Unitary Cl- channels activated by cytoplasmic Ca2+ in canine ventricular myocytes

TMEM16A Plays an Insignificant Role in Myocardium Remodeling but May Promote Angiogenesis of Heart During Pressure-overload

Background: Cardiac hypertrophy (CH) occurs with an increase in myocardium mass as an adaptive compensation to increased stress. Prolonged CH causes decompensated heart failure (HF). Enhanced angiogenesis by vascular endothelial growth factor (VEGF) is observed in hypertrophied hearts; impaired angiogenesis by angiotensin II (AngII) is observed in failing hearts. Angiogenesis is executed by vascular endothelial cells (ECs). Abnormal Ca²⁺ homeostasis is a hallmark feature of hypertrophied and failing hearts. Ca²⁺-activated chloride channel transmembrane protein 16A (TMEM16A) is expressed in cardiomyocytes and ECs but its role in heart under stress remains unknown.

Methods: Pressure-overload-induced CH and HF mouse models were established. Echocardiography was performed to evaluate cardiac parameters. Quantitative real-time PCR, traditional and simple western assays were used to quantify molecular expression. Whole-cell patch-clamp experiments were used to detect TMEM16A current (I_TMEM16A) and action potential duration (APD) of cardiomyocytes. VEGF and AngII were used separately in ECs culture to simulate enhanced or impaired angiogenesis, respectively. TMEM16A low-expressed and over-expressed ECs were obtained by siRNA or lentivirus transfection. Wound healing, tube formation and ECs spheroids sprouting assays were performed to assess migration and angiogenesis.

Results: Neither TMEM16A molecular expression levels nor whole-cell I_TMEM16A density varied significantly during the development of CH and HF. I_TMEM16A comprises transient outward current, but doesn’t account for APD prolongation in hypertrophied or failing cardiomyocytes. In cultured ECs, TMEM16A knockdown inhibited migration and angiogenesis, TMEM16A overexpression showed opposite result. Promotion of migration and angiogenesis by VEGF was decreased in TMEM16A low-expressed ECs but was increased in TMEM16A over-expressed ECs. Inhibition of migration and angiogenesis by AngII was enhanced in TMEM16A low-expressed ECs but was attenuated in TMEM16A over-expressed ECs.

Conclusion: TMEM16A contributes insignificantly in myocardium remodeling during pressure-overload. TMEM16A is a positive regulator of migration and angiogenesis under normal condition or simulated stress. TMEM16A may become a new target for upregulation of angiogenesis in ischemic disorders like ischemic heart disease.

Molecular mechanisms of activation and regulation of ANO1-Encoded Ca2+-Activated Cl- channels

Ca2+-activated Cl- channels (CaCCs) perform a multitude of functions including the control of cell excitability, regulation of cell volume and ionic homeostasis, exocrine and endocrine secretion, fertilization, amplification of olfactory sensory function, and control of smooth muscle cell contractility. CaCCs are the translated products of two members (ANO1 and ANO2, also known as TMEM16A and TMEM16B) of the Anoctamin family of genes comprising ten paralogs. This review focuses on recent progress in understanding the molecular mechanisms involved in the regulation of ANO1 by cytoplasmic Ca2+, post-translational modifications, and how the channel protein interacts with membrane lipids and protein partners. After first reviewing the basic properties of native CaCCs, we then present a brief historical perspective highlighting controversies about their molecular identity in native cells. This is followed by a summary of the fundamental biophysical and structural properties of ANO1. We specifically address whether the channel is directly activated by internal Ca2+ or indirectly through the intervention of the Ca2+-binding protein Calmodulin (CaM), and the structural domains responsible for Ca2+- and voltage-dependent gating. We then review the regulation of ANO1 by internal ATP, Calmodulin-dependent protein kinase II-(CaMKII)-mediated phosphorylation and phosphatase activity, membrane lipids such as the phospholipid phosphatidyl-(4,5)-bisphosphate (PIP2), free fatty acids and cholesterol, and the cytoskeleton. The article ends with a survey of physical and functional interactions of ANO1 with other membrane proteins such as CLCA1/2, inositol trisphosphate and ryanodine receptors in the endoplasmic reticulum, several members of the TRP channel family, and the ancillary Κ+ channel β subunits KCNE1/5.

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.

Ca2+-activated Cl- current is antiarrhythmic by reducing both spatial and temporal heterogeneity of cardiac repolarization

The role of Ca²⁺-activated Cl^- current (I_Cl(Ca)) in cardiac arrhythmias is still controversial. It can generate delayed afterdepolarizations in Ca²⁺-overloaded cells while in other studies incidence of early afterdepolarization (EAD) was reduced by I_Cl(Ca). Therefore our goal was to examine the role of I_Cl(Ca) in spatial and temporal heterogeneity of cardiac repolarization and EAD formation. Experiments were performed on isolated canine cardiomyocytes originating from various regions of the left ventricle; subepicardial, midmyocardial and subendocardial cells, as well as apical and basal cells of the midmyocardium. I_Cl(Ca) was blocked by 0.5mmol/L 9-anthracene carboxylic acid (9-AC). Action potential (AP) changes were tested with sharp microelectrode recording. Whole-cell 9-AC-sensitive current was measured with either square pulse voltage-clamp or AP voltage-clamp (APVC). Protein expression of TMEM16A and Bestrophin-3, ion channel proteins mediating I_Cl(Ca), was detected by Western blot. 9-AC reduced phase-1 repolarization in every tested cell. 9-AC also increased AP duration in a reverse rate-dependent manner in all cell types except for subepicardial cells. Neither I_Cl(Ca) density recorded with square pulses nor the normalized expressions of TMEM16A and Bestrophin-3 proteins differed significantly among the examined groups of cells. The early outward component of I_Cl(Ca) was significantly larger in subepicardial than in subendocardial cells in APVC setting. Applying a typical subepicardial AP as a command pulse resulted in a significantly larger early outward component in both subepicardial and subendocardial cells, compared to experiments when a typical subendocardial AP was applied. Inhibiting I_Cl(Ca) by 9-AC generated EADs at low stimulation rates and their incidence increased upon beta-adrenergic stimulation. 9-AC increased the short-term variability of repolarization also. We suggest a protective role for I_Cl(Ca) against risk of arrhythmias by reducing spatial and temporal heterogeneity of cardiac repolarization and EAD formation.

Characterization of Cardiac Anoctamin1 Ca²⁺-Activated Chloride Channels and Functional Role in Ischemia-Induced Arrhythmias

Anoctamin1 (ANO1) encodes a Ca(2+)-activated chloride (Cl(-)) channel (CaCC) in variety tissues of many species. Whether ANO1 expresses and functions as a CaCC in cardiomyocytes remain unknown. The objective of this study is to characterize the molecular and functional expression of ANO1 in cardiac myocytes and the role of ANO1-encoded CaCCs in ischemia-induced arrhythmias in the heart. Quantitative real-time RT-PCR, immunofluorescence staining assays, and immunohistochemistry identified the molecular expression, location, and distribution of ANO1 in mouse ventricular myocytes (mVMs). Patch-clamp recordings combined with pharmacological analyses found that ANO1 was responsible for a Ca(2+)-activated Cl(-) current (I(Cl.Ca)) in cardiomyocytes. Myocardial ischemia led to a significant increase in the current density of I(Cl.Ca), which was inhibited by a specific ANO1 inhibitor, T16A(inh)-A01, and an antibody targeting at the pore area of ANO1. Moreover, cardiomyocytes isolated from mice with ischemia-induced arrhythmias had an accelerated early phase 1 repolarization of action potentials (APs) and a deeper "spike and dome" compared to control cardiomyocytes from non-ischemia mice. Application of the antibody targeting at ANO1 pore prevented the ischemia-induced early phase 1 repolarization acceleration and caused a much shallower "spike and dome". We conclude that ANO1 encodes CaCC and plays a significant role in the phase 1 repolarization of APs in mVMs. The ischemia-induced increase in ANO1 expression may be responsible for the increased density of I(Cl.Ca) in the ischemic heart and may contribute, at least in part, to ischemia-induced arrhythmias.

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.

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.

Integrative systems models of cardiac excitation-contraction coupling

Excitation-contraction coupling in the cardiac myocyte is mediated by a number of highly integrated mechanisms of intracellular Ca²(+) transport. The complexity and integrative nature of heart cell electrophysiology and Ca²(+) cycling has led to an evolution of computational models that have played a crucial role in shaping our understanding of heart function. An important emerging theme in systems biology is that the detailed nature of local signaling events, such as those that occur in the cardiac dyad, have important consequences at higher biological scales. Multiscale modeling techniques have revealed many mechanistic links between microscale events, such as Ca²(+) binding to a channel protein, and macroscale phenomena, such as excitation-contraction coupling gain. Here, we review experimentally based multiscale computational models of excitation-contraction coupling and the insights that have been gained through their application.

Cardiomyocytes with disrupted CFTR function require CaMKII and Ca(2+)-activated Cl(-) channel activity to maintain contraction rate

The physiological role of the cystic fibrosis transmembrane conductance regulator (CFTR) in cardiomyocytes remains unclear. Using spontaneously beating neonatal ventricular cardiomyocytes from wild-type (WT) or CFTR knockout (KO) mice, we examined the role of CFTR in the modulation of cardiomyocyte contraction rate. Contraction rates of spontaneously beating myocytes were captured by video imaging. Real-time changes in intracellular ([Ca(2+)](i)) and protein kinase A (PKA) activity were measured by fura-2 and fluorescence resonance energy transfer, respectively. Acute inhibition of CFTR in WT cardiomyocytes using the CFTR inhibitor CFTR(inh)-172 transiently inhibited the contraction rate. By contrast, cardiomyocytes from CFTR KO mice displayed normal contraction rates. Further investigation revealed that acute inhibition of CFTR activity in WT cardiomyocytes activated L-type Ca(2+) channels, leading to a transient increase of [Ca(2+)](i) and inhibition of PKA activity. Additionally, we found that contraction rate normalization following acute CFTR inhibition in WT cardiomyocytes or chronic deletion in cardiomyocytes from CFTR KO mice requires the activation of Ca(2+)/calmodulin-dependent kinase II (CaMKII) and Ca(2+)-activated Cl(-) channels (CaCC) because simultaneous addition of myristoylated-autocamtide-2-related inhibitory peptide or niflumic acid and CFTR(inh)-172 to WT cardiomyocytes or treatment of cardiomyoctes from CFTR KO mice with these agents caused sustained attenuation of contraction rates. Our results demonstrate that regulation of cardiomyocyte contraction involves CFTR. They also reveal that activation of CaMKII and CaCC compensates for loss of CFTR function. Increased dependence on CaMKII upon loss of CFTR function might leave cystic fibrosis patients at increased risk of heart dysfunction and disease.

Human ClCa1 modulates anionic conduction of calcium-dependent chloride currents

Proteins of the CLCA gene family including the human ClCa1 (hClCa1) have been suggested to constitute a new family of chloride channels mediating Ca(2+)-dependent Cl- currents. The present study examines the relationship between the hClCa1 protein and Ca(2+)-dependent Cl- currents using heterologous expression of hClCa1 in HEK293 and NCIH522 cell lines and whole cell recordings. By contrast to previous reports claiming the absence of Cl- currents in HEK293 cells, we find that HEK293 and NCIH522 cell lines express constitutive Ca(2+)-dependent Cl- currents and show that hClCa1 increases the amplitude of Ca(2+)-dependent Cl- currents in those cells. We further show that hClCa1 does not modify the permeability sequence but increases the Cl- conductance while decreasing the G(SCN-)/G(Cl-) conductance ratio from approximately 2-3 to approximately 1. We use an Eyring rate theory (two barriers, one site channel) model and show that the effect of hClCa1 on the anionic channel can be simulated by its action on lowering the first and the second energy barriers. We conclude that hClCa1 does not form Ca(2+)-dependent Cl- channels per se or enhance the trafficking/insertion of constitutive channels in the HEK293 and NCIH522 expression systems. Rather, hClCa1 elevates the single channel conductance of endogenous Ca(2+)-dependent Cl- channels by lowering the energy barriers for ion translocation through the pore.

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.

Anoctamin/TMEM16 family members are Ca2+-activated Cl- channels

Ca(2+)-activated Cl- channels (CaCCs) perform many important functions in cell physiology including secretion of fluids from acinar cells of secretory glands, amplification of olfactory transduction, regulation of cardiac and neuronal excitability, mediation of the fast block to polyspermy in amphibian oocytes, and regulation of vascular tone. Although a number of proteins have been proposed to be responsible for CaCC currents, the anoctamin family (ANO, also known as TMEM16) exhibits characteristics most similar to those expected for the classical CaCC. Interestingly, this family of proteins has previously attracted the interest of both developmental and cancer biologists. Some members of this family are up-regulated in a number of tumours and functional deficiency in others is linked to developmental defects.

Expression, localization, and functional properties of Bestrophin 3 channel isolated from mouse heart

Bestrophins are a novel family of proteins that encode calcium-activated chloride channels. In this study we establish that Bestrophin transcripts are expressed in the mouse and human heart. Native mBest3 protein expression and localization in heart was demonstrated by using a specific polyclonal mBest3 antibody. Immunostaining of isolated cardiac myocytes indicates that mBest3 is present at the membrane. Using the patch-clamp technique, we characterized the biophysical and pharmacological properties of mBest3 cloned from heart. Whole cell chloride currents were evoked in both HEK293 and COS-7 cells expressing mBest3 by elevation of intracellular calcium. mBest3 currents displayed a K(D) for Ca(2+) of approximately 175 nM. The calcium-activated chloride current was found to be time and voltage independent and displayed slight outward rectification. The anion permeability sequence of the channel was SCN(-)>I(-)>Cl(-), and the current was inhibited by niflumic acid and DIDS in the micromolar range. In addition, we generated a site-specific mutation (F80L) in the putative pore region of mBest3 that significantly altered the ion conduction and pharmacology of this channel. Our functional and mutational studies examining the biophysical properties of mBest3 indicate that it functions as a pore-forming chloride channel that is activated by physiological levels of calcium. This study reports novel findings regarding the molecular expression, tissue localization, and functional properties of mBest3 cloned from heart.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Single Cl- channels activated by Ca2+ in Drosophila S2 cells are mediated by bestrophins

Mutations in human bestrophin-1 (VMD2) are genetically linked to several forms of retinal degeneration but the underlying mechanisms are unknown. Bestrophin-1 (hBest1) has been proposed to be a Cl⁻ channel involved in ion and fluid transport by the retinal pigment epithelium (RPE). To date, however, bestrophin currents have only been described in overexpression systems and not in any native cells. To test whether bestrophins function as Ca²⁺-activated Cl⁻ (CaC) channels physiologically, we used interfering RNA (RNAi) in the Drosophila S2 cell line. S2 cells express four bestrophins (dbest1–4) and have an endogenous CaC current. The CaC current is abolished by several RNAi constructs to dbest1 and dbest2, but not dbest3 or dbest4. The endogenous CaC current was mimicked by expression of dbest1 in HEK cells, and the rectification and relative permeability of the current were altered by replacing F81 with cysteine. Single channel analysis of the S2 bestrophin currents revealed an ∼2-pS single channel with fast gating kinetics and linear current–voltage relationship. A similar channel was observed in CHO cells transfected with dbest1, but no such channel was seen in S2 cells treated with RNAi to dbest1. This provides definitive evidence that bestrophins are components of native CaC channels at the plasma membrane.

Ionic mechanisms of cardiac cell swelling induced by blocking Na+/K+ pump as revealed by experiments and simulation

Although the Na(+)/K(+) pump is one of the key mechanisms responsible for maintaining cell volume, we have observed experimentally that cell volume remained almost constant during 90 min exposure of guinea pig ventricular myocytes to ouabain. Simulation of this finding using a comprehensive cardiac cell model (Kyoto model incorporating Cl(-) and water fluxes) predicted roles for the plasma membrane Ca(2+)-ATPase (PMCA) and Na(+)/Ca(2+) exchanger, in addition to low membrane permeabilities for Na(+) and Cl(-), in maintaining cell volume. PMCA might help maintain the [Ca(2+)] gradient across the membrane though compromised, and thereby promote reverse Na(+)/Ca(2+) exchange stimulated by the increased [Na(+)](i) as well as the membrane depolarization. Na(+) extrusion via Na(+)/Ca(2+) exchange delayed cell swelling during Na(+)/K(+) pump block. Supporting these model predictions, we observed ventricular cell swelling after blocking Na(+)/Ca(2+) exchange with KB-R7943 or SEA0400 in the presence of ouabain. When Cl(-) conductance via the cystic fibrosis transmembrane conductance regulator (CFTR) was activated with isoproterenol during the ouabain treatment, cells showed an initial shrinkage to 94.2 +/- 0.5%, followed by a marked swelling 52.0 +/- 4.9 min after drug application. Concomitantly with the onset of swelling, a rapid jump of membrane potential was observed. These experimental observations could be reproduced well by the model simulations. Namely, the Cl(-) efflux via CFTR accompanied by a concomitant cation efflux caused the initial volume decrease. Then, the gradual membrane depolarization induced by the Na(+)/K(+) pump block activated the window current of the L-type Ca(2+) current, which increased [Ca(2+)](i). Finally, the activation of Ca(2+)-dependent cation conductance induced the jump of membrane potential, and the rapid accumulation of intracellular Na(+) accompanied by the Cl(-) influx via CFTR, resulting in the cell swelling. The pivotal role of L-type Ca(2+) channels predicted in the simulation was demonstrated in experiments, where blocking Ca(2+) channels resulted in a much delayed cell swelling.

Simulation of Ca2+-activated Cl- current of cardiomyocytes in rabbit pulmonary vein: implications of subsarcolemmal Ca2+ dynamics

In recent studies, we recorded transiently activated outward currents by the application of three-step voltage pulses to induce a reverse mode of Na+-Ca2+ exchange (NCX). We found that these currents were mediated by a Ca2+-activated Cl- current. Based on the recent reports describing the atrial Ca2+ transients, the Ca2+ transient at the subsarcolemmal space was initiated and then diffused into the cytosolic space. Because the myocardium in the pulmonary vein is an extension of the atrium, the Ca2+-activated Cl- current may reflect the subsarcolemmal Ca2+ dynamics. We tried to predict the subsarcolemmal Ca2+ dynamics by simulating these current traces. According to recent reports on the geometry of atrial myocytes, we assumed that there were three compartments of sarcoplasmic reticulum (SR): a network SR, a junctional SR and a central SR. Based on these structures, we also divided the cytosolic space into three compartments: the junctional, subsarcolemmal and cytosolic spaces. Geometry information and cellular capacitance suggested that there were essentially no T-tubules in these cells. The basic physical data, such as the compartmental volumes, the diffusion coefficients and the stability coefficients of the Ca2+ buffers, were obtained from the literature. In the simulation, we incorporated the NCX, the L-type Ca2+ channel, the rapid activating outward rectifier K+ channel, the Na+-K+ pump, the SR Ca2+-pump, the ryanodine receptor, the Ca2+-activated Cl- channel and the dynamics of Na+, K+, Ca2+ and Cl-. In these conditions, we could successfully reconstruct the Ca2+-activated Cl- currents. The simulation allowed estimation of the Ca2+ dynamics of each compartment and the distribution of the Ca2+-activated Cl- channel and the NCX in the sarcolemma on the junctional or subsarcolemmal space.

An outwardly rectifying chloride channel in human atrial cardiomyocytes

INTRODUCTION

Among a range of chloride channels, outwardly rectifying Cl- channels have been reported in the heart of various species. Although the anionic current carried by this channel has been subjected to intense electrophysiological investigations, paradoxically no examination of single-channel currents has been reported for human cardiomyocytes.

METHODS AND RESULTS

Using the cell-attached and cell-free configurations of the patch-clamp technique, we have characterized the properties of an outwardly rectifying chloride current (ORCC) at the unitary level in freshly isolated human atrial cardiomyocytes. In excised inside-out patches, the channel presented a nonlinear I/V relationship with a conductance of 76.5 +/- 14.7 pS in the positive voltage range and 8.1 +/- 2 pS in the negative voltage range, indicating an outward rectification. Preincubation with the protein kinase C activator phorbol 12-myristate 13-acetate (PMA) significantly increased the number of spontaneously active channels observed. The channel was Cl- selective (Cl- to Na+ permeability ratio, PCl/PNa= 18) with the permeability sequence I- > Br- > Cl- > F- > gluconate. It was blocked by the classical Cl- channels blockers glibenclamide, NPPB, SITS, and DIDS. Channel activity was not dependent upon internal calcium concentration. In the cell-attached configuration, ORCC channel activation was observed under perfusion of a hypotonic solution.

CONCLUSION

Human atrial myocytes express an outwardly rectifying Cl- channel that is sensitive to PKC activation. This channel shares biophysical and pharmacological properties with the swelling-activated chloride current implicated in cardiac pathologies such as myocardial ischemia and dilated cardiopathies.

Expression and functional phenotype of mouse ERG K+ channels in the inner ear: potential role in K+ regulation in the inner ear

An outcome of the intricate K+ regulation in the cochlear duct is the endocochlear potential (EP), approximately 80 mV, the "battery" that runs hair-cell transduction; however, the detailed molecular mechanisms for the generation of the EP remain unclear. We provide strong evidence indicating that the intermediate cells (ICs) of the stria vascularis (StV) express outward K+ current that rectifies inwardly at positive potentials. The channel belongs to the ether-a-go-go-related gene (erg) family of K+ channels. We cloned an ERG1a channel in the mouse inner ear (MERG1a). The cellular distribution of MERG1a in the cochlea displayed the highest levels of immunoreactivity in the ICs and modest reactivity in the marginal cells as well as in several extrastrial cells (e.g., hair cells). Functional expression of the StV-specific MERG1a channel reveals a current that activates at relatively negative potentials (approximately-50 mV) and shows rapid inactivation reflected as inward rectification at depolarized potentials. The current was sensitive to the methanesulfonanilide drug E-4031 (IC50, approximately 165 nM) and the recombinant peptide rBeKm-1 (IC50, approximately 16 nM), and the single-channel conductance in symmetrical K+ was approximately 14 pS. The site of expression of MERG1a and its functional phenotype (e.g., modulation of the current by external K+ make it one of the most likely candidates for establishing the high throughput of K+ ions across ICs to generate EP. In addition, the property of the channel that produces marked K+ extrusion in increased external K+ may be important in shaping the dynamics of K+ cycling in the inner ear.

Would modulation of intracellular Ca2+ be antiarrhythmic?

Under several types of conditions, reversal of steps of excitation-contraction coupling (RECC) can give rise to nondriven electrical activity. In this review we explore those conditions for several cardiac cell types (SA, atrial, Purkinje, ventricular cells). We find that abnormal spontaneous Ca2+ release from intracellular Ca2+ stores, aberrant Ca2+ influx from sarcolemmal channels or abnormal Ca2+ surges in nonuniform muscle can be the initiators of the RECC. Often, with such increases in Ca2+, spontaneous Ca2+ waves occur and lead to membrane depolarizations. Because the change in membrane voltage is produced by Ca2+-dependent changes in ion channel function, we also review here what is known about the molecular interaction of Ca2+ and several Ca2+-dependent processes, including the intracellular Ca2+ release channels implicated in the genetic basis of some forms of human arrhythmias. Finally, we review what is known about the effectiveness of several agents in modifying such Ca2+-dependent arrhythmias.

Calcium-activated chloride channels

Calcium-activated chloride channels (CaCCs) play important roles in cellular physiology, including epithelial secretion of electrolytes and water, sensory transduction, regulation of neuronal and cardiac excitability, and regulation of vascular tone. This review discusses the physiological roles of these channels, their mechanisms of regulation and activation, and the mechanisms of anion selectivity and conduction. Despite the fact that CaCCs are so broadly expressed in cells and play such important functions, understanding these channels has been limited by the absence of specific blockers and the fact that the molecular identities of CaCCs remains in question. Recent status of the pharmacology and molecular identification of CaCCs is evaluated.

Intracellular calcium release and cardiac disease

Intracellular calcium release channels are present on sarcoplasmic and endoplasmic reticuli (SR, ER) of all cell types. There are two classes of these channels: ryanodine receptors (RyR) and inositol 1,4,5-trisphosphate receptors (IP3R). RyRs are required for excitation-contraction (EC) coupling in striated (cardiac and skeletal) muscles. RyRs are made up of macromolecular signaling complexes that contain large cytoplasmic domains, which serve as scaffolds for proteins that regulate the function of the channel. These regulatory proteins include calstabin1/calstabin2 (FKBP12/FKBP12.6), a 12/12.6 kDa subunit that stabilizes the closed state of the channel and prevents aberrant calcium leak from the SR. Kinases and phosphatases are targeted to RyR2 channels and modulate RyR2 function in response to extracellular signals. In the classic fight or flight stress response, phosphorylation of RyR channels by protein kinase A reduces the affinity for calstabin and activates the channels leading to increased SR calcium release. In heart failure, a cardiac insult causes a mismatch between blood supply and metabolic demands of organs. The chronically activated fight or flight response leads to leaky channels, altered calcium signaling, and contractile dysfunction and cardiac arrhythmias.

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.

Calstabin deficiency, ryanodine receptors, and sudden cardiac death

Altered cardiac ryanodine receptor (RyR2) function has an important role in heart failure and genetic forms of arrhythmias. RyR2 constitutes the major intracellular Ca2+ release channel in the cardiac sarcoplasmic reticulum (SR). The peptidyl-prolyl isomerase calstabin2 (FKBP12.6) is a component of the RyR2 macromolecular signaling complex. Calstabin2 binding to RyR2 is regulated by PKA phosphorylation of Ser2809 in RyR2. PKA phosphorylation of RyR2 decreases the binding affinity for calstabin2 and increases RyR2 open probability and sensitivity to Ca2+-dependent activation. In heart failure, a majority of studies have found that RyR2 becomes chronically PKA hyper-phosphorylated which depletes calstabin2 from the channel complex. Calstabin2 dissociation causes a diastolic SR Ca2+ leak contributing to depressed intracellular Ca2+ cycling and decreased cardiac contractility. Missense mutations linked to genetic forms of exercise-induced arrhythmias and sudden cardiac death also cause decreased calstabin2-binding affinity and leaky RyR2 channels. We review the importance of calstabin2 for RyR2 function and excitation-contraction coupling, and discuss new observations that implicate dysregulation of calstabin2 binding as a central mechanism for abnormal calcium cycling in heart failure and triggered arrhythmias.

Rate dependence and regulation of action potential and calcium transient in a canine cardiac ventricular cell model

BACKGROUND

Computational biology is a powerful tool for elucidating arrhythmogenic mechanisms at the cellular level, where complex interactions between ionic processes determine behavior. A novel theoretical model of the canine ventricular epicardial action potential and calcium cycling was developed and used to investigate ionic mechanisms underlying Ca2+ transient (CaT) and action potential duration (APD) rate dependence.

METHODS AND RESULTS

The Ca2+/calmodulin-dependent protein kinase (CaMKII) regulatory pathway was integrated into the model, which included a novel Ca2+-release formulation, Ca2+ subspace, dynamic chloride handling, and formulations for major ion currents based on canine ventricular data. Decreasing pacing cycle length from 8000 to 300 ms shortened APD primarily because of I(Ca(L)) reduction, with additional contributions from I(to1), I(NaK), and late I(Na). CaT amplitude increased as cycle length decreased from 8000 to 500 ms. This positive rate-dependent property depended on CaMKII activity.

CONCLUSIONS

CaMKII is an important determinant of the rate dependence of CaT but not of APD, which depends on ion-channel kinetics. The model of CaMKII regulation may serve as a paradigm for modeling effects of other regulatory pathways on cell function.

A mathematical treatment of integrated Ca dynamics within the ventricular myocyte

We have developed a detailed mathematical model for Ca2+ handling and ionic currents in the rabbit ventricular myocyte. The objective was to develop a model that: 1), accurately reflects Ca-dependent Ca release; 2), uses realistic parameters, particularly those that concern Ca transport from the cytosol; 3), comes to steady state; 4), simulates basic excitation-contraction coupling phenomena; and 5), runs on a normal desktop computer. The model includes the following novel features: 1), the addition of a subsarcolemmal compartment to the other two commonly formulated cytosolic compartments (junctional and bulk) because ion channels in the membrane sense ion concentrations that differ from bulk; 2), the use of realistic cytosolic Ca buffering parameters; 3), a reversible sarcoplasmic reticulum (SR) Ca pump; 4), a scheme for Na-Ca exchange transport that is [Na]i dependent and allosterically regulated by [Ca]i; and 5), a practical model of SR Ca release including both inactivation/adaptation and SR Ca load dependence. The data describe normal electrical activity and Ca handling characteristics of the cardiac myocyte and the SR Ca load dependence of these processes. The model includes a realistic balance of Ca removal mechanisms (e.g., SR Ca pump versus Na-Ca exchange), and the phenomena of rest decay and frequency-dependent inotropy. A particular emphasis is placed upon reproducing the nonlinear dependence of gain and fractional SR Ca release upon SR Ca load. We conclude that this model is more robust than many previously existing models and reproduces many experimental results using parameters based largely on experimental measurements in myocytes.

Direct block of the cystic fibrosis transmembrane conductance regulator Cl(-) channel by niflumic acid

Niflumic acid is widely used to inhibit Ca(2+) -activated Cl(-) channels. However, the chemical structure of niflumic acid resembles that of diphenylamine-2-carboxylate, a drug that inhibits the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. To investigate how niflumic acid inhibits CFTR Cl(-) channel, we studied recombinant wild-type human CFTR in excised inside-out membrane patches. When added to the intracellular solution, niflumic acid caused a concentration- and voltage-dependent decrease of CFTR Cl(-) current with half-maximal inhibitory concentration (K(i)) of 253 microM and Hill co-efficient of approximately 1, at -50 mV. Niflumic acid inhibition of single CFTR Cl(-) channels was characterized by a very fast, flickery block that decreased dramatically current amplitude without altering open-probability. Consistent with these data, spectral analysis of CFTR Cl(-) currents suggested that channel block by niflumic acid was described by the closed <--> open <--> blocked kinetic scheme with blocker on rate (k(on)) = 13.9 x 10(6) M(-1)s(-1), off rate (k(off))=3348 s(-1) and dissociation constant (K(d)) = 241 microM, at -50 mV. Based on these data, we tested the effects of niflumic acid on transepithelial Cl(-) secretion and cyst growth using type I MDCK epithelial cells. Niflumic acid (200 microM) inhibited cAMP-stimulated, bumetanide-sensitive short-circuit current by 55%. Moreover, the drug potently retarded cyst growth. We conclude that niflumic acid is an open-channel blocker of CFTR that inhibits Cl(-) permeation by plugging the channel pore. It or related agents might be of value in the development of new therapies for autosomal dominant polycystic kidney disease.

Cloning and Expression of a Small-Conductance Ca2+-Activated K+ Channel From the Mouse Cochlea: Coexpression with α9/α10 Acetylcholine Receptors

Functional interactions between ligand-gated, voltage-, and Ca2+-activated ion channels are essential to the properties of excitable cells and thus to the working of the nervous system. The outer hair cells in the mammalian cochlea receive efferent inputs from the brain stem through cholinergic nerve fibers that form synapses at their base. The acetylcholine released from these efferent fibers activates fast inhibitory postsynaptic currents mediated, to some extent, by small-conductance Ca2+-activated K+ channels (SK) that had not been cloned. Here we report the cloning, characterization, and expression of a complete SK2 cDNA from the mouse cochlea. The cDNAs of the mouse cochlea α9 and α10 acetylcholine receptors were also obtained, sequenced, and coexpressed with the SK2 channels. Human cultured cell lines transfected with SK2 yielded Ca2+-sensitive K+ current that was blocked by dequalinium chloride and apamin, known blockers of SK channels. Xenopus oocytes injected with SK2 in vitro transcribed RNA, under conditions where only outward K+ currents could be recorded, expressed an outward current that was sensitive to EGTA, dequalinium chloride, and apamin. In HEK-293 cells cotransfected with cochlear SK2 plus α9/α10 receptors, acetylcholine induced an inward current followed by a robust outward current. The results indicate that SK2 and the α9/α10 acetylcholine receptors are sufficient to partly recapitulate the native hair cell efferent synaptic response.

Ca2+-activated Cl- current reduces transmural electrical heterogeneity within the rabbit left ventricle

OBJECTIVE

Various cationic membrane channels contribute to the heterogeneity of action potential configuration between the transmural layers of the left ventricle. The role of anionic membrane channels is less intensively studied. We investigated the role of the Ca2+-activated Cl- current, ICl(Ca), in transmural electrical heterogeneity.

METHODS AND RESULTS

We determined the density of ICl(Ca) and its physiological role in subepicardial and subendocardial ventricular myocytes of rabbit using the patch-clamp technique. ICl(Ca) was measured as the 4,4'diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) sensitive current. The current-voltage relationships and the densities of ICl(Ca) were similar in subepicardial and subendocardial myocytes. However, the functional role of ICl(Ca) exhibited striking differences. In subendocardial myocytes, blockade of ICl(Ca) by DIDS increased action potential duration (APD) significantly at all measured stimulus frequencies (3.33-0.2 Hz). In subepicardial myocytes, ICl(Ca) blockade increased APD only at 3.33 Hz, but not at the lower stimulus frequencies. At 1 Hz, ICl(Ca) blockade in subepicardial myocytes only caused an APD increase when the transient outward K+ current, Ito1, was blocked.

CONCLUSIONS

The densities and gating properties of ICl(Ca) are similar in subepicardial and subendocardial myocytes. ICl(Ca) contributes to APD shortening in subendocardial, but not in subepicardial myocytes except at 3.33 Hz. These differences in functional expression of ICl(Ca) reduce the electrical heterogeneity in rabbit left ventricle.

Identification of an N-terminal amino acid of the CLC-3 chloride channel critical in phosphorylation-dependent activation of a CaMKII-activated chloride current

CLC-3, a member of the CLC family of chloride channels, mediates function in many cell types in the body. The multifunctional calcium-calmodulin-dependent protein kinase II (CaMKII) has been shown to activate recombinant CLC-3 stably expressed in tsA cells, a human embryonic kidney cell line derivative, and natively expressed channel protein in a human colonic tumour cell line T84. We examined the CaMKII-dependent regulation of CLC-3 in a smooth muscle cell model as well as in the human colonic tumour cell line, HT29, using whole-cell voltage clamp. In CLC-3-expressing cells, we observed the activation of a Cl(-) conductance following intracellular introduction of the isolated autonomous CaMKII into the voltage-clamped cell via the patch pipette. The CaMKII-dependent Cl(-) conductance was not observed following exposure of the cells to 1 microm autocamtide inhibitory peptide (AIP), a selective inhibitor of CaMKII. Arterial smooth muscle cells express a robust CaMKII-activated Cl(-) conductance; however, CLC-3(-/-) cells did not. The N-terminus of CLC-3, which contains a CaMKII consensus sequence, was phosphorylated by CaMKII in vitro, and mutation of the serine at position 109 (S109A) abolished the CaMKII-dependent Cl(-) conductance, indicating that this residue is important in the gating of CLC-3 at the plasma membrane.

Involvement of transient receptor potential-like channels in responses to mGluR-I activation in midbrain dopamine neurons

We investigated the involvement of store-operated channels (SOCs) and transient receptor potential (TRP) channels in the response to activation of the group I metabotropic glutamate receptor subtype 1 (mGluR1) with the agonist (S)-3,5-dihydroxyphenylglycine (DHPG, puff application) in dopamine neurons in rat brain slices. The mGluR1-induced conductance reversed polarity close to 0 mV and at more positive potentials when extracellular potassium concentrations were increased, indicating the involvement of a cationic channel. DHPG currents but not intracellular calcium responses were reduced by low extracellular sodium concentrations but were not affected by sodium channel blockers, tetrodotoxin and saxitoxin or by inhibition of the h-current with cesium. Abolition of calcium responses with intracellular BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; 10 mm) did not affect current responses, indicating they were not calcium activated. Extracellular application of non-selective SOCs and TRP channel blockers 2-aminoethoxydiphenylborane (2-APB), SKF96365, ruthenium red and flufenamic acid (but not gadolinium) reduced DHPG current and calcium responses. Intracellular application of ruthenium red and 2-APB did not affect DHPG currents, indicating that IP3 and ryanodine receptors did not mediate their actions. Single-cell PCR revealed the presence of TRPC1 and 5 mRNA in most dopamine neurons and subtypes 3, 4 and 6 in some. Store depletion evoked calcium entry indicative of SOCs, providing the first functional observation of such channels in native central neurons. Store depletion with either cyclopiazonic acid or ryanodine abolished calcium but not current responses to DHPG. The electrophysiological and pharmacological properties of the mGluR1-induced inward current are consistent with the involvement of TRP channels whereas calcium responses are dependent on the function of SOCs in voltage clamp recordings.

Characterization of Ca2+-activated Cl- currents in mouse kidney inner medullary collecting duct cells

Ca2+-activated Cl- (ClCa) channels were characterized biophysically and pharmacologically in a mouse kidney inner medullary collecting duct cell line, IMCD-K2. Whole cell recording was performed with symmetrical N-methyl-d-glucamine chloride (NMDG)-Cl in the intracellular and extracellular solutions, and the intracellular Ca2+ concentration ([Ca2+]i) was adjusted with Ca2+-EGTA buffers. The amplitude of the current was dependent on [Ca2+]i. [Ca2+]i <800 nM strongly activated outwardly rectifying Cl- currents, whereas high Ca2+ (21 microM) elicited time-independent currents that did not rectify. The currents activated at low [Ca2+] exhibited time-dependent activation and deactivation. The affinity of the channel for Ca2+ was voltage dependent. The EC50 for Ca2+ was approximately 0.4 microM at +100 mV and approximately 1.0 microM at -100 mV. The Cl- channel blocker niflumic acid in the bath equally inhibited both inward and outward currents reversibly, with a Ki = 7.6 microM. DIDS, diphenylamine-2-carboxylic acid, and anthracene-9-carboxylic acid reversibly inhibited outward currents in a voltage-dependent manner. DTT slowly inhibited the currents, but tamoxifen did not. Comparing the biophysical and pharmacological properties, we conclude that IMCD-K2 cells express the same type of ClCa channels as those we have described in detail in Xenopus laevis oocytes (Qu Z and Hartzell HC. J Biol Chem 276: 18423-18429, 2001).

Multiple conductance states of single Ca2+-activated Cl- channels in rabbit pulmonary artery smooth muscle cells

Ca2+-activated Cl- channels contribute to agonist-evoked contraction and spontaneous activity in some smooth muscle preparations. Patch pipette techniques were used to study the properties of single Ca2+-activated Cl- channels in freshly dispersed rabbit pulmonary artery myocytes. In the cell-attached recording mode, two conductance states of 3.5 and 1.8 pS were recorded either spontaneously or in response to increasing [Ca2+]i. With inside-out patches, the 3.5 pS channel current predominated at 50 nM [Ca2+]i, but at 500 nM [Ca2+]i most channels opened to the 1.8 pS level and an additional 1.2 pS channel conductance was resolved. At 1 microM [Ca2+]i all of the Cl- channels opened either to the 1.8 pS or 1.2 pS level. In 0 [Ca2+]i, no channel activity was observed at -100 mV to +100 mV, but with 10-250 nM [Ca2+]i the total single channel open probability (NP(o)) increased with depolarisation. This voltage dependence was not seen at higher values of [Ca2+]i. The plot of NPo vs. [Ca2+]i yielded Ca2+ affinity constants of 8 and 250 nM and Hill slopes of 1.3 and 2.3 at +100 and -100 mV, respectively. The distribution of open times was fitted by two exponentials of about 5 and 30 ms, which were neither voltage nor Ca2+ dependent. Replacement of external Cl- by I- shifted the reversal potential by about -30 mV and lengthened the longer of the two mean open times without significant effects on other kinetic parameters. Based on these data, a model for the activation of Ca2+-activated Cl- channels is proposed.

An integrative model of the cardiac ventricular myocyte incorporating local control of Ca2+ release

The local control theory of excitation-contraction (EC) coupling in cardiac muscle asserts that L-type Ca(2+) current tightly controls Ca(2+) release from the sarcoplasmic reticulum (SR) via local interaction of closely apposed L-type Ca(2+) channels (LCCs) and ryanodine receptors (RyRs). These local interactions give rise to smoothly graded Ca(2+)-induced Ca(2+) release (CICR), which exhibits high gain. In this study we present a biophysically detailed model of the normal canine ventricular myocyte that conforms to local control theory. The model formulation incorporates details of microscopic EC coupling properties in the form of Ca(2+) release units (CaRUs) in which individual sarcolemmal LCCs interact in a stochastic manner with nearby RyRs in localized regions where junctional SR membrane and transverse-tubular membrane are in close proximity. The CaRUs are embedded within and interact with the global systems of the myocyte describing ionic and membrane pump/exchanger currents, SR Ca(2+) uptake, and time-varying cytosolic ion concentrations to form a model of the cardiac action potential (AP). The model can reproduce both the detailed properties of EC coupling, such as variable gain and graded SR Ca(2+) release, and whole-cell phenomena, such as modulation of AP duration by SR Ca(2+) release. Simulations indicate that the local control paradigm predicts stable APs when the L-type Ca(2+) current is adjusted in accord with the balance between voltage- and Ca(2+)-dependent inactivation processes as measured experimentally, a scenario where common pool models become unstable. The local control myocyte model provides a means for studying the interrelationship between microscopic and macroscopic behaviors in a manner that would not be possible in experiments.

Presence of a calcium-activated chloride current in mouse ventricular myocytes

The properties of several components of outward K(+) currents, including the pharmacological and kinetics profiles as well as the respective molecular correlates, have been identified in mouse cardiac myocytes. Surprisingly little is known with regard to the Ca(2+)-activated ionic currents. We studied the Ca(2+)-activated transient outward currents in mouse ventricular myocytes. We have identified a 4-aminopyridine (4-AP)- and tetraethyl ammonium-resistant transient outward current that is Ca(2+) dependent. The current is carried by Cl(-) and is critically dependent on Ca(2+) influx via voltage-gated Ca(2+) channels and the sarcoplasmic reticulum Ca(2+) store. The current can be blocked by the anion transport blockers niflumic acid and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. Single channel recordings reveal small conductance channels (approximately 1 pS in 140 mM Cl(-)) that can be blocked by anion transport blockers. Ensemble-averaged current faithfully mirrors the transient kinetics observed at the whole level. Niflumic acid (in the presence of 4-AP) leads to prolongation of the early repolarization. Thus this current may contribute to early repolarization of action potentials in mouse ventricular myocytes.

The large conductance potassium channel beta-subunit can interact with and modulate the functional properties of a calcium-activated chloride channel, CLCA1

We have recently compared the biophysical and pharmacological properties of native Ca(2+)-activated Cl(-) currents in murine portal vein with mCLCA1 channels cloned from murine portal vein myocytes (Britton, F. C., Ohya, S., Horowitz, B., and Greenwood, I. A. (2002) J. Physiol. (Lond.) 539, 107-117). These channels shared a similar relative permeability to various anions, but the expressed channel current lacked the marked time dependence of the native current. In addition, the expressed channel showed a lower Ca(2+) sensitivity than the native channel. As non-pore-forming regulatory beta-subunits alter the kinetics and increase the Ca(2+) sensitivity of Ca(2+)-dependent K(+) channels (BK channels) we investigated whether co-expression of beta-subunits with CLCA1 would alter the kinetics/Ca(2+) sensitivity of mCLCA1. Internal dialysis of human embryonic kidney cells stably expressing CLCA1 with 500 nM Ca(2+) evoked a significantly larger current when the beta-subunit KCNMB1 was co-expressed. In a small number of co-transfected cells marked time dependence to the activation kinetics was observed. Interaction studies using the mammalian two-hybrid technique demonstrated a physical association between CLCA1 and KCNMB1 when co-expressed in human embryonic kidney cells. These data suggest that activation of CLCA1 can be modified by accessory subunits.

A Ca(2+)-inhibited non-selective cation conductance contributes to pacemaker currents in mouse interstitial cell of Cajal

Interstitial cells of Cajal (ICC) provide pacemaker activity in some smooth muscles. The nature of the pacemaker conductance is unclear, but studies suggest that pacemaker activity is due to a voltage-independent, Ca(2+)-regulated, non-selective cation conductance. We investigated Ca(2+)-regulated conductances in murine intestinal ICC and found that reducing cytoplasmic Ca(2+) activates whole-cell inward currents and single-channel currents. Both the whole-cell currents and single-channel currents reversed at 0 mV when the equilibrium potentials of all ions present were far from 0 mV. Recordings from on-cell patches revealed oscillations in unitary currents at the frequency of pacemaker currents in ICC. Voltage-clamping cells to -60 mV did not change the oscillatory activity of channels in on-cell patches. Depolarizing cells with high external K(+) caused loss of resolvable single-channel currents, but the oscillatory single-channel currents were restored when the patches were stepped to negative potentials. Unitary currents were also resolved in excised patches. The single-channel conductance was 13 pS, and currents reversed at 0 mV. The channels responsible were strongly activated by 10(-7) M Ca(2+), and 10(-6) M Ca(2+) reduced activity. The 13 pS channels were strongly activated by the calmodulin inhibitors calmidazolium and W-7 in on-cell and excised patches. Calmidazolium and W-7 also activated a persistent inward current under whole-cell conditions. Murine ICC express Ca(2+)-inhibited, non-selective cation channels that are periodically activated at the same frequency as pacemaker currents. This conductance may contribute to the pacemaker current and generation of electrical slow waves in GI muscles.

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.

Regulation of a human chloride channel. a paradigm for integrating input from calcium, type ii calmodulin-dependent protein kinase, and inositol 3,4,5,6-tetrakisphosphate

We have studied the regulation of Ca(2+)-dependent chloride (Cl(Ca)) channels in a human pancreatoma epithelial cell line (CFPAC-1), which does not express functional cAMP-dependent cystic fibrosis transmembrane conductance regulator chloride channels. In cell-free patches from these cells, physiological Ca(2+) concentrations activated a single class of 1-picosiemens Cl(-)-selective channels. The same channels were also stimulated by a purified type II calmodulin-dependent protein kinase (CaMKII), and in cell-attached patches by purinergic agonists. In whole-cell recordings, both Ca(2+)- and CaMKII-dependent mechanisms contributed to chloride channel stimulation by Ca(2+), but the CaMKII-dependent pathway was selectively inhibited by inositol 3,4,5,6-tetrakisphosphate (Ins(3,4,5,6)P(4)). This inhibitory effect of Ins(3,4,5,6)P(4) on Cl(Ca) channel stimulation by CaMKII was reduced by raising [Ca(2+)] and prevented by inhibition of protein phosphatase activity with 100 nm okadaic acid. These data provide a new context for understanding the physiological relevance of Ins(3,4,5,6)P(4) in the longer term regulation of Ca(2+)-dependent Cl(-) fluxes in epithelial cells.

Beat dependent alteration of Ca2+-activated Cl- current during rapid stimulation in rabbit ventricular myocytes

The transient outward currents (Ito) play an important role in action potential repolarization in cardiac myocytes. Two components of Ito have been identified as 4-AP-sensitive but Ca2+-insensitive Ito carried by K, and Ca2+-sensitive but 4-AP insensitive Ito carried by Cl- (I(Cl(Ca))). It is known that the amplitudes of Ito change depending on the stimulation frequency. In this study we investigated the beat dependent alteration of I(Cl(Ca)) during rapid stimulation using the whole cell patch clamp technique in rabbit ventricular myocytes. The cells were internally perfused with a solution containing 0.1 microM free Ca2+ to develop I(Cl(Ca)) and all internal K+ was replaced with Cs+ to block 4-AP-sensitive Ito and other K+ currents. By applying depolarizing pulses at a high frequency of 2.5 Hz, the amplitudes of I(Cl(Ca)) gradually increased as the number of pulses increased following a transient decrease in the 2nd pulse and reached a plateau level at the 20th pulse. The shape of the current-voltage curve of I(Cl(Ca)) was not overly different for different numbers of preceding pulses. The recovery from inactivation of I(Cl(Ca)) could be fitted to a single exponential curve and full recovery was achieved after > 1 sec with a time constant of 368 ms. The ramp clamp experiments showed that the conductance of the background I(Cl(Ca)) increased with the preceding pulse numbers, indicating that the resting level of [Ca2]i increased with the pulses applied. From these results, we conclude that beat dependent alteration of I(Cl(Ca)) is determined by not only its apparent kinetic property, but also the resting level of [Ca2+]i during rapid stimulation.

Anion permeation in Ca(2+)-activated Cl(-) channels

Ca(2+)-activated Cl channels (Cl(Ca)Cs) are an important class of anion channels that are opened by increases in cytosolic [Ca(2+)]. Here, we examine the mechanisms of anion permeation through Cl(Ca)Cs from Xenopus oocytes in excised inside-out and outside-out patches. Cl(Ca)Cs exhibited moderate selectivity for Cl over Na: P(Na)/P(Cl) = 0.1. The apparent affinity of Cl(Ca)Cs for Cl was low: K(d) = 73 mM. The channel had an estimated pore diameter >0.6 nm. The relative permeabilities measured under bi-ionic conditions by changes in E(rev) were as follows: C(CN)(3) > SCN > N(CN)(2) > ClO(4) > I > N(3) > Br > Cl > formate > HCO(3) > acetate = F > gluconate. The conductance sequence was as follows: N(3) > Br > Cl > N(CN)(2) > I > SCN > COOH > ClO(4) > acetate > HCO(3) = C(CN)(3) > gluconate. Permeant anions block in a voltage-dependent manner with the following affinities: C(CN)(3) > SCN = ClO(4) > N(CN)(2) > I > N(3) > Br > HCO(3) > Cl > gluconate > formate > acetate. Although these data suggest that anionic selectivity is determined by ionic hydration energy, other factors contribute, because the energy barrier for permeation is exponentially related to anion hydration energy. Cl(Ca)Cs exhibit weak anomalous mole fraction behavior, implying that the channel may be a multi-ion pore, but that ions interact weakly in the pore. The affinity of the channel for Ca(2+) depended on the permeant anion at low [Ca(2+)] (100-500 nM). Apparently, occupancy of the pore by a permeant anion increased the affinity of the channel for Ca(2+). The current was strongly dependent on pH. Increasing pH on the cytoplasmic side decreased the inward current, whereas increasing pH on the external side decreased the outward current. In both cases, the apparent pKa was voltage-dependent with apparent pKa at 0 mV = approximately 9.2. The channel may be blocked by OH(-) ions, or protons may titrate a site in the pore necessary for ion permeation. These data demonstrate that the permeation properties of Cl(Ca)Cs are different from those of CFTR or ClC-1, and provide insights into the nature of the Cl(Ca)C pore.

Modulation of potassium channels in the hearts of transgenic and mutant mice with altered polyamine biosynthesis

Inward rectification of cardiac I(K1)channels was modulated by genetic manipulation of the naturally occurring polyamines. Ornithine decarboxylase (ODC) was overexpressed in mouse heart under control of the cardiac alpha -myosin heavy chain promoter (alpha MHC). In ODC transgenic hearts, putrescine and cadaverine levels were highly elevated ( identical with 35-fold for putrescine), spermidine was increased 3.6-fold, but spermine was essentially unchanged. I(K1)density was reduced by identical with 38%, although the voltage-dependence of rectification was essentially unchanged. Interestingly, the fast component of transient outward (I(to,f)) current was increased, but the total outward current amplitude was unchanged. I(K1)and I(to)currents were also studied in myocytes from mutant Gyro (Gy) mice in which the spermine synthase gene is disrupted, leading to a complete loss of spermine. I(K1)current densities were not altered in Gy myocytes, but the steepness of rectification was reduced indicating a role for spermine in controlling rectification. Intracellular dialysis of myocytes with putrescine, spermidine and spermine caused reduction, no change and increase of the steepness of rectification, respectively. Taken together with kinetic analysis of I(K1)activation these results are consistent with spermine being a major rectifying factor at potentials positive to E(K), spermidine dominating at potentials around and negative to E(K), and putrescine playing no significant role in rectification in the mouse heart.

Mathematical analysis of canine atrial action potentials: rate, regional factors, and electrical remodeling

Dogs have been used extensively to study atrial arrhythmias, but there are no published mathematical models of the canine atrial action potential (AP). To obtain insights into the ionic mechanisms governing canine atrial AP properties, we incorporated formulations of K(+), Na(+), Ca(2+), and Cl(-) currents, based on measurements in canine atrial myocytes, into a mathematical model of the AP. The rate-dependent behavior of model APs corresponded to experimental measurements and pointed to a central role for L-type Ca(2+) current inactivation in rate adaptation. Incorporating previously described regional ionic current variations into the model largely reproduced AP forms characteristic of the corresponding right atrial regions (appendage, pectinate muscle, crista terminalis, and atrioventricular ring). When ionic alterations induced by tachycardia-dependent remodeling were incorporated, the model reproduced qualitatively the AP features constituting the cellular substrate for atrial fibrillation. We conclude that this ionic model of the canine atrial AP agrees well with experimental measurements and gives potential insights into mechanisms underlying functionally important electrophysiological phenomena in canine atrium.