher at more positive potentials. Kinetics of (gp)Cl(or) were more rapid with epinephrine activation than with forskolin activation. Epinephrine increased P(o) at the resting membrane potential for (gp)Cl(L13). Secretagogue activation of these Cl(-) channels may contribute to stimulation of electrogenic K(+) secretion across colonic epithelium by increasing basolateral membrane Cl(-) conductance that permits Cl(-) exit after uptake via Na(+)-K(+)-2Cl(-) cotransport.

P2Y6 receptor mediates colonic NaCl secretion via differential activation of cAMP-mediated transport

Extracellular nucleotides are important regulators of epithelial ion transport. Here we investigated nucleotide-mediated effects on colonic NaCl secretion and the signal transduction mechanisms involved. Basolateral UDP induced a sustained activation of Cl(-) secretion, which was completely inhibited by 293B, a specific inhibitor of cAMP-stimulated basolateral KCNQ1/KCNE3 K(+) channels. We therefore speculated that a basolateral P2Y(6) receptor could increase cAMP. Indeed UDP elevated cAMP in isolated crypts. We identified an epithelial P2Y(6) receptor using crypt [Ca(2+)](i) measurements, RT-PCR, and immunohistochemistry. To investigate whether the rat P2Y(6)elevates cAMP, we coexpressed the P2Y(1) or P2Y(6) receptor together with the cAMP-regulated cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel in Xenopus oocytes. A two-electrode voltage clamp was used to monitor nucleotide-induced Cl(-) currents. In oocytes expressing the P2Y(1) receptor, ATP transiently activated the endogenous Ca(2+)-activated Cl(-) current, but not CFTR. In contrast, in oocytes expressing the P2Y(6)receptor, UDP transiently activated the Ca(2+)-activated Cl(-) current and subsequently CFTR. CFTR Cl(-) currents were identified by their halide conductance sequence. In summary we find a basolateral P2Y(6) receptor in colonic epithelial cells stimulating sustained NaCl secretion by way of a synergistic increase of [Ca(2+)](i) and cAMP. In support of these data P2Y(6) receptor stimulation differentially activates CFTR in Xenopus oocytes.

Substance P-evoked Cl(-) secretion in guinea pig distal colonic epithelia: interaction with PGE(2)

Interaction between substance P (SP) and PGE(2) on Cl(-) secretion in the guinea pig distal colonic epithelia was investigated. A short-circuit current (I(sc)) was measured as an index of ion transport. Mucosa preparations deprived of muscle and submucosa of distal colon were mounted in the Ussing flux chamber and treated with TTX and piroxicam to remove the influences of neuronal activity and endogenous PG synthesis, respectively. Although SP (10(-7) M) itself evoked little increase in I(sc), exogenous PGE(2) concentration dependently enhanced the response of SP. The effect of PGE(2) on the SP-evoked response was mimicked by forskolin and 8-bromoadenosine cAMP. Depletion of Ca2+ from the bathing solution reduced the PGE(2)-dependent response of SP. Effects of PGE(2), SP, and SP in the presence of PGE(2) on intracellular Ca2+ concentration ([Ca2+](i)) in isolated crypt cells were measured by the confocal microscope fluorescence imaging system. SP, but not PGE(2), temporally evoked an increase in [Ca2+](i) but declined to the baseline within 3 min. A return of the SP-evoked increase in [Ca2+](i) was slower in the presence of PGE(2) than SP alone. These results suggest that PGE(2) synergistically enhances SP-evoked Cl(-) secretion via an interaction between the intracellular cAMP and [Ca2+](i) in the epithelial cells. In conclusion, SP and PGE(2) could cooperatively induce massive Cl(-) secretion in guinea pig distal colon at epithelial levels.

Ca(2+)-dependent and -independent Cl(-) secretion stimulated by the nitric oxide donor, GEA 3162, in rat colonic epithelium

The lipophilic nitric oxide-liberating drug, 1,2,3,4-oxatriazolium,5-amino-3-(3,4-dichlorophenyl)-chloride (GEA 3162), concentration-dependently induced a Cl(-) secretion in rat colon. At a low concentration (5 x 10(-5) M), the action was Ca(2+)-dependent, whereas at a high concentration (5 x 10(-4) M), the response was independent from extracellular Ca(2+). Fura-2 experiments at isolated colonic crypts revealed that GEA 3162 induced an increase of the cytoplasmic Ca(2+) concentration due to an influx of extracellular Ca(2+), probably mediated by an activation of a nonselective cation conductance as demonstrated by whole-cell patch-clamp studies. After depolarization of the basolateral membrane, GEA 3162 (5 x 10(-4) M) stimulated a current, which was suppressed by glibenclamide but was resistant against blockade of protein kinases by staurosporine, suggesting an activation of apical Cl(-) channels directly by the nitric oxide (NO) donor. After permeabilizing the apical membrane with the ionophore, nystatin, GEA 3162 (5 x 10(-4) M) activated basolateral K(+) conductances and the Na(+)-K(+)-ATPase. Thus, the lipophilic NO donor GEA 3162 stimulates a Cl(-) secretion in a Ca(2+)-dependent and -independent manner.

Methods for the study of ionic currents and Ca2+-signals in isolated colonic crypts

Isolated epithelial cells from intestinal mucosae are a suitable object for the study of the regulation of ion transport in the gut. This regulation possesses a great importance for human and veterinary medicine, as diarrheal diseases, which often are caused by an inadequate activation of intestinal anion secretion, are one of the major lethal diseases of children or young animals. The aim of this paper is to describe a method for the isolation of intact colonic crypts, e.g. for the subsequent investigation of the regulation of anion secretion by the intracellular second messenger, Ca(2+) using electrophysiological and imaging techniques.

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.

The flavonol quercetin activates basolateral K(+) channels in rat distal colon epithelium

1. The flavonol quercetin has been shown to activate a Cl(-) secretion in rat colon. Unlike the secretory activity of the related isoflavone genistein, quercetin's secretory activity does not depend on cyclic AMP; instead, it depends on Ca(2+). We investigated the possible involvement of Ca(2+) dependent basolateral K(+) channels using apically permeabilized rat distal colon epithelium mounted in Ussing chambers. 2. In intact epithelium, quercetin induced an increase in short-circuit current (I(sc)), which was diminished by the Cl(-) channel blockers NPPB and DPC, but not by glibenclamide, DIDS or anthracene-9-carboxylic acid. The effect of the flavonol was also inhibited by several serosally applied K(+) channel blockers (Ba(2+), quinine, clotrimazole, tetrapentylammonium, 293B), whereas other K(+) channel blockers failed to influence the quercetin-induced increase in I(sc) (tetraethylammonium, charybdotoxin). 3. The apical membrane was permeabilized by mucosal addition of nystatin and a serosally directed K(+) gradient was applied. The successful permeabilization was confirmed by experiments demonstrating the failure of bumetanide to inhibit the carbachol-induced current. 4. In apically permeabilized epithelium, quercetin induced a K(+) current (I(K)), which was neither influenced by ouabain nor by bumetanide. Whereas DPC, NPPB, charybdotoxin and 293B failed to inhibit this I(K), quinine, Ba(2+), clotrimazole and tetrapentylammonium were effective blockers of this current. 5. We conclude from these results that at least part of the quercetin-induced Cl(-) secretion can be explained by an activation of basolateral K(+) channels.

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.

Function and spatial distribution of ion channels and transporters in cell migration

Cell migration plays a central role in many physiological and pathophysiological processes, such as embryogenesis, immune defense, wound healing, or the formation of tumor metastases. Detailed models have been developed that describe cytoskeletal mechanisms of cell migration. However, evidence is emerging that ion channels and transporters also play an important role in cell migration. The purpose of this review is to examine the function and subcellular distribution of ion channels and transporters in cell migration. Topics covered will be a brief overview of cytoskeletal mechanisms of migration, the role of ion channels and transporters involved in cell migration, and ways by which a polarized distribution of ion channels and transporters can be achieved in migrating cells. Moreover, a model is proposed that combines ion transport with cytoskeletal mechanisms of migration.

Role of CFTR in the colon

In contrast to the airways, the defects in colonic function in cystic fibrosis (CF) patients are closely related to the defect in CFTR. The gastrointestinal phenotype of CF transgenic mice closely resembles the phenotype in CF patients, which clearly indicates the crucial role of CFTR in colonic Cl- secretion and the absence of an effective compensation. In the colon, stimulation of CFTR Cl- channels involves cAMP- or cGMP-dependent phosphorylation. Exocytosis is not involved. Activation of CFTR leads to coactivation of basolateral KVLQT1-type K+ channels and inhibition of luminal Na+ channels (ENaC). In contrast to cultured cells, Ca2+ does not activate luminal Cl- channels in intact enterocytes. It activates basolateral SK4-type K+ channels and luminal K+ channels, which provide additional driving force for Cl- exit. The magnitude of Cl- secretion, however, completely depends on the presence of at least a residual CFTR function in the luminal membrane. These findings have been clearly demonstrated by Ussing chamber experiments in colon epithelium biopsies of CF and normal individuals: Colonic Cl- secretion in CF patients is variable and reflects the genotype; a complete defect of CFTR is paralleled by the absence of Cl- secretion and unmasks Ca(2+)-regulated K+ channels in the luminal membrane; overabsorption of Na+ in CF reflects the absence of ENaC inhibition by CFTR; and the functional status of CF colon can be mimicked by the complete suppression of cAMP stimulation in enterocytes of healthy individuals.

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.

Involvement of calmodulin and protein kinase C in the regulation of K+ transport by carbachol across the rat distal colon

The cholinergic agonist carbachol stimulates the apical H+-K+-ATPase and apical as well as basolateral K+ channels in the rat distal colon. The effect of carbachol was tested in the presence of different inhibitors of the Ca2+ signaling pathway in order to characterize the intracellular mechanisms involved. Both carbachol-stimulated Rb+-efflux as well as carbachol-stimulated mucosal Rb+-uptake were dependent on the presence of serosal Ca2+. The Ca2+-calmodulin antagonist calmidazolium (10(-7) mol l(-1)) inhibited the stimulation of mucosal and serosal Rb+ efflux by carbachol. A similar effect had KN-62 (10(-5) mol l(-1)), an inhibitor of the Ca2+-calmodulin-dependent kinase II, suggesting the regulation of basolateral and apical K+ channels by this kinase. Staurosporine (10(-6) mol l(-1)), which potently inhibits protein kinase C, did not alter the effect of carbachol on Rb+ efflux, although the stimulation of apical Rb+ efflux by carbachol seemed to be less prolonged, indicating that protein kinase C is not involved in the regulation of K+ permeability. In contrast, mucosal Rb+ uptake, which is determined by the ouabain- and vanadate-sensitive K+ transport via the apical H+-K+-ATPase, was decreased to nearly one third of control values in the presence of calmidazolium. Both calmidazolium and staurosporine, but not KN-62, prevented the stimulatory action of carbachol on the H+-K+-ATPase, suggesting a synergistic control of this ion pump by both Ca2+-calmodulin and protein kinase C.

Quantification and distribution of Ca(2+)-activated maxi K(+) channels in rabbit distal colon

The Ca(2+)-activated maxi K(+) channel is an abundant channel type in the distal colon epithelium, but nothing is known regarding the actual number and precise localization of these channels. The aim of this study has therefore been to quantify the maxi K(+) channels in colon epithelium by binding of iberiotoxin (IbTX), a selective peptidyl ligand for maxi K(+) channels. In isotope flux measurements 75% of the total K(+) channel activity in plasma membranes from distal colon epithelium is inhibited by IbTX (K(0.5) = 4.5 pM), indicating that the maxi K(+) channel is the predominant channel type in this epithelium. Consistent with the functional studies, the radiolabeled double mutant (125)I-IbTX-D19Y/Y36F binds to the colon epithelium membranes with an equilibrium dissociation constant of approximately 10 pM. The maximum receptor concentration values (in fmol/mg protein) for (125)I-IbTX-D19Y/Y36F binding to colon epithelium are 78 for surface membranes and 8 for crypt membranes, suggesting that the maxi K(+) channels are predominantly expressed in the Na(+)-absorbing surface cells, as compared with the Cl(-)-secreting crypt cells. However, aldosterone stimulation of this tissue induced by a low-Na(+) diet does not change the total number of maxi K(+) channels.

The regulation of epithelial cell cAMP- and calcium-dependent chloride channels

This chapter has focused on two types of chloride conductance found in epithelial cells. The leap from the Ussing chamber to patch-clamp studies has identified yet other conductances present which have also been electrophysiologically characterized. In the case of the swelling activated wholecell chloride current, a physiological function is apparent and a single-channel basis found, but its genetic identity remains unknown (see reviews by Frizzell and Morris, 1994; and Strange et al., 1996). The outwardly rectified chloride channel has been the subject of considerable electrophysiological interest over the past 10 years and is well characterized at the single-channel level, but its physiological function remains controversial (reviewed by Frizzell and Morris, 1994; Devidas and Guggino, 1997). Yet other conductances related to the CLC gene family also appear to be present in epithelial cells of the kidney (reviewed by Jentsch, 1996; Jentsch and Gunter, 1997) where physiological functions for some isoforms are emerging. Clearly, there remain many unknowns. Chief among these is the molecular basis of GCa2+Cl and many of other the conductances. As sequences become available it is expected that the wealth of information gained by investigation into CFTR function will provide a conceptual blueprint for similar studies in these later channel clones.

Thromboxane A2 receptor linked with the Ca2+ pathway in rat colonic crypt cells

To clarify the presence of thromboxane A2 (TXA2) receptor in the colonic epithelium, we examined the effect of 9,11-epithio-11, 12-methano-thromboxane A2 (STA2), a stable analogue of TXA2, on intracellular free Ca2+ concentration ([Ca2+]i) of indo-1-loaded single cells in isolated rat colonic crypts by laser confocal microscopy. STA2 increased [Ca2+]i in a concentration-dependent manner with a transient peak phase and a subsequent plateau phase. The EC50 values at peak and plateau phases were 1 and 32 nM, respectively. The STA2-induced increase in [Ca2+]i was completely blocked by two selective TXA2 receptor antagonists, KW-3635 and ONO-3708. These antagonists did not affect both the basal [Ca2+]i and the carbaco-induced increase in [Ca2+]i. Prostaglandin E2 did not increase [Ca2+]i. These results indicate that the STA2-elicited increase in [Ca2+]i is mediated specifically by a TXA2 receptor in colonic crypt cells This is the first report showing the presence of a TXA2 receptor that is associated with Ca2+ mobilization in the colon.

Modulation of chloride, potassium and bicarbonate transport by muscarinic receptors in a human adenocarcinoma cell line

1. Short-circuit current (I(SC)) responses to carbachol (CCh) were investigated in Colony 1 epithelia, a subpopulation of the HCA-7 adenocarcinoma cell line. In Krebs-Henseleit (KH) buffer, CCh responses consisted of three I(SC) components: an unusual rapid decrease (the 10 s spike) followed by an upward spike at 30 s and a slower transient increase (the 2 min peak). This response was not potentiated by forskolin; rather, CCh inhibited cyclic AMP-stimulated I(SC). 2. In HCO3- free buffer, the decrease in forskolin-elevated I(SC) after CCh was reduced, although the interactions between CCh and forskolin remained at best additive rather than synergistic. When Cl- anions were replaced by gluconate, both Ca2+- and cyclic AMP-mediated electrogenic responses were significantly inhibited. 3. Basolateral Ba2+ (1-10 mM) and 293B (10 microM) selectively inhibited forskolin stimulation of I(SC), without altering the effects of CCh. Under Ba2+- or 293B-treated conditions, CCh responses were potentiated by pretreatment with forskolin. 4. Basolateral charybdotoxin (50 nM) significantly increased the size of the 10 s spike of CCh responses in both KH and HCO3- free medium, without affecting the 2 min peak. The enhanced 10 s spike was inhibited by prior addition of 5 mM apical Ba2+. Charybdotoxin did not affect forskolin responses. 5. In epithelial layers prestimulated with forskolin, the muscarinic antagonists atropine and 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, both at 100 nM) abolished subsequent 10 microM CCh responses. Following addition of p-fluoro hexahydro-sila-difenidol (pF-HHSiD, 10 microM) or pirenzepine (1 microM), qualitative changes in the CCh response time-profile also indicated a rightward shift of the agonist concentration-response curve; however, 1 microM gallamine had no effect. These results suggest that a single M3-like receptor subtype mediates the secretory response to CCh. 6. It is concluded that CCh and forskolin activate discrete populations of basolateral K+ channels gated by either Ca2+ or cyclic AMP, but that the Cl- permeability of the apical membrane may limit their combined effects on electrogenic Cl- secretion. In addition, CCh activates a Ba2+-sensitive apical K+ conductance leading to electrogenic K+ transport. Both agents may also modulate HCO3- secretion through a mechanism at least partially dependent on carbonic anhydrase.

Mechanisms of carbachol-induced alterations in K+ transport across the rat colon

The effect of carbachol, an agonist of the Ca2+ pathway, on K+ transport in rat proximal and distal colon was studied by measuring unidirectional fluxes, uptake, and efflux of Rb+, a marker for K+, in mucosa-submucosa preparations. Unidirectional ion flux measurements revealed that carbachol stimulated K+ secretion in the proximal colon by a marked increase in the serosa-to-mucosa flux (J(Rb)sm) and a more moderate rise in the mucosa-to-serosa flux (J(Rb)ms). In the distal colon carbachol had no effect on J(Rb)ms but J(Rb)sm was reduced after a transient increase finally resulting in an inhibition of K+ secretion. Carbachol caused a stimulation of mucosal Rb+ uptake in the distal colon, which was diminished in the presence of inhibitors of the apical H+-K+-ATPase, vanadate and ouabain. In contrast, in the proximal colon the serosal Rb+ uptake was enhanced by carbachol, an effect, which could be prevented by bumetanide, an inhibitor of the basolateral Na+-K+-2Cl(-)-cotransporter. Efflux experiments revealed that carbachol caused a transient increase of apical and basolateral Rb+ permeability in both colonic segments. In the distal colon, stimulated K+ efflux to the serosal side was reduced by quinine, efflux to the mucosal side was blocked by tetraethylammonium. In the proximal colon, carbachol-activated apical and basolateral K+ efflux were inhibited by Ba2+. In conclusion, these data suggest that in the distal colon carbachol stimulates the H+-K+-ATPase and the basolateral K+ efflux through quinine-sensitive K+ channels, whereas in the proximal colon carbachol induces K+ secretion due to a stimulation of the basolateral Na+-K+-2Cl(-)-cotransporter and an increased efflux to the luminal side via Ba2+-sensitive apical K+ channels.

Cholinergic ion secretion in human colon requires coactivation by cAMP

Cl- secretion in the colon can be activated by an increase of either intracellular Ca2+ or cAMP. In this study we examined a possible interdependence of the two second-messenger pathways in human colonic epithelium. When measured in a modified Ussing chamber, carbachol (CCH; 100 micromol/l, basolateral), via an increase in cytosolic Ca2+ concentration ([Ca2+]i), activated a transient lumen-negative equivalent short-circuit current (Isc) [change (Delta) in Isc = -79.4 +/- 7.5 microA/cm2]. Previous studies indicated that intracellular Ca2+ directly acts on basolateral K+ channels, thus enhancing driving force for luminal Cl- exit. Increased intracellular cAMP (by basolateral addition of 100 micromol/l IBMX and 1 micromol/l forskolin) activated a sustained lumen-negative current (DeltaIsc = -42.4 +/- 7.2 microA/cm2) that was inhibited by basolateral trans-6-cyano-4-(N-ethylsulfonyl-N-methylamino)-3-hydroxy-2, 2-dimethyl&2-chromane (10 micromol/l), a blocker of KvLQT1 channels. In the presence of elevated cAMP, the CCH-activated currents were augmented (DeltaIsc = 167.7 +/- 32.7 microA/cm2), suggesting cooperativity of the Ca2+- and cAMP-mediated responses. Inhibition of endogenous cAMP production by indomethacin (10 micromol/l) significantly reduced CCH-activated currents and even reversed the polarity in 70% of the experiments. The transient lumen-positive Isc was probably due to activation of apical K+ channels because it was blocked by luminal Ba2+ (5 mmol/l) and tetraethylammonium (10 mmol/l). In the presence of indomethacin (10 micromol/l, basolateral), an increase of cAMP activated a sustained negative Isc. Under these conditions, CCH induced a large further increase in lumen-negative Isc (DeltaIsc = -100.0 +/- 21.0 microA/cm2). We conclude that CCH acting via [Ca2+]i can induce Cl- secretion only in the presence of cAMP, i.e., when luminal Cl- channels are already activated. The activation of a luminal and basolateral K+ conductance by CCH may be essential for transepithelial KCl secretion in human colon.

K+ and Cl- conductances in the distal colon of the rat

1. K+ and Cl- conductances and their putative regulation have been characterized in the rat colonic epithelium by Ussing-chamber experiments, whole-cell and single-channel patch-clamp recordings. 2. The apical Cl- conductance is under the control of intracellular cAMP. An increase in the concentration of this second messenger induces transepithelial Cl- secretion due to the activation of an apical 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB)- and glibenclamide-sensitive Cl- conductance. 3. In addition to the apical Cl- conductance, the basolateral membrane is equipped with Cl- channels. They are stimulated by cell swelling and play a role in cell volume regulation and transepithelial Cl- absorption. 4. The basolateral K+ conductance is under the dominant control of intracellular Ca2+. An increase in the cytosolic Ca2+ concentration leads to the opening of basolateral K+ channels, which causes a hyperpolarization of the cell membrane, indirectly supporting Cl- secretion owing to an increase in the driving force for Cl- exit. The predominant effect of cAMP on the basolateral K+ conductance is an inhibitory one, probably due to a decrease in the intracellular Ca2+ concentration. 5. The apical K+ conductance, which is involved in transepithelial K+ secretion, is stimulated by an increase in the intracellular Ca2+ concentration. 6. The differential regulation of apical and basolateral ion conductances in the epithelium of the rat distal colon provides an interesting example for the mechanisms underlying vectorial transport of ions across polarized cells.

Inhibition of a K+ conductance by the phosphatase inhibitor calyculin A in rat distal colon

Basal membrane permeability of epithelial cells from the lower third and the middle of rat colonic crypts is dominated by a K+ conductance as shown by ion replacement experiments. Calyculin A, an inhibitor of protein phosphatases, induced a depolarization of these cells. The depolarization was concomitant with an inhibition of membrane current. The current inhibited by calyculin A had a reversal potential identical with the theoretical K+ equilibrium potential indicating that the drug inhibits a basal K+ conductance. The efficiency of calyculin A was comparable with that of other well-known K+ channel blockers such as Ba2+, tetraethylammonium or quinine. In the intact tissue, calyculin A exerted an inhibitory action on forskolin-induced anion secretion, an effect which may be explained by the decrease in the driving force for Cl- exit after inhibition of cellular K+ conductance. Together with previous results, these data suggest an inhibition of epithelial K+ conductance by phosphorylation.

The antifungal antibiotic, clotrimazole, inhibits chloride secretion by human intestinal T84 cells via blockade of distinct basolateral K+ conductances. Demonstration of efficacy in intact rabbit colon and in an in vivo mouse model of cholera

The antifungal antibiotic clotrimazole (CLT) blocks directly and with high potency the Ca2+-activated K+ channels of human erythrocytes, erythroleukemia cells, and ferret vascular smooth muscle cells. We recently reported that CLT inhibits Cl- secretion in human intestinal T84 cells, likely by affecting K+ transport (Rufo, P.A., L. Jiang, S.J. Moe, C. Brugnara, S.L. Alper, and W.I. Lencer. 1996. J. Clin. Invest. 98:2066-2075). To determine if CLT had direct effects on K+ conductances in T84 cells, we selectively permeabilized apical membranes of confluent T84 cell monolayers using the ionophore amphotericin B. This technique permits direct measurement of basolateral K+ transport. We found that CLT and a stable des-imidazolyl derivative inhibited directly two pharmacologically distinct basolateral membrane K+conductances, but had no effect on apical membrane Cl- conductances. The effects of CLT on Cl- secretion were also examined in intact tissue. CLT inhibited forskolin-induced Cl- secretion in rabbit colonic mucosal sheets mounted in Ussing chambers by 91%. CLT also inhibited cholera toxin-induced intestinal Cl- secretion in intact mice by 94%. These data provide direct evidence that CLT blocks Cl- secretion in intestinal T84 cells by inhibition of basolateral K+ conductances, and show that CLT inhibits salt and water secretion from intact tissue in vitro and in vivo. The results further support the suggestion that CLT and its metabolites may show clinical efficacy in the treatment of secretory diarrheas of diverse etiologies.