Regulatory volume decrease of cardiac myocytes induced by beta-adrenergic activation of the Cl- channel in guinea pig

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.

Myocardial electrotonic response to submaximal exercise in dogs with healed myocardial infarctions: evidence for β-adrenoceptor mediated enhanced coupling during exercise testing

INTRODUCTION

Autonomic neural activation during cardiac stress testing is an established risk-stratification tool in post-myocardial infarction (MI) patients. However, autonomic activation can also modulate myocardial electrotonic coupling, a known factor to contribute to the genesis of arrhythmias. The present study tested the hypothesis that exercise-induced autonomic neural activation modulates electrotonic coupling (as measured by myocardial electrical impedance, MEI) in post-MI animals shown to be susceptible or resistant to ventricular fibrillation (VF).

METHODS

Dogs (n = 25) with healed MI instrumented for MEI measurements were trained to run on a treadmill and classified based on their susceptibility to VF (12 susceptible, 9 resistant). MEI and ECGs were recorded during 6-stage exercise tests (18 min/test; peak: 6.4 km/h @ 16%) performed under control conditions, and following complete β-adrenoceptor (β-AR) blockade (propranolol); MEI was also measured at rest during escalating β-AR stimulation (isoproterenol) or overdrive-pacing.

RESULTS

Exercise progressively increased heart rate (HR) and reduced heart rate variability (HRV). In parallel, MEI decreased gradually (enhanced electrotonic coupling) with exercise; at peak exercise, MEI was reduced by 5.3 ± 0.4% (or -23 ± 1.8Ω, P < 0.001). Notably, exercise-mediated electrotonic changes were linearly predicted by the degree of autonomic activation, as indicated by changes in either HR or in HRV (P < 0.001). Indeed, β-AR blockade attenuated the MEI response to exercise while direct β-AR stimulation (at rest) triggered MEI decreases comparable to those observed during exercise; ventricular pacing had no significant effects on MEI. Finally, animals prone to VF had a significantly larger MEI response to exercise.

CONCLUSIONS

These data suggest that β-AR activation during exercise can acutely enhance electrotonic coupling in the myocardium, particularly in dogs susceptible to ischemia-induced VF.

Steady-state solutions of cell volume in a cardiac myocyte model elaborated for membrane excitation, ion homeostasis and Ca2+ dynamics

The cell volume continuously changes in response to varying physiological conditions, and mechanisms underlying volume regulation have been investigated in both experimental and theoretical studies. Here, general formulations concerning cell volume change are presented in the context of developing a comprehensive cell model which takes Ca(2+) dynamics into account. Explicit formulas for charge conservation and steady-state volumes of the cytosol and endoplasmic reticulum (ER) are derived in terms of membrane potential, amount of ions, Ca(2+)-bound buffer molecules, and initial cellular conditions. The formulations were applied to a ventricular myocyte model which has plasma-membrane Ca(2+) currents with dynamic gating mechanisms, Ca(2+)-buffering reactions with diffusive and non-diffusive buffer proteins, and Ca(2+) uptake into or release from the sarcoplasmic reticulum (SR) accompanied by compensatory cationic or anionic currents through the SR membrane. Time-dependent volume changes in cardiac myocytes induced by varying extracellular osmolarity or by action potential generation were successfully simulated by the novel formulations. Through application of bifurcation analysis, the existence and uniqueness of steady-state solutions of the cell volume were validated, and contributions of individual ion channels and transporters to the steady-state volume were systematically analyzed. The new formulas are consistent with previous fundamental theory derived from simple models of minimum compositions. The new formulations may be useful for examination of the relationship between cell function and volume change in other cell types.

Cell volume control in phospholemman (PLM) knockout mice: do cardiac myocytes demonstrate a regulatory volume decrease and is this influenced by deletion of PLM?

In addition to modulatory actions on Na+-K+-ATPase, phospholemman (PLM) has been proposed to play a role in cell volume regulation. Overexpression of PLM induces ionic conductances, with 'PLM channels' exhibiting selectivity for taurine. Osmotic challenge of host cells overexpressing PLM increases taurine efflux and augments the cellular regulatory volume decrease (RVD) response, though a link between PLM and cell volume regulation has not been studied in the heart. We recently reported a depressed cardiac contractile function in PLM knockout mice in vivo, which was exacerbated in crystalloid-perfused isolated hearts, indicating that these hearts were osmotically challenged. To address this, the present study investigated the role of PLM in osmoregulation in the heart. Isolated PLM wild-type and knockout hearts were perfused with a crystalloid buffer supplemented with mannitol in a bid to prevent perfusate-induced cell swelling and maintain function. Accordingly, and in contrast to wild-type control hearts, contractile function was improved in PLM knockout hearts with 30 mM mannitol. To investigate further, isolated PLM wild-type and knockout cardiomyocytes were subjected to increasing hyposmotic challenges. Initial validation studies showed the IonOptix video edge-detection system to be a simple and accurate 'real-time' method for tracking cell width as a marker of cell size. Myocytes swelled equally in both genotypes, indicating that PLM, when expressed at physiological levels in cardiomyocytes, is not essential to limit water accumulation in response to a hyposmotic challenge. Interestingly, freshly isolated adult cardiomyocytes consistently failed to mount RVDs in response to cell swelling, adding to conflicting reports in the literature. A proposed perturbation of the RVD response as a result of the cell isolation process was not restored, however, with short-term culture in either adult or neonatal cardiomyocytes.

Simulation analysis of intracellular Na+ and Cl− homeostasis during β1-adrenergic stimulation of cardiac myocyte

To quantitatively understand intracellular Na+ and Cl- homeostasis as well as roles of Na+/K+ pump and cystic fibrosis transmembrane conductance regulator Cl- channel (ICFTR) during the beta1-adrenergic stimulation in cardiac myocyte, we constructed a computer model of beta1-adrenergic signaling and implemented it into an excitation-contraction coupling model of the guinea-pig ventricular cell, which can reproduce membrane excitation, intracellular ion changes (Na+, K+, Ca2+ and Cl-), contraction, cell volume, and oxidative phosphorylation. An application of isoproterenol to the model cell resulted in the shortening of action potential duration (APD) after a transient prolongation, the increases in both Ca2+ transient and cell shortening, and the decreases in both Cl- concentration and cell volume. These results are consistent with experimental data. Increasing the density of ICFTR shortened APD and augmented the peak amplitudes of the L-type Ca2+ current (ICaL) and the Ca2+ transient during the beta1-adrenergic stimulation. This indirect inotropic effect was elucidated by the increase in the driving force of ICaL via a decrease in plateau potential. Our model reproduced the experimental data demonstrating the decrease in intracellular Na+ during the beta-adrenergic stimulation at 0 or 0.5 Hz electrical stimulation. The decrease is attributable to the increase in Na+ affinity of Na+/K+ pump by protein kinase A. However it was predicted that Na+increases at higher beating rate because of larger Na+ influx through forward Na+/Ca2+ exchange. It was demonstrated that dynamic changes in Na+ and Cl- fluxes remarkably affect the inotropic action of isoproterenol in the ventricular myocytes.

A model of the guinea-pig ventricular cardiac myocyte incorporating a transverse-axial tubular system

A model of the guinea-pig cardiac ventricular myocyte has been developed that includes a representation of the transverse-axial tubular system (TATS), including heterogeneous distribution of ion flux pathways between the surface and tubular membranes. The model reproduces frequency-dependent changes of action potential shape and intracellular ion concentrations and can replicate experimental data showing ion diffusion between the tubular lumen and external solution in guinea-pig myocytes. The model is stable at rest and during activity and returns to rested state after perturbation. Theoretical analysis and model simulations show that, due to tight electrical coupling, tubular and surface membranes behave as a homogeneous whole during voltage and current clamp (maximum difference 0.9 mV at peak tubular INa of -38 nA). However, during action potentials, restricted diffusion and ionic currents in TATS cause depletion of tubular Ca2+ and accumulation of tubular K+ (up to -19.8% and +3.4%, respectively, of bulk extracellular values, at 6 Hz). These changes, in turn, decrease ion fluxes across the TATS membrane and decrease sarcoplasmic reticulum (SR) Ca2+ load. Thus, the TATS plays a potentially important role in modulating the function of guinea-pig ventricular myocyte in physiological conditions.

Reactive oxygen species inhibit hyposmotic stress-dependent volume regulation in cultured rat cardiomyocytes

Cells have developed compensatory mechanisms to restore cell volume, and the ability to resist osmotic swelling or shrinkage parallels their resistance to necrosis or apoptosis. There are several mechanisms by which cells adapt to hyposmotic stress including that of regulatory volume decrease. In ischemia and reperfusion, cardiomyocytes are exposed to hyposmotic stress, but little is known as to how their volume is controlled. Exposure of cultured neonatal rat cardiomyocytes to hyposmotic media induced a rapid swelling without any compensatory regulatory volume decrease. The hyposmotic stress increased the production of reactive oxygen species, mainly through NADPH oxidase. Adenoviral overexpression of catalase inhibited the hyposmosis-dependent OH(*) production, induced the regulatory volume decrease mechanism, and prevented cell death. These results suggest that hyposmotic stress of cardiomyocytes stimulates production of reactive oxygen species which are closely linked to volume regulation and cell death.

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.

Involvement of chloride channels in IGF-I-induced proliferation of porcine arterial smooth muscle cells

OBJECTIVE

The existence of Cl- channels in vascular smooth muscle cells (VSMCs) has been increasingly investigated, but the biological functions are not yet clear. Insulin-like growth factor (IGF)-I affects proliferation and migration of VSMCs, and dysregulation of this axis may be involved in atherogenesis and intimal hyperplasia. We examined the effects of Cl- channel blockers on IGF-I-induced proliferation in porcine VSMCs. The siRNA approach was used to support the role of ClC-2, a member of the volume-regulated Cl- channel family, in cell proliferation of VSMCs.

METHODS AND RESULTS

The IGF-I-induced VSMC proliferation was significantly suppressed by the Cl- channel blockers NPPB and IAA94 but not by DIDS. IGF-I-induced cell proliferation parallels a significant increase in the endogenous expression of ClC-2 mRNA and protein. Inhibitors of PI3-kinase, LY294002 and wortmannin, significantly attenuated the IGF-I-upregulated ClC-2 expression and cell proliferation. We observed ClC-2-like Cl- current, and this current was augmented by IGF-I. SiRNA specifically targeted to ClC-2 abolished IGF-I-induced cell proliferation.

CONCLUSION

Our data demonstrate that ClC-2 plays a role in IGF-1-induced regulation of VSMC proliferation in cardiovascular diseases.

Ischemia-induced enhancement of CFTR expression on the plasma membrane in neonatal rat ventricular myocytes

Pathophysiological functions of cardiac cystic fibrosis transmembrane conductance regulator (cCFTR) in ischemia are not well known. Using neonatal rat ventricular cardiomyocytes in primary culture in this study, we thus examined whether the CFTR protein is expressed and is functioning as a cAMP-activated anion channel on the plasma membrane under ischemic conditions. After the cells were subjected to simulated ischemia (O(2) and glucose deprivation), an up-regulation of the CFTR expression was transiently observed in the membrane fraction by Western blot. A peak expression of mature CFTR protein was found at 3 h of ischemia, and thereafter the signal diminished gradually. In contrast, the results of Northern blot indicated that the expression level of CFTR mRNA changed little until 3 h of ischemia, whereas the level slightly decreased after 8 h of ischemia. An immunohistochemical examination showed, in agreement with the results of Western blot analysis, that the expression of CFTR protein on the plasma membrane became most prominent at 3 h of ischemia, whereas the plasmalemmal CFTR signal was markedly reduced after 8 h of ischemia. Whole-cell recordings showed that the cardiomyocytes responded to cAMP with an activation of time- and voltage-independent currents that contained an anion-selective component sensitive to CFTR Cl(-) channel blockers (NPPB and glibenclamide) but not to a stilbene-derivative conventional Cl(-) channel blocker (SITS). This cAMP-activated Cl(-) channel current was found to be enhanced after an application of ischemic stress for 3 to 4 h. These findings indicate that a plasmalemmal expression of CFTR is transiently enhanced under glucose-free hypoxic conditions presumably because of a posttranslational control.

Volume-sensitive Cl- current in bovine adrenocortical cells: comparison with the ACTH-induced Cl- current

In a previous study performed on zona fasciculata (ZF) cells isolated from calf adrenal glands, we identified an ACTH-induced Cl- current involved in cell membrane depolarization. In the present work, we describe a volume-sensitive Cl- current and compare it with the ACTH-activated Cl- current. Experiments were performed using the whole-cell patch-clamp recording method, video microscopy and cortisol-secretion measurements. In current-clamp experiments, hypotonic solutions induced a membrane depolarization to -22 mV. This depolarization, correlated with an increase in the membrane conductance, was sensitive to different Cl- channel inhibitors. In voltage-clamp experiments, hypotonic solution induced a membrane current that slowly decayed and reversed at -21 mV. This ionic current displayed no time dependence and showed a slight outward rectification. It was blocked to variable extent by different conventional Cl- channel inhibitors. Under hypotonic conditions, membrane depolarizations were preceded by an increase in cell volume that was not detected under ACTH stimulation. It was concluded that hypotonic solution induced cell swelling, which activated a Cl- current involved in membrane depolarization. Although cell volume change was not observed in the presence of ACTH, biophysical properties and pharmacological profile of the volume-sensitive Cl- current present obvious similarities with the ACTH-activated Cl- current. As compared to ACTH, hypotonic solutions failed to trigger cortisol production that was weakly stimulated in the presence of high-K+ solution. This shows that in ZF cells, membrane depolarization is not a sufficient condition to fully activate secretory activities.

Suppression of cell proliferation with induction of p21 by Cl(-) channel blockers in human leukemic cells

The existence of Cl(-) channels in lymphocytes and neutrophils has been increasingly recognized, but the biological functions are not yet clear. We examined the effects of Cl(-) channel blockers on the cell proliferation and the cell cycle of human leukemic cell lines. The growth of leukemic cells was suppressed most efficiently by NPPB (5-nitro-2-(3-phenylpropylamino) benzoic acid), partially by 9-AC (9-anthracenecarboxylic acid) and tamoxifen, but not by stilbene compounds. NPPB increased the G0/G1 population and induced the expression of p21, one of the critical molecules for G1/S checkpoint. Antisense oligonucleotide for a NPPB-sensitive and stilbene-insensitive Cl(-) channel, ClC-2, sufficiently suppressed the ClC-2 protein synthesis, but did not affect the growth of leukemic cells. These findings suggest that NPPB-sensitive and stilbene-insensitive Cl(-) channels other than ClC-2 play important roles in cell cycles and cell proliferation of human leukemic cells.

Impaired regulatory volume decrease in freshly isolated cholangiocytes from cystic fibrosis mice: implications for cystic fibrosis transmembrane conductance regulator effect on potassium conductance

Various K(+) and Cl(-) channels are important in cell volume regulation and biliary secretion, but the specific role of cystic fibrosis transmembrane conductance regulator in cholangiocyte cell volume regulation is not known. The goal of this research was to study regulatory volume decrease (RVD) in bile duct cell clusters (BDCCs) from normal and cystic fibrosis (CF) mouse livers. Mouse BDCCs without an enclosed lumen were prepared as described (Cho, W. K. (2002) Am. J. Physiol. 283, G1320-G1327). The isotonic solution consisted of HEPES buffer with 40% of the NaCl replaced with isomolar amounts of sucrose, whereas hypotonic solution was the same as isotonic solution without sucrose. The cell volume changes were indirectly assessed by measuring cross-sectional area (CSA) changes of the BDCCs using quantitative videomicroscopy. Exposure to hypotonic solutions increased relative CSAs of normal BDCCs to 1.20 +/- 0.01 (mean +/- S.E., n = 50) in 10 min, followed by RVD to 1.07 +/- 0.01 by 40 min. Hypotonic challenge in CF mouse BDCCs also increased relative CSA to 1.20 +/- 0.01 (n = 53) in 10 min but without significant recovery. Coadministration of the K(+)-selective ionophore valinomycin restored RVD in CF mouse BDCCs, suggesting that the impaired RVD was likely from a defect in K(+) conductance. Moreover, this valinomycin-induced RVD in CF mice was inhibited by 5-nitro-2'-(3-phenylpropylamino)-benzoate, indicating that it is not from nonspecific effects. Neither cAMP nor calcium agonists could reverse the impaired RVD seen in CF cholangiocytes. Our conclusion is that CF mouse cholangiocytes have defective RVD from an impaired K(+) efflux pathway, which could not be reversed by cAMP nor calcium agonists.

Cell-Volume Regulation by Swelling-Activated Chloride Current in Guinea-Pig Ventricular Myocytes

The cell-volume regulation by swelling-activated Cl- current (I(Cl,swell)) was studied in guinea pig ventricular myocytes, using a microscopic video-image analysis. We have previously shown that in ventricular cells depolarized in high-K+ ([K+]o>45 mM) solution, an activation of the cyclic AMP-dependent Cl- current (I(Cl,cAMP)) leads to cell swelling. We first investigated the mechanism underlying the I(Cl,cAMP)-independent recovery (shrinkage) of the swollen cells. They shrank when the membrane potential (Vm) was made negative to the equilibrium potential of Cl- (ECl) by lowering [K+]o or [Cl-]o in the high-K+ solution. This shrinkage was attenuated by the inhibitors (DIDS, glibenclamide, furosemide) of swelling-activated Cl- current (I(Cl,swell)). These findings suggested an involvement of I(Cl,swell) in the observed isosmotic cell shrinkage. On the other hand, an application of hyposmotic (70% of control) solution to the cells at normal [K+]o (ECl>Vm) induced a cell swelling, and the swollen cells underwent a slight but definite spontaneous cell shrinkage during hyposmotic challenge, indicating the operation of the mechanism of regulatory volume decrease (RVD). This RVD was pronounced at low [Cl-]o, at which ECl was much more positive than Vm. On the contrary, when the hyposmotic solution was applied to the cells at high [K+]o, at which ECl was negative to Vm, the cells swelled vigorously and monotonically without showing RVD, the swelling being much greater than that seen at normal [K+]o. Both the RVD at normal [K+]o and the extra cell swelling at high [K+]o were suppressed by DIDS. These results suggest that I(Cl,swell) activated by cell swelling can shrink or inflate the cardiac cells under hyposmotic as well as isosmotic conditions, depending on Vm and ECl.

Phloretin differentially inhibits volume-sensitive and cyclic AMP-activated, but not Ca-activated, Cl(-) channels

Some phenol derivatives are known to block volume-sensitive Cl(-) channels. However, effects on the channel of the bisphenol phloretin, which is a known blocker of glucose uniport and anion antiport, have not been examined. In the present study, we investigated the effects of phloretin on volume-sensitive Cl(-) channels in comparison with cyclic AMP-activated CFTR Cl(-) channels and Ca(2+)-activated Cl(-) channels using the whole-cell patch-clamp technique. Extracellular application of phloretin (over 10 microM) voltage-independently, and in a concentration-dependent manner (IC(50) approximately 30 microM), inhibited the Cl(-) current activated by a hypotonic challenge in human epithelial T84, Intestine 407 cells and mouse mammary C127/CFTR cells. In contrast, at 30 microM phloretin failed to inhibit cyclic AMP-activated Cl(-) currents in T84 and C127/CFTR cells. Higher concentrations (over 100 microM) of phloretin, however, partially inhibited the CFTR Cl(-) currents in a voltage-dependent manner. At 30 and 300 microM, phloretin showed no inhibitory effect on Ca(2+)-dependent Cl(-) currents induced by ionomycin in T84 cells. It is concluded that phloretin preferentially blocks volume-sensitive Cl(-) channels at low concentrations (below 100 microM) and also inhibits cyclic AMP-activated Cl(-) channels at higher concentrations, whereas phloretin does not inhibit Ca(2+)-activated Cl(-) channels in epithelial cells.

Chloride channel antagonists perturb growth and morphology of Neurospora crassa

The chloride channel antagonists anthracene-9-carboxylic acid, ethacrynic acid and niflumic acid were found to be fungistatic and morphogenic when tested against the ascomycete Neurospora crassa. Potency increased with decreasing pH, suggesting that the protonated forms of the compounds were active. Niflumic acid produced the most pronounced growth aberrations which may reflect an ability to acidify the cytoplasm and block the plasma membrane anion channel of N. crassa.

Changes in cell volume induced by activation of the cyclic amp-dependent chloride channel in guinea-pig cardiac myocytes

The effects of the activation of cyclic AMP-dependent Cl- current (ICl,cAMP) on cell volume were studied at various [K+]o under isosmotic conditions in guinea-pig ventricular myocytes. The area of the cell image obtained with videomicroscopy was used as an index of cell volume. I(Cl,cAMP) was activated by adrenaline (5.5 microM). Measurements of the membrane potential (Vm) were performed by the gramicidin-perforated patch-clamp method. At 5.4 mM [K+]o with low [Cl-]o, where Vm was negative to the predicted equilibrium potential of Cl- (ECl), adrenaline sizably decreased the cell area. At high [K+]o with normal [Cl-]o, where Vm was positive to ECl, adrenaline increased the cell area; at 145.4 mM [K+]o the cell area was increased to 110% of control on average (n = 22). The cells swollen in this manner shrank when [Cl-]o was reduced to a low level in the presence of adrenaline. The results indicate that the induction of Cl- influxes (outward I(Cl,cAMP)) or effluxes (inward I(Cl,cAMP)) can lead to a cell swelling or shrinkage, respectively. The addition of BaCl2 (1 mm), a blocker of K+ channels, attenuated the adrenaline-dependent cell swelling, supporting the view that Cl- fluxes must be accompanied by cofluxes of K+ ions to affect the cell volume. The adrenaline-dependent cell swelling was inhibited by antagonizing beta-adrenergic stimulation with acetylcholine or by blocking I(Cl,cAMP) channels with glibenclamide, confirming the involvement of I(Cl,cAMP) in the adrenaline response. The results show that the activation of I(Cl,cAMP) can shrink or inflate the cardiac cells under isosmotic conditions, depending on Vm and ECl.

Beta-adrenergic stimulation of volume-sensitive chloride transport in lamprey erythrocytes

We measured the effects of a beta-adrenergic agonist, isoproterenol, on chloride transport and volume regulation of lamprey (Lampetra fluviatilis) erythrocytes in isotonic (288 mosm L(-1)) and hypotonic (192 mosm L(-1)) medium. Isoproterenol at a high concentration (10(-5) M) did not influence chloride transport in isotonic medium but markedly increased chloride fluxes in hypotonic conditions: unidirectional flux increased from 100 mmol kg dcw(-1) h(-1) in the absence to 350 mmol kg dcw(-1) h(-1) (dcw=dry cell weight) in the presence of isoproterenol. Simultaneously, the half-time for volume recovery decreased from 27 to 9 min. Isoproterenol caused an increase in cellular cyclic AMP (cAMP) concentration. The stimulation of chloride transport in hypotonic conditions could be induced by application of the permeable cAMP analogue, 8-bromo-cyclic AMP, suggesting that the effect of beta-adrenergic stimulation on chloride transport occurs downstream of cAMP production. As isoproterenol did not affect unidirectional rubidium fluxes in hypotonic conditions, the transport pathway influenced by beta-adrenergic stimulation is most likely the swelling-activated chloride channel. Because the beta-adrenergic agonist only influenced the transport in hypotonic conditions despite the fact that cAMP concentration also increased in isotonic conditions, the activation may involve a volume-dependent conformational change in the chloride channel.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

A Novel Anionic Inward Rectifier in Native Cardiac Myocytes

Abstract —Although the cationic inward rectifiers (Kir and hyperpolarization-activated I f channels) have been well characterized in cardiac myocytes, the expression and physiological role of anionic inward rectifiers in heart are unknown. In the present study, we report the functional and molecular identification of a novel chloride (Cl − ) inward rectifier (Cl.ir) in mammalian heart. Under conditions in which cationic inward rectifier channels were blocked, membrane hyperpolarization (−40 to −140 mV) activated an inwardly rectifying whole-cell current in mouse atrial and ventricular myocytes. Under isotonic conditions, the current activated slowly with a biexponential time course (time constants averaging 179.7±23.4 [mean±SEM] and 2073.6±287.6 ms at −120 mV). Hypotonic cell swelling accelerated the activation and increased the current amplitude whereas hypertonic cell shrinkage inhibited the current. The inwardly rectifying current was carried by Cl − ( I Cl.ir ) and had an anion permeability sequence of Cl − > I − ≫aspartate. I Cl.ir was blocked by 9-anthracene-carboxylic acid and cadmium but not by stilbene disulfonates and tamoxifen. A similar I Cl.ir was also observed in guinea pig cardiac myocytes. The properties of I Cl.ir are consistent with currents generated by expression of ClC-2 Cl − channels. Reverse transcription polymerase chain reaction and Northern blot analysis confirmed transcriptional expression of ClC-2 in both atrial and ventricular tissues and isolated myocytes of mouse and guinea pig hearts. These results indicate that a novel I Cl.ir is present in mammalian heart and support a potentially important role of ClC-2 channels in the regulation of cardiac electrical activity and cell volume under physiological and pathological conditions. The full text of this article is available at http://www.circresaha.org.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Changes in cell volume induced by ion channel flux in guinea-pig cardiac myocytes

1. The cell width of guinea-pig ventricular myocytes was measured using an optic device during patch-clamp experiments and the relationship between the ion channel flux and changes in cell volume was examined. 2. On superfusing myocytes with 50, 70, 150 and 200% osmotic solutions, the relative cell width changed to 121.1 (n = 4), 110.8 (n = 27), 87.1 (n = 6) and 82.6% (n = 6) of control, respectively. Changes in cell length were less than 2% in these test solutions. 3. The application of 300 nmol/L isoprenaline to myocytes swollen in the 70% hypotonic solution induced a decrease in cell width from 111.2 to 106.2% (n = 13). The application of isoprenaline in the isotonic solution also induced a decrease in cell width to 96.5% in eight of 13 cells. A membrane depolarization of 2-4 mV accompanied the isoprenaline-induced decrease in volume. In the remaining five cells, neither an obvious isoprenaline-induced decrease in volume nor membrane depolarization was observed. Under ruptured whole-cell voltage clamp conditions, the activation of inward isoprenaline-induced Cl- current decreased cell width. 4. Cell width was seen to either decrease or increase when a large outward or inward K+ current, respectively, was induced by shifting the holding potential or by applying 200 mumol/L pinacidil. Under gramicidin-perforated whole-cell clamp conditions, the cell width did not change, even when a large inward K+ current was induced. 5. When the test solution was applied to half of an elongated myocyte by using a micropipette, the cell width increased or decreased in the part exposed to the hypotonic or hypertonic test solutions, respectively. In contrast, in the other half of the elongated myocyte, the cell width responded in the opposite direction. 6. It is concluded that a continuous ionic flux through ion channels is capable of inducing changes in cell volume by generating a localized osmotic gradient across the cardiac sarcolemma.

Inhibition of volume-regulated anion channels by expression of the cystic fibrosis transmembrane conductance regulator

1. To investigate whether the cystic fibrosis transmembrane conductance regulator (CFTR) interacts with volume regulated anion channels (VRACs), we measured the volume-activated chloride current (ICl,swell) using the whole-cell patch-clamp technique in calf pulmonary artery endothelial (CPAE) cells and in COS cells transiently transfected with wild-type (WT) CFTR and the deletion mutant DeltaF508 CFTR. 2. ICl,swell was significantly reduced in CPAE cells expressing WT CFTR to 66.5 +/- 8.8 % (n = 13; mean +/- s. e.m.) of the control value (n = 11). This reduction was independent of activation of the CFTR channel. 3. Expression of DeltaF508 CFTR resulted in two groups of CPAE cells. In the first group IBMX and forskolin could activate a Cl- current. In these cells ICl,swell was reduced to 52.7 +/- 18.8 % (n = 5) of the control value (n = 21). In the second group IBMX and forskolin could not activate a current. The amplitude of ICl,swell in these cells was not significantly different from the control value (112.4 +/- 13.7 %, n = 11; 21 control cells). 4. Using the same method we showed that expression of WT CFTR in COS cells reduced ICl,swell to 62.1 +/- 11.9 % (n = 14) of the control value (n = 12) without any changes in the kinetics of the current. Non-stationary noise analysis suggested that there is no significant difference in the single channel conductance of VRAC between CFTR expressing and non-expressing COS cells. 5. We conclude that expression of WT CFTR down-regulates ICl, swell in CPAE and COS cells, suggesting an interaction between CFTR and VRAC independent of activation of CFTR.

Cardiac cell volume: crystal clear or murky waters? A comparison with other cell types

The osmolarity of bodily fluids is strictly controlled so that most cells do not experience changes in osmotic pressure under normal conditions, but osmotic changes can occur in pathological states such as ischemia, septic shock, and diabetic coma. The primary effect of a change in osmolarity is to acutely alter cell volume. If the osmolarity around a cell is decreased, the cell swells, and if increased, it shrinks. In order to tolerate changes in osmolarity, cells have evolved volume regulatory mechanisms activated by osmotic challenge to normalise cell volume and maintain normal function. In the heart, osmotic stress is encountered during a period of myocardial ischemia when metabolites such as lactate accumulate intracellularly and to a certain degree extracellularly, and cause cell swelling. This swelling may be exacerbated further on reperfusion when the hyperosmotic extracellular milieu is replaced by normosmotic blood. In this review, we describe the theory and mechanisms of volume regulation, and draw on findings in extracardiac tissues, such as kidney, whose responses to osmotic change are well characterised. We then describe cell volume regulation in the heart, with particular emphasis on the effect of myocardial ischemia. Finally, we describe the consequences of osmotic cell swelling for the cell and for the heart, and discuss the implications for antiarrhythmic drug efficacy. Using computer modelling, we have summated the changes induced by cell swelling, and predict that swelling will shorten the action potential. This finding indicates that cell swelling is an important component of the response to ischemia, a component modulating the excitability of the heart.