A Novel Anionic Inward Rectifier in Native Cardiac Myocytes

Chloride ions in health and disease

Chloride is a key anion involved in cellular physiology by regulating its homeostasis and rheostatic processes. Changes in cellular Cl- concentration result in differential regulation of cellular functions such as transcription and translation, post-translation modifications, cell cycle and proliferation, cell volume, and pH levels. In intracellular compartments, Cl- modulates the function of lysosomes, mitochondria, endosomes, phagosomes, the nucleus, and the endoplasmic reticulum. In extracellular fluid (ECF), Cl- is present in blood/plasma and interstitial fluid compartments. A reduction in Cl- levels in ECF can result in cell volume contraction. Cl- is the key physiological anion and is a principal compensatory ion for the movement of the major cations such as Na+, K+, and Ca2+. Over the past 25 years, we have increased our understanding of cellular signaling mediated by Cl-, which has helped in understanding the molecular and metabolic changes observed in pathologies with altered Cl- levels. Here, we review the concentration of Cl- in various organs and cellular compartments, ion channels responsible for its transportation, and recent information on its physiological roles.

Arrhythmogenic Remodeling in the Failing Heart

Chronic heart failure is a clinical syndrome with multiple etiologies, associated with significant morbidity and mortality. Cardiac arrhythmias, including ventricular tachyarrhythmias and atrial fibrillation, are common in heart failure. A number of cardiac diseases including heart failure alter the expression and regulation of ion channels and transporters leading to arrhythmogenic electrical remodeling. Myocardial hypertrophy, fibrosis and scar formation are key elements of arrhythmogenic structural remodeling in heart failure. In this article, the mechanisms responsible for increased arrhythmia susceptibility as well as the underlying changes in ion channel, transporter expression and function as well as alterations in calcium handling in heart failure are discussed. Understanding the mechanisms of arrhythmogenic remodeling is key to improving arrhythmia management and the prevention of sudden cardiac death in patients with heart failure.

Novel roles for voltage-gated T-type Ca2+ and ClC-2 channels in phagocytosis and angiogenic factor balance identified in human iPSC-derived RPE

Human-induced pluripotent stem cell (hiPSC)-derived retinal pigment epithelium (RPE) is a powerful tool for pathophysiological studies and preclinical therapeutic screening, as well as a source for clinical cell transplantation. Thus, it must be validated for maturity and functionality to ensure correct data readouts and clinical safety. Previous studies have validated hiPSC-derived RPE as morphologically characteristic of the tissue in the human eye. However, information concerning the expression and functionality of ion channels is still limited. We screened hiPSC-derived RPE for the polarized expression of a panel of L-type (Ca_V 1.1, Ca_V 1.3) and T-type (Ca_V 3.1, Ca_V 3.3) Ca²⁺ channels, K⁺ channels (Maxi-K, Kir4.1, Kir7.1), and the Cl^- channel ClC-2 known to be expressed in native RPE. We also tested the roles of these channels in key RPE functions using specific inhibitors. In addition to confirming the native expression profiles and function of certain channels, such as L-type Ca²⁺ channels, we show for the first time that T-type Ca²⁺ channels play a role in both phagocytosis and vascular endothelial growth factor (VEGF) secretion. Moreover, we demonstrate that Maxi-K and Kir7.1 channels are involved in the polarized secretion of VEGF and pigment epithelium-derived factor (PEDF). Furthermore, we show a novel localization for ClC-2 channel on the apical side of hiPSC-derived RPE, with an overexpression at the level of fluid-filled domes, and demonstrate that it plays an important role in phagocytosis, as well as VEGF and PEDF secretion. Taken together, hiPSC-derived RPE is a powerful model for advancing fundamental knowledge of RPE functions.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Molecular identification of HSPA8 as an accessory protein of a hyperpolarization-activated chloride channel from rat pulmonary vein cardiomyocytes

Pulmonary veins (PVs) are the major origin of atrial fibrillation. Recently, we recorded hyperpolarization-activated Cl^- current (I _{Cl, h}) in rat PV cardiomyocytes. Unlike the well-known chloride channel protein 2 (CLCN2) current, the activation curve of I _{Cl, h} was hyperpolarized as the Cl^- ion concentration ([Cl^-] _i ) increased. This current could account for spontaneous activity in PV cardiomyocytes linked to atrial fibrillation. In this study, we aimed to identify the channel underlying I _{Cl, h} Using RT-PCR amplification specific for Clcn2 or its homologs, a chloride channel was cloned from rat PV and detected in rat PV cardiomyocytes using immunocytochemistry. The gene sequence and electrophysiological functions of the protein were identical to those previously reported for Clcn2, with protein activity observed as a hyperpolarization-activated current by the patch-clamp method. However, the [Cl^-] _i dependence of activation was entirely different from the observed I _{Cl, h} of PV cardiomyocytes; the activation curve of the Clcn2-transfected cells shifted toward positive potential with increased [Cl^-] _i , whereas the I _{Cl, h} of PV and left ventricular cardiomyocytes showed a leftward shift. Therefore, we used MS to explore the possibility of additional proteins interacting with CLCN2 and identified an individual 71-kDa protein, HSPA8, that was strongly expressed in rat PV cardiomyocytes. With co-expression of HSPA8 in HEK293 and PC12 cells, the CLCN2 current showed voltage-dependent activation and shifted to negative potential with increasing [Cl^-] _i Molecular docking simulations further support an interaction between CLCN2 and HSPA8. These findings suggest that CLCN2 in rat heart contains HSPA8 as a unique accessory protein.

Injection of mRNA isolated from trophozoites of Giardia intestinalis induces expression of three types of chloride currents in Xenopus laevis oocytes

Giardia lamblia is one of the most important worldwide causes of intestinal infections, yet little is known about its cellular physiology, especially the diversity of ionic channels that this parasite expresses. In this work, we show that injection of mRNA isolated from trophozoites of Giardia, into Xenopus laevis oocytes, induces expression of three types of chloride currents (here referred to as ICl-G1, ICl-G2, and ICl-G3), which have different biophysical and pharmacological properties. ICl-G1 currents show inward rectification and voltage dependence are enhanced by hypotonicity, show a selectivity sequence of (I > Br > Cl > F), and are inhibited by NPPB, DIDS, SITS, 9AC, DPC, and Zinc. These findings suggest that ICl-G1 is the result of expression of chloride channels related to ClC2. ICl-G2 currents show outward rectification and are dependent of intracellular calcium, its selectivity sequence is (Cl > Br > I > F) and are inhibited by NPPB, DIDS, SITS, 9AC, DPC, niflumic acid, tannic acid, and benzbromarone. These findings suggest that they are produced by calcium dependent chloride channels (CaCC). The third type of currents (ICl-G3) appears only after a hypoosmotic challenge, and has similar properties to those described for ICl-swell, such as outward rectification, instant activation, and slow inactivation at large depolarizing voltages. They were blocked by NPPB, DIDS, 9AC, NIf, DCPIB, and tamoxifen. Our results indicate that Giardia intestinalis has at least three types of anion conductances.

Cell Volume-Activated and Volume-Correlated Anion Channels in Mammalian Cells: Their Biophysical, Molecular, and Pharmacological Properties

There are a number of mammalian anion channel types associated with cell volume changes. These channel types are classified into two groups: volume-activated anion channels (VAACs) and volume-correlated anion channels (VCACs). VAACs can be directly activated by cell swelling and include the volume-sensitive outwardly rectifying anion channel (VSOR), which is also called the volume-regulated anion channel; the maxi-anion channel (MAC or Maxi-Cl); and the voltage-gated anion channel, chloride channel (ClC)-2. VCACs can be facultatively implicated in, although not directly activated by, cell volume changes and include the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, the Ca2+-activated Cl- channel (CaCC), and the acid-sensitive (or acid-stimulated) outwardly rectifying anion channel. This article describes the phenotypical properties and activation mechanisms of both groups of anion channels, including accumulating pieces of information on the basis of recent molecular understanding. To that end, this review also highlights the molecular identities of both anion channel groups; in addition to the molecular identities of ClC-2 and CFTR, those of CaCC, VSOR, and Maxi-Cl were recently identified by applying genome-wide approaches. In the last section of this review, the most up-to-date information on the pharmacological properties of both anion channel groups, especially their half-maximal inhibitory concentrations (IC50 values) and voltage-dependent blocking, is summarized particularly from the standpoint of pharmacological distinctions among them. Future physiologic and pharmacological studies are definitely warranted for therapeutic targeting of dysfunction of VAACs and VCACs.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Research and progress on ClC‑2 (Review)

Chloride channel 2 (ClC-2) is one of the nine mammalian members of the ClC family. The present review discusses the molecular properties of ClC‑2, including CLCN2, ClC‑2 promoter and the structural properties of ClC‑2 protein; physiological properties; functional properties, including the regulation of cell volume. The effects of ClC‑2 on the digestive, respiratory, circulatory, nervous and optical systems are also discussed, in addition to the mechanisms involved in the regulation of ClC‑2. The review then discusses the diseases associated with ClC‑2, including degeneration of the retina, Sjögren's syndrome, age‑related cataracts, degeneration of the testes, azoospermia, lung cancer, constipation, repair of impaired intestinal mucosa barrier, leukemia, cystic fibrosis, leukoencephalopathy, epilepsy and diabetes mellitus. It was concluded that future investigations of ClC‑2 are likely to be focused on developing specific drugs, activators and inhibitors regulating the expression of ClC‑2 to treat diseases associated with ClC‑2. The determination of CLCN2 is required to prevent and treat several diseases associated with ClC‑2.

Effect of Different Formulations of Magnesium Chloride Used As Anesthetic Agents on the Performance of the Isolated Heart of Octopus vulgaris

Magnesium chloride (MgCl2) is commonly used as a general anesthetic in cephalopods, but its physiological effects including those at cardiac level are not well-characterized. We used an in vitro isolated perfused systemic heart preparation from the common octopus, Octopus vulgaris, to investigate: (a) if in vivo exposure to MgCl2 formulations had an effect on cardiac function in vitro and, if so, could this impact recovery and (b) direct effects of MgCl2 formulations on cardiac function. In vitro hearts removed from animals exposed in vivo to 3.5% MgCl2 in sea water (20 min) or to a mixture of MgCl2+ ethanol (1.12/1%; 20 min) showed cardiac function (heart rate, stroke volume, cardiac output) comparable to hearts removed from animals killed under hypothermia. However, 3.5% MgCl2 (1:1, sea water: distilled water, 20 min) produced a significant impairment of the Frank-Starling response as did 45 min exposure to the MgCl2+ ethanol mixture. Perfusion of the isolated heart with MgCl2± ethanol formulations produced a concentration-related bradycardia (and arrest), a decreased stroke volume and cardiac output indicating a direct effect on the heart. The cardiac effects of MgCl2 are discussed in relation to the involvement of magnesium, sodium, chloride, and calcium ions, exposure time and osmolality of the formulations and the implications for the use of various formulations of MgCl2 as anesthetics in octopus. Overall, provided that the in vivo exposure to 3.5% MgCl2 in sea water or to a mixture of MgCl2+ ethanol is limited to ~20 min, residual effects on cardiac function are unlikely to impact post-anesthetic recovery.

Potential role of cardiac chloride channels and transporters as novel therapeutic targets

The heart and blood vessels express a range of anion currents (e.g. ICl.PKA) and symporter/antiporters (e.g. Cl(-)/HCO3(-) exchanger) that translocate chloride (Cl(-)). They have been proposed to contribute to a variety of physiological processes including cellular excitability, cell volume homeostasis and apoptosis. Additionally there is evidence that Cl(-) currents or transporters may play a role in cardiac pathophysiology. Arrhythmogenesis, the process of cardiac ischaemic preconditioning, and the adaptive remodelling process in myocardial hypertrophy and heart failure have all been linked to such channels or transporters. We have explored the possibility that selective targeting of one or more of these may provide benefit in cardiovascular disease. Existing evidence points to an emerging role of cardiac cell anion channels as potential therapeutic targets, the 'disease-specificity' of which may represent a substantial improvement on current targets. However, the limitations of current techniques hitherto applied (such as developmental compensation in gene-modified animals) and pharmacological agents (which do not at present possess sufficient selectivity for the adequate probing of function) have thus far hindered translation to the introduction of new therapy.

Chloride channelopathies of ClC-2

Chloride channels (ClCs) have gained worldwide interest because of their molecular diversity, widespread distribution in mammalian tissues and organs, and their link to various human diseases. Nine different ClCs have been molecularly identified and functionally characterized in mammals. ClC-2 is one of nine mammalian members of the ClC family. It possesses unique biophysical characteristics, pharmacological properties, and molecular features that distinguish it from other ClC family members. ClC-2 has wide organ/tissue distribution and is ubiquitously expressed. Published studies consistently point to a high degree of conservation of ClC-2 function and regulation across various species from nematodes to humans over vast evolutionary time spans. ClC-2 has been intensively and extensively studied over the past two decades, leading to the accumulation of a plethora of information to advance our understanding of its pathophysiological functions; however, many controversies still exist. It is necessary to analyze the research findings, and integrate different views to have a better understanding of ClC-2. This review focuses on ClC-2 only, providing an analytical overview of the available literature. Nearly every aspect of ClC-2 is discussed in the review: molecular features, biophysical characteristics, pharmacological properties, cellular function, regulation of expression and function, and channelopathies.

Phenomics of cardiac chloride channels

Forward genetic studies have identified several chloride (Cl-) channel genes, including CFTR, ClC-2, ClC-3, CLCA, Bestrophin, and Ano1, in the heart. Recent reverse genetic studies using gene targeting and transgenic techniques to delineate the functional role of cardiac Cl- channels have shown that Cl- channels may contribute to cardiac arrhythmogenesis, myocardial hypertrophy and heart failure, and cardioprotection against ischemia reperfusion. The study of physiological or pathophysiological phenotypes of cardiac Cl- channels, however, is complicated by the compensatory changes in the animals in response to the targeted genetic manipulation. Alternatively, tissue-specific conditional or inducible knockout or knockin animal models may be more valuable in the phenotypic studies of specific Cl- channels by limiting the effect of compensation on the phenotype. The integrated function of Cl- channels may involve multiprotein complexes of the Cl- channel subproteome. Similar phenotypes can be attained from alternative protein pathways within cellular networks, which are influenced by genetic and environmental factors. The phenomics approach, which characterizes phenotypes as a whole phenome and systematically studies the molecular changes that give rise to particular phenotypes achieved by modifying the genotype under the scope of genome/proteome/phenome, may provide more complete understanding of the integrated function of each cardiac Cl- channel in the context of health and disease.

Pathological impact of hyperpolarization-activated chloride current peculiar to rat pulmonary vein cardiomyocytes

Pulmonary veins (PVs) are believed to be a crucial origin of atrial fibrillation. We recently reported that rat PV cardiomyocytes exhibit arrhythmogenic automaticity in response to norepinephrine. Herein, we further characterized the electrophysiological properties underlying the potential arrhythmogenicity of PV cardiomyocytes. Patch clamping studies revealed a time dependent hyperpolarization-activated inward current in rat PV cardiomyocytes, but not in left atrial (LA) myocytes. The current was Cs(+) resistant, and was not affected by removal of external Na(+) or K(+). The current was inhibited with Cd(2+), and the reversal potential was sensitive to changes in [Cl(-)] on either side of the membrane in a manner consistent with a Cl(-) selective channel. Cl(-) channel blockers attenuated the current, and slowed or completely inhibited the norepinephrine-induced automaticity. The biophysical properties of the hyperpolarization-activated Cl(-) current in rat PVs were different from those of ClC-2 currents previously reported: (i) the voltage-dependent activation of the Cl(-) current in rat PVs was shifted to negative potentials as [Cl(-)]i increased, (ii) the Cl(-) current was enhanced by extracellular acidification, and (iii) extracellular hyper-osmotic stress increased the current, whereas hypo-osmotic cell swelling suppressed the current. qPCR analysis revealed negligible ClC-2 mRNA expression in the rat PV. These findings suggest that rat PV cardiomyocytes possess a peculiar voltage-dependent Cl(-) channel, and that the channel may play a functional role in norepinephrine-induced automaticity.

Classification of potassium and chlorine ionic currents in retinal ganglion cell line (RGC-5) by whole-cell patch clamp

Retinal ganglion cell line (RGC-5) has been widely used as a valuable model for studying pathophysiology and physiology of retinal ganglion cells in vitro. However, the electrophysiological characteristics, especially a thorough classification of ionic currents in the cell line, remain to be elucidated in details. In the present study, we determined the resting membrane potential (RMP) in RGC-5 cell line and then identified different types of ionic currents by using the whole-cell patch-clamp technique. The RMP recorded in the cell line was between −30 and −6 mV (−17.6 ± 2.6 mV, n = 10). We observed the following voltage-gated ion channel currents: (1) inwardly rectifying Cl− current (ICl,ir), which could be blocked by Zn2+; (2) Ca2+-activated Cl− current (ICl,Ca), which was sensitive to extracellular Ca2+ and could be inhibited by disodium 4,4’-diisothiocyanatostilbene-2,2’-disulfonate; (3) inwardly rectifying K+ currents (IK1), which could be blocked by Ba2+; (4) a small amount of delayed rectifier K+ current (IK). On the other hand, the voltage-gated sodium channels current (INa) and transient outward potassium channels current (IA) were not observed in this cell line. These results further characterize the ionic currents in the RGC-5 cell line and are beneficial for future studies especially on ion channel (patho)physiology and pharmacology in the RGC-5 cell line.

New molecular mechanisms for cardiovascular disease: cardiac hypertrophy and cell-volume regulation

Cardiac hypertrophy is an increase in the muscle volume of the ventricle due to the enlargement of cardiac cells. Physiological cardiac hypertrophy is the normal response to healthy exercise, and pathological hypertrophy is the response to increased stress such as hypertension. Intracellular and extracellular aniosmotic conditions also change cell volume. Since persistent cell swelling or cell shrinkage during aniosmotic conditions results in cell death, the ability to regulate cell volume is important for the maintenance of cellular homeostasis. Cell swelling activates a regulatory volume decrease (RVD) response in which solute leakage pathways are stimulated and solute with water exits cells, reducing the cell volume towards the original value. In cardiac cells, one of the essential factors for cell-volume regulation is the volume-regulated anion channel (VRAC). However, the relationship between cardiac hypertrophy and cell-volume regulation is not clear. In this review, we introduce our recent findings showing that the impairment of VRAC current is exhibited in ventricular cells from mice with cardiac hypertrophy induced by transverse aortic constriction. Similar results were shown in caveolin-3-deficient mice, which develop cardiac hypertrophy without pressure overload. These results suggest that VRAC will be a new target for protection from the development of cardiac hypertrophy.

Functional role of CLC-2 chloride inward rectifier channels in cardiac sinoatrial nodal pacemaker cells

A novel Cl(-) inward rectifier channel (Cl,ir) encoded by ClC-2, a member of the ClC voltage-gated Cl(-) channel gene superfamily, has been recently discovered in cardiac myocytes of several species. However, the physiological role of Cl,ir channels in the heart remains unknown. In this study we tested the hypothesis that Cl,ir channels may play an important role in cardiac pacemaker activity. In isolated guinea-pig sinoatrial node (SAN) cells, Cl,ir current was activated by hyperpolarization and hypotonic cell swelling. RT-PCR and immunohistological analyses confirmed the molecular expression of ClC-2 in guinea-pig SAN cells. Hypotonic stress increased the diastolic depolarization slope and decreased the maximum diastolic potential, action potential amplitude, APD(50), APD(90), and the cycle-length of the SAN cells. These effects were largely reversed by intracellular dialysis of anti-ClC-2 antibody, which significantly inhibited Cl,ir current but not other pacemaker currents, including the hyperpolarization-activated non-selective cationic "funny" current (I(f)), the L-type Ca(2+) currents (I(Ca,L)), the slowly-activating delayed rectifier I(Ks) and the volume-regulated outwardly-rectifying Cl(-) current (I(Cl,vol)). Telemetry electrocardiograph studies in conscious ClC-2 knockout (Clcn2(-/-)) mice revealed a decreased chronotropic response to acute exercise stress when compared to their age-matched Clcn2(+/+) and Clcn2(+/-) littermates. Targeted inactivation of ClC-2 does not alter intrinsic heart rate but prevented the positive chronotropic effect of acute exercise stress through a sympathetic regulation of ClC-2 channels. These results provide compelling evidence that ClC-2-encoded endogenous Cl,ir channels may play an important role in the regulation of cardiac pacemaker activity, which may become more prominent under stressed or pathological conditions.

Phenomics of cardiac chloride channels: the systematic study of chloride channel function in the heart

Recent studies have identified several chloride (Cl-) channel genes in the heart, including CFTR, ClC-2, ClC-3, CLCA, Bestrophin, and TMEM16A. Gene targeting and transgenic techniques have been used to delineate the functional role of cardiac Cl- channels in the context of health and disease. It has been shown that Cl- channels may contribute to cardiac arrhythmogenesis, myocardial hypertrophy and heart failure, and cardioprotection against ischaemia-reperfusion. The study of physiological or pathophysiological phenotypes of cardiac Cl- channels, however, may be complicated by the compensatory changes in the animals in response to the targeted genetic manipulation. Alternatively, tissue-specific conditional or inducible knockout or knockin animal models may be more valuable in the phenotypic studies of specific Cl- channels by limiting the effect of compensation on the phenotype. The integrated function of Cl- channels may involve multi-protein complexes of the Cl- channel subproteome and similar phenotypes can be attained from alternative protein pathways within cellular networks, which are influenced by genetic and environmental factors. Therefore, the phenomics approach, which characterizes phenotypes as a whole phenome and systematically studies the molecular changes that give rise to particular phenotypes achieved by modifying the genotype (such as gene knockouts or knockins) under the scope of genome/proteome/phenome, may provide a more complete understanding of the integrated function of each cardiac Cl- channel in the context of health and disease.

Functional characteristics and molecular identification of swelling-activated chloride conductance in adult rabbit heart ventricles

Outwardly rectifying swelling-activated chloride conductance (ICl,Swell) in rabbit heart plays a critical role in cardioprotection following ischemic preconditioning (IP). But the functional characterization and molecular basis of this chloride conductance in rabbit heart ventricular myocytes is not clear. Candidate chloride channel clones (e.g. ClC-2, ClC-3, ClC-4 and ClC-5) were determined using RT-PCR and Western blot analysis. Whole cell ICl,Swell was recorded from isolated rabbit ventricular myocytes using patch clamp techniques during hypo-osmotic stress. The inhibitory effects of 4,4' isothiocyanato-2,2-disulfonic acid (DIDS), 5-nitro-2(3-phenylroylamino) benzoic acid (NPPB) and indanyloxyacetic acid 94 (IAA-94) on ICl,Swell were examined. The expected size of PCR products for ClC-2, ClC-3 and ClC-4 but not for ClC-5 was obtained. ClC-2 and ClC-3 expression was confirmed by automated fluorescent DNA sequencing. RT-PCR and Western blot showed that ClC-4 was expressed in abundance and ClC-2 was expressed at somewhat lower levels. The biological and pharmacological properties of I(Cl,Swell), including outward rectification, activation due to cell volume change, sensitivity to DIDS, IAA-94 and NPPB were identical to those known properties of ICl,Swell in exogenously expressed systems and other mammals hearts. It was concluded that ClC-3 or ClC-4 might be responsible for the outwardly rectifying part of ICl,Swell and may be the molecular targets of cardioprotection associated with ischemic preconditioning or hypo-osmotic shock.

Properties and possible function of a hyperpolarisation-activated chloride current in Drosophila

A chloride current, ICl,H, slowly activating on hyperpolarisation was investigated in Drosophila melanogaster larval muscles using the two-electrode voltage clamp. Sizeable currents were observed after the intracellular chloride concentration([Cl–]i) had been elevated by diffusion of Cl– from the electrodes. The time course of ICl,H was rather variable and required two exponentials to be accurately described. The reversal potential, –40 to –20 mV in Cl–-loaded fires, shifted on lowering external[Cl–] in the positive direction. Steady-state activation of ICl,H was characterised by V0.5 of≈–120 mV and a slope factor, k, of ≈10 mV at a[Cl–]i ≈35 mmol l–1. Raising[Cl–]i to ≈50 mmol l–1 caused a negative shift of V0.5 equivalent to the change of ECl and led to a nearly threefold increase in maximal steady-state conductance. ICl,H was resistant to 10 mmol l–1 Zn2+ and 1 mmol l–1Cd2+ but was greatly reduced by 1 mmol l–19-anthracenecarboxylic acid (9-AC). ICl,H was affected by changes of extracellular pH and increased on lowering extracellular osmolality. 9-AC also decreased muscle fibre resting conductance by approximately 20% and increased muscle contractions. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis confirmed the expression of all three ClC genes in muscle, and immunohistochemistry indicated location of Drosophila melanogaster chloride channel-2(DmClC-2) at the Z-lines. We conclude that DmClC-2 accounts for the channels underlying ICl,H, and in part for the resting chloride conductance. DmClC-2 may serve general homeostatic mechanisms such as pH- and osmo-regulation or may support muscle function on high motor activity or during a particular neurohormonal state of the animal.

Differential pharmacology of the cardiac anionic background current I(AB)

A novel anionic background conductance (I(AB)) in cardiac ventricular myocytes has recently been identified but at present there is comparatively little information on its pharmacological modulation. This study investigated the effects of on I(AB) of four pyrethroid agents tefluthrin (a selective activator of this current), tetramethrin, fenpropathrin and alpha-cypermethrin in addition to other well known chloride channel modulators (chlorotoxin, gadolinium and picrotoxin). Guinea-pig ventricular myocytes were isolated using an enzymatic and mechanical dispersion procedure and all electrophysiological measurements were made using the whole-cell patch-clamp technique. In contrast to other anion conductances (stretch- or volume-regulated chloride current (I(Cl,vol)), a cAMP-dependent Cl(-) current (I(Cl,cAMP))) I(AB) was augmented by tefluthrin, fenpropathrin, alpha-cypermethrin (but not tetramethrin). I(AB) was insensitive to chlorotoxin, gadolinium and picrotoxin. Thus, I(AB) exhibits a distinct pharmacological profile from other known cardiac anion conductances.

Anion channels in astrocytes: biophysics, pharmacology, and function

The chloride/anion channels that have been so far identified in cultured astrocytes and those that have been confirmed in situ by a combination of mRNA identification, immunocytochemistry, and biophysical studies are reviewed. It is emphasized that we are just beginning to describe such channels and analyze their functions in astrocytes. The best-studied anion channels studied so far are those known as volume-regulated anion channels (VRACs). These, as for most channels, have been mainly studied in cultured astrocytes, but some correlative studies have been done in situ, because these channels have been emphasized as release routes for transmitters; namely, excitatory amino acids and ATP. They are activated by cell shape changes and cell swelling, and the release of amino acids and ATP and chloride currents, measured by whole cell clamping, by these processes has been well described, as is also their activation by low concentrations of extracellular ATP. However, the identity of these channels in astrocytes, as in all other cells, remains elusive. The potential involvement of VRACs in pathological states such as stroke, metastasis, and spreading depression is also discussed.

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.

Association between Hsp90 and the ClC-2 chloride channel upregulates channel function

The voltage-dependent ClC-2 chloride channel has been implicated in a variety of physiological functions, including fluid transport across specific epithelia. ClC-2 is activated by hyperpolarization, weakly acidic external pH, intracellular Cl−, and cell swelling. To add more insight into the mechanisms involved in ClC-2 regulation, we searched for associated proteins that may influence ClC-2 activity. With the use of immunoprecipitation of ClC-2 from human embryonic kidney-293 cells stably expressing the channel, followed by electrophoretic separation of coimmunoprecipitated proteins and mass spectrometry identification, Hsp70 and Hsp90 were unmasked as possible ClC-2 interacting partners. Association of Hsp90 with ClC-2 was confirmed in mouse brain. Inhibition of Hsp90 by two specific inhibitors, geldanamycin or radicicol, did not affect total amounts of ClC-2 but did reduce plasma membrane channel abundance. Functional experiments using the whole cell configuration of the patch-clamp technique showed that inhibition of Hsp90 reduced ClC-2 current amplitude and impaired the intracellular Cl− concentration [Cl−]-dependent rightward shift of the fractional conductance. Geldanamycin and radicicol increased both the slow and fast activation time constants in a chloride-dependent manner. Heat shock treatment had the opposite effect. These results indicate that association of Hsp90 with ClC-2 results in greater channel activity due to increased cell surface channel expression, facilitation of channel opening, and enhanced channel sensitivity to intracellular [Cl−]. This association may have important pathophysiological consequences, enabling increased ClC-2 activity in response to cellular stresses such as elevated temperature, ischemia, or oxidative reagents.

Acidic extracellular pH-activated outwardly rectifying chloride current in mammalian cardiac myocytes

Extracellular acidic pH was found to induce an outwardly rectifying Cl- current (I(Cl,acid)) in mouse ventricular cells, with a half-maximal activation at pH 5.9. The current showed the permeability sequence for anions to be SCN- > Br- > I- > Cl- > F- > aspartate, while it exhibited a time-dependent activation at large positive potentials. Similar currents were also observed in mouse atrial cells and in atrial and ventricular cells from guinea pig. Some Cl- channel blockers (DIDS, niflumic acid, and glibenclamide) inhibited ICl,acid, whereas tamoxifen had little effect on it. Unlike volume-regulated Cl- current (ICl,vol) and CFTR Cl- current (ICl,CFTR), ICl,acid was independent of the presence of intracellular ATP. Activation of ICl,acid appeared to be also independent of intracellular Ca2+ and G protein. ICl,acid and ICl,vol could develop in an additive fashion in acidic hypotonic solutions. Isoprenaline-induced ICl,CFTR was inhibited by acidification in a pH-dependent manner in guinea pig ventricular cells. Our results support the view that ICl,acid and ICl,vol stem from two distinct populations of anion channels and that the ICl,acid channels are present in cardiac cells. ICl,acid may play a role in the control of action potential duration or cell volume under pathological conditions, such as ischemia-related cardiac acidosis.

Molecular mechanisms of regulation of fast-inactivating voltage-dependent transient outward K+ current in mouse heart by cell volume changes

The K(v)4.2/4.3 channels are the primary subunits that contribute to the fast-inactivating, voltage-dependent transient outward K(+) current (I(to,fast)) in the heart. I(to,fast) is the critical determinant of the early repolarization of the cardiac action potential and plays an important role in the adaptive remodelling of cardiac myocytes, which usually causes cell volume changes, during myocardial ischaemia, hypertrophy and heart failure. It is not known, however, whether I(to,fast) is regulated by cell volume changes. In this study we investigated the molecular mechanism for cell volume regulation of I(to,fast) in native mouse left ventricular myocytes. Hyposmotic cell swelling caused a marked increase in densities of the peak I(to,fast) and a significant shortening in phase 1 repolarization of the action potential duration. The voltage-dependent gating properties of I(to,fast) were, however, not altered by changes in cell volume. In the presence of either protein kinase C (PKC) activator (12,13-dibutyrate) or phosphatase inhibitors (calyculin A and okadaic acid), hyposmotic cell swelling failed to further up-regulate I(to,fast). When expressed in NIH/3T3 cells, both K(v)4.2 and K(v)4.3 channels were also strongly regulated by cell volume in the same voltage-independent but PKC- and phosphatase-dependent manner as seen in I(to,fast) in the native cardiac myocytes. We conclude that K(v)4.2/4.3 channels in the heart are regulated by cell volume through a phosphorylation/dephosphorylation pathway mediated by PKC and serine/threonine phosphatase(s). These findings suggest a novel role of K(v)4.2/4.3 channels in the adaptive electrical and structural remodelling of cardiac myocytes in response to myocardial hypertrophy, ischaemia and reperfusion.

Inward-rectifier K+ current in guinea-pig ventricular myocytes exposed to hyperosmotic solutions

Superfusion of heart cells with hyperosmotic solution causes cell shrinkage and inhibition of membrane ionic currents, including delayed-rectifer K+ currents. To determine whether osmotic shrinkage also inhibits inwardly-rectifying K+ current (I(K1)), guinea-pig ventricular myocytes in the perforated-patch or ruptured-patch configuration were superfused with a Tyrode's solution whose osmolarity (T) relative to isosmotic (1T) solution was increased to 1.3-2.2T by addition of sucrose. Hyperosmotic superfusate caused a rapid shrinkage that was accompanied by a negative shift in the reversal potential of Ba(2+)-sensitive I(K1), an increase in the amplitude of outward I(K1), and a steepening of the slope of the inward I(K1)-voltage (V) relation. The magnitude of these effects increased with external osmolarity. To evaluate the underlying changes in chord conductance (G(K1)) and rectification, G(K1)-V data were fitted with Boltzmann functions to determine maximal G(K1) (G(K1)max) and voltage at one-half G(K1)max (V(0.5)). Superfusion with hyperosmotic sucrose solutions led to significant increases in G(K1)max (e.g., 28 +/- 2% with 1.8T), and significant negative shifts in V(0.5) (e.g., -6.7 +/- 0.6 mV with 1.8T). Data from myocytes investigated under hyperosmotic conditions that do not induce shrinkage indicate that G(K1)max and V(0.5) were insensitive to hyperosmotic stress per se but sensitive to elevation of intracellular K+. We conclude that the effects of hyperosmotic sucrose solutions on I(K1) are related to shrinkage-induced concentrating of intracellular K+.

Inwardly rectifying chloride channel activity in intestinal pacemaker cells

Cl(-) channels are proposed to play a role in gut pacemaker activity, but little is known about the characteristics of Cl(-) channels in interstitial cells of Cajal (ICC), the intestinal pacemaker cells. The objective of the present study was to identify whole cell Cl(-) currents in ICC associated with previously observed single-channel activity and to characterize its inward rectification. Whole cell patch-clamp studies showed that ICC express an inwardly rectifying Cl(-) current that was not sensitive to changes in cation composition of the extracellular solutions. Currents were not affected by replacing all cations with N-methyl-d-glucamine (NMDG(+)). Whole cell currents followed the Cl(-) equilibrium potential and were inhibited by DIDS and 9-anthracene carboxylic acid. Ramp protocols of single-channel activity showed that inward rectification was due to reduction in single-channel open probability, not a reduction in single-channel conductance. Single-channel data led to the hypothesis that strong cooperation exists between 30-pS channels that show less cooperation at potentials positive to the reversal potential. Hence, an inwardly rectifying Cl(-) channel plays a prominent role in determining pacemaker activity in the gut.

Functional role of anion channels in cardiac diseases

In comparison to cation (K+, Na+, and Ca2+) channels, much less is currently known about the functional role of anion (Cl-) channels in cardiovascular physiology and pathophysiology. Over the past 15 years, various types of Cl- currents have been recorded in cardiac cells from different species including humans. All cardiac Cl- channels described to date may be encoded by five different Cl- channel genes: the PKA- and PKC-activated cystic fibrosis tansmembrane conductance regulator (CFTR), the volume-regulated ClC-2 and ClC-3, and the Ca2+-activated CLCA or Bestrophin. Recent studies using multiple approaches to examine the functional role of Cl- channels in the context of health and disease have demonstrated that Cl- channels might contribute to: 1) arrhythmogenesis in myocardial injury; 2) cardiac ischemic preconditioning; and 3) the adaptive remodeling of the heart during myocardial hypertrophy and heart failure. Therefore, anion channels represent very attractive novel targets for therapeutic approaches to the treatment of heart diseases. Recent evidence suggests that Cl- channels, like cation channels, might function as a multiprotein complex or functional module. In the post-genome era, the emergence of functional proteomics has necessitated a new paradigm shift to the structural and functional assessment of integrated Cl- channel multiprotein complexes in the heart, which could provide new insight into our understanding of the underlying mechanisms responsible for heart disease and protection.

Selective block of swelling-activated Cl- channels over cAMP-dependent Cl- channels in ventricular myocytes

The objective of this study on guinea-pig and rabbit ventricular myocytes was to evaluate the sensitivities of swelling-activated Cl- current (ICl(swell)) and cAMP-dependent cystic fibrosis transmembrane regulator (CFTR) Cl- current (ICl(CFTR)) to block by dideoxyforskolin and verapamil. The currents were recorded from whole-cell configured myocytes that were dialysed with a Cs+-rich pipette solution and superfused with either isosmotic Na+-, K+-, Ca2+-free solution that contained 140 mM sucrose or hyposmotic sucrose-free solution. Forskolin-activated ICl(CFTR) was inhibited by reference blocker anthracene-9-carboxylic acid but unaffected by < or = 200 microM dideoxyforskolin and verapamil. However, dideoxyforskolin and verapamil had strong inhibitory effects on outwardly-rectifying, inactivating, distilbene-sensitive ICl(swell); IC50 values were approximately 30 microM, and blocks were voltage-independent and reversible. The results establish that dideoxyforskolin and verapamil can be used to distinguish between ICl(CFTR) and ICl(swell) in heart cells, and expand the pharmacological characterization of cardiac ICl(swell).

Contribution of a time-dependent and hyperpolarization-activated chloride conductance to currents of resting and hypotonically shocked rat hepatocytes

Hepatocellular Cl- flux is integral to maintaining cell volume and electroneutrality in the face of the many transport and metabolic activities that describe the multifaceted functions of these cells. Although a significant volume-regulated Cl- current (VRAC) has been well described in hepatocytes, the Cl- channels underlying the large resting anion conductance have not been identified. We used a combination of electrophysiological and molecular approaches to describe potential candidates for this conductance. Anion currents in rat hepatocytes and WIF-B and HEK293T cells were measured under patch electrode-voltage clamp. With K+-free salts of Cl- comprising the major ions externally and internally, hyperpolarizing steps between -40 and -140 mV activated a time-dependent inward current in hepatocytes. Steady-state activation was half-maximal at -63 mV and 28-38% of maximum at -30 to -45 mV, previously reported hepatocellular resting potentials. Gating was dependent on cytosolic Cl-, shifting close to 58 mV/10-fold change in Cl- concentration. Time-dependent inward Cl- currents and a ClC-2-specific RT-PCR product were also observed in WIF-B cells but not HEK293T cells. All cell types exhibited typical VRAC in response to dialysis with hypertonic solutions. DIDS (0.1 mM) inhibited the hepatocellular VRAC but not the inward time-dependent current. Antibodies against the COOH terminus of ClC-2 reacted with a protein between 90 and 100 kDa in liver plasma membranes. The results demonstrate that rat hepatocytes express a time-dependent inward Cl- channel that could provide a significant depolarizing influence in the hepatocyte.

Involvement of anion channel(s) in the modulation of the transient outward K+ channel in rat ventricular myocytes

The cardiac Ca2+-independent transient outward K+ current ( Ito), a major repolarizing ionic current, is markedly affected by Cl− substitution and anion channel blockers. We reexplored the mechanism of the action of anions on Ito by using whole cell patch-clamp in single isolated rat cardiac ventricular myocytes. The transient outward current was sensitive to blockade by 4-aminopyridine (4-AP) and was abolished by Cs+ substitution for intracellular K+. Replacement of most of the extracellular Cl− with less permeant anions, aspartate (Asp−) and glutamate (Glu−), markedly suppressed the current. Removal of external Na+ or stabilization of F-actin with phalloidin did not significantly affect the inhibitory action of less permeant anions on Ito. In contrast, the permeant Cl− substitute Br− did not markedly affect the current, whereas F− substitution for Cl− induced a slight inhibition. The Ito elicited during Br− substitution for Cl− was also sensitive to blockade by 4-AP. The ability of Cl− substitutes to induce rightward shifts of the steady-state inactivation curve of Ito was in the following sequence: NO3− > Cl− ≈ Br− > gluconate− > Glu− > Asp−. Depolymerization of actin filaments with cytochalasin D (CytD) induced an effect on the steady-state inactivation of Ito similar to that of less permeant anions. Fluorescent phalloidin staining experiments revealed that CytD-pretreatment significantly decreased the intensity of FITC-phalloidin staining of F-actin, whereas Asp− substitution for Cl− was without significant effect on the intensity. These results suggest that the Ito channel is modulated by anion channel(s), in which the actin cytoskeleton may be implicated.

Altered properties of volume-sensitive osmolyte and anion channels (VSOACs) and membrane protein expression in cardiac and smooth muscle myocytes from Clcn3-/- mice

ClC-3, a member of the large superfamily of ClC voltage-dependent Cl(-) channels, has been proposed as a molecular candidate responsible for volume-sensitive osmolyte and anion channels (VSOACs) in some cells, including heart and vascular smooth muscle. However, the reported presence of native VSOACs in at least two cell types from transgenic ClC-3 disrupted (Clcn3(-/-)) mice casts considerable doubt on this proposed role for ClC-3. We compared several properties of native VSOACs and examined mRNA transcripts and membrane protein expression profiles in cardiac and pulmonary arterial smooth muscle cells from Clcn3(+/+) and Clcn3(-/-) mice to: (1) test the hypothesis that native VSOACs are unaltered in cells from Clcn3(-/-) mice, and (2) test the possibility that targeted inactivation of the Clcn3 gene using a conventional murine global knock-out approach may result in compensatory changes in expression of other membrane proteins. Our experiments demonstrate that VSOAC currents in myocytes from Clcn3(+/+) and Clcn3(-/-) mice are remarkably similar in terms of activation and inactivation kinetics, steady-state current densities, rectification, anion selectivity (I(-) > Cl(-)>> Asp(-)) and sensitivity to block by glibenclamide, niflumic acid, DIDS and extracellular ATP. However, additional experiments revealed several significant differences in other fundamental properties of native VSOACs recorded from atrial and smooth muscle cells from Clcn3(-/-) mice, including: differences in regulation by endogenous protein kinase C, differential sensitivity to block by anti-ClC-3 antibodies, and differential sensitivities to [ATP](i) and free [Mg(2+)](i). These results suggest that in response to Clcn3 gene deletion, there may be compensatory changes in expression of other proteins that alter VSOAC channel subunit composition or associated regulatory subunits that give rise to VSOACs with different properties. Consistent with this hypothesis, in atria from Clcn3(-/-) mice compared to Clcn3(+/+) mice, quantitative analysis of ClC mRNA expression levels revealed significant increases in transcripts for ClC-1, ClC-2, and ClC-3, and protein expression profiles obtained using two-dimensional polyacrylamide gel electrophoresis revealed complex changes in at least 35 different unidentified membrane proteins in cells from Clcn3(-/-) mice. These findings emphasize that caution needs to be exercised in simple attempts to interpret the phenotypic consequences of conventional global Clcn3 gene inactivation.

Contribution of chloride channels to volume regulation of cortical astrocytes

The objective of this study was to determine the relative contribution of Cl(-) channels to volume regulation of cultured rat cortical astrocytes after hypotonic cell swelling. Using a Coulter counter, we showed that cortical astrocytes regulate their cell volume by approximately 60% within 45 min after hypotonic challenge. This volume regulation was supported when Cl(-) was replaced with Br(-), NO(3)(-), methanesulfonate(-), or acetate(-) but was inhibited when Cl(-) was replaced with isethionate(-) or gluconate(-). Additionally, substitution of Cl(-) with I(-) completely blocked volume regulation. Volume regulation was unaffected by furosemide or bumetanide, blockers of KCl transport, but was inhibited by Cl(-) channel blockers, including 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), and niflumic acid. Surprisingly, the combination of Cd(2+) with NPPB, DIDS, or niflumic acid inhibited regulation to a greater extent than any of these drugs alone. Volume regulation did not differ among astrocytes cultured from different brain regions, as cerebellar and hippocampal astrocytes exhibited behavior identical to that of cortical astrocytes. These data suggest that Cl(-) flux through ion channels rather than transporters is essential for volume regulation of cultured astrocytes in response to hypotonic challenge.

Swelling-activated chloride channels in cardiac physiology and pathophysiology

Characteristics and functions of the cardiac swelling-activated Cl current (I(Cl,swell)) are considered in physiologic and pathophysiologic settings. I(Cl,swell) is broadly distributed throughout the heart and is stimulated not only by osmotic and hydrostatic increases in cell volume, but also by agents that alter membrane tension and direct mechanical stretch. The current is outwardly rectifying, reverses between the plateau and resting potentials (E(m)), and is time-independent over the physiologic voltage range. Consequently, I(Cl,swell) shortens action potential duration, depolarizes E(m), and acts to decrease cell volume. Because it is activated by stimuli that also activate cation stretch-activated channels, I(Cl,swell) should be considered as a potential effector of mechanoelectrical feedback. I(Cl,swell) is activated in ischemic and non-ischemic dilated cardiomyopathies and perhaps during ischemia and reperfusion. I(Cl,swell) plays a role in arrhythmogenesis, myocardial injury, preconditioning, and apoptosis of myocytes. As a result, I(Cl,swell) potentially is a novel therapeutic target.

Electrophysiological response of rat atrial myocytes to acidosis

The effect of acidosis on the electrical activity of isolated rat atrial myocytes was investigated using the patch-clamp technique. Reducing the pH of the bathing solution from 7.4 to 6.5 shortened the action potential. Acidosis had no significant effect on transient outward or inward rectifier currents but increased steady-state outward current. This increase was still present, although reduced, when intracellular Ca(2+) was buffered by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA); BAPTA also inhibited acidosis-induced shortening of the action potential. Ni(2+) (5 mM) had no significant effect on the acidosis-induced shortening of the action potential. Acidosis also increased inward current at -80 mV and depolarized the resting membrane potential. Acidosis activated an inwardly rectifying Cl(-) current that was blocked by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), which also inhibited the acidosis-induced depolarization of the resting membrane potential. It is concluded that an acidosis-induced increase in steady-state outward K(+) current underlies the shortening of the action potential and that an acidosis-induced increase in inwardly rectifying Cl(-) current underlies the depolarization of the resting membrane potential during acidosis.

Electrophysiological response of rat ventricular myocytes to acidosis

The effects of acidosis on the action potential, resting potential, L-type Ca(2+) (I(Ca)), inward rectifier potassium (I(K1)), delayed rectifier potassium (I(K)), steady-state (I(SS)), and inwardly rectifying chloride (I(Cl,ir)) currents of rat subepicardial (Epi) and subendocardial (Endo) ventricular myocytes were investigated using the patch-clamp technique. Action potential duration was shorter in Epi than in Endo cells. Acidosis (extracellular pH decreased from 7.4 to 6.5) depolarized the resting membrane potential and prolonged the time for 50% repolarization of the action potential in Epi and Endo cells, although the prolongation was larger in Endo cells. At control pH, I(Ca), I(K1), and I(SS) were not significantly different in Epi and Endo cells, but I(K) was larger in Epi cells. Acidosis did not alter I(Ca), I(K1), or I(K) but decreased I(SS); this decrease was larger in Endo cells. It is suggested that the acidosis-induced decrease in I(SS) underlies the prolongation of the action potential. I(Cl,ir) at control pH was Cd(2+) sensitive but 4,4'-disothiocyanato-stilbene-2,2'-disulfonic acid resistant. Acidosis increased I(Cl,ir); it is suggested that the acidosis-induced increase in I(Cl,ir) underlies the depolarization of the resting membrane potential.

High-resolution scanning patch-clamp: new insights into cell function

Cell specialization is often governed by the spatial distribution of ion channels and receptors on the cell surface. So far, little is known about functional ion channel localization. This is due to a lack of satisfactory methods for investigating ion channels in an intact cell and simultaneously determining the channels' positions accurately. We have developed a novel high-resolution scanning patch-clamp technique that enables the study of ion channels, not only in small cells, such as sperm, but in submicrometer cellular structures, such as epithelial microvilli, fine neuronal dendrites, and, particularly, T-tubule openings of cardiac myocytes. In cardiac myocytes, as in most excitable cells, action potential propagation depends essentially on the properties of ion channels that are functionally and spatially coupled. We found that the L-type calcium and chloride channels are distributed and colocalized in the region of T-tubule openings, but not in other regions of the myocyte. In addition, chloride channels were found in narrowly defined regions of Z-grooves. This finding suggests a new synergism between these types of channels that may be relevant for action potential propagation along the T-tubule system and excitation-contraction coupling.

"Sleepy" inward rectifier channels in guinea-pig cardiomyocytes are activated only during strong hyperpolarization

K(+) channels of isolated guinea-pig cardiomyocytes were studied using the patch-clamp technique. At transmembrane potentials between -120 and -220 mV we observed inward currents through an apparently novel channel. The novel channel was strongly rectifying, no outward currents could be recorded. Between -200 and -160 mV it had a slope conductance of 42.8 +/- 3.0 pS (S.D.; n = 96). The open probability (P(o)) showed a sigmoid voltage dependence and reached a maximum of 0.93 at -200 mV, half-maximal activation was approximately -150 mV. The voltage dependence of P(o) was not affected by application of 50 microM isoproterenol. The open-time distribution could be described by a single exponential function, the mean open time ranged between 73.5 ms at -220 mV and 1.4 ms at -160 mV. At least two exponential components were required to fit the closed time distribution. Experiments with different external Na(+), K(+) and Cl(-) concentrations suggested that the novel channel is K(+) selective. Extracellular Ba(2+) ions gave rise to a voltage-dependent reduction in P(o) by inducing long closed states; Cs(+) markedly reduced mean open time at -200 mV. In cell-attached recordings the novel channel frequently converted to a classical inward rectifier channel, and vice versa. This conversion was not voltage dependent. After excision of the patch, the novel channel always converted to a classical inward rectifier channel within 0-3 min. This conversion was not affected by intracellular Mg(2+), phosphatidylinositol (4,5)-bisphosphate or spermine. Taken together, our findings suggest that the novel K(+) channel represents a different "mode" of the classical inward rectifier channel in which opening occurs only at very negative potentials.

Ion permeation and selectivity in ClC-type chloride channels

Voltage-gated anion channels are present in almost every living cell and have many physiological functions. Recently, a novel gene family encoding voltage-gated chloride channels, the ClC family, was identified. The knowledge of primary amino acid sequences has allowed for the study of these anion channels in heterologous expression systems and made possible the combination of site-directed mutagenesis and high-resolution electrophysiological measurements as a means of gaining insights into the molecular basis of channel function. This review focuses on one particular aspect of chloride channel function, the selective transport of anions through biological membranes. I will describe recent experiments using a combination of cellular electrophysiology, molecular genetics, and recombinant DNA technology to study the molecular basis of ion permeation and selection in ClC-type chloride channels. These novel tools have provided new insights into basic mechanisms underlying the function of these biologically important channels.

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.

Inward rectifiers in the heart: an update on I(K1)

The cardiac inward rectifier potassium current (I(K1)), present in all ventricular and atrial myocytes, has been suggested to play a major role in repolarization of the action potential and stabilization of the resting potential. The molecular basis is now ascribed to members of the Kir2 sub-family of inward rectifier K channel genes, and the availability of recombinant expression systems has led to elucidation of the mechanism of inward rectification, as well as additional regulatory mechanisms involving intracellular pH and phosphorylation. In vivo manipulation of the genes encoding I(K1)and regulatory proteins now promise to provide new insights to the role of this conductance in the heart. This review details recent advances and considers the prospects for further elucidation of the role of this conductance in cardiac electrical activity.

CLH-3, a ClC-2 anion channel ortholog activated during meiotic maturation in C. elegans oocytes

BACKGROUND

ClC anion channels are ubiquitous and have been identified in organisms as diverse as bacteria and humans. Despite their widespread expression and likely physiological importance, the function and regulation of most ClCs are obscure. The nematode Caenorhabditis elegans offers significant experimental advantages for defining ClC biology. These advantages include a fully sequenced genome, cellular and molecular manipulability, and genetic tractability.

RESULTS

We show by patch clamp electrophysiology that C. elegans oocytes express a hyperpolarization- and swelling-activated Cl(-) current with biophysical characteristics strongly resembling those of mammalian ClC-2. Double-stranded RNA-mediated gene interference (RNAi) and single-oocyte RT-PCR demonstrated that the channel is encoded by clh-3, one of six C. elegans ClC genes. CLH-3 is inactive in immature oocytes but can be triggered by cell swelling. However, CLH-3 plays no apparent role in oocyte volume homeostasis. The physiological signal for channel activation is the induction of oocyte meiotic maturation. During meiotic maturation, the contractile activity of gonadal sheath cells, which surround oocytes and are coupled to them via gap junctions, increases dramatically. These ovulatory sheath cell contractions are initiated prematurely in animals in which CLH-3 expression is disrupted by RNAi.

CONCLUSIONS

The inwardly rectifying Cl(-) current in C. elegans oocytes is due to the activity of a ClC channel encoded by clh-3. Functional and structural similarities suggest that CLH-3 and mammalian ClC-2 are orthologs. CLH-3 is activated during oocyte meiotic maturation and functions in part to modulate ovulatory contractions of gap junction-coupled gonadal sheath cells.

Cardiovascular actions of magnesium

Intracellular magnesium is an important modulator of calcium and potassium channels in cardiac myocytes. Hypomagnesemia is common in hospitalized patients and may contribute significantly to cardiac morbidity and mortality, particularly in states associated with myocardial ischemia. Therefore, it is important to maintain the plasma magnesium concentration within the normal range in asymptomatic patients and in patients with cardiac disease as prophylaxis against the occurrence of significant arrhythmias.

Molecular distribution of volume-regulated chloride channels (ClC-2 and ClC-3) in cardiac tissues

The molecular identification of cardiac chloride channels has provided probes to investigate their distribution and abundance in heart. In this study, the molecular expression and distribution of volume-regulated chloride channels ClC-2 and ClC-3 in cardiac tissues were analyzed and quantified. Total RNA was isolated from atria and ventricles of several species (dog, guinea pig, and rat) and subjected to a quantitative RT-PCR strategy. ClC-2 and ClC-3 mRNA expression were calculated relative to beta-actin expression within these same tissues. The transcriptional levels of ClC-3 mRNA were between 1.8 and 10.2% of beta-actin expression in atria and between 3.4 and 8.6% of beta-actin in ventricles (n = 3 for each tissue). The levels of ClC-2 in both atria and ventricles were significantly less than those measured for ClC-3 (n = 3; P < 0.05). ClC-2 mRNA levels were between 0.04-0.08% and 0.03-0.18% of beta-actin expression in atria and ventricles, respectively (n = 3 for each tissue). Immunoblots of atrial and ventricular wall protein extracts demonstrated ClC-2- and ClC-3-specific immunoreactivity at 97 and 85 kDa, respectively. Immunohistochemical localization in guinea pig cardiac muscle demonstrates a ubiquitous distribution of ClC-2 and ClC-3 channels in the atrial and ventricular wall. Confocal analysis detected colocalization of ClC-2 and ClC-3 in sarcolemmal membranes and distinct ClC-3 immunoreactivity in cytoplasmic regions. The molecular expression of ClC-2 and ClC-3 in cardiac tissue is consistent with the proposed role of these chloride channels in the regulation of cardiac cell volume and the modulation of cardiac electrical activity.