A conserved pore-lining glutamate as a voltage- and chloride-dependent gate in the ClC-2 chloride channel

ClC-2 is a ubiquitously expressed, two-pore homodimeric Cl- channel opened by hyperpolarisation. Little is known about its gating mechanisms. Crystallographic and functional studies in other ClC channels suggest that a conserved glutamate residue carboxylate side-chain can close protopores by interacting with a Cl--binding site in the pore. Competition for this site is thought to provide the molecular basis for gating by extracellular Cl-. We now show that ClC-2 gating depends upon intra- but not extracellular Cl- and that neutralisation of E217, the homologous pore glutamate, leads to loss of sensitivity to intracellular Cl- and voltage. Experiments testing for transient activation by extracellular protons demonstrate that E217 is not available for protonation in the closed channel state but becomes so after opening by hyperpolarisation. The results suggest that E217 is a hyperpolarisation-dependent protopore gate in ClC-2 and that access of intracellular Cl- to a site normally occupied by its side-chain in the pore stabilises the open state. A remaining hyperpolarisation-dependent gate might correspond to that closing both pores simultaneously in other ClC channels.

Extracellular conserved cysteine forms an intersubunit disulphide bridge in the KCNK5 (TASK-2) K+ channel without having an essential effect upon activity

The functional channel unit of K(+) channels with two pore regions in tandem is thought to be a homodimer and it has been suggested that this dimeric structure occurs by interaction of an extracellular domain, the self-interacting domain. Interaction and functional assembly have been studied in some detail for KCNK1. It is proposed that a disulphide bond between highly conserved C69 residues of the self-interacting domain is formed which is essential for channel activity. We mutated C51, the equivalent residue in the pH-dependent KCNK5, to study its effect on channel function. Western analysis of proteins from cells expressing epitope-tagged KCNK5 and KCNK5-C51S was consistent with reduction-sensitive self-association of monomers dependent upon the presence of C51. Patch-clamp analysis of heterologously expressed KCNK5-C51S, however, revealed it was functional and indistinguishable in rectification properties and pH dependence from the non-mutated channel. The same result was found with KCNK5-C115S. It is concluded that the proposed disulphide bond between cysteine 51 residues of KCNK5 subunits does occur and preserves a dimeric structure in the detergent solubilized complex. Functional assays, on the other hand, suggest that such a disulphide bridge is not essential for correct functional expression.

Effect of an N-terminus deletion on voltage-dependent gating of the ClC-2 chloride channel

ClC-2, a chloride channel widely expressed in mammalian tissues, is activated by hyperpolarisation and extracellular acidification. Deletion of amino acids 16-61 in rat ClC-2 abolishes voltage and pH dependence in two-electrode voltage-clamp experiments in amphibian oocytes. These results have been interpreted in terms of a ball-and-chain type of mechanism in which the N-terminus would behave as a ball that is removed from an inactivating site upon hyperpolarisation. We now report whole-cell patch-clamp measurements in mammalian cells showing hyperpolarization-activation of rClC-2Delta16-61 differing only in presenting faster opening and closing kinetics than rClC-2. The lack of time and voltage dependence observed previously was reproduced, however, in nystatin-perforated patch experiments. The behaviour of wild-type rClC-2 did not differ between conventional and nystatin-perforated patches. Similar results were obtained with ClC-2 from guinea-pig. One possible explanation of the results is that some diffusible component is able to lock the channel in an open state but does so only to the mutated channel. Alternative explanations involving the osmotic state of the cell and cytoskeleton structure are also considered. Low extracellular pH activates the wild-type channel but not rClC-2Delta16-61 when expressed in oocytes, a result that had been interpreted to suggest that protons affect the ball-and-chain mechanism. In our experiments no difference was seen in the effect of extracellular pH upon rClC-2 and rClC-2Delta16-61 in either recording configuration, suggesting that protons act independently from possible effects of the N-terminus on gating. Our observations of voltage-dependent gating of the N-terminal deleted ClC-2 are an argument against a ball-and-chain mechanism for this channel.

ClC-2 in guinea pig colon: mRNA, immunolabeling, and functional evidence for surface epithelium localization

The principal function of the colon in fluid homeostasis is the absorption of NaCl and water. Apical membrane Na(+) channels, Na(+)/H(+) and Cl(-)/HCO exchangers, have all been postulated to mediate NaCl entry into colonocytes. The identity of the basolateral exit pathway for Cl(-) is unknown. We have previously demonstrated the presence of the ClC-2 transcript in the guinea pig intestine. Now we explore in more detail, the tissue and cellular distribution of chloride channel ClC-2 in the distal colon by in situ hybridization and immunohistochemistry. The patch-clamp technique was used to characterize Cl(-) currents in isolated surface epithelial cells from guinea pig distal colon and these were compared with those mediated by recombinant guinea pig (gp)ClC-2. ClC-2 mRNA and protein were found in the surface epithelium of the distal colon. Immunolocalization revealed that, in addition to some intracellular labeling, ClC-2 was present in the basolateral membranes but absent from the apical pole of colonocytes. Isolated surface epithelial cells exhibited hyperpolarization-activated chloride currents showing a Cl(-) > I(-) permeability and Cd(2+) sensitivity. These characteristics, as well as some details of the kinetics of activation and deactivation, were very similar to those of recombinant gpClC-2 measured in parallel experiments. The presence of active ClC-2 type currents in surface colonic epithelium, coupled to a basolateral location for ClC-2 in the distal colon, suggests a role for ClC-2 channel in mediating basolateral membrane exit of Cl(-) as an essential step in a NaCl absorption process.

Distribution of aquaporins in the colon of Octodon degus, a South American desert rodent

Octodon degus is a desert rodent of northern Chile, adapted to survive with a limited supply of water. This rodent has a high degree of fecal dehydration, related to colon water absorption. With the hypothesis that aquaporins (AQPs) might be present in the colon epithelium of O. degus and involved in fluid absorption, we studied colon water absorption in vivo and the distribution of AQPs and Na(+) transporters by immunocytochemistry. AQP-1 was found in apical and basolateral membranes of surface-absorptive and crypt epithelial cells. AQP-8 was found in the cytoplasm of enterocytes of surface colon. AQP-3 immunolabeling, on the other hand, was absent from the epithelium but present in a subepithelial fibroblast layer, pericryptal cells, and muscularis mucosae. The hydration state did not modify the amount of immunostaining for any of the AQPs. Colon water absorption was markedly decreased by the mercurial agent p-chloromercuribenzenesulfonic acid and was not affected by water deprivation. The NHE3 isoform of Na(+)/H(+) exchanger and alpha-1 subunit of the Na(+)-K(+)-ATPase were found in apical and basolateral membranes of surface-absorptive cells, respectively. These results suggest that colon water absorption is mostly transcellular and mediated by water channels like AQP-1. Apical Na(+)/H(+) exchanger and basolateral Na(+)-K(+)-ATPase in surface cells could be part of the Na(+) absorption pathway. It is hypothesized that this transport is necessary to provide an osmotic gradient for water absorption. The roles of AQP-8 and AQP-3 in water absorption remain to be established.

A voltage-independent K+ conductance activated by cell swelling in Ehrlich cells is modulated by a G-protein-mediated process

Cell swelling following hypoosmotic stress leads to the activation of volume-sensitive ion channels that allow a K+ and Cl- efflux accompanied by water loss. A Ca2+-insensitive K+ channel (I(K,vol)) has been described in Ehrlich cells that can be activated by hypotonicity and leukotriene D4 and is inhibited by clofilium. We have studied the activation and deactivation by osmotic stimuli of this channel. A G-protein appears to be involved in these processes since GTP-gamma-S accelerates deactivation, while GDP-beta-S blocks the channel in the open state, a result mimicked by pertussis toxin (PTX). In addition, PTX accelerates the onset of I(K,vol). We propose that I(K,vol) is tonically inhibited by a PTX-sensitive G-protein.

Modulation of the two-pore domain acid-sensitive K+ channel TASK-2 (KCNK5) by changes in cell volume

The molecular identity of K(+) channels involved in Ehrlich cell volume regulation is unknown. A background K(+) conductance is activated by cell swelling and is also modulated by extracellular pH. These characteristics are most similar to those of newly emerging TASK (TWIK-related acid-sensitive K(+) channels)-type of two pore-domain K(+) channels. mTASK-2, but not TASK-1 or -3, is present in Ehrlich cells and mouse kidney tissue from where the full coding sequences were obtained. Heterologous expression of mTASK-2 cDNA in HEK-293 cells generated K(+) currents in the absence intracellular Ca(2+). Exposure to hypotonicity enhanced mTASK-2 currents and osmotic cell shrinkage led to inhibition. This occurred without altering voltage dependence and with only slight decrease in pK(a) in hypotonicity but no change in hypertonicity. Replacement with other cations yields a permselectivity sequence for mTASK-2 of K(+) > Rb(+) Cs(+) > NH(4)(+) > Na(+) congruent with Li(+), similar to that for the native conductance (I(K, vol)). Clofilium, a quaternary ammonium blocker of I(K, vol), blocked the mTASK-2-mediated K(+) current with an IC(50) of 25 microm. The presence of mTASK-2 in Ehrlich cells, its functional similarities with I(K, vol), and its modulation by changes in cell volume suggest that this two-pore domain K(+) channel participates in the regulatory volume decrease phenomenon.

K+ conductance activated during regulatory volume decrease. The channels in Ehrlich cells and their possible molecular counterpart

K+ currents activated by hypotonic cell swelling have been studied in Ehrlich ascites tumour cells by the whole-cell recording mode of the patch-clamp technique. K+ together with Cl- currents developed in the absence of added intracellular Ca2+ and with strong buffering of internal Ca2+ in experiments conducted at 37 degrees C. Manipulation of the extracellular medium with other cations suggests a selectivity sequence of K+ > Rb+ > NH4+ > or = Na+ approximately equals Li+ approximately equals Cs+. The current-voltage relationship of the volume-sensitive K+ current was well fitted with the Goldman-Hodgkin-Katz current equation between -130 and 20 mV at both physiological and high K+ extracellular solutions. The class III antiarrhytmic drug clofilium blocked the volume-sensitive K+ current in a voltage-independent manner. Clofilium was also found to be a strong inhibitor of the regulatory volume decrease (RVD) response of Ehrlich cells. The leukotriene D4 (LTD4) can activate the same current in isotonicity, consistent with a role for this compound in the signalling process of volume regulation. It is suggested that K+ channels activated by cell swelling belong to the so-called background K+ channel group. These are voltage-independent channels which underlie the resting potential of many cells and have recently been identified as belonging to a family of K+ channels with two pore domains in tandem (2P-4TM). Preliminary experiments show the presence of the TASK-2 channel, a member of the 2P-4TM family inhibited by acid extracellular pH, in Ehrlich cells and suggest that it might underlie the swelling-induced K+ current.

Aquaporin-2, a regulated water channel, is expressed in apical membranes of rat distal colon epithelium

Aquaporin-2 (AQP-2) is the vasopressin-regulated water channel expressed in the apical membrane of principal cells in the collecting duct and is involved in the urinary concentrating mechanism. In the rat distal colon, vasopressin stimulates water absorption through an unknown mechanism. With the hypothesis that AQP-2 could contribute to this vasopressin effect, we studied its presence in rat colonic epithelium. We used RT-PCR, in situ hybridization, immunoblotting, and immunocytochemistry to probe for AQP-2 expression. An AQP-2 amplicon was obtained through RT-PCR of colon epithelium RNA, and in situ hybridization revealed AQP-2 mRNA in colonic crypts and, to a lesser extent, in surface absorptive epithelial cells. AQP-2 protein was localized to the apical membrane of surface absorptive epithelial cells, where it colocalized with H(+)-K(+)-ATPase but not with Na(+)-K(+)-ATPase. AQP-2 was absent from the small intestine, stomach, and liver. Water deprivation increased the hybridization signal and the protein level (assessed by Western blot analysis) for AQP-2 in distal colon. This was accompanied by increased p-chloromercuriphenylsulfonic acid-sensitive water absorption. These results indicate that AQP-2 is present in the rat distal colon, where it might be involved in a water-sparing mechanism. In addition, these results support the idea that AQP-2, and probably other aquaporins, are involved in water absorption in the colon.

Cloning, cellular distribution and functional expression of small intestinal epithelium guinea pig ClC-5 chloride channel

We report the cloning of a guinea pig ClC-5 chloride channel (gpClC-5) from distal small intestinal epithelial cells by RT-PCR and RACE. The transcript is shown to be present in duodenum, jejunum and ileum epithelium by RT-PCR and Northern analysis. This is confirmed by in situ hybridisation which also shows the transcript to be homogeneously distributed in the crypt and villus regions. Expression of gpClC-5 in HEK-293 cells generated markedly outwardly rectified chloride currents with a perm-selectivity sequence of NO(3)(-)>Cl(-)>Br(-)>I(-)>F(-)>gluconate(-). The possible role of gpClC-5 in this epithelial location is discussed.

Splice variants of a ClC-2 chloride channel with differing functional characteristics

We identified two ClC-2 clones in a guinea pig intestinal epithelial cDNA library, one of which carries a 30-bp deletion in the NH(2) terminus. PCR using primers encompassing the deletion gave two products that furthermore were amplified with specific primers confirming their authenticity. The corresponding genomic DNA sequence gave a structure of three exons and two introns. An internal donor site occurring within one of the exons accounts for the deletion, consistent with alternative splicing. Expression of the variants gpClC-2 and gpClC-2Delta77-86 in HEK-293 cells generated inwardly rectifying chloride currents with similar activation characteristics. Deactivation, however, occurred with faster kinetics in gpClC-2Delta77-86. Site-directed mutagenesis suggests that a protein kinase C-mediated phosphorylation consensus site lost in gpClC-2Delta77-86 is not responsible for the observed change. The deletion-carrying variant is found in most tissues examined, and it appears more abundant in proximal colon, kidney, and testis. The presence of a splice variant of ClC-2 modified in its NH(2)-terminal domain could have functional consequences in tissues where their relative expression levels are different.

K+ currents activated by leukotriene D4 or osmotic swelling in Ehrlich ascites tumour cells

K+ and Cl- currents activated by hypoosmotic cell swelling (IK,vol and Icl,vol) or after addition of leukotriene D4 (LTD4) to cells in isotonic medium were studied in Ehrlich ascites tumour cells. IK,vol and Icl,vol were not affected by strong buffering of intracellular Ca2+ or by additional removal of extracellular Ca2+. In isotonic media, 5 nmol/l LTD4 activated large K+ but not Cl- currents. The LTD4-activated IK was, as has been shown previously for IK,vol, insensitive to charybdotoxin (ChTX) but was blocked by the antiarrhythmic drug clofilium. The current/voltage (I/V) relation for the LTD4-activated IK was, as recently demonstrated for IK,vol, well fitted by the Goldman-Hodgkin-Katz current equation between -130 mV and 30 mV in both physiological and K+-rich extracellular solutions. LTD4 had no additional effect on the magnitude of IK in Ehrlich cells already activated by the hypoosmotic stimulus. Nevertheless, the onset time for IK after hypoosmotic cell swelling was significantly less in the presence of LTD4. The similar I/V relation, pharmacological sensitivity and lack of additivity suggest that hypoosmotic swelling and addition of LTD4 activate the same K+ channels in Ehrlich cells. The influence of [Ca2+]i appears, however, to differ between IK,vol and the IK activated by LTD4 in that the latter was reduced significantly by strong buffering of [Ca2+]i. This might reflect the involvement of some additional factor in the hypoosmotic activation of K+ channels besides the stimulation mediated by LTD4.

Characterisation of a cell swelling-activated K+-selective conductance of ehrlich mouse ascites tumour cells

The K+ and Cl- currents activated by hypotonic cell swelling were studied in Ehrlich ascites tumour cells using the whole-cell recording mode of the patch-clamp technique. Currents were measured in the absence of added intracellular Ca2+ and with strong buffering of Ca2+. K+ current activated by cell swelling was measured as outward current at the Cl- equilibrium potential (ECl) under quasi-physiological gradients. It could be abolished by replacing extracellular Na+ with K+, thereby cancelling the driving force. Replacement with other cations suggested a selectivity sequence of K+ > Rb+ > NH4 approximately Na+ approximately Li+; Cs+ appeared to be inhibitory. The current-voltage relationship of the volume-sensitive K+ current was well fitted with the Goldman-Hodgkin-Katz current equation between -130 and +20 mV with a permeability coefficient of around 10(-6) cm s(-1) with both physiological and high-K+ extracellular solutions. The class III antiarrhythmic drug clofilium blocked the volume-sensitive K+ current in a voltage-independent manner with an IC50 of 32 microM. Clofilium was also found to be a strong inhibitor of the regulatory volume decrease response of Ehrlich cells. Cell swelling-activated K+ currents of Ehrlich cells are voltage and calcium insensitive and are resistant to a range of K+ channel inhibitors. These characteristics are similar to those of the so-called background K+ channels. Noise analysis of whole-cell current was consistent with a unitary conductance of 5.5 pS for the single channels underlying the K+ current evoked by cell swelling, measured at 0 mV under a quasi-physiological K+ gradient.

Effect of the inactivating "ball" peptide of Shaker B on intermediate conductance Ca(2+)-dependent inwardly rectifying K+ channels of HeLa cells

The patch-clamp technique was used to study the effect of intracellularly added inactivating "ball" peptide (BP) of the Shaker B K+ channel upon Ca(2+)-dependent inwardly rectifying K+ channels of the intermediate conductance type expressed in HeLa cells. Intracellular BP caused only moderate inhibition of outward K+ currents when assayed at an intracellular Ca2+ concentration of 100 nmol/l. Increasing intracellular Ca2+ levels led in itself to some voltage-dependent blockade of K+ currents, which was absent when high extracellular K+ was used. An additional strong blockade by intracellular BP was nevertheless observed both in Na(+)- and K(+)-rich extracellular solutions. A non-inactivating BP analogue had no effect. At this higher intracellular Ca2+ concentration the inhibition of these intermediate conductance Ca(2+)-dependent channels by BP was voltage-dependent, being absent at hyperpolarizing potentials, and could be relieved by increasing extracellular K+. These data suggest that BP acts at an internal pore site in Ca(2+)-dependent intermediate conductance K+ channels of HeLa cells, and that these might possess a receptor site for the peptide similar to that of other K+ channels such as Ca(2+)-activated maxi-K+ channels.

Separate taurine and chloride efflux pathways activated during regulatory volume decrease

Organic osmolyte and halide permeability pathways activated in epithelial HeLa cells by cell swelling were studied by radiotracer efflux techniques and single-cell volume measurements. The replacement of extracellular Cl- by anions that are more permeant through the volume-activated Cl- channel, as indicated by electrophysiological measurements, significantly decreased taurine efflux. In the presence of less-permeant anions, an increase in taurine efflux was observed. Simultaneous measurement of the 125I, used as a tracer for Cl-, and [3H]taurine efflux showed that the time courses for the two effluxes differed. In Cl--rich medium the increase in I- efflux was transient, whereas that for taurine was sustained. Osmosensitive Cl- conductance, assessed by measuring changes in cell volume, increased rapidly after hypotonic shock. The influx of taurine was able to counteract Cl- conductance-dependent cell shrinkage but only approximately 4 min after triggering cell swelling. This taurine-induced effect was blocked by DIDS. Differences in anion sensitivity, the time course of activation, and sensitivity to DIDS suggest that the main cell swelling-activated permeability pathways for taurine and Cl- are separate.

Modulation by extracellular and intracellular iodide of volume-activated Cl- current in HeLa cells

The patch-clamp technique was used to study the effect of extracellular and intracellular iodide on the properties of the volume-activated anion current in HeLa cells. Upon hypotonic challenge, HeLa cells responded by activating an outwardly rectifying Cl- current. Replacement of extracellular Cl- by I-, a more permeable anion, increased the peak outward and inward current, reduced the magnitude of deactivation observed at depolarized potentials and shifted the half-maximal (V0.5) deactivation voltage towards more positive values. On the other hand, when internal Cl- was replaced by I- the volume-activated current was not observed in normal, Cl--rich hypotonic extracellular solution. However, switching to a hypotonic extracellular solution containing a mixture of Cl- and I- resulted in the activation of the volume-sensitive current. Furthermore, once the current was activated, I- could be excluded from the external solution without significantly affecting the current properties. These results suggest that the permeant anion plays a crucial role in the gating mechanism of the volume-activated Cl- current, influencing the swelling-dependent activation and the voltage-dependent deactivation processes.

Swelling-activated potassium currents of Ehrlich ascites tumour cells

The K+ and Cl- currents activated by Ca2+-ionophore treatment or by hypotonic cell swelling have been studied in Ehrlich ascites tumour cells by the patch-clamp technique. A charybdotoxin-inhibitable K+ current was activated by increasing intracellular Ca2+ concentration. In contrast, the K+ current activated by cell swelling was insensitive to charybdotoxin as well as to apamin, suggesting that channels different from those sensitive to Ca2+ are responsible for regulatory volume adjustments in these cells. The magnitude of the K+ and Cl- currents activated by hypotonic challenge was markedly temperature-dependent, possibly reflecting the temperature-dependence of enzymes involved in the intracellular signalling of cell volume regulation.

Outwardly rectifying Cl- channel in guinea pig small intestinal villus enterocytes: effect of inhibitors

Previous studies in enterocytes isolated from the villus region of small intestinal epithelium have identified a macroscopic current carried by Cl-. In this work a single-channel patch-clamp study was carried out in the same cells, and a spontaneously active, outwardly rectifying Cl- channel was identified and proposed to underlie the whole cell current. The channel had conductances of 62 and 19 pS at 80 and -80 mV, respectively, in symmetrical Cl- solutions in excised patches. Similar activity was seen in cell-attached patches, but only outward currents could be discerned in this configuration. The activity of the channel, measured as open probability, was independent of intracellular calcium levels and voltage. The selectivity sequence for different anions was SCN- > I- > Br- > Cl- > F- > (gluconate, glutamate, SO4(2-)). The channel was inhibited by 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), verapamil, and 4-hydroxytamoxifen (but not by tamoxifen), with potencies similar to those observed for Cl- channels previously described in other cells. Inhibition by trinitrophenyladenosine 5'-triphosphate was also observed but only at depolarized potentials. At 50 mV the half-maximal inhibitory concentration was 18 nM. It is proposed that this channel plays a role in transepithelial Cl- transport and certain regulatory Cl- fluxes.

Modulation by extracellular Cl- of volume-activated organic osmolyte and halide permeabilities in HeLa cells

Organic osmolyte and halide permeability pathways activated in epithelial HeLa cells by osmotically induced cell swelling were studied using electrophysiological and radiotracer efflux techniques. On hypotonic challenge, HeLa cells responded by activating an efflux pathway for [3H]taurine and a swelling-induced outwardly rectifying Cl- channel. Removal of extracellular Cl-, or its replacement by a less permeable anion, enhanced taurine efflux and decreased the inward current (Cl- efflux). The effect of Cl- removal on taurine efflux was not a consequence of changes in membrane potential. The degree of deactivation of the Cl- current at depolarized potentials was also Cl- dependent, suggesting that external Cl- is necessary for channel activity. The Cl- channel inhibitors 1,9-dideoxyforskolin, tamoxifen, and 4,4'- diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) inhibited swelling-activated taurine efflux, with DIDS being the most potent, at variance with the sensitivity of the Cl- channel. DIDS effect was dependent on external Cl-; concentrations of DIDS that inhibited 50% of taurine efflux were 0.2 and 4 microM at low and high Cl-, respectively. The results could be interpreted on the basis of separate pathways for swelling-activated taurine efflux and Cl- current differentially affected by Cl-. Alternatively, taurine and Cl- flux might occur through a common channel, with the two solutes interacting within the pore and being affected differentially by Cl- replacement.

Possible thiol group involvement in intracellular pH effect on low-conductance Ca(2+)-dependent K+ channels

We have studied the effect of intracellular pH (pHi) shifts on the activity of Ca(2+)-dependent, inwardly rectifying K+ channels of HeLa cells. Recordings of macroscopic currents in symmetrical 145 mK K+ and internal pH of 7.4 gave moderate inward rectification of the current. At pH 6.4, inward rectification was more marked, whereas it switched to outward rectification at pH 8.2. In excised inside-out membrane patches, similar changes in pHi did not affect the single-channel conductance of the channels underlying the Ca(2+)-dependent K+ currents. At neutral pH, the open state probability (Po) was independent of voltage in the range from -70 to 70 mV. At alkaline pH, Po became voltage dependent, decreasing at negative potentials and increasing with depolarization compared with pH 7.4. These changes accounted for the pH-dependent changes in rectification of the macroscopic current. The possibility that voltage dependence might arise from the ionization of a thiol group was tested by using thiol-directed reagents. The decrease in Po produced by intracellular alkalinization at negative potential was reverted by treatment with N-ethylmaleimide, 5,5'-dithiobis(2-nitrobenzoic acid), and 2,2'-dithiodipyridine. The effect of intracellular alkalinization is speculated to occur through ionization of a cysteine group(s) within the field of the membrane affecting gating.

Ammonium inhibition of Ca(2+)-dependent inwardly rectifying K+ currents in HeLa cells

The patch-clamp technique was used to examine the effect of ammonium on inwardly rectifying Ca(2+)-dependent K+ channels of HeLa cells. Ammonium added extracellularly blocked macroscopic K+ currents with an IC50 of 1.6 mM. Inhibition was fully reversible and voltage-independent and is not likely to be secondary to an increase in intracellular pH as this would have the opposite effect. In these experiments ammonium appeared to have an intracellular site of action. Intracellular ammonium rapidly and reversibly inhibits the activity of single Ca(2+)-dependent K+ channels underlying the macroscopic currents. This compound is often employed to manipulate intracellular pH but its use would not be indicated when full activity of the type of channels studied here needs to be unaltered.

Calcium-activated chloride currents and non-selective cation channels in a novel cystic fibrosis-derived human pancreatic duct cell line

The modulation of ion fluxes across the plasma membrane of epithelial cells is central for fluid secretion and absorption. Their disruption can lead to pathological states. An example is cystic fibrosis (CF), a disease characterized by abnormal functioning of the cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-modulated chloride channel. Here we report the characterization of calcium-activated, DIDS sensitive chloride current and non-selective calcium-activated cation channels in a novel human pancreatic duct cell line (YHV-1) derived from a non-delta F508 mutation CF patient bearing a severe phenotype. Southern blot analysis of the CFTR gene indicates a distinct electrophoretic pattern for the region spanned by exons 15-24, a result presumably related to a mutation which has yet to be identified. In contrast to large calcium-activated chloride currents there were no cAMP-dependent CFTR-type chloride currents. Non-selective cation channels were blocked by intracellular ATP and activated by intracellular calcium and cAMP. We propose the cell line YHV-1 as a suitable model for studying pancreatic ion and fluid secretion alterations in CF.

Impaired cell volume regulation in intestinal crypt epithelia of cystic fibrosis mice

Cystic fibrosis is a disease characterized by abnormalities in the epithelia of the lungs, intestine, salivary and sweat glands, liver, and reproductive systems, often as a result of inadequate hydration of their secretions. The primary defect in cystic fibrosis is the altered activity of a cAMP-activated Cl- channel, the cystic fibrosis transmembrane conductance regulator (CFTR) channel. However, it is not clear how a defect in the CFTR Cl- channel function leads to the observed pathological changes. Although much is known about the structural properties and regulation of the CFTR, little is known of its relationship to cellular functions other than the cAMP-dependent Cl- secretion. Here we report that cell volume regulation after hypotonic challenge is also defective in intestinal crypt epithelial cells isolated from CFTR -/- mutant mice. Moreover, the impairment of the regulatory volume decrease in CFTR -/- crypts appears to be related to the inability of a K+ conductance to provide a pathway for the exit of this cation during the volume adjustments. This provides evidence that the lack of CFTR protein may have additional consequences for the cellular function other than the abnormal cAMP-mediated Cl- secretion.

Activation of an Na+/K+/2Cl- cotransport system by phosphorylation in crypt cells isolated from guinea pig distal colon

BACKGROUND & AIMS

K+ secretion is believed to require the presence of a basolateral Na+/K+/2Cl- cotransporter. The aim of this study was to identify this transport system in epithelial cells from guinea pig colon and to study its possible regulation by phosphorylation.

METHODS

Cells were selectively isolated from crypt or surface epithelium of proximal or distal colon. Radioisotopes were used to measure K+, Na+, or Cl- influx. Bumetanide was used to discriminate for influx mediated by Na+/K+/2Cl- cotransport.

RESULTS

Under basal conditions, no bumetanide-sensitive K+ influx was observed. Pretreatment with the protein-phosphatase inhibitor calyculin A (50% effective concentration, 23 nmol/L) or ionomycin showed a bumetanide-sensitive K+ influx specifically in distal colon crypt cells. Okadaic acid and protein kinases C or A activators did not have effect. Bumetanide-sensitive K+ uptake was abolished by the removal of external Na+ or Cl- and occurred by cotransport in a 1Na+/1K+/2Cl- stoichiometry.

CONCLUSIONS

Evidence is presented for the presence of an Na+/K+/2Cl- cotransporter in crypt cells from distal colon epithelium. The activity of this transporter is proposed to be regulated by a phosphorylation/dephosphorylation cycle, controlled by a type I protein phosphatase. It is possible that this phosphatase(s) is modulated by intracellular Ca2+.