Regions involved in the opening of CIC-2 chloride channel by voltage and cell volume

Nonequilibrium gating and voltage dependence of the ClC-0 Cl- channel

The gating of ClC-0, the voltage-dependent Cl- channel from Torpedo electric organ, is strongly influenced by Cl- ions in the external solution. Raising external Cl- over the range 1-600 mM favors the fast-gating open state and disfavors the slow-gating inactivated state. Analysis of purified single ClC-0 channels reconstituted into planar lipid bilayers was used to identify the role of Cl- ions in the channel's fast voltage-dependent gating process. External, but not internal, Cl- had a major effect on the channel's opening rate constant. The closing rate was more sensitive to internal Cl- than to external Cl-. Both opening and closing rates varied with voltage. A model was derived that postulates (a) that in the channel's closed state, Cl- is accessible to a site located at the outer end of the conduction pore, where it binds in a voltage-independent fashion, (b) that this closed conformation can open, whether liganded by Cl- or not, in a weakly voltage-dependent fashion, (c) that the Cl(-)-liganded closed channel undergoes a conformational change to a different closed state, such that concomitant with this change, Cl- ion moves inward, conferring voltage-dependence to this step, and (d) that this new Cl(-)-liganded closed state opens with a very high rate. According to this picture, Cl- movement within the pre-open channel is the major source of voltage dependence, and charge movement intrinsic to the channel protein contributes very little to voltage-dependent gating of ClC-0. Moreover, since the Cl- activation site is probably located in the ion conduction pathway, the fast gating of ClC-0 is necessarily coupled to ion conduction, a nonequilibrium process.

Two physically distinct pores in the dimeric ClC-0 chloride channel

The Torpedo chloride channel ClC-0 is the prototype of a large family of chloride channels that have roles in transepithelial transport and in regulating electrical excitability and cell volume. ClC-0 opens in bursts with two identical conductance levels of approximately 8pS. Hyperpolarization slowly increases the probability of bursts ('slow gating'), and depolarization increases channel opening within bursts ('fast gating'). Replacing serine 123 by threonine changes rectification, ion selectivity and gating, but retains the typical bursting behaviour with two identical independent albeit reduced, conductance states (approximately 1.5 pS). Coexpression with wild-type ClC-0, either as covalently linked concatamers or as independent proteins, leads to bursting channels with two different pores. Our experiments strongly suggest that conductance, ion selectivity and 'fast' gating are determined only by the single subunit forming a single pore, independent from the attached pore; in contrast, 'slow' gating is a function of both subunits. Thus ClC-0 is a homodimer with two largely independent pores.

P2Y1 purinergic receptors in sensory neurons: contribution to touch-induced impulse generation

Somatic sensation requires the conversion of physical stimuli into the depolarization of distal nerve endings. A single cRNA derived from sensory neurons renders Xenopus laevis oocytes mechanosensitive and is found to encode a P2Y1 purinergic receptor. P2Y1 mRNA is concentrated in large-fiber dorsal root ganglion neurons. In contrast, P2X3 mRNA is localized to small-fiber sensory neurons and produces less mechanosensitivity in oocytes. The frequency of touch-induced action potentials from frog sensory nerve fibers is increased by the presence of P2 receptor agonists at the peripheral nerve ending and is decreased by the presence of P2 antagonists. P2X-selective agents do not have these effects. The release of ATP into the extracellular space and the activation of peripheral P2Y1 receptors appear to participate in the generation of sensory action potentials by light touch.

Distinct voltage-dependent gating behaviours of a swelling-activated chloride current in human epithelial cells

1. The swelling-activated chloride current is critical in the homeostatic regulatory volume decrease (RVD) of both excitable and non-excitable cells. Although not activated by voltage, it displays kinetic behaviour similar to voltage-gated Shaker-type potassium currents. We have studied the voltage-dependent properties of this current in single T84 human cell line epithelial cells using whole-cell patch clamp methodology. 2. An external anion permeability sequence of I- > Cl- > methanesulphonate (MeSO3-) was observed for the swelling-activated current. Extracellular application of the chloride channel blocker DIDS (100 microM) resulted in approximately 50% block of the current in a voltage-dependent manner. 3. At positive membrane potentials, the swelling-activated chloride current undergoes time-dependent inactivation. Following such inactivation, recovery of both the inward and outward components of the macroscopic current was found to be voltage dependent. The time constants describing these two individual recovery processes were identical over a range of membrane potentials. In addition, the magnitude of current recovery was directly dependent upon the degree prior inactivation of current at positive voltage. 4. We further observed that the swelling-activated current undergoes a form of steady-state, voltage-dependent inactivation that appears to differ from the inactivation observed at positive potentials. This steady-state inactivation occurred over the physiological voltage range, with a membrane potential at half-maximal inactivation (V1/2) of -72 mV, and differed from the time-dependent inactivation observed at positive membrane potentials, which occurred with a V1/2 of 40 mV. These observations demonstrate two distinct forms of voltage-dependent inactivation, probably reflecting two separate gating processes at the level of the channel. 5. These latter properties are thus anticipated to regulate voltage-dependent chloride efflux under cell swelling conditions and further influence RVD and membrane excitability in cells generating action potentials.

Alteration of GABAA receptor function following gene transfer of the CLC-2 chloride channel

The effect of GABAA receptor activation varies from inhibition to excitation depending on the state of the transmembrane anionic concentration gradient (delta anion). delta anion was genetically altered in cultured dorsal root ganglion neurons via adenoviral vector-mediated expression of ClC-2, a Cl- channel postulated to regulate the Cl- concentration in neurons in which GABAA receptor activation is predominantly inhibitory. ClC-2 expression was verified by the presence of the appropriate mRNA, protein, and membrane conductance. CIC-2 expression resulted in a large negative shift in the Cl- equilibrium potential (ECl) that attenuated the GABA-mediated membrane depolarization and prevented GABAA receptor-mediated action potentials. These results establish that gene transfer of transmembrane ion channels to neurons can be used to demonstrate their physiological function, and that delta anion can be genetically manipulated to alter the function of neuronal GABAA receptors in situ.

Cell swelling activates ATP-dependent voltage-gated chloride channels in M-1 mouse cortical collecting duct cells

In the present study we used whole-cell patch clamp recordings to investigate swelling-activated Cl-currents (ICl-swell) in M-1 mouse cortical collecting duct (CCD) cells. Hypotonic cell swelling reversibly increased the whole-cell Cl- conductance by about 30-fold. The I-V relationship was outwardly-rectifying and ICl-swell displayed a characteristic voltage-dependence with relatively fast inactivation upon large depolarizing and slow activation upon hyperpolarizing voltage steps. Reversal potential measurements revealed a selectivity sequence SCN- > I- > Br- > Cl- > > gluconate. ICl-swell was inhibited by tamoxifen, NPPB (5-nitro-2(3-phenylpropylamino)-benzoate), DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid), flufenamic acid, niflumic acid, and glibenclamide, in descending order of potency. Extracellular cAMP had no significant effect. ICl-swell was Ca2+ independent, but current activation depended on the presence of a high-energy gamma-phosphate group from intracellular ATP or ATP gamma S. Moreover, it depended on the presence of intracellular Mg2+ and was inhibited by staurosporine, which indicates that a phosphorylation step is involved in channel activation. Increasing the cytosolic Ca2+ concentration by using ionomycin stimulated Cl- currents with a voltage dependence different from that of ICl-swell. Analysis of whole-cell current records during early onset of ICl-swell and during final recovery revealed discontinuous step-like changes of the whole-cell current level which were not observed under nonswelling conditions. A single-channel I-V curve constructed using the smallest resolvable current transitions detected at various holding potentials and revealed a slope conductance of 55, 15, and 8 pS at +120, 0, and -120 mV, respectively. The larger current steps observed in these recordings had about 2, 3, or 4 times the size of the putative single-channel current amplitude, suggesting a coordinated gating of several individual channels or channel subunits. In conclusion we have functionally characterized ICl-swell in M-1 CCD cells and have identified the underlying single channels in whole-cell current recordings.

A short CIC-2 mRNA transcript is produced by exon skipping

CIC-2 is a voltage- and volume-regulated chloride channel expressed in many tissues. We have shown that CIC-2 in rat lung airways is significantly down-regulated after birth [Murray,C.B. et al. (1995) Am. J. Respir. Cell Mol. Biol., 12, 597-604]. During PCR amplification from rat lung cDNA, a second transcript was identified which is 60 bp shorter than the full length sequence. The peptide translated from this 60 bp sequence contains many positively charged amino acid residues. Rat genomic DNA sequencing showed that the 60 bp sequence is an intact exon. A 71% pyrimidine content and an AAG 3'-end splice site in the intron immediately upstream from the 60 bp sequence were identified which may account for the alternative splicing of the following exon. Human genomic sequence analyses demonstrated similar intron-exon arrangement. A high CT content and an AAG 3' acceptor site were conserved in the intron corresponding to the rat upstream intron. The presence of the full length short form transcript was confirmed in rat kidney by RT-PCR, and the ratio of the long and the short form transcripts varied significantly according to the tissues examined, with the lowest long/short form ratio found in the lung among the tissues studied. Our data demonstrated that the alternatively spliced short form (CIC-2S) is transcribed in many rat tissues, the ratio of the long/short form transcripts is lower in the lung compared with the brain, and the genomic organization in this area is conserved in rat and human.

Volume-sensitive chloride currents in primary cultures of human fetal vas deferens epithelial cells

Using the patch-clamp technique, we have identified a large, outwardly rectifying, Cl--selective whole-cell current in primary cultures of human vas deferens epithelial cells. Whole-cell currents were time- and voltage-dependent and displayed inactivation following depolarising pulses >/= 60 mV. Currents were equally permeable to bromide (PBr/PCl = 1.05 +/- 0.04), iodide (PI/PCl = 1. 06 +/- 0.07) and Cl-, but significantly less permeable to gluconate (PGluc /PCl = 0.23 +/- 0.03). Currents spontaneously increased with time after establishing a whole-cell recording, but could be inhibited by exposure to a hypertonic bath solution which reduced inward currents by 68 +/- 4%. Subsequent exposure of the cells to a hypotonic bath solution led to a 418 +/- 110% increase in inward current, indicating that these currents are regulated by osmolarity. 4,4'-Diisothiocyanatostilbene-2,2'-disulphonic acid (100 microM) produced a rapid and reversible voltage-dependent block (60 +/- 5% and 10 +/- 7% inhibition of current, measured at +/- 60 mV, respectively). Dideoxyforskolin (50 microM) also reduced the volume-sensitive Cl- current, but with a much slower time course, by 41 +/- 13% and 32 +/- 16% (measured at +/- 60 mV, respectively). Tamoxifen (10 microM) had no effect on the whole-cell Cl- current. These results suggest that vas deferens epithelial cells possess a volume-sensitive Cl- conductance which has biophysical and pharmacological properties broadly similar to volume-sensitive Cl- currents previously described in a variety of cell types.

Chloride channels: a molecular perspective

Plasma membrane Cl- channels perform a variety of functions, including control of excitability in neurons and muscle, cell volume regulation and transepithelial transport. Structurally, three classes of Cl- channels have been identified: ligand-gated, postsynaptic Cl- channels (e.g. GABA and glycine receptors); the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels (which belong to the traffic ATPase superfamily); and the CLC family of Cl- channels. Recent developments of note include further characterization of the expanding CLC Cl- channel family, advances in understanding the regulation of the CFTR Cl- channel and its emergent role as a regulator of other channels, clarification of issues related to swelling-activated Cl- channels, and the discovery that several co-transporter molecules are now known to induce Cl- currents in Xenopus oocytes.

Cl channel blockers inhibit the volume-activated efflux of Cl and taurine in cultured neurons

The effects of the Cl channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), 1,9-dideoxyforskolin (DDF), dipyridamole, and niflumic acid and of the polyunsaturated fatty acids arachidonic, linolenic, and linoleic acids on regulatory volume decrease (RVD) and associated 125I and [3H]taurine fluxes in cultured rat cerebellar granule neurons were examined. Dose-response curves of NPPB, DDF, and dipyridamole showed 20-100% inhibition of RVD and osmolyte fluxes. Niflumic acid was less potent, requiring 150-600 microM to show effects of this magnitude. The polyunsaturated fatty acids (5-20 microM) inhibited 80-90% RVD and osmolyte fluxes, with arachidonic acid exhibiting the most potent effect. The volume-associated taurine efflux was somewhat higher in younger neurons, but the pharmacological sensitivity was essentially the same in immature and mature cells. The effects of all tested drugs on 125I and [3H]taurine fluxes were remarkably similar, indicating a close pharmacological sensitivity of the transport mechanism for the two osmolytes. This is in line with the suggestion of a common pathway for the volume-associated release of Cl and amino acids functioning as osmolytes.

Volume-activated chloride channels in neuroblastoma cells are blocked by the antiestrogen toremifene

The presence of volume-activated chloride channels has been examined in neuroblastoma C1300 cells using the whole-cell configuration of the patch-clamp technique. Chloride channels could not be detected under isotonic conditions. However, hypotonic challenge induced slowly developed inward and outward anionic currents that exhibited outward rectification and inactivation at the most depolarizing potentials, features that were similar to the currents described in other cell preparations where volume-activated Cl- channels have been associated with the expression of P-glycoprotein. This hypotonicity-activated Cl- currents could be reversibly blocked by extracellular exposure to toremifene, a novel synthetic antioestrogen. The fact that toremifene and its analog tamoxifen, have been shown to block P-glycoprotein-associated chloride channels and to reverse P-glycoprotein associated multidrug resistance in a number of cell lines suggest that P-glycoprotein could be involved in the generation of hypotonic-induced chloride conductance in neuroblastoma cells.

P-glycoprotein regulates a volume-activated chloride current in bovine non-pigmented ciliary epithelial cells

1. The whole-cell patch clamp technique was used to investigate the swelling-activated currents in bovine non-pigmented ciliary epithelial (NPCE) cells. 2. Exposure to hypotonic solution activated a current that was blocked by 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB). The I-V relationship was shifted in the direction expected for a Cl- current when the external Cl- was replaced by gluconate (permeability ratio P(gluconate)/PCl = 0.17). The inhibition of the current evoked by voltage clamp steps of +80 mV yielded an IC50 for NPPB of 13.4 microM. 3. The current was found to be dependent on ATP. With ATP in the patch pipette the current could be repeatedly activated by exposure to hypotonic solution but when ATP was omitted the current ran down with time. 4. The development of this current was associated with visible cell swelling and inhibitors of regulatory volume decrease in these cells, e.g. tamoxifen, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (SITS) and 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS), also inhibited this current. 5. The volume-activated current was additionally blocked by NPPB, verapamil, quinidine and dideoxyforskolin. 6. The current was independent of external calcium and exhibited slight outward rectification and time-dependent inactivation at strong depolarizing potentials. 7. Disrupting the cytoskeleton and microtubules with cytochalasin B and colchicine had no effect on the activation of the Cl- current. 8. An antibody (C219) to the MDR1 gene product, P-glycoprotein, caused a functional block of the swelling-activated Cl- current when added to the patch pipette. 9. Immunofluorescence studies using the monoclonal antibodies C219 and JSB-1 demonstrated the presence of P-glycoprotein in the ciliary epithelial cells. The immunofluorescence was stronger on the non-pigmented than on the pigmented cells. 10. It is concluded that swelling in NPCE cells activates a Ca(2+)-independent, ATP-dependent Cl- current and that the activity of this current is associated with P-glycoprotein. 11. It is suggested that this Cl- current contributes to regulatory volume decrease and may participate in the secretory activity of these cells.

Salt tolerance in plants and microorganisms: toxicity targets and defense responses

Salt tolerance of crops could be improved by genetic engineering if basic questions on mechanisms of salt toxicity and defense responses could be solved at the molecular level. Mutant plants accumulating proline and transgenic plants engineered to accumulate mannitol or fructans exhibit improved salt tolerance. A target of salt toxicity has been identified in Saccharomyces cerevisiae: it is a sodium-sensitive nucleotidase involved in sulfate activation and encoded by the HAL2 gene. The major sodium-extrusion system of S. cerevisiae is a P-ATPase encoded by the ENA1 gene. The regulatory system of ENA1 expression includes the protein phosphatase calcineurin and the product of the HAL3 gene. In Escherichia coli, the Na(+)-H+ antiporter encoded by the nhaA gene is essential for salt tolerance. No sodium transport system has been identified at the molecular level in plants. Ion transport at the vacuole is of crucial importance for salt accumulation in this compartment, a conspicuous feature of halophytic plants. The primary sensors of osmotic stress have been identified only in E. coli. In S. cerevisiae, a protein kinase cascade (the HOG pathway) mediates the osmotic induction of many, but not all, stress-responsive genes. In plants, the hormone abscisic acid mediates many stress responses and both a protein phosphatase and a transcription factor (encoded by the ABI1 and ABI3 genes, respectively) participate in its action.

Cloning and functional expression of rat CLC-5, a chloride channel related to kidney disease

We have cloned a novel member of the CLC chloride channel family from rat brain, rCLC-5. The cDNA predicts a 83-kDa protein belonging to the branch including CLC-3 and CLC-4, with which it shares approximately 80% identity. Expression of rCLC-5 in Xenopus oocytes elicits novel anion currents. They are strongly outwardly rectifying and have a conductivity sequence of NO3- > Cl- > Br- > I- >> glutamate-. Although CLC-5 has consensus sites for phosphorylation by protein kinase A, raising the intracellular cAMP concentration had no effect on these currents. Currents were also unchanged when rCLC-5 was coexpressed with rCLC-3 and rCLC-4, either singly or in combination. rCLC-5 is expressed predominantly in kidney and also in brain, lung, and liver. Along the nephron, rCLC-5 message is detectable in all tubule segments investigated, but expression in the glomerulus and the S2 segment of the proximal tubule is low.

ClC-6 and ClC-7 are two novel broadly expressed members of the CLC chloride channel family

We cloned two novel members of the CLC chloride channel family from rat and human brain. ClC-6 is a 97-kDa protein, and ClC-7 a 89-kDa protein roughly 45% identical with ClC-6. Together they define a new branch of this gene family. Both genes are very broadly expressed, e.g. in brain, testes, muscle and kidney. In mouse embryos, both genes are expressed as early as day 7. While the human gene for ClC-6 is located on human chromosome 1p36 and shares this region with hClC-Ka and hClC-Kb, ClC-7 is on 16p13. ClC-6 has a highly conserved glycosylation site between transmembrane domains D8 and D9, while ClC-7 is the only known eukaryotic ClC protein which lacks this site. Hydropathy analysis indicates that domain D4 cannot serve as a transmembrane domain. Both ClC-6 and ClC-7 cannot be expressed as chloride channels in Xenopus oocytes, either singly or in combination.

Tyrosine protein kinase inhibitors prevent activation of cardiac swelling-induced chloride current

The effect of tyrosine protein kinase inhibitors on the swelling-induced chloride current (ICl-swelling) of dog atrial myocytes was studied using the whole-cell patch-clamp recording technique. Currents were measured during hyperpolarizing voltage ramps with potassium currents blocked by cesium. Osmolarity was varied using mannitol. Exposure to hypotonic solution (approximately 249 mosmol/kg) activated ICl-swelling. Hypertonic solution (approximately 363 mosmol/kg) was used to shrink swollen cells and turn off ICl-swelling. In studies on the acute effect of tyrosine protein kinase inhibitors each cell was swollen three separate times. Control, treatment, and washout ICl-swelling were compared. Genistein (50-80 microM) prevented reactivation of ICl-swelling without affecting cell size. The effect of genistein partially subsided upon washout. The effect of genistein on ICl-swelling was not mimicked by 80 microM daidzein, a related compound that does not inhibit tyrosine protein kinases. When intracellular adenosine 5'-O-(3-thiotriphosphate (ATP[gamma S]) was used, genistein did not prevent the reactivation of ICl-swelling. Intracellular ATP[gamma S] did not result in a persistent activation of ICl-swelling when cell size was returned to control. Acute exposure to 1 microM herbimycin A or 100 microM tyrphostin 51 did not prohibit the activation of ICl-swelling. A 24-h exposure to 1 microM herbimycin A did inhibit ICl-swelling. The results provide important clues regarding the activation mechanism for ICl-swelling and suggest that a tyrosine protein phosphorylation may be necessary, but not sufficient, for activation of ICl-swelling.

Cl- channels in basolateral renal medullary vesicles. X. Cloning of a Cl- channel from rabbit outer medulla

These experiments were intended to identify candidate cDNAs which might encode basolateral membrane Cl- channels of the mTAL using a homology-based cloning strategy. We prepared a cDNA library using a 1.8 to 3.2 kb mRNA fraction from rabbit outer medulla that induces a Cl- conductance in cellular membranes of Xenopus laevis oocytes. The cDNA library was screened with two 32P-oligonucleotide probes corresponding to highly conserved sequences in other Cl- channels. We isolated two cDNAs: rbClC-Ka and rbClC-Kb. The protein sequences deduced from these two cDNAs had 99% homology. Using RT-PCR technology, cultured mouse mTAL cells were found to contain mRNA corresponding to those two cDNAs. Expression of the mRNAs corresponding to these two cDNAs was kidney-specific and was greater in rabbit renal medulla than rabbit renal cortex. Finally, by using RT-PCR technology in combination with microdissected glomeruli or tubule segments, we found mRNA for rbClC-Ka in glomeruli, proximal convoluted tubules, mTAL and cortical collecting tubules.

Volume-sensitive chloride channel activity does not depend on endogenous P-glycoprotein

To determine whether endogenous P-glycoprotein, the MDR1 gene product that functions as a drug transport pump, is a volume-sensitive Cl- channel molecule or a protein kinase C-mediated regulator of the Cl- channel, whole-cell patch-clamp and molecular biological experiments were carried out in a human small intestinal epithelial cell line. Endogenous expression of P-glycoprotein was confirmed by Northern blot analysis, reverse transcription-polymerase chain reaction, Western blot analysis, and immunostaining. The P-glycoprotein expression was abolished by the antisense (but not sense) oligonucleotide for the MDR1 gene, whereas the magnitude of the Cl- current activated by osmotic swelling was not distinguishable between both antisense- and sense-treated cells. The volume-sensitive Cl- currents were not specifically affected by the anti-P-glycoprotein monoclonal antibodies, MRK16, C219, and UIC2. An inhibitor of P-glycoprotein-mediated pump activity, verapamil, was found to never affect the Cl- current. A substrate for the P-glycoprotein-mediated drug pump, vincristine or daunomycin, did not prevent swelling-induced activation of the Cl- current. Furthermore, the Cl- current was not affected by an activator of protein kinase C (12-O-tetradecanoylphorbol-13-acetate or 1-oleoyl-2-acetyl-sn-glycerol). Thus, it is concluded that the endogenous P-glycoprotein molecule is not itself a volume-sensitive Cl- channel nor a protein kinase C-mediated regulator of the channel in the human epithelial cells.

Molecular cloning and characterization of a novel truncated from (ClC-2 beta) of ClC-2 alpha (ClC-2G) in rabbit heart

Two cDNAs encoding rabbit heart Cl- channels (rabClC-2 beta and rabClC-2 alpha) were isolated by a PCR cloning strategy. RabClC-2 beta is a novel cDNA consisting of 2998 bp and encoding the 822-amino acid protein, while rabClC-2 alpha is identical to previously reported ClC-2G. RabClC-2 beta is 68 amino acids truncated from NH2-terminus of rabClC-2 alpha, but all 13 putative hydrophobic domains are conserved in rabClC-2 beta. Although rabClC-2 alpha was suggested to be activated by extracellular hypotonicity, expression of rabClC-2 beta in Xenopus oocytes induced large Cl- currents even in the absence of extracellular hypotonicity. Induction of external hypotonicity did not further increase the amplitude of membrane currents. On the other hand, as similar to rabClC-2 alpha, rabClC-2 beta current was augmented by PKA activation. Thus, different RNA processing of the same gene appears to provide two highly homologous PKA-activated Cl- channels with or without responsiveness to cell swelling in rabbit heart.

Chimeric Na+/H+ exchangers: an epithelial membrane-bound N-terminal domain requires an epithelial cytoplasmic C-terminal domain for regulation by protein kinases

All cloned members of the mammalian Na+/H+ exchanger gene family encode proteins that consist of two functionally distinct domains: a membrane-bound N terminus and a cytoplasmic C terminus, which are required for ion transport and regulation of transport, respectively. Despite their similarity in structure, three members of this family, designated NHE1, NHE2, and NHE3, exhibit different kinetic mechanisms in response to growth factors and protein kinases. For instance, growth factors stimulate NHE1 by a change in the affinity constant for intracellular H+, K'(Hi+), and regulate NHE2 and NHE3 by a change in Vmax. We have constructed chimeric Na+/H+ exchangers by exchanging the N and C termini among three cloned rabbit Na+/H+ exchangers (NHE1 to NHE3) to determine which domain is responsible for the above Vmax-vs.-K'(H(i)+) effect of the Na+/H+ isoforms. All of the chimeras had functional exchange activity and basal kinetic properties similar to those of wild-type exchangers. Studies with serum showed that the N terminus is responsible for the Vmax-vs.-K'(H(i)+) stimulation of the Na+/H+ exchanger isoforms. Moreover, phorbol 12-myristate 13-acetate and fibroblast growth factor altered Na+/H+ exchange only in chimeras that had an epithelial N-terminal domain matched with an epithelial C-terminal domain. Therefore, the protein kinase-induced regulation of Na+/H+ exchangers is mediated through a specific interaction between the N- and C-termini, whcih is restricted so that epithelial N- and epithelial N-and C-terminal portions of the exchangers are required for regulation.

Basolateral membrane chloride permeability of A6 cells: implication in cell volume regulation

The permeability to Cl- of the basolateral membrane (blm) was investigated in renal (A6) epithelial cells, assessing their role in transepithelial ion transport under steady-state conditions (isoosmotic) and following a hypoosmotic shock (i.e. in a regulatory volume decrease, RVD). Three different complementary studies were made by measuring: (1) the Cl- transport rates (delta F/Fo s-1 (x10(-3))), where F is the fluorescence of N-(6-methoxyquinoyl) acetoethyl ester, MQAE, and Fo the maximal fluorescence (x10(-3)) of both membranes by following the intracellular Cl- activities (ai Cl-, measured with MQAE) after extracellular Cl- substitution (2) the blm 86Rb and 36Cl uptakes and (3) the cellular potential and Cl- current using the whole-cell patch-clamp technique to differentiate between the different Cl- transport mechanisms. The permeability of the blm to Cl- was found to be much greater than that of the apical membranes under resting conditions: aiCl- changes were 5.3 +/- 0.7 mM and 25.5 +/- 1.05 mM (n = 79) when Cl- was substituted by NO3(-) in the media bathing apical and basolateral membranes. The Cl- transport rate of the blm was blocked by bumetanide (100 microM) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 50 microM) but not by N-phenylanthranilic acid (DPC, 100 microM). 86Rb and 36Cl uptake experiments confirmed the presence of a bumetanide- and a NPPB-sensitive Cl- pathway, the latter being approximately three times more important than the former (Na/K/2Cl cotransporter). Appli-cation of a hypoosmotic medium to the serosal side of the cell increased delta F/Fo s-1 (x10(-3)) after extracellular Cl- substitution (1.03 +/- 0.10 and 2.45 +/- 0.17 arbitrary fluorescent units s-1 for isoosmotic and hypoosmotic conditions respectively, n = 11); this delta F/Fo s-1 (x10(-3)) increase was totally blocked by serosal NPPB application; on the other hand, cotransporter activity was decreased by the hypoosmotic shock. Cellular Ca2+ depletion had no effect on delta F/Fo s-1 (x10(-3)) under isoosmotic conditions, but blocked the delta F/Fo s-1 (x10(-3)) increase induced by a hypoosmotic stress. Under isotonic conditions the measured cellular potential at rest was -37.2 +/- 4.0 mV but reached a maximal and transient depolarization of -25.1 +/- 3.7 mV (n = 9) under hypoosmotic conditions. The cellular current at a patch-clamping cellular potential of -85 mV (close to the Nernst equilibrium potential for K+) was blocked by NPPB and transiently increased by hypoosmotic shock (≈50% maximum increase). This study demonstrates that the major component of Cl- transport through the blm of the A6 monolayer is a conductive pathway (NPPB-sensitive Cl- channels) and not a Na/K/2Cl cotransporter. These channels could play a role in transepithelial Cl- absorption and cell volume regulation. The increase in the blm Cl- conductance, inducing a depolarization of these membranes, is proposed as one of the early events responsible for the stimulation of the 86Rb efflux involved in cell volume regulation.

A voltage-gated chloride channel in ascidian embryos modulated by both the cell cycle clock and cell volume

1. Eggs of the ascidian Boltenia villosa have an inwardly rectifying Cl- current whose amplitude varies by more than 10-fold during each cell cycle, the largest amplitude being at exit from M-phase. We examined whether this current was also sensitive to changes in cell volume. 2. Cell swelling, produced by direct inflation through a whole-cell recording pipette, greatly increased the amplitude of the Cl- current at all stages of the cell cycle in activated eggs. Swelling was much less effective in unfertilized eggs. 3. The increase in Cl- current amplitude continued for 10-20 min after an increase in diameter that was complete in 10 s, suggesting the involvement of a second messenger system in the response. 4. Treatment of unfertilized eggs with 6-dimethylaminopurine (DMAP), an inhibitor of cell cycle-dependent protein kinases, increased the amplitude of the Cl- current and its sensitivity to swelling to levels characteristic of fertilized eggs. 5. Osmotically produced swelling also increased Cl- current amplitude in unfertilized eggs. 6. We propose that dephosphorylation renders the Cl- channel functional, and that swelling or activation of the egg increases the sensitivity of the channel to dephosphorylation, perhaps by disrupting its links to the cytoskeleton.

Functional role of follicular cells in the generation of osmolarity-dependent Cl- currents in Xenopus follicles

1. Osmolarity-dependent (osmo-dependent) ionic currents from follicle-enclosed Xenopus oocytes (follicles) were studied using the two-microelectrode voltage-clamp technique, combined with intra-oocyte pressure injection of sucrose or polyethylene glycols (PEGs). 2. Intra-oocyte injections of sucrose or PEG (3-25 nmol) generated inward membrane currents (follicles held at -60 mV) associated with an increase in membrane conductance. These currents were carried mainly by chloride ions (ICl(osm)), and were strongly attenuated by increasing the tonicity of the external medium, or by external application of La3+ (0.1-1 mM). 3. The ability to generate ICl(osm) depended on the molecular weight of the injected PEG. Injections of PEG 200 or 300 generated ICl(osm) in 95% of the follicles tested, PEG 600 generated comparable currents in only 20% of the follicles, while similar injections of PEG 1000 did not elicit ICl(osm). 4. Octanol (1-1.5 mM), a gap junction channel blocker, reversibly inhibited 50-90% of the ICl(osm) generated by injections of sucrose or PEG 300. Moreover, sucrose or PEG injections did not elicit ICl(osm) in defolliculated oocytes. 5. It is concluded that an increase in the internal osmolarity of the follicular cells activates a mechanism, probably involving cellular swelling, which leads to the opening of ICl(osm) channels most probably located in the follicular cell membrane.

Antisense oligonucleotides suppress cell-volume-induced activation of chloride channels

Cell volume regulation is an essential feature of most cells. After swelling in hypotonic media, the simultaneous activation of potassium and chloride channels is believed to be the initial, time-determining step in cell volume regulation. The activation of both pathways is functionally linked and enables the cells to lose ions and water, subsequently leading to cell shrinkage and readjustment of the initial volume. NIH 3T3 fibroblasts efficiently regulate their volume after swelling and bear chloride channels that are activated by decreasing extracellular osmolarity. The chloride current elicited in these cells after swelling is reminiscent of the current found in oocytes expressing an outwardly rectifying chloride current termed ICln. Introduction of antisense oligodeoxynucleotides complementary to the first 30 nucleotides of the coding region of the ICln channel into NIH 3T3 fibroblasts suppresses the activation of the swelling-induced chloride current. The experiments directly demonstrate an unambiguous link between a volume-activated chloride current and a cloned protein involved in chloride transport.

Failure of P-glycoprotein (MDR1) expressed in Xenopus oocytes to produce swelling-activated chloride channel activity

1. P-glycoprotein, the protein product of the multidrug resistance (MDR1) gene, has ATP-dependent transporter activity. It has been suggested that P-glycoprotein may also function as a volume-regulated chloride channel or chloride channel regulator. To assess the chloride channel function of P-glycoprotein, we examined swelling-activated chloride conductances in Xenopus oocytes injected with human MDR1 cRNA. 2. Functional expression of P-glycoprotein in Xenopus oocytes was confirmed using Western blot analysis and by assessing transport of the P-glycoprotein substrate, calcein AM. 3. Endogenous, swelling-activated chloride conductances were virtually absent by the time P-glycoprotein expression was confirmed. Thus, this expression system afforded the advantage of assessing putative MDR1-associated chloride currents in the absence of background currents. 4. The currents activated by hypotonic shock (50%) in both MDR1-injected and control (water-injected) oocytes were not significantly different. The swelling response was due in part to the activation of a potassium-selective conductance which could be inhibited by barium. No chloride-selective currents were activated by hypotonic shock in the presence or absence of barium. Therefore, we conclude that P-glycoprotein expression does not produce a swelling-activated chloride conductance in the Xenopus oocyte expression system.

Drug-transport and volume-activated chloride channel functions in human erythroleukemia cells: relation to expression level of P-glycoprotein

The characteristics of volume-activated chloride currents, drug transport function and levels of P-glycoprotein (PgP) expression were compared between two human chronic erythroleukemia cell lines: a parental (K562) cell line and a derivative obtained by vinblastine selection (K562 VBL400). Parental K562 cells showed no detectable P-glycoprotein expression, measured at the protein level (immunofluorescence labeling with monoclonal antibodies), and had very low levels of MDR-1 mRNA expression (RT-PCR analysis), when compared with levels measured in K562 VBL400. Differences in Pgp-mediated transport were estimated by comparing the rates of Fluo3 accumulation. The higher drug-transport function of K562 VBL400 cells (e.g., lower Fluo3 accumulation) correlated with their elevated levels of MDR-1. The rate of dye transport was sensitive to verapamil but was not affected by the tonicity of the extracellular medium. In contrast to the clear differences in transport function, the characteristics of chloride currents induced by cell swelling were indistinguishable between the two cell lines. Currents measured in the whole-cell configuration were outwardly rectifying, had a higher permeability to iodide than to chloride (SCN- > I- > Cl- > gluconate), were potently blocked by NPPB and were unresponsive to verapamil. The percentage of responding cells and the mean current density were nearly identical in both cell lines. In addition, activation of the volume-sensitive current was not prevented during whole-cell recordings obtained with pipettes containing high concentration of cytotoxic drugs (vincristine or vinblastine). These results do not lend support to the previously reported association between Pgp expression and volume-sensitive chloride channels, and suggest that a different protein is responsible for this type of chloride channel in K562 cells.

Absence of the skeletal muscle sarcolemma chloride channel ClC-1 in myotonic mice

The voltage-dependent chloride channel ClC-1 stabilizes resting membrane potential in skeletal muscle. Mutations in the ClC-1 gene are responsible for both human autosomal recessive generalized myotonia and autosomal dominant myotonia congenita. To understand the tissue distribution and subcellular localization of ClC-1 and to evaluate its role in an animal model of myotonia, antibodies were raised against the carboxyl terminus of this protein. Expression of the 130-kDa ClC-1 protein is unique to skeletal muscle, consistent with its mRNA tissue distribution. Immunolocalization shows prominent ClC-1 antigen in the sarcolemma of both type I and II muscle fibers. Sarcolemma localization is confirmed by Western analysis of skeletal muscle subcellular fractions. The ADR myotonic mouse (phenotype ADR, genotype adr/adr), in which defective ClC-1 mRNA has been identified, is shown here to be absent in ClC-1 protein expression, whereas other skeletal muscle sarcolemma protein expression appears normal. Immunohistochemistry of skeletal muscle from ADR and other mouse models of human muscle disease demonstrate that the absence of ClC-1 chloride channel is a defect specific to ADR mice.

PKC-sensitive Cl- channels associated with ciliary epithelial homologue of pICln

Swelling activates and protein kinase C (PKC) downregulates Cl- channels in cultured nonpigmented ciliary epithelial (NPE) cells. We now report that the PKC inhibitor staurosporine upregulates whole cell Cl- currents isosmotically. The kinetics and current-voltage relationship are similar to those of volume-activated Cl- channels of these cells. These properties are inconsistent with cloned ClC-0, ClC-1, ClC-2, and MDR1 channels but could reflect the cystic fibrosis transmembrane conductance regulator (CFTR) channel or the Cl- channel regulator pICln. CFTR mRNA was undetectable by Northern analysis of cultured NPE cells or ciliary body tissue. In contrast, a human pICln probe obtained by polymerase chain reaction cloning and showing 90% identity with the rat cDNA clone detected high levels of transcripts in NPE cells. The level was low in tissue, where the NPE message was diluted by RNA from other cells. We conclude that NPE cells display staurosporine-activated Cl- channels [gSt(Cl)] likely identical with the volume-activated channels. The same cells expressing gSt(Cl) transcribe mRNA for a novel homologue (pHCBICln) of pICln that may regulate Cl- transport into the aqueous humor.

Gating of the voltage-dependent chloride channel CIC-0 by the permeant anion

Chloride channels of the ClC family are important for the control of membrane excitability, cell volume regulation, and possibly transepithelial transport. Although lacking the typical voltage-sensor found in cation channels, gating of ClC channels is clearly voltage-dependent. For the prototype Torpedo channel ClC-0 (refs 11-15) we now show that channel opening is strongly facilitated by external chloride. Other less permeable anions can substitute for chloride with less efficiency. ClC-0 conductance shows an anomalous mole fraction behaviour with Cl-/NO3- mixtures, suggesting a multi-ion pore. Gating shows a similar anomalous behaviour, tightly linking permeation to gating. Eliminating a positive charge at the cytoplasmic end of domain D12 changes kinetics, concentration dependence and halide selectivity of gating, and alters pore properties such as ion selectivity, single-channel conductance and rectification. Taken together, our results strongly suggest that in these channels voltage-dependent gating is conferred by the permeating ion itself, acting as the gating charge.

P-glycoprotein and cell volume-activated chloride channels

The multidrug resistance P-glycoprotein (P-gp) is an active drug transporter which can expel hydrophobic compounds from cells. Expression of P-gp has many effects on cells and tissues and the physiological function, or functions, of P-gp are still unclear. Recently, expression of P-gp has been associated with altered activity of chloride channels which play a role in regulating cell volume of response to osmotic shock or nutrient uptake. The nature and physiological role of this association has been a subject of some debate. In this article, mechanisms by which P-gp might influence cell volume-activated chloride currents is discussed, and the potential physiological role of this regulation considered.

Volume-activated chloride currents associated with the multidrug resistance P-glycoprotein

The ability to regulate volume is an important property of most, if not all cells. In epithelial cells, amongst others, cell volume-activated chloride channels are central to this response. The molecular identities of these channels are not yet known. Expression of the human multidrug resistance P-glycoprotein (P-gp) has been associated with cell volume-regulated chloride currents, although the nature of this association is the subject of debate. Recent data indicate that P-gp acts by regulating the activation of an endogenous channel protein. In this review, evidence associating P-gp with cell volume-activated chloride currents, and the possible mechanisms by which this might be achieved, are discussed.

Cloning, functional expression, and characterization of a PKA-activated gastric Cl- channel

cDNA encoding a Cl- channel was isolated from a rabbit gastric library, sequenced, and expressed in Xenopus oocytes. The predicted protein (898 amino acids, relative molecular mass 98,433 Da) was overall 93% similar to the rat brain ClC-2 Cl- channel. However, a 151-amino acid stretch toward the COOH-terminus was 74% similar to ClC-2 with six amino acids deleted. Two new potential protein kinase A (PKA) phosphorylation sites (also protein kinase C phosphorylation sites) were introduced. cRNA-injected Xenopus oocytes expressed a Cl- channel that was active at pHtrans 3 and had a linear current-voltage (I-V) curve and a slope conductance of 29 +/- 1 pS at 800 mM CsCl. A fivefold Cl- gradient caused a rightward shift in the I-V curve with a reversal potential of +30 +/- 3 mV, indicating anion selectivity. The selectivity was I- > Cl- > NO3-. The native and recombinant Cl- channel were both activated in vitro by PKA catalytic subunit and ATP. The electrophysiological and regulatory properties of the cloned and the native channel were similar. The cloned protein may be the Cl- channel involved in gastric HCl secretion.

Maitotoxin increases voltage independent chloride and sodium currents in GH4C1 rat pituitary cells

Maitotoxin (MTX) is a 3,424 dalton polyether marine toxin that causes influx of calcium through type L voltage-dependent calcium channels (L-VDCC) in GH4C1 rat pituitary cells, presumably as the result of membrane depolarization. In this study we have investigated the ionic conductances responsible for MTX-induced depolarization under voltage clamp conditions using the perforated and ruptured patch methods. MTX-induced steady-state voltage independent currents of nearly 400 pS/pF within seconds of addition to the bath. Ion substitution experiments demonstrated these currents are consistent with the conductance of sodium and chloride, but not calcium, ions. MTX induction of the voltage-independent chloride conductance in GH4C1 cells occurred concurrently without modification of L-VDCC currents. Pretreatment with nimodipine eliminated voltage activation of L-VDCC, and reduced by two thirds the voltage independent current. Analysis as a function of time of MTX exposure revealed that the first 60 sec of MTX-induced currents were not affected by nimodipine pretreatment, but subsequent additional currents were prevented. This indicates that the initial currents induced by MTX occur independently of L-VDCC mediated calcium entry, but full activation of these currents by MTX likely requires the involvement L-VDCC. Taken together this work identifies a voltage-independent sodium/chloride conductance as an initial action of MTX, one that may promote the sequence of ionic events leading to activation of L-VDCC and massive calcium entry.

Membrane mechanisms and intracellular signalling in cell volume regulation

Recent work on selected aspects of the cellular and molecular physiology of cell volume regulation is reviewed. First, the physiological significance of the regulation of cell volume is discussed. Membrane transporters involved in cell volume regulation are reviewed, including volume-sensitive K+ and Cl- channels, K+, Cl- and Na+, K+, 2Cl- cotransporters, and the Na+, H+, Cl-, HCO3-, and K+, H+ exchangers. The role of amino acids, particularly taurine, as cellular osmolytes is discussed. Possible mechanisms by which cells sense their volumes, along with the sensors of these signals, are discussed. The signals are mechanical changes in the membrane and changes in macromolecular crowding. Sensors of these signals include stretch-activated channels, the cytoskeleton, and specific membrane or cytoplasmic enzymes. Mechanisms for transduction of the signal from sensors to transporters are reviewed. These include the Ca(2+)-calmodulin system, phospholipases, polyphosphoinositide metabolism, eicosanoid metabolism, and protein kinases and phosphatases. A detailed model is presented for the swelling-initiated signal transduction pathway in Ehrlich ascites tumor cells. Finally, the coordinated control of volume-regulatory transport processes and changes in the expression of organic osmolyte transporters with long-term adaptation to osmotic stress are reviewed briefly.

Properties of voltage-gated chloride channels of the ClC gene family

We review the properties of ClC chloride channels, members of an expanding gene family originally discovered by the cloning of the ClC-0 chloride channel from Torpedo electric organ. There are at least nine different ClC genes in mammals, several of which seem to be expressed ubiquitously, while others are expressed in a highly specific manner (e.g. the muscle-specific ClC-1 channel and the kidney-specific ClC-K channels). The newly cloned rat ClC-4 is strongly expressed in liver and brain, but also in heart, muscle, kidney and spleen. ClC chloride channels are structurally unrelated to other channel proteins and have twelve putative transmembrane domains. They function as multimers with probably four subunits. Functional characterization is most advanced with ClC-0, ClC-1 (mutations which cause myotonia) and ClC-2, a swelling-activated chloride channel. Many of the new ClC family members cannot yet be expressed functionally.

Effects of P-glycoprotein expression on cyclic AMP and volume-activated ion fluxes and conductances in HT-29 colon adenocarcinoma cells

The tissue distribution of P-glycoprotein (Pgp) and the structurally related cystic fibrosis transmembrane conductance regulator (CFTR) is apparently mutually exclusive, particularly in epithelial; where one protein is expressed the other is not. To study the possible function(s) of Pgp and its potential effects on CFTR expression in epithelia, HT-29 colon adenocarcinoma cells, which constitutively express CFTR, were pharmacologically adapted to express the classical multidrug resistance (MDR) phenotype (Pgp+). Concomitant with the appearance of Pgp and MDR phenotype (drug resistance, reduced drug accumulation and increased drug efflux), CFTR levels and cAMP-stimulated Cl conductances were markedly decreased compared to wild-type HT-29 (Pgp-) cells (as shown using the whole cell patch clamp technique). Removal of drug pressure led to the gradual decrease in Pgp levels and MDR phenotype, as evidenced by increased rhodamine 123 accumulation (Pgp-Rev). Concomitantly, CFTR levels and cAMP-stimulated Cl- conductances increased. The cell responses of Pgp/Rev cells were heterogeneous with respect to both Pgp and CFTR functions. We also studied the possible contribution to Pgp to hypotonically activated (HCS) ion conductances. K+ and Cl- effluxes from Pgp- cells were markedly increased by HCS. This increase was twice as high as that induced by the cation ionophore gramicidin; it was blocked by the Cl- channel blocker DIDS (4,4'-disothiocyano-2,2'-disulfonic stilbene) and required extracellular Ca2+. In Pgp+ cells, the HCS-induced fluxes were not significantly different from those of Pgp- cells. Verapamil (10 microM), which caused 80% reversal of Pgp-associated drug extrusion, failed to inhibit the HCS-evoked Cl- efflux of Pgp+ cells. Similarly, HCS increased Cl- conductance to the same extent in Pgp-, Pgp+ and Pgp-Rev cells. Verapamil (100 microM), but not 1,9-dideoxyforskolin (50 and 100 microM), partially inhibited the HCS-evoked whole cell current (WCC) in all three lines. Since the inhibition by verapamil was not detected in the presence of the K+ channel blocker Ba2+ (3 mM), it is suggested that verapamil affects K+ and not Cl- conductance. We conclude that hypotonically activated Cl- and K+ conductances are similar in HT-29 cells irrespective of Pgp expression. Expression of high levels of Pgp in HT-29 cells confers no physiologically significant capacity for cell volume regulation.

Purification, reconstitution, and subunit composition of a voltage-gated chloride channel from Torpedo electroplax

The voltage-gated Cl- channel from Torpedo electroplax was purified in functional form by an immunoaffinity procedure. Channel activity was assayed by 36Cl- uptake into reconstituted liposomes and by direct recording after insertion into planar lipid bilayers. The purified channel displays the same "double-barreled" gating kinetics observed with native membranes, as well as the correct single-channel permeation characteristics. Preparations of active channels consist of a 90-kDa polypeptide, as expected from the known cDNA sequence. No associated subunits are present in the purified material. Direct protein sequencing confirms the absence of a cleavable signal sequence and demonstrates an N-terminus at Ser-2 of the cDNA-derived sequence. This "ClC-0" protein is lightly glycosylated, losing only approximately 2 kDa of sugar upon treatment with endoglycosidase H or N-glycanase. Most if not all of this glycosylation is found on Asn-365. This result necessitates revision of current transmembrane topology proposals, which have placed this residue on the cytoplasmic side of the membrane. Sedimentation in sucrose density gradients under activity-preserving conditions suggests the ClC-0 channel is slightly larger than the Na/K-ATPase alpha/beta-protomer (approximately equal to 150 kDa) and substantially smaller than the reduced form of the nicotinic acetylcholine receptor (approximately equal to 300 kDa). The detergent-solubilized ClC-0 channel, which invariably displays two Cl- diffusion pores in the active complex, is therefore built most likely as a homodimer of the 90-kDa protein purified here.

Role of the cytoskeleton in the regulation of Cl- channels in human embryonic skeletal muscle cells

The effects of volume change and cytoskeleton manipulation on the Cl- channels in human embryonic skeletal muscle cells were studied. Trypsination, used for production of myoballs, changes the channel properties only a little. When the external osmolarity was reduced from 300 to 270 mosmol/l, the specific Cl- conductance, gCl, (at -80 mV) of myoballs increased from 5.1 +/- 1.9 to 30.4 +/- 12.2 microS/cm2 (SD; n = 6) within 15 min. Concomitantly, the kinetics of Cl- currents, elicited by clamping the membrane potential from a negative to positive values, changed from activation and subsequent slow inactivation to instantaneous activation with fast inactivation. G protein activation, protein kinase action or [Ca2+]i elevation seemed not to be involved in these effects. Similar changes were produced in the absence of a transmembrane osmotic gradient by 500 nM intracellular cytochalasin D (gCl = 34.3 +/- 10.3 microS/cm2; n = 6) or 12.5 microM colchicine (gCl = 15.4 +/- 1.4 microS/cm2; n = 5). When the external osmolarity was increased to 418 mosmol/l, 1 microM cytochalasin D did not affect gCl. In four of six cell-attached patches the open probability of the intermediate Cl- channel was increased after reduction of the bath osmolarity. In inside-out patches, the drugs increased the open probability of the channels. It is concluded that the Cl- channels are under control of the cytoskeleton.

Characterization of the Cl- conductance in the granular duct cells of mouse mandibular glands

We have previously shown that mouse mandibular granular ducts contain a hyperpolarization-activated Cl- conductance. We now show that the instantaneous current/voltage (I/V) relation of this Cl- conductance is inwardly rectifying with a slope conductance of 15.4 +/- 1.8 nS (n = 4) at negative potentials and of 6.7 +/- 0.9 nS (n = 4) at positive potentials. Thus, the inward rectification seen in the steady-state I/V relation is due, not only to voltage activation of the Cl- conductance, but also to the intrinsic conductance properties of the channel. We show further that the ductal Cl- conductance is not activated by including ATP (10 mmol/l) in the pipette solution. Finally, we show that the conductance is not blocked by the addition of any of the following compounds to the extracellular solution: anthracene-9-carboxylate (A9C, 1 mmol/l), diphenylamine-2-carboxylate (DPC, 1 mmol/l), 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB, 100 mumol/l), 4,4'-diisothiocyanato-stilbene-2,2'-disulphonate (DIDS, 100 mumol/l), indanyloxyacetic acid (IAA-94, 100 mumol/l), verapamil (100 mumol/l), glibenclamide (100 mumol/l) and Ba2+ (5 mmol/l). The properties of the ductal Cl- conductance most nearly resemble those of the ClC-2 channel. Both channel types have instantaneous I/V relations that are slightly inwardly rectifying, are activated by hyperpolarization with a time-course in the order of hundreds of milliseconds, have a selectivity sequence of Br- > Cl- > I-, and are insensitive to DIDS. The only identified difference between the two is that the ClC-2 channel is 50% blocked both by DPC and A9C (1 mmol/l), whereas the ductal Cl- conductance is insensitive to these compounds.

Potassium channels and their evolving gates

Potassium channels allow potassium ions to flow across the membrane and play a key role in maintaining membrane potential. Recent research has begun to reveal how these channels transport potassium in preference to other ions, how their activity is controlled, and how they are related to other channels.

Permeation properties and modulation of volume-activated Cl(-)-currents in human endothelial cells

1. We have studied the permeation and pharmacological properties of a recently described volume-activated, calcium-insensitive, small-conductance Cl(-)-channel in endothelial cells from human umbilical vein. 2. The relative permeability for various anions was I- > Cl- approximately Br- > F- > gluconate- (1.63 +/- 0.36: 1:0.95 +/- 0.16:0.46 +/- 0.04:0.19 +/- 0.07, n = 10). 3. 5-Nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) induced a fast and reversible block of the current (Ki = 29 mumol l-1). 4. Extracellular ATP induced a low-affinity block of the current, that showed a small voltage-dependence (K1 = 4.9 mmol l-1 at +80 mV and K1 = 8.2 mmol l-1 at -80 mV). 5. Extracellularly applied arachidonic acid (10 mumol l-1) irreversibly blocked the current in 5 out of 9 cells. This block seems to be non-specific, because other ionic currents, e.g. inwardly rectifying K+ currents, were blocked as well. 6. Tamoxifen induced a high affinity block of the current (K1 = 2.9 mumol l-1). Block and reversal of block were however much slower than with NPPB. 7. Cytotoxic compounds, which are substrates of the P-glycoprotein multidrug transporter, loaded into endothelial cells via the patch pipette, exerted only minor effects on the volume-activated current. Vinblastine and colcemid did not affect the volume-activated current, whereas daunomycin and vincristine induced a slow 'run-down' of the current. 8. The similarity between permeation and pharmacological properties of volume-activated Cl--currents in endothelial cells and those in many other cell types may suggest that they all belong to the same family of volume-activated small-conductance Cl--channels. Evidence that they belong to the class of P-glycoprotein associated Cl--channels is however only marginal, whereas their biophysical characteristics differ significantly from those of the CIC-2 volume-activated Cl--channels.

Molecular physiology of anion channels

Anion channels have diverse functions, ranging from regulation of cell volume to transepithelial transport and control of excitability. Three well established structural classes of plasma membrane chloride channels now exist: the ligand-gated chloride channels, the cAMP-stimulated cystic fibrosis transmembrane conductance regulator channel, and the voltage-gated (or swelling-activated) members of the CLC chloride channel family. Genetic defects leading to inherited disease are known for each of these classes. A combination of mutagenesis and biophysical analysis has been used to correlate their structure with function. Recently, the role of several molecules has been questioned; rather than being chloride channels themselves, they may be activators of endogenous channels in the cells used for expression.

Two highly homologous members of the ClC chloride channel family in both rat and human kidney

We have cloned two closely related putative Cl- channels from both rat kidney (designated rClC-K1 and rClC-K2) and human kidney (hClC-Ka and hClC-Kb) by sequence homology to the ClC family of voltage-gated Cl- channels. While rClC-K1 is nearly identical to ClC-K1, a channel recently isolated by a similar strategy, rClC-K2 is 80% identical to rClC-K1 and is encoded by a different gene. hClC-Ka and hClC-Kb show approximately 90% identity, while being approximately 80% identical to the rat proteins. All ClC-K gene products are expressed predominantly in the kidney. While rClC-K1 is expressed strongly in the cortical thick ascending limb and the distal convoluted tubule, with minor expression in the S3 segment of the proximal tubule and the cortical collecting tubule, rClC-K2 is expressed in all segments of the nephron examined, including the glomerulus. Since they are related more closely to each other than to the rat proteins, hClC-Ka and hClC-Kb cannot be regarded as strict homologs of rClC-K1 or rClC-K2. After injection of ClC-K cRNAs into oocytes, corresponding proteins were made and glycosylated, though no additional Cl- currents were detectable. Glycosylation occurs between domains D8 and D9, leading to a revision of the transmembrane topology model for ClC channels.