Inhibition of the inward-rectifying Cl- channel in rat choroid plexus by a decrease in extracellular pH

Mechanisms of cerebrospinal fluid secretion by the choroid plexus epithelium: Application to various intracranial pathologies

The choroid plexus (CP) is a small yet highly active epithelial tissue located in the ventricles of the brain. It secretes most of the CSF that envelops the brain and spinal cord. The epithelial cells of the CP have a high fluid secretion rate and differ from many other secretory epithelia in the organization of several key ion transporters. One striking difference is the luminal location of, for example, the vital Na+-K+-ATPase. In recent years, there has been a renewed focus on the role of ion transporters in CP secretion. Several studies have indicated that increased membrane transport activity is implicated in disorders such as hydrocephalus, idiopathic intracranial hypertension, and posthemorrhagic sequelae. The importance of the CP membrane transporters in regulating the composition of the CSF has also been a focus in research in recent years, particularly as a regulator of breathing and hemodynamic parameters such as blood pressure. This review focuses on the role of the fundamental ion transporters involved in CSF secretion and its ion composition. It gives a brief overview of the established factors and controversies concerning ion transporters, and finally discusses future perspectives related to the role of these transporters in the CP epithelium.

Cerebrospinal fluid pH regulation

The cerebrospinal fluid (CSF) fills the brain ventricles and the subarachnoid space surrounding the brain and spinal cord. The fluid compartment of the brain ventricles communicates with the interstitial fluid of the brain across the ependyma. In comparison to blood, the CSF contains very little protein to buffer acid-base challenges. Nevertheless, the CSF responds efficiently to changes in systemic pH by mechanisms that are dependent on the CO2/HCO3- buffer system. This is evident from early studies showing that the CSF secretion is sensitive to inhibitors of acid/base transporters and carbonic anhydrase. The CSF is primarily generated by the choroid plexus, which is a well-vascularized structure arising from the pial lining of the brain ventricles. The epithelial cells of the choroid plexus host a range of acid/base transporters, many of which participate in CSF secretion and most likely contribute to the transport of acid/base equivalents into the ventricles. This review describes the current understanding of the molecular mechanisms in choroid plexus acid/base regulation and the possible role in CSF pH regulation.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

Transport across the choroid plexus epithelium

The choroid plexus epithelium is a secretory epithelium par excellence. However, this is perhaps not the most prominent reason for the massive interest in this modest-sized tissue residing inside the brain ventricles. Most likely, the dominant reason for extensive studies of the choroid plexus is the identification of this epithelium as the source of the majority of intraventricular cerebrospinal fluid. This finding has direct relevance for studies of diseases and conditions with deranged central fluid volume or ionic balance. While the concept is supported by the vast majority of the literature, the implication of the choroid plexus in secretion of the cerebrospinal fluid was recently challenged once again. Three newer and promising areas of current choroid plexus-related investigations are as follows: 1) the choroid plexus epithelium as the source of mediators necessary for central nervous system development, 2) the choroid plexus as a route for microorganisms and immune cells into the central nervous system, and 3) the choroid plexus as a potential route for drug delivery into the central nervous system, bypassing the blood-brain barrier. Thus, the purpose of this review is to highlight current active areas of research in the choroid plexus physiology and a few matters of continuous controversy.

Differential cellular expression of FXYD1 (phospholemman) and FXYD2 (gamma subunit of Na, K-ATPase) in normal human tissues: a study using high density human tissue microarrays

FXYD proteins have been proposed to function as regulators of Na, K-ATPase function by lowering affinities of the system for potassium and sodium. However, their distribution in normal human tissues has not been studied. We have therefore used immunohistochemistry and semi-quantitative histomorphometric analysis to determine the relative expression at the protein level and distribution of FXYD1 (phospholemman) and FXYD2 (gamma subunit of Na, K-ATPase) in human Tissue MicroArrays (TMAs). Expression of FXYD1 was abundant in heart, kidney, placenta, skeletal muscle, gastric and anal mucosa, small intestine and colon. Lower FXYD1 expression was detected in uterine, intestinal and bladder smooth muscle, choroid plexus, liver, gallbladder, spleen, breast, prostate and epididymis. The tissue distribution of FXYD2 was less extensive compared to that of FXYD1. There was an abundant expression in kidney and choroid plexus and moderate expression in placenta, amniotic membranes, breast epithelium, salivary glands, pancreas and uterine endometrium. Weaker FXYD2 expression was detected in the adrenal medulla, liver, gallbladder, bladder and pancreas. The common denominator in the distribution of FXYD1 and FXYD2 was expression in highly active transport epithelia of the kidney, choroid plexus, placenta and salivary glands. This study reveals, in human tissues, the specific expression of FXYD proteins, which may associate with Na, K-ATPase in selected cell types and modulate its catalytic properties.

Ion channel diversity, channel expression and function in the choroid plexuses

Knowledge of the diversity of ion channel form and function has increased enormously over the last 25 years. The initial impetus in channel discovery came with the introduction of the patch clamp method in 1981. Functional data from patch clamp experiments have subsequently been augmented by molecular studies which have determined channel structures. Thus the introduction of patch clamp methods to study ion channel expression in the choroid plexus represents an important step forward in our knowledge understanding of the process of CSF secretion.

Two K⁺ conductances have been identified in the choroid plexus: Kv1 channel subunits mediate outward currents at depolarising potentials; Kir 7.1 carries an inward-rectifying conductance at hyperpolarising potentials. Both K⁺ channels are localised at the apical membrane where they may contribute to maintenance of the membrane potential while allowing the recycling of K⁺ pumped in by Na⁺-K⁺ ATPase. Two anion conductances have been identified in choroid plexus. Both have significant HCO₃^- permeability, and may play a role in CSF secretion. One conductance exhibits inward-rectification and is regulated by cyclic AMP. The other is carried by an outward-rectifying channel, which is activated by increases in cell volume. The molecular identity of the anion channels is not known, nor is it clear whether they are expressed in the apical or basolateral membrane. Recent molecular evidence indicates that choroid plexus also expresses the non-selective cation channels such as transient receptor potential channels (TRPV4 and TRPM3) and purinoceptor type 2 (P2X) receptor operated channels. In conclusion, good progress has been made in identifying the channels expressed in the choroid plexus, but determining the precise roles of these channels in CSF secretion remains a challenge for the future.

Water and solute secretion by the choroid plexus

The cerebrospinal fluid (CSF) provides mechanical and chemical protection of the brain and spinal cord. This review focusses on the contribution of the choroid plexus epithelium to the water and salt homeostasis of the CSF, i.e. the secretory processes involved in CSF formation. The choroid plexus epithelium is situated in the ventricular system and is believed to be the major site of CSF production. Numerous studies have identified transport processes involved in this secretion, and recently, the underlying molecular background for some of the mechanisms have emerged. The nascent CSF consists mainly of NaCl and NaHCO(3), and the production rate is strictly coupled to the rate of Na(+) secretion. In contrast to other secreting epithelia, Na(+) is actively pumped across the luminal surface by the Na(+),K(+)-ATPase with possible contributions by other Na(+) transporters, e.g. the luminal Na(+),K(+),2Cl(-) cotransporter. The Cl(-) and HCO(3) (-) ions are likely transported by a luminal cAMP activated inward rectified anion conductance, although the responsible proteins have not been identified. Whereas Cl(-) most likely enters the cells through anion exchange, the functional as well as the molecular basis for the basolateral Na(+) entry are not yet well-defined. Water molecules follow across the epithelium mainly through the water channel, AQP1, driven by the created ionic gradient. In this article, the implications of the recent findings for the current model of CSF secretion are discussed. Finally, the clinical implications and the prospects of future advances in understanding CSF production are briefly outlined.

Identification and functional characterization of ClC-2 chloride channels in trabecular meshwork cells

In the eye, trabecular meshwork (TM) cell volume may be an important determinant of aqueous humor outflow. Among their functions, ClC-2 chloride channels are thought to be involved in regulation of cellular volume and intracellular [Cl(-)]. We characterized the properties and modulation of an inwardly rectifying chloride current activated in these cells. Patch-clamp recordings revealed inwardly rectifying chloride currents activated by membrane hyperpolarization in primary cultures of both bovine (BTM) and human (HTM) TM cells. Electrophysiological properties and anion permeability sequence (Cl(-)>Br(-)>I(-)>F(-)) were in agreement with previous data for ClC-2 in other cells. The currents were blocked by Cd(2+) and enhanced by extracellular acidification, 8Br-cAMP and cell swelling, while extracellular alkalinization decreased them. RT-PCR experiments using total RNA revealed the molecular expression of ClC-2 channels. Previously we reported the involvement of swelling-activated chloride channels (Cl(swell)) and Ca(2+)-activated K(+) channels (BK(Ca)) in cell volume and outflow facility regulation. However, in the present analysis, cell volume experiments in calcein-loaded cells and outflow facility studies performed in bovine anterior segments revealed that ClC-2 channels do not make a significant contribution to the recovery of cellular volume or to the regulation of the outflow facility. Nevertheless, ClC-2 modulation by different stimuli may contribute to intracellular [Cl(-)] regulation and other cellular functions yet to be determined in the TM.

Molecular mechanisms of cerebrospinal fluid production

The epithelial cells of the choroid plexuses secrete cerebrospinal fluid (CSF), by a process which involves the transport of Na(+), Cl(-) and HCO(3)(-) from the blood to the ventricles of the brain. The unidirectional transport of ions is achieved due to the polarity of the epithelium, i.e. the ion transport proteins in the blood-facing (basolateral) membrane are different to those in the ventricular (apical) membrane. The movement of ions creates an osmotic gradient which drives the secretion of H(2)O. A variety of methods (e.g. isotope flux studies, electrophysiological, RT-PCR, in situ hybridization and immunocytochemistry) have been used to determine the expression of ion transporters and channels in the choroid plexus epithelium. Most of these transporters have now been localized to specific membranes. For example, Na(+)-K(+)ATPase, K(+) channels and Na(+)-2Cl(-)-K(+) cotransporters are expressed in the apical membrane. By contrast the basolateral membrane contains Cl(-)- HCO(3) exchangers, a variety of Na(+) coupled HCO(3)(-) transporters and K(+)-Cl(-) cotransporters. Aquaporin 1 mediates water transport at the apical membrane, but the route across the basolateral membrane is unknown. A model of CSF secretion by the mammalian choroid plexus is proposed which accommodates these proteins. The model also explains the mechanisms by which K(+) is transported from the CSF to the blood.

Biophysical and pharmacological characterization of hypotonically activated chloride currents in cortical astrocytes

Rat cortical astrocytes regulate their cell volume in response to hypotonic challenge. This regulation is believed to depend largely on the release of chloride or organic osmolytes through anion channels. Using whole-cell recordings, we identified weakly outwardly rectifying chloride currents that could be activated in response to hypotonic challenge. These currents exhibited the following permeability sequence upon replacement of chloride in the bathing solution with various anions: I- > NO3- > Cl- > Gluc- > or = MeS- > Ise-. Interestingly, extracellular I-, albeit showing the greatest permeability, blocked the currents with an IC50 of approximately 50 mM. Currents were almost completely inhibited by 123 microM NPPB and partially inhibited by 200 microM niflumic acid or 200 microM DIDS. Additionally, the total number of Cl- ions effluxed through the hypotonically activated channels was markedly similar to the total solute efflux during volume regulation. We therefore propose the hypotonically activated chloride channel as a major contributor to volume regulation of astrocytes. To examine potential candidate chloride channel genes expressed by astrocytes, we employed RT-PCR to demonstrate the presence of transcripts for ClC-2, 3, 4, 5, and 7, as well as for VDAC and CFTR in cultured astrocytes. Moreover, we performed immunostaining with antibodies against each of these channels and showed the strongest expression of ClC-2 and ClC-3, strong expression of ClC-5 and VDAC, weak expression of ClC-7 and very weak expression of ClC-4 and CFTR. Intriguingly, although we found at least seven Cl- channel proteins from three different gene families in astrocytes, none appeared to be active in resting cells.

Conformation-dependent regulation of inward rectifier chloride channel gating by extracellular protons

We have investigated the gating properties of the inward rectifier chloride channel (Cl(ir)) from mouse parotid acinar cells by external protons (H(+)(o)) using the whole-cell patch-clamp technique. Increasing the pH(o) from 7.4 to 8.0 decreased the magnitude of Cl(ir) current by shifting the open probability to more negative membrane potentials with little modification of the activation kinetics. The action of elevated pH was independent of the conformational state of the channel. The effects of low pH on Cl(ir) channels were dependent upon the conformational state of the channel. That is, application of pH 5.5 to closed channels essentially prevented channel opening. In contrast, application of pH 5.5 to open channels actually increased the current. These results are consistent with the existence of two independent protonatable sites: (1) a site with a pK near 7.3, the titration of which shifts the voltage dependence of channel gating; and (2) a site with pK = 6.0. External H(+) binds to this latter site (with a stoichiometry of two) only when the channels are closed and prevent channel opening. Finally, block of channels by Zn(2+) and Cd(2+) was inhibited by low pH media. We propose that mouse parotid Cl(ir) current has a bimodal dependence on the extracellular proton concentration with maximum activity near pH 6.5: high pH decreases channel current by shifting the open probability to more negative membrane potentials and low pH also decreases the current but through a proton-dependent stabilization of the channel closed state.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

Inward-rectifying anion channels are expressed in the epithelial cells of choroid plexus isolated from ClC-2 'knock-out' mice

Choroid plexus epithelial cells express inward-rectifying anion channels which have a high HCO(3)(-) permeability. These channels are thought to have an important role in the secretion of cerebrospinal fluid. The possible relationship between these channels and the ClC-2 Cl(-) channel was investigated in the present study. RT-PCR, using specific ClC-2 primers, amplified a 238 bp fragment of mRNA from rat choroid plexus, which was 99 % identical to the 5' sequence of rat ClC-2. A 2005 bp clone was isolated from a rat choroid plexus cDNA library using a probe for ClC-2. The clone showed greater than 99 % identity with the sequence of rat ClC-2. Inward-rectifying anion channels were observed in whole-cell recordings of choroid plexus epithelial cells isolated from ClC-2 knock-out mice. The mean inward conductance was 19.6 plus minus 3.6 nS (n = 8) in controls (3 heterozygote animals), and 22.5 plus minus 3.1 nS (n = 10) in three knock-out animals. The relative permeability of the conductances to I(-) and Cl(-) (P(I) : P(Cl)) was determined. I(-) was more permeant than Cl(-) in both heterozygotes (P(I):P(Cl) = 4.0 +/- 0.9, n = 3) and knock-out animals (P(I) : P(Cl) = 4.1 +/- 1.4, n = 3). These results indicate that rat choroid plexus expresses the ClC-2 variant that was originally reported in other tissues. ClC-2 does not contribute significantly to inward-rectifying anion conductance in mouse choroid plexus, which must therefore express a novel inward-rectifying anion channel.

pH-sensitive inwardly rectifying chloride current in cultured rat cortical astrocytes

The effect of pH(o) on plasma membrane chloride current of cultured rat cortical astrocytes was investigated using the whole-cell patch-clamp technique. In the presence of intra- and extracellular solutions with symmetrical high Cl(-) content and K(+) channel inhibitors, the cells exhibited an inwardly rectifying current. The current activated slowly at potentials negative to -40 mV and did not display time-dependent inactivation. The current was inhibited by 0.1 mM Cd(2+), 0.1 mM Zn(2+), 1 mM 9-anthracene-carboxylic acid, and 0.2 mM 5-nitro-2-(3-phenylpropylamino)benzoic acid, but not by 10 mM Ba(2+) or 3 mM Cs(+). Reversal potential of the current followed the chloride equilibrium potential and was not influenced by changes in K(+) or Na(+) concentration. The inwardly rectifying chloride current was augmented by extracellular acidosis and reduced by alkalosis. The pH sensitivity was most pronounced in the physiologically relevant pH(o) range of 6.9--7.9. Lowering pH to 6.4 induced no additional increase in steady-state current amplitude compared with pH(o) 6.9, but it substantially slowed the activation kinetics. According to its kinetic and pharmacological properties this chloride current is similar to that found in cultured rat astrocytes after long-term treatment with dibutyryl-cAMP, however, in our cultures it was consistently expressed without any treatment with the drug. Considering that astrocytes possess carbonic anhydrase and Cl(-)/HCO3(-) antiporter, this current may participate in the regulation of the interstitial and astrocyte pH.

pH modulation of an inward rectifier chloride current in cultured rat cortical astrocytes

The effects of changes in extra- and intracellular pH in the pathophysiological range (6.0-8.0) on astroglial plasma membrane ionic currents were investigated with the whole-cell patch-clamp technique. In cultured rat neocortical type-1 astrocytes differentiated by a long-term treatment with dibutyryl cyclic-AMP, exposure to an extracellular pH of 6.4 induced, as compared with the control extracellular pH at 7.3, a sustained and reversible increase in the holding current at -60mV. The rise in current was accompanied by a decrease in the apparent input resistance. Ion substitution experiments indicated that extracellular pH 6.4 upregulated the resting Cl(-) conductance, whereas an opposite effect could be observed at extracellular pH 8.0. Recordings of isolated Cl(-) currents showed that this modulation occurred on the previously identified hyperpolarization-activated, inwardly rectifying Cl(-) current, I(Clh). Extracellular acidification to pH 6.4 shifted the voltage dependence of I(Clh) activation by approximately 20mV towards more positive potentials, whereas a approximately 20mV opposite shift was observed upon exposure to extracellular pH 8.0. These effects were paralleled by an increase (extracellular pH 6.4) or decrease (extracellular pH 8.0) in the maximal conductance. Decreasing (6.0) or increasing (8.0) the intracellular pH shifted the steady-state activation of I(Clh) towards more negative or positive potentials, respectively, leaving unchanged the current sensitivity to extracellular pH modifications. The modulation of the inward rectifier Cl(-) current expressed by differentiated cultured neocortical astrocytes indicates that extra- and intracellular changes in pH occurring in a pathophysiological range may contribute to regulating Cl(-) accumulation in astroglial cells.

Mechanisms of CSF secretion by the choroid plexus

The epithelial cells of the choroid plexus secrete cerebrospinal fluid (CSF), by a process that involves the movement of Na(+), Cl(-) and HCO(3)(-) from the blood to the ventricles of the brain. This creates the osmotic gradient, which drives the secretion of H(2)O. The unidirectional movement of the ions is achieved due to the polarity of the epithelium, i.e., the ion transport proteins in the blood-facing (basolateral) are different to those in the ventricular (apical) membranes. Saito and Wright (1983) proposed a model for secretion by the amphibian choroid plexus, in which secretion was dependent on activity of HCO(3)(-) channels in the apical membrane. The patch clamp method has now been used to study the ion channels expressed in rat choroid plexus. Two potassium channels have been observed that have a role in maintaining the membrane potential of the epithelial cell, and in regulating the transport of K(+) across the epithelium. An inward-rectifying anion channel has also been identified, which is closely related to ClC-2 channels, and has a significant HCO(3)(-) permeability. This channel is expressed in the apical membrane of the epithelium where it may play an important role in CSF secretion. A model of CSF secretion by the mammalian choroid plexus is proposed that accommodates these channels and other data on the expression of transport proteins in the choroid plexus.

pH and external Ca(2+) regulation of a small conductance Cl(-) channel in kidney distal tubule

A single channel characterization of the Cl(-) channels in distal nephron was undertaken using vesicles prepared from plasma membranes of isolated rabbit distal tubules. The presence in this vesicle preparation of ClC-K type Cl(-) channels was first established by immunodetection using an antibody raised against ClC-K isoforms. A ClC-K1 based functional characterization was next performed by investigating the pH and external Ca(2+) regulation of a small conductance Cl(-) channel which we identified previously by channel incorporation experiments. Acidification of the cis (external) solution from pH 7.4 to 6.5 led to a dose-dependent inhibition of the channel open probability P(O). Similarly, changing the trans pH from 7.4 to 6.8 resulted in a 4-fold decrease of the channel P(O) with no effect on the channel conductance. Channel activity also appeared to be regulated by cis (external) Ca(2+) concentration, with a dose-dependent increase in channel activity as a function of the cis Ca(2+) concentration. It is concluded on the basis of these results that the small conductance Cl(-) channel present in rabbit distal tubules is functionally equivalent to the ClC-K1 channel in the rat. In addition, the present work constitutes the first single channel evidence for a chloride channel regulated by external Ca(2+).

Regulation of a hyperpolarization-activated chloride current in murine respiratory ciliated cells

1. The properties of a hyperpolarization-activated Cl- current (Ihyp-act) in murine ciliated respiratory cells have been studied using whole cell patch clamping. 2. The current-voltage relationship was inwardly rectifying which was due to voltage-dependent gating of the channel. 3. Inward current was markedly sensitive to the extracellular Cl- concentration, an effect that was not related to changes in transmembrane Cl- gradient. Decreasing extracellular Cl- concentration to 6 mM caused a 70 % reduction in inward current with the dose-response relationship exhibiting a Hill coefficient of approximately 2.0 and an IC50 of 29 mM. 4. External anion replacement gave a selectivity sequence of Br- >= I- > Cl- > gluconate = aspartate. The more permeant halides significantly increased current density while the less permeant anions decreased current density, indicating that an extracellular anion is important for channel activity. 5. The conductance was unaffected by exposure to anisotonic pipette solutions or to increases in intracellular cAMP; however, current density was reduced dose dependently by increases in intracellular calcium concentration from 0.1 to 0.5 microM. These results indicate that Ihyp-act is unlikely to be involved in either volume regulation or cAMP/Ca2+-stimulated fluid secretion. 6. Decreasing extracellular pH to 5.0 irreversibly inhibited Ihyp-act. However, the current was fully active over the pH range 5.4-9.0 making it unlikely that it is modulated by extracellular pH under physiological conditions. 7. We speculate that Ihyp-act may have a role in basal Cl- absorption, acting as a Cl- sensor to maintain optimal volume and composition of airway surface liquid.

The chloride channel ClC-2 contributes to the inwardly rectifying Cl- conductance in cultured porcine choroid plexus epithelial cells

1. The contribution of ClC-2 protein to the inwardly rectifying Cl- conductance in cultured porcine choroid plexus epithelial cells was investigated using Western analysis and whole-cell current recordings. 2. Inwardly rectifying currents were elicited by hyperpolarizing voltage at a potential more negative than -50 mV in the presence of intracellular protein kinase A (PKA). The relative halide selectivity estimated from the shift in the reversal potential (Erev) was I- > Br- > Cl- > F-. 3. Extracellular vasoactive intestinal peptide (VIP) activated the same currents in a dose-dependent manner with a half-maximal concentration of 167.3 nM. H-89 (a PKA inhibitor) interfered with the current activation by VIP. 4. The Cl- channel was inhibited by external Cd2+, Ba2+or H+, but only weakly inhibited by known Cl- channel blockers including glibenclamide, NPPB, DIDS and anthracene-9-carboxylic acid (9AC). 5. A specific antibody to ClC-2 detected a 79 kDa protein in porcine choroid plexus cells, which was reduced in cells treated with antisense oligodeoxynucleotide for ClC-2. Both PKA and VIP failed to activate the inwardly rectifying Cl- currents in cells transfected with the antisense oligodeoxynucleotide, while they activated the currents in cells transfected with GFP alone or the control oligodeoxynucleotide randomized from antisense oligonucleotide. 6. It is concluded that ClC-2 protein contributes to the inwardly rectifying Cl- conductance in porcine choroid plexus epithelial cells.

Identification of an acid-activated Cl(-) channel from human skeletal muscles

ClC-4 gene was isolated as a putative Cl(-) channel. Due to a lack of functional expression of ClC-4, its physiological role remains unknown. We isolated a human ClC-4 clone (hClC-4sk) from human skeletal muscles and stably transfected it to Chinese hamster ovary cells. Whole cell patch-clamp studies showed that the hClC-4sk channel was activated by external acidic pH and inhibited by DIDS. It passed a strong outward Cl(-) current with a permeability sequence of I(-) > Cl(-) > F(-). The hClC-4sk has consensus sites for phosphorylation by protein kinase A (PKA); however, stimulation of PKA had no effect on the currents. hClC-4sk mRNA was expressed in excitable tissues, such as heart, brain, and skeletal muscle. These functional characteristics of hClC-4sk provide a clue to its physiological role in excitable cells.