Molecular and functional distributions of chloride conductances in rabbit ventricle

Arrhythmia and sudden death associated with elevated cardiac chloride channel activity

The identification and analysis of several cationic ion channels and their associated genes have greatly improved our understanding of the molecular and cellular mechanisms of cardiac arrhythmia. Our objective in this study was to examine the involvement of anionic ion channels in cardiac arrhythmia. We used a transgenic mouse model to overexpress the human cystic fibrosis transmembrane conductance regulator (CFTR) gene, which encodes a cAMP-regulated chloride channel. We used RNase protection and in situ hybridization assays to determine the level of CFTR expression, and radiotelemetry and in vivo electrophysiological study in combination with pharmacological intervention to analyse the cardiac function. Cardiac CFTR overexpression leads to stress-related sudden death in this model. In vivo intracardiac electrophysiological studies performed in anaesthetized mice showed no significant differences in baseline conduction parameters including atrial-His bundle (AH) or His bundle-ventricular (HV) conduction intervals, atrioventricular (AV) Wenckebach or 2:1 AV block cycle length and AV nodal functional refractory period. However, following isoproterenol administration, there was marked slowing of conduction parameters, including high-grade AV block in transgenic mice, with non-sustained ventricular tachycardia easily inducible using programmed stimulation or burst pacing. Our sudden death mouse model can be a valuable tool for investigation of the role of chloride channels in arrhythmogenesis and, potentially, for future evaluation of novel anti-arrhythmic therapeutic strategies and pharmacological agents.

Contributions of sustained INa and IKv43 to transmural heterogeneity of early repolarization and arrhythmogenesis in canine left ventricular myocytes

The roles of sustained components of I(Na) and I(Kv43) in shaping the action potentials (AP) of myocytes isolated from the canine left ventricle (LV) have not been studied in detail. Here we investigate the hypothesis that these two currents can contribute substantially to heterogeneity of early repolarization and arrhythmic risk. Quantitative data from voltage-clamp and expression profiling experiments were used to complete meaningful modifications to an existing "local control" model of canine midmyocardial myocyte excitation-contraction coupling for epicardial and endocardial cells. We include 1) heterogeneous I(Kv43), I(Ks), and I(SERCA) density; 2) modulation of I(Kv43) by Kv channel interacting protein type 2 (KChIP2) channel subunits; 3) a possible Ca(2+)-dependent open-state inactivation of I(Kv43); and 4) a sustained component of the inward Na(+) current, I(NaL). The resulting simulations illustrate ways in which KChIP2- and Ca(2+)-dependent control of I(Kv43) can result in a sustained outward current that can neutralize I(NaL) in a rate- and myocyte subtype-dependent manner. Both these currents appear to play significant roles in modulating AP duration and rate dependence in midmyocardial myocytes. Furthermore, an increased ratio of I(Kv43) to I(NaL) is capable of protecting epicardial myocytes from the early afterdepolarizations resulting from the SCN5A-I1768V mutation-induced increase in I(NaL). Experimentally observed transmural differences in Ca(2+) handling, including greater sarcoplasmic reticulum Ca(2+) content and faster Ca(2+) transient decay rates on the epicardium, were recapitulated in our simulations. By design, these models allow upward integration into organ models or may be used as a basis for further investigations into cellular heterogeneities.

Antagonistic regulation of swelling-activated Cl- current in rabbit ventricle by Src and EGFR protein tyrosine kinases

Regulation of swelling-activated Cl(-) current (I(Cl,swell)) is complex, and multiple signaling cascades are implicated. To determine whether protein tyrosine kinase (PTK) modulates I(Cl,swell) and to identify the PTK involved, we studied the effects of a broad-spectrum PTK inhibitor (genistein), selective inhibitors of Src (PP2, a pyrazolopyrimidine) and epidermal growth factor receptor (EGFR) kinase (PD-153035), and a protein tyrosine phosphatase (PTP) inhibitor (orthovanadate). I(Cl,swell) evoked by hyposmotic swelling was increased 181 +/- 17% by 100 microM genistein, and the genistein-induced current was blocked by the selective I(Cl,swell) blocker tamoxifen (10 microM). Block of Src with PP2 (10 microM) stimulated tamoxifen-sensitive I(Cl,swell) by 234 +/- 27%, mimicking genistein, whereas the inactive analog of PP2, PP3 (10 microM), had no effect. Moreover, block of PTP by orthovanadate (1 mM) inhibited I(Cl,swell) and prevented its stimulation by PP2. In contrast with block of Src, block of EGFR kinase with PD-153035 (20 nM) inhibited I(Cl,swell). Several lines of evidence argue that the PP2-stimulated current was I(Cl,swell): 1) the stimulation was volume dependent, 2) the current was blocked by tamoxifen, 3) the current outwardly rectified with both symmetrical and physiological Cl(-) gradients, and 4) the current reversed near the Cl(-) equilibrium potential. To rule out contributions of other currents, Cd(2+) (0.2 mM) and Ba(2+) (1 mM) were added to the bath. Surprisingly, Cd(2+) suppressed the decay of I(Cl,swell), and Cd(2+) plus Ba(2+) eliminated time-dependent currents between -100 and +100 mV. Nevertheless, these divalent ions did not eliminate I(Cl,swell) or prevent its stimulation by PP2. The results indicate that tyrosine phosphorylation controls I(Cl,swell), and regulation of I(Cl,swell) by the Src and EGFR kinase families of PTK is antagonistic.

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.

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.

The sodium channel beta-subunit SCN3b modulates the kinetics of SCN5a and is expressed heterogeneously in sheep heart

1. Cardiac sodium channels are composed of a pore-forming alpha-subunit, SCN5a, and one or more auxiliary beta-subunits. The aim of this study was to investigate the role of the recently discovered member of the beta-subunit family, SCN3b, in the heart. 2. Northern blot and Western blot studies show that SCN3b is highly expressed in the ventricles and Purkinje fibres but not in atrial tissue. This is in contrast to the uniform expression of SCN1b throughout the heart. 3. Co-expression of SCN3b with the cardiac-specific alpha-subunit SCN5a in Xenopus oocytes resulted in a threefold increase in the level of functional sodium channel expression, similar to that observed when SCN1b was co-expressed with SCN5a. These results suggest that both SCN1b and SCN3b improve the efficiency with which the mature channel is targeted to the plasma membrane. 4. When measured in cell-attached oocyte macropatches, SCN3b caused a significant depolarising shift in the half-voltage of steady-state inactivation compared to SCN5a alone or SCN5a + SCN1b. The half-voltage of steady-state activation was not significantly different between SCN5a alone and SCN5a + SCN3b or SCN5a + SCN1b. 5. The rates of inactivation for SCN5a co-expressed with either subunit were not significantly different from that for SCN5a alone. However, recovery from inactivation at -90 mV was significantly faster for SCN5a + SCN1b compared to SCN5a + SCN3b, and both were significantly faster than SCN5a alone. 6. Thus, SCN1b and SCN3b have distinctive effects on the kinetics of activation and inactivation, which, in combination with the different patterns of expression of SCN3b and SCN1b, could have important consequences for the integrated electrical activity of the intact heart.

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

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