Swelling-induced Cl- current in guinea-pig atrial myocytes: inhibition by glibenclamide

Expression and functional maintenance of volume-regulated anion channels in myometrial smooth muscles of pregnant mice

Pregnancy causes changes in the uterus, such as increased cell volume and altered water content. However, the mechanisms that protect the structure and maintain the function of uterine smooth muscle cells against these changes during pregnancy have not been clarified. This study focused on the volume-regulated anion channel (VRAC), which opens with cell swelling under low osmotic pressure and releases Cl- ions and various organic osmolytes to resist cell swelling and regulates a wide range of biological processes such as cell death. In this study, myometrial smooth muscle (MSM) tissues and cells (MSMCs) were collected from non-pregnant and pregnant mice. Using western blotting and immunocytochemistry, leucine-rich repeat containing protein 8A (LRRC8A), an essential membrane protein that constitutes part of the VRAC, was determined to be diffused throughout MSMCs including in the cell membrane. Patch-clamp experiments were performed to investigate the electrophysiology of swelling-induced Cl- currents (ICl, swell) mediated by the VRAC. No significant changes between non-pregnancy and pregnancy groups were observed in either the expression density of LRRC8A or the current density of ICl, swell, however the presence of LRRC8A on the cell membrane was significantly increased in the third trimester of pregnancy compared to the non-pregnancy. This study suggests that the VRAC may play a role, such as maintaining cellular homeostasis in the pregnant MSM.

Alternative Targets for Modulators of Mitochondrial Potassium Channels

Mitochondrial potassium channels control potassium influx into the mitochondrial matrix and thus regulate mitochondrial membrane potential, volume, respiration, and synthesis of reactive oxygen species (ROS). It has been found that pharmacological activation of mitochondrial potassium channels during ischemia/reperfusion (I/R) injury activates cytoprotective mechanisms resulting in increased cell survival. In cancer cells, the inhibition of these channels leads to increased cell death. Therefore, mitochondrial potassium channels are intriguing targets for the development of new pharmacological strategies. In most cases, however, the substances that modulate the mitochondrial potassium channels have a few alternative targets in the cell. This may result in unexpected or unwanted effects induced by these compounds. In our review, we briefly present the various classes of mitochondrial potassium (mitoK) channels and describe the chemical compounds that modulate their activity. We also describe examples of the multidirectional activity of the activators and inhibitors of mitochondrial potassium channels.

Mitochondrial Potassium Channels as Druggable Targets

Mitochondrial potassium channels have been described as important factors in cell pro-life and death phenomena. The activation of mitochondrial potassium channels, such as ATP-regulated or calcium-activated large conductance potassium channels, may have cytoprotective effects in cardiac or neuronal tissue. It has also been shown that inhibition of the mitochondrial Kv1.3 channel may lead to cancer cell death. Hence, in this paper, we examine the concept of the druggability of mitochondrial potassium channels. To what extent are mitochondrial potassium channels an important, novel, and promising drug target in various organs and tissues? The druggability of mitochondrial potassium channels will be discussed within the context of channel molecular identity, the specificity of potassium channel openers and inhibitors, and the unique regulatory properties of mitochondrial potassium channels. Future prospects of the druggability concept of mitochondrial potassium channels will be evaluated in this paper.

The COX-2 selective blocker etodolac inhibits TNFα-induced apoptosis in isolated rabbit articular chondrocytes

Chondrocyte apoptosis contributes to the disruption of cartilage integrity in osteoarthritis (OA). Recently, we reported that activation of volume-sensitive Cl⁻ current (I_Cl,vol) mediates cell shrinkage, triggering apoptosis in rabbit articular chondrocytes. A cyclooxygenase (COX) blocker is frequently used for the treatment of OA. In the present study, we examined in vitro effects of selective blockers of COX on the TNFα-induced activation of I_Cl,vol in rabbit chondrocytes using the patch-clamp technique. Exposure of isolated chondrocytes to TNFα resulted in an obvious increase in membrane Cl⁻ conductance. The TNFα-evoked Cl⁻ current exhibited electrophysiological and pharmacological properties similar to those of I_Cl,vol. Pretreatment of cells with selective COX-2 blocker etodolac markedly inhibited I_Cl,vol activation by TNFα as well as subsequent apoptotic events such as apoptotic cell volume decrease (AVD) and elevation of caspase-3/7 activity. In contrast, a COX-1 blocker had no effect on the decrease in cell volume or the increase in caspase-3/7 activity induced by TNFα. Thus, the COX-2-selective blocker had an inhibitory effect on TNFα-induced apoptotic events, which suggests that this drug would have efficacy for the treatment of OA.

17β-Oestradiol inhibits doxorubicin-induced apoptosis via block of the volume-sensitive Cl(-) current in rabbit articular chondrocytes

BACKGROUND AND PURPOSE Chondrocyte apoptosis contributes to disruption of cartilage integrity in osteoarthritis. Recent evidence suggested that the volume-sensitive organic osmolyte/anion channel [volume-sensitive (outwardly rectifying) Cl(-) current (I(Cl,vol) )] plays a functional role in the development of cell shrinkage associated with apoptosis (apoptotic volume decrease) in several cell types. In this study, we investigated the cellular effects of 17β-oestradiol on doxorubicin-induced apoptotic responses in rabbit articular chondrocytes. EXPERIMENTAL APPROACH Whole-cell membrane currents and cross-sectional area were measured from chondrocytes using a patch-clamp method and microscopic cell imaging, respectively. Caspase-3/7 activity was assayed as an index of apoptosis. KEY RESULTS Addition of doxorubicin (1 µM) to isosmotic bath solution rapidly activated the Cl(-) current with properties similar to those of I(Cl,vol) in chondrocytes. Doxorubicin also gradually decreased the cross-sectional area of chondrocytes, followed by enhanced caspase-3/7 activity; both of these responses were totally abolished by the I(Cl,vol) blocker DCPIB (20 µM). Pretreatment of chondrocytes with 17β-oestradiol (1 nM) for short (approximately 10 min) and long (24 h) periods almost completely prevented the doxorubicin-induced activation of I(Cl,vol) and subsequent elevation of caspase-3/7 activity. These effects of 17β-oestradiol were significantly attenuated by the oestrogen receptor blocker ICI 182780 (10 µM), as well as the phosphatidyl inositol-3-kinase (PI3K) inhibitors wortmannin (100 nM) and LY294002 (20 µM). Testosterone (10 nM) had no effect on the doxorubicin-induced Cl(-) current. CONCLUSIONS AND IMPLICATIONS 17β-Oestradiol prevents the doxorubicin-induced cell shrinkage mediated through activation of I(Cl,vol) and subsequent induction of apoptosis signals, through a membrane receptor-dependent PI3K pathway in rabbit articular chondrocytes.

alpha-Adrenoceptor-mediated depletion of phosphatidylinositol 4, 5-bisphosphate inhibits activation of volume-regulated anion channels in mouse ventricular myocytes

BACKGROUND AND PURPOSE

Volume-regulated anion channels (VRACs) play an important role in cell-volume regulation. alpha(1)-Adrenoceptor stimulation by phenylephrine (PE) suppressed the hypotonic activation of VRAC current in mouse ventricular cells and regulatory volume decrease (RVD) was also absent in PE-treated cells. We examined whether the effects of alpha(1)-adrenoceptor stimuli on VRAC current were modulated by phosphatidylinositol signalling.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp method was used to record the hypotonicity-induced VRAC current in mouse ventricular cells. RVD was analyzed by videomicroscopic measurement of cell images.

KEY RESULTS

The attenuation of VRAC current by PE was suppressed by alpha(1A)-adrenoceptor antagonists (prazosin and WB-4101), anti-G(q) protein antibody and a specific phosphoinositide-specific phospholipase C (PLC) inhibitor (U-73122), but not by antagonists for alpha(1B)-, alpha(1D)- or beta-adrenoceptor, or protein kinase C inhibitors. The inhibition of VRAC by PE was antagonized by intracellular excess phosphatidylinositol 4,5-bisphosphate (PIP(2)), while intracellular anti-PIP(2) antibody (PIP(2) Ab) inhibited the activation of VRAC currents. When cells were loaded with phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) with or without PIP(2) Ab, PE little affected the VRAC current. Extracellular m-3M3FBS (an activator of PLC) suppressed VRAC in the absence of PE, and this effect was reversed by intracellular excess PIP(2).

CONCLUSIONS AND IMPLICATIONS

Our results indicate that the stimulation of alpha(1A)-adrenoceptors by PE inhibited the activation of cardiac VRAC current via PIP(3) depletion brought about by PLC-dependent reduction of membrane PIP(2) level.

Therapeutic potential of cystic fibrosis transmembrane conductance regulator (CFTR) inhibitors in polycystic kidney disease

In the common genetic disorder autosomal dominant polycystic kidney disease (ADPKD), kidney function is disrupted by multiple fluid-filled epithelial cysts. Cyst growth in ADPKD involves fluid accumulation within the cyst lumen driven by cystic fibrosis transmembrane conductance regulator (CFTR)-mediated transepithelial Cl- secretion. This suggests that inhibitors of the CFTR Cl- channel might retard cyst growth. This review considers how knowledge of CFTR structure and function and its role in transepithelial salt and water movements provides insight into the mechanism of action of CFTR inhibitors. Some small molecules, termed open-channel blockers, inhibit directly the CFTR Cl- channel by physically obstructing the CFTR pore and preventing Cl- flow. By contrast, other small molecules, termed allosteric inhibitors, bind to CFTR at a site remote from the channel pore and interfere with conformational changes that open the pore. The application of high-throughput screening to CFTR drug discovery has led to the identification of new inhibitors of the CFTR Cl- channel including the thiazolidinone CFTR(inh)-172 and the glycine hydrazide GlyH-101. The demonstration that CFTR inhibitors retard cyst expansion and kidney enlargement in mouse models of ADPKD provides proof of concept for the use of small-molecule CFTR inhibitors in the treatment of ADPKD.

Regulatory role of tyrosine phosphorylation in the swelling-activated chloride current in isolated rabbit articular chondrocytes

Articular chondrocytes are exposed in vivo to the continually changing osmotic environment and thus require volume regulatory mechanisms. The present study was designed to investigate (i) the functional role of the swelling-activated Cl(-) current (I(Cl,swell)) in the regulatory volume decrease (RVD) and (ii) the regulatory role of tyrosine phosphorylation in I(Cl,swell), in isolated rabbit articular chondrocytes. Whole-cell membrane currents were recorded from chondrocytes in isosmotic, hyposmotic and hyperosmotic external solutions under conditions where Na(+), K(+) and Ca(2+) currents were minimized. The cell surface area was also measured using microscope images from a separate set of chondrocytes and was used as an index of cell volume. The isolated chondrocytes exhibited a RVD during sustained exposure to hyposmotic solution, which was mostly inhibited by the I(Cl,swell) blocker 4-(2-butyl-6,7-dichloro-2-cyclopentyl-indan-1-on-5-yl)oxobutyric acid (DCPIB) at 20 microM. Exposure to a hyposmotic solution activated I(Cl,swell), which was also largely inhibited by 20 microM DCPIB. I(Cl,swell) in rabbit articular chondrocytes had a relative taurine permeability (P(tau)/P(Cl)) of 0.21. Activation of I(Cl,swell) was significantly reduced by the protein tyrosine kinase (PTK) inhibitor genistein (30 microM) but was only weakly affected by its inactive analogue daidzein (30 microM). Intracellular application of protein tyrosine phosphatase (PTP) inhibitor sodium orthovanadate (250 and 500 microM) resulted in a gradual activation of a Cl(-) current even in isosmotic solutions. This Cl(-) current was almost completely inhibited by 4,4-diisothiocyanatostilbene-2,2-disulfonate (DIDS, 500 microM) and was also largely suppressed by exposure to hyperosmotic solution, thus indicating a close similarity to I(Cl,swell). Pretreatment of chondrocytes with genistein significantly prevented the activation of the Cl(-) current by sodium orthovanadate, suggesting that the basal activity of endogenous PTK is required for the activation of this Cl(-) current. Our results provide evidence to indicate that activation of I(Cl,swell) is involved in RVD in isolated rabbit articular chondrocytes and is facilitated by tyrosine phosphorylation.

CARDIAC-SPECIFIC OVEREXPRESSION OF THE HUMAN SHORT CLC-3 CHLORIDE CHANNEL ISOFORM IN MICE

1. ClC-3 has been proposed as a molecular candidate responsible for volume-sensitive outwardly rectifying anion channels (VSOAC) in cardiac and smooth muscle cells. To further test this hypothesis, we produced a novel line of transgenic mice with cardiac-specific overexpression of the human short ClC-3 isoform (hsClC-3). 2. Northern and western blot analyses demonstrated that mRNA and protein levels of the short isoform (sClC-3) in the heart were significantly increased in hsClC-3-overexpressing (OE) mice compared with wild-type (WT) mice. Heart weight : bodyweight ratios for OE mice were significantly smaller compared with age-matched WT mice. 3. Electrocardiogram recordings indicated no difference at rest, whereas echocardiographic recordings revealed consistent reductions in left ventricular diastolic diameter, left ventricular posterior wall thickness at end of diastole and interventricular septum thickness in diastole in OE mice. 4. The VSOAC current densities in atrial cardiomyocytes were significantly increased by ClC-3 overexpression compared with WT cells. No differences in VSOAC current properties in OE and WT atrial myocytes were observed in terms of outward rectification, anion permeability (I(-) > Cl(-) > Asp(-)) and inhibition by 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid and glibenclamide. The VSOAC in atrial myocytes from both groups were totally abolished by phorbol-12,13-dibutyrate (a protein kinase C activator) and by intracellular dialysis of an N-terminal anti-ClC-3 antibody. 5. Cardiac cell volume measurements revealed a significant acceleration of the rate of regulatory volume decrease (RVD) in OE myocytes compared with WT. 6. In conclusion, enhanced VSOAC currents and acceleration of the time-course of RVD in atrial myocytes of OE mice is strong evidence supporting an essential role of sClC-3 in native VSOAC function in mouse atrial myocytes.

Modulation of human cardiovascular outward rectifying chloride channel by intra- and extracellular ATP

The macroscopic volume-regulated anion current (VRAC) is regulated by both intracellular and extracellular ATP, which has important implications in signaling and regulation of cellular excitability. The outwardly rectifying Cl(-) channel (ORCC) is a major contributor to the VRAC. This study investigated the effects of intracellular and extracellular ATP on the ORCCs expressed in the human cardiovascular system. With inside-out single-channel patch-clamp techniques, ORCCs were recorded from myocytes isolated from human atrium and septal ventricle and from primary cells originating from human coronary artery endothelium and human coronary artery smooth muscle. ORCCs from all of these tissues had similar biophysical properties, i.e., they were outwardly rectifying in symmetrical Cl(-) solutions, exhibited a slope conductance of approximately 90-100 pS at positive potentials and approximately 22 pS at negative potentials, and had a high open probability that was independent of voltage or time. The presence of ATP at the cytosolic face of the membrane increased the number of patches that contained functional ORCC but had no effect on gating. In contrast, "extracellular" ATP (in pipette solution) had no effect on the proportion of patches in which ORCC was detected but strongly reduced the open probability by increasing the closed dwell time. The potency order for nucleotides to affect gating was ATPgammaS > ATP = UTP > ADP > AMP, which suggests that a negatively charged phosphate group is involved in ORCC block. Our findings are consistent with a role of ORCC in the human cardiovasculature (atrium, ventricle, and coronary arteries). Regulation of ORCC by extracellular ATP suggests that this channel may have an important role in maintaining electrical activity and membrane potential under conditions in which extracellular ATP levels are elevated, such as with ATP release from nerve endings or during pathophysiological conditions.

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

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

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

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

Structure and pharmacology of swelling-sensitive chloride channels, I(Cl,swell)

Since several years, the interest for chloride channels and more particularly for the enigmatic swelling-activated chloride channel (I(Cl,swell)) is increasing. Despite its well-characterized electrophysiological properties, the I(Cl,swell) structure and pharmacology are not totally elucidated. These channels are involved in a variety of cell functions, such as cardiac rhythm, cell proliferation and differentiation, cell volume regulation and cell death through apoptosis. This review will consider different aspects regarding structure, electrophysiological properties, pharmacology, modulation and functions of these swelling-activated chloride channels.

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

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

Swelling-activated chloride channels in cardiac physiology and pathophysiology

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

ACTH-induced Cl(-) current in bovine adrenocortical cells: correlation with cortisol secretion

ACTH has been shown to depolarize bovine adrenal zona fasciculata cells by inhibiting a K(+) current. The effects of this hormone on such cells have been reexamined using perforated and standard patch recording methods. In current clamp experiments, ACTH (10 nM) induced a membrane depolarization to -36 +/- 1 mV (n = 56), which was mimicked by forskolin (10 microM) or by 8-(4-chlorophenylthio)-cAMP (8 mM). ACTH-induced membrane depolarizations were associated in the majority of cells with an increase in membrane conductance. In the other cells, these membrane responses could occur without change or could be correlated with a transient or with a continuous Cs(+)-sensitive decrease in membrane conductance. The depolarizations associated with an increase in membrane conductance were depressed by Cl(-) current inhibitors diphenylamine-2-carboxylic acid (DPC; 1 mM), anthracene-9-carboxylic acid (9-AC; 1 mM), DIDS (400 microM), verapamil (100 microM), and glibenclamide (20 microM). In voltage-clamped Cs(+)-loaded cells, ACTH activated a time-independent current that displayed an outward rectification and reversed at -21.5 mV +/- 2 (n = 6). This current, observed in the presence of internal EGTA (5 mM), was depressed in low Cl(-) external solution and was inhibited by DPC, 9-AC, DIDS, 5-nitro-2-(3-phenylpropylamino)benzoic acid, verapamil, and glibenclamide. ACTH-stimulated cortisol secretion was blocked by Cl(-) channel inhibitors DIDS (400 microM) and DPC (1 mM). The present results reveal that, in addition to inhibiting a K(+) current, ACTH activates in bovine zona fasciculata cells a Ca(2+)-insensitive, cAMP-dependent Cl(-) current. This Cl(-) current is involved in the ACTH-induced membrane depolarization, which seems to be a crucial step in stimulating steroidogenesis.

Mechanisms responsible for cell volume regulation during hyperkalemic cardioplegic arrest

BACKGROUND

Cardioplegia has been shown to induce significant cell swelling. This study tested the hypothesis that (1) the [K+][Cl-] product of the cardioplegia solution is the main determinant of myocyte swelling, and (2) reperfusion myocyte shrinkage results from a rectifying Cl- conductance.

METHODS

Rabbit ventricular myocytes were superfused with 37 degrees C Krebs-Henseleit solution for 10 minutes. Then cells underwent 20 minutes of superfusion with standard St. Thomas' solution ([K+][Cl-] product = 2566 mmol/L2) and two solutions with lower [K+][Cl-] product (1500 and 700 mmol/L2) at 9 degrees C. Cells were then resuperfused with 37 degrees C Krebs-Henseleit solution for 30 minutes. Cell volume was measured by videomicroscopy.

RESULTS

Cells superfused with St. Thomas' having [K+][Cl-] products of 2,566, 1,500, and 700 mmol/L2 swelled by 9.18%+/-3.57%, 5.51%+/-1.08%, and 1.49%+/-1.56%, respectively. Reexposure to Krebs-Henseleit solution caused these cells to shrink by 5.79%+/-1.41%, 8.72% +/-3.68%, and 13.46%+/-5.60%, respectively. This shrinkage was blocked by Cl- channel blockers given at the onset of superfusion.

CONCLUSIONS

Lowering the [K+][Cl-] product of St. Thomas' solution attenuated myocyte edema. Myocyte shrinkage during reexposure to Krebs-Henseleit solution resulted from the volume-activated Cl- channel.

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.

The sulphonylurea glibenclamide inhibits voltage dependent potassium currents in human atrial and ventricular myocytes

1 It was the aim of our study to investigate the effects of the sulphonylurea glibenclamide on voltage dependent potassium currents in human atrial myocytes. 2 The drug blocked a fraction of the quasi steady state current (ramp response) which was activated positive to -20 mV, was sensitive to 4-aminopyridine (500 microM) and was different from the ATP dependent potassium current IK(ATP). 3 Glibenclamide dose dependently inhibited both, the peak as well as the late current elicited by step depolarization positive to -20 mV. The IC50 for reduction in charge area of total outward current was 76 microM. 4 The double-exponential inactivation time-course of the total outward current was accelerated in the presence of glibenclamide with a tau(fast) of 12.7+/-1.5 ms and a tau(slow) of 213+/-25 ms in control and 5.8+/-1.9 ms (P<0.001) and 101+/-20 ms (P<0.05) under glibenclamide (100 microM). 5 Our data suggest, that both repolarizing currents in human atrial myocytes, the transient outward current (Ito1) and the ultrarapid delayed rectifier current (IKur) were inhibited by glibenclamide. 6 In human ventricular myocytes glibenclamide inhibited Ito1 without affecting the late current. 7 Our data suggest that glibenclamide inhibits human voltage dependent cardiac potassium currents at concentrations above 10 microM.

Effects of anion channel antagonists in canine colonic myocytes: comparative pharmacology of Cl-, Ca2+ and K+ currents

1. Volume-Sensitive, Outwardly Rectifying (VSOR) Cl- currents were measured in canine colonic myocytes by whole-cell patch clamp. Decreasing extracellular osmolarity 50 milliosmoles l-1 activated current that was carried by Cl- and 5 - 7 times greater in the outward direction. 2. Niflumic acid, an inhibitor of Ca2+-activated Cl- channels, did not inhibit VSOR Cl- current. Glibenclamide, an antagonist of CFTR, and anthracene-9-carboxylate (9-AC) inhibited current less than 25% at 100 microM. 3. DIDS (4, 4-diisothiocyanato-stilbene-2,2'disulphonate) inhibited VSOR Cl- current more potently than SITS (4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulphonate). IC50s were 0.84 and 226 microM, respectively. 4. VSOR Cl- current was strongly inhibited by tamoxifen ([Z]-1-[p-dimethylaminoethoxy-phenyl]-1,2-diphenyl-1-butene), an anti-oestrogen compound (IC50=0.57 microM). 5. Gd3+ antagonized VSOR Cl- current more potently than La3+. The IC50 for Gd3+ was 23 microM. In contrast, 100 microM La3+ inhibited current only 35+/-7%. 6. Antagonists of VSOR Cl- current had non-specific effects. These compounds blocked voltage-dependent K+ and Ca2+ currents in colonic myocytes. Tamoxifen (10 microM) and DIDS (10 microM) inhibited L-type Ca2+ current 87+/-7 and 31+/-5%, respectively. Additionally, in the presence of 300 nM charybdotoxin, tamoxifen (1 microM) and DIDS (10 microM) inhibited delayed rectifier K+ current 38+/-8 and 10+/-2%, respectively. 7. The pharmacology of VSOR Cl- channels overlaps with voltage-dependent cation channels. DIDS and tamoxifen inhibited VSOR Cl- equally. However, because DIDS had much less effect on L-type Ca2+ and delayed rectifier K+ channels than did tamoxifen, it might be useful in experiments to investigate the physiological and pathophysiological role of this conductance in whole tissues.

High intracellular chloride delays the activation of the volume-sensitive chloride conductance in mouse L-fibroblasts

1. The relationship between cell volume and volume-sensitive Cl- conductance during hyposmotic cell swelling of patched cells and the effects of intracellular chloride on the conductance have been studied in mouse L-fibroblasts. To this end, swelling-activated current and cell volume were measured simultaneously in cells dialysed with low-Cl- (16 mM) or high-Cl- (130 mM) solutions using the whole-cell patch-clamp technique and videomicroscopy. 2. The increase in cell volume of patched cells and the volume-sensitive conductance saturated during a 4-5 min exposure to mildly hyposmotic solutions (15-20 % less than isosmotic). The swelling of patched cells varied considerably and was greater than the swelling of intact cells. No correlation between the maximal values of the volume-sensitive conductance and the maximal volumes of swollen cells was evident for cells dialysed with the low-Cl- solutions. 3. The amplitude of the volume-sensitive conductance decreased with a reduction in either extracellular or intracellular Cl- concentration; the size of the maximal conductance was not modulated by intracellular Cl- ions. 4. The activation of the volume-sensitive conductance was slower in high-Cl- cells than in low-Cl- cells whether it was induced by hypotonic cell swelling or by cell inflation; in low-Cl- cells the conductance saturated before the cell volume had reached its maximal value. 5. It is concluded that in patched cells an increase in cell volume triggers activation of the volume-sensitive Cl- conductance but does not determine its amplitude and that the rate of activation of the conductance is affected by the intracellular Cl- concentration.

Characterization of cell volume-sensitive chloride currents in freshly prepared and cultured pancreatic acinar cells from early postnatal rats

1. In freshly prepared and cultured exocrine pancreatic acinar cells from 5- to 7-day-old rats a chloride-selective membrane conductance could be activated by intracellular application of GTPgammaS (40-100 microM), by application of positive pressure (5 cmH2O) to the pipette interior or by challenging the cells with a hyposmolar bath solution. Hyperosmolar bath solutions inhibited the cell volume-sensitive chloride currents. 2. The anion permeability sequence of the cell volume-sensitive chloride conductance was I- > Cl- approximately Br- > F- > methanesulphonate- > glutamate-. I- had a higher permeability but lower conductance than Cl-. The permeability ratio for Pglutamate/PCl was 0.12. 3. The cell volume-sensitive chloride conductance showed outward rectification. Membrane depolarization to high positive voltages (>= +60 mV) caused a time-dependent decay in outward currents. 4. DIDS (4, 4'-diisothiocyanatostilbene-2,2'-disulphonic acid) and SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid) reversibly inhibited the cell volume-sensitive chloride current in a voltage-dependent manner. NPPB (5-nitro-2-(3-phenylpropylamino)-benzoic acid), quinidine, quinine and tamoxifen caused voltage-independent current inhibition. 5. Combined fura-2 and whole-cell current measurements showed that activation of the cell volume-sensitive chloride current does not involve cytosolic Ca2+ signals. Furthermore, there is no evidence that Ca2+-activated chloride currents play a significant role in cultured pancreatic acinar cells from 5- to 7-day-old rats. 6. Polymerase chain reaction followed by DNA sequence analysis indicated the presence of mRNA homologous to the ClC-3 chloride channel in pancreatic tissue from 5-day-old rats.

Non-receptor-mediated activation of IK(ATP) and inhibition of IK(ACh) by diadenosine polyphosphates in guinea-pig atrial myocytes

1. The effects of diadenosine polyphosphates (APnA, where n = 4-6) were studied on beating frequency of perfused guinea-pig hearts and on muscarinic K+ current (IK(ACh)) and ATP-regulated K+ current (IK(ATP)) in atrial myocytes from guinea-pig hearts using whole-cell voltage clamp. 2. Bradycardia induced by APnA in perfused hearts was completely inhibited by 8-cyclopentyl- 1,3-dipropylxanthine (CPX, 20 microM), a selective antagonist at A1 adenosine receptors, and was augmented by dipyridamole (Dipy), an inhibitor of cellular adenosine (Ado) uptake. 3. Whereas exposure of atrial myocytes to Ado (100 microM) within about 1 s induced a significant whole-cell IK(ACh), APnA up to 1 mM applied for some tens of seconds failed to activate IK(ACh). If present for periods > 2 min, APnA caused inhibition of agonist-evoked IK(ACh) and activation of a weakly inward rectifying K+ current, which was identified as IK(ATP) by its sensitivity to glibenclamide and its current-voltage curve. 4. The actions of extracellular APnA on IK(ACh) and IK(ATP) were mimicked by intracellular loading of compounds via the patch clamp pipette and by intracellular loading of AMP. 5. The results from isolated myocytes exclude APnA acting as A1 agonists. It is suggested that myocytes can take up APnA, which are degraded to AMP. In the presence of ATP, AMP is converted to ADP, a physiological activator of ATP-regulated K+ channels, by adenylate kinase. A similar mechanism resulting in a reduction of the [GTP]/[GDP] ratio might be responsible for inhibition of IK(ACh). 6. In the perfused heart and other multicellular cardiac preparations the actions of APnA are mediated by Ado via A1 receptors. It is suggested that APnA in multicellular cardiac tissue are hydrolysed by an ectohydrolase to yield AMP which is converted to Ado by ectonucleotidases.