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

Extracellular chloride regulation of Kv2.1, contributor to the major outward Kv current in mammalian outer hair cells

Outer hair cells (OHC) function as both receptors and effectors in providing a boost to auditory reception. Amplification is driven by the motor protein prestin, which is under anionic control. Interestingly, we now find that the major, 4-AP-sensitive, outward K(+) current of the OHC (I(K)) is also sensitive to Cl(-), although, in contrast to prestin, extracellularly. I(K) is inhibited by reducing extracellular Cl(-) levels, with a linear dependence of 0.4%/mM. Other voltage-dependent K(+) (Kv) channel conductances in supporting cells, such as Hensen and Deiters' cells, are not affected by reduced extracellular Cl(-). To elucidate the molecular basis of this Cl(-)-sensitive I(K), we looked at potential molecular candidates based on Cl(-) sensitivity and/or similarities in kinetics. For I(K), we identified three different Ca(2+)-independent components of I(K) based on the time constant of inactivation: a fast, transient outward current, a rapidly activating, slowly inactivating current (Ik(1)), and a slowly inactivating current (Ik(2)). Extracellular Cl(-) differentially affects these components. Because the inactivation time constants of Ik(1) and Ik(2) are similar to those of Kv1.5 and Kv2.1, we transiently transfected these constructs into CHO cells and found that low extracellular Cl(-) inhibited both channels with linear current reductions of 0.38%/mM and 0.49%/mM, respectively. We also tested heterologously expressed Slick and Slack conductances, two intracellularly Cl(-)-sensitive K(+) channels, but found no extracellular Cl(-) sensitivity. The Cl(-) sensitivity of Kv2.1 and its robust expression within OHCs verified by single-cell RT-PCR indicate that these channels underlie the OHC's extracellular Cl(-) sensitivity.

Arrhythmogenic autoantibodies against calcium channel lead to sudden death in idiopathic dilated cardiomyopathy

AIMS

Calcium channel plays an important role in the autoimmune pathogenesis of idiopathic dilated cardiomyopathy (DCM). Autoantibodies have emerged as a new upstream target of sudden death in DCM. We sought to validate the hypothesis that autoantibodies against l-type calcium channel (CC-AAbs) are arrhythmogenic and lead to sudden death in patients with DCM.

METHODS AND RESULTS

We investigated sudden death and ventricular arrhythmias in 80 patients with DCM in a prospective, case follow-up survey. During a follow-up of 32 (SD 8) months, CC-AAbs-positive patients not only had a higher incidence of ventricular tachycardia (VT) but also a higher incidence of sudden death than CC-AAbs-negative patients (for VT: 59.0 vs. 24.4%, P = 0.002 and for sudden death: 20.5 vs. 4.9%, P = 0.045). Further univariate and multivariate analyses showed that the occurrence of CC-AAbs was the strongest independent predictor for sudden death (odds ratio: 10.20, 95% confidence interval: 2.43-36.78, P = 0.0027). Experimental studies in ex vivo systems using affinity-purified CC-AAbs from patients demonstrated that CC-AAbs were able to induce VT by prolongation of action potential duration (APD) and triggered activity by early afterdepolarization (EAD).

CONCLUSION

Our results demonstrate for the first time to our knowledge that there is a high incidence of sudden death and VT in CC-AAbs-positive patients with DCM. Furthermore, experimental data from ex vivo systems suggest that CC-AAbs might induce VT by prolongation of APD and triggered activity by EAD.

Changes in the calcium current among different transmural regions contributes to action potential heterogeneity in rat heart

To clarify the transmural heterogeneity of action potential (AP) time course, we examined the regulation of L-type Ca(2+) current (I(Ca,L)) by voltage and Ca(2+)-dependent mechanisms. Currents were recorded using patch clamp of single rat subepicardial (EPI) and subendocardial (ENDO) of left ventricular, right ventricular (RV) and septal (SEP) cardiomyocytes. Voltage clamp commands were derived from ENDO and EPI APs or rectangular voltage pulses. During rectangular pulses, peak I(Ca,L) was significantly greater in EPI than in other cells. The inactivation of I(Ca,L) by Ca(2+)-dependent mechanisms (suppressed by ryanodine and BAPTA) was present in all cells but greater in extent in ENDO and SEP cells. Activation and inactivation curves for all regions show subtle differences that are Ca(2+) sensitive, with Ca(2+) inactivation shifting the activation variables negative by approximately 7 mV and inactivation variables positive by 2-7 mV (EPI being least, RV greatest). In AP-clamps, the peak I(Ca,L) was significantly smaller in ENDO than in EPI cells, while the integrated current was significantly larger in ENDO than in EPI cells. The results are discussed with regard to the interplay of AP time course and net Ca(2+) influx.

Assessing the therapeutic and toxicological effects of cesium chloride following administration to nude mice bearing PC-3 or LNCaP prostate cancer xenografts

PURPOSE

The purpose of this study was to assess the therapeutic and toxicological effects of cesium chloride (CsCl) administration in mice bearing prostate cancer tumors.

METHODS

Three CsCl dose titration studies were completed in tumor-bearing and non-tumor-bearing athymic nude mice. All mice were administered either vehicle (controls), 150, 300, 600, 800, 1,000, or 1,200 mg/kg of CsCl once daily by oral gavage for 30 consecutive days. Body mass was measured daily, food and water consumption were measured every 2 days, and tumor volume was measured twice weekly. Histopathological analysis was conducted on tissues collected from each of the studies. Serum AST/ALT and creatinine were also measured.

RESULTS

Administration of 800-1,200 mg/kg CsCl reduced PC-3 tumor growth but had no effect on LNCaP tumors. Administration of 800-1,200 mg/kg CsCl also resulted in increased water consumption, bladder crystal development, and higher prevalence of cardiac fibrin clots. An observed loss in body mass was dependent on the xenograft type and concentration of CsCl administered. CsCl did not affect serum AST/ALT and creatinine levels.

CONCLUSIONS

CsCl may have a therapeutic effect against prostate cancer, but one cannot overlook the acute toxicities also described.

Effects of monocarboxylic acid-derived Cl−channel blockers on depolarization-activated potassium currents in rat ventricular myocytes

The effects of monocarboxylic acid-derived Cl(-) channel blockers on cardiac depolarization-activated K(+) currents were investigated. Membrane currents in rat ventricular myocytes were recorded using the whole-cell configuration of the patch-clamp technique. 5-Nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) and niflumic acid (NFA) induced an outward current at 0 mV. Both NPPB and NFA failed to induce any current when used intracellularly or after K(+) in the bath and pipette solutions was replaced by equimolar Cs(+). Voltage pulse protocols revealed that NPPB and NFA enhanced the steady-state K(+) current but inhibited the transient outward K(+) current. Genistein, a tyrosine kinase (PTK) inhibitor, inhibited NPPB- and NFA-induced outward current. Another PTK inhibitor, lavendustin A, produced a comparable effect. In contrast, the inactive analogue of genistein, daidzein, was ineffective. Orthovanadate, a tyrosine phosphatase inhibitor, markedly slowed the deactivation of the outward current induced by NPPB and NFA. The protein kinase A (PKA) inhibitor H-89 inhibited NPPB-induced outward current at 0 mV. In contrast, the protein kinase C (PKC) inhibitor H-7 was without significant effect on the action of NPPB. Pretreatment of the myocytes with genistein or H-89 prevented the enhancing effect of NPPB. Increasing intracellular Cl(-) from 22 to 132 mm slightly reduced NPPB-induced outward current at 0 mV. These results demonstrate that the monocarboxylic acid-derived Cl(-) channel blockers NPPB and NFA enhance cardiac steady-state K(+) current, and suggest that the enhancing effect of the Cl(-) channel blockers is mediated by stimulation of PKA and PTK signalling pathways.

Effect of the stable thromboxane derivative, carbocyclic thromboxane A2, on membrane potential of rat myenteric neurones in culture

The effects of carbocyclic thromboxane A(2) (cTXA(2); 10(-6) mol L(-1)) on membrane potential and cytosolic Ca(2+) concentration were measured with the whole-cell patch-clamp or the fura-2 method, respectively, at rat myenteric ganglia. cTXA(2) caused a hyperpolarization of myenteric neurones from -19.3 +/- 2.5 to -29.3 +/- 2.3 mV. In addition, the eicosanoid potentiated the carbachol-induced depolarization from 4.2 +/- 1.0 mV under control conditions to 11.1 +/- 1.1 mV in the presence of the cTXA(2) (n = 9). The hyperpolarization was abolished by internal application of CsCl (140 mmol L(-1)), a non-selective blocker of K(+) channels, or EGTA (11 mmol L(-1)in the pipette solution), a chelator of intracellular Ca(2+). A similar inhibition was observed in the presence of charybdotoxin (10(-7) mol L(-1)). Fura-2 imaging experiments revealed a cTXA(2)-evoked increase in the intracellular Ca(2+) concentration as indicated by a rise in the fura-2 ratio signal. This response was mediated by a release of Ca(2+) from intracellular stores as sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase blockade with cyclopiazonic acid (5 x 10(-5) mol L(-1)) completely abolished the response to cTXA(2). A similar inhibition was observed after blockade of phospholipase C with U-73122 (10(-5) mol L(-1)). These results suggest an activation of Ca(2+)-activated K(+) channels by cTXA(2) after stimulation of phospholipase C.

Effect of Cl−channel blockers on aconitine-induced arrhythmias in rat heart

The effects of Cl- channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and niflumic acid (NFA) on aconitine-induced arrhythmias were investigated. Left ventricular pressure and electrocardiogram were monitored in Langendorff-perfused rat hearts. Whole-cell patch-clamp and current-clamp techniques were used to measure sodium current (I(Na)) and action potential (AP), respectively, in single rat cardiac ventricular myocytes. Addition of the Na+ channel agonist aconitine (0.1 microM) to the perfusion solution produced polymorphic ventricular arrhythmias with a latent period of 25.5 +/- 6.3 s. NPPB could reverse aconitine-induced arrhythmias. A similar effect was observed by using NFA. NPPB and NFA reversibly depressed the upstroke of the AP in a dose-dependent manner with IC50 values of approximately 12.3 and approximately 73.1 microM, respectively, without significantly affecting the resting potential of rat ventricular myocytes. Both Cl- channel blockers inhibited I(Na) and induced a leftward shift of the steady-state inactivation of I(Na). In conclusion, the results of this study demonstrate that NPPB as well as NFA can suppress aconitine-induced arrhythmias in rat hearts mainly by inhibiting cardiac I(Na).

Isolation and characterization of I(Kr) in cardiac myocytes by Cs+ permeation

Isolation of the rapidly activating delayed rectifier potassium current (I(Kr)) from other cardiac currents has been a difficult task for quantitative study of this current. The present study was designed to separate I(Kr) using Cs+ in cardiac myocytes. Cs+ have been known to block a variety of K+ channels, including many of those involved in the cardiac action potential such as inward rectifier potassium current I(K1) and the transient outward potassium current I(to). However, under isotonic Cs+ conditions (135 mM Cs+), a significant membrane current was recorded in isolated rabbit ventricular myocytes. This current displayed the voltage-dependent onset of and recovery from inactivation that are characteristic to I(Kr). Consistently, the current was selectively inhibited by the specific I(Kr) blockers. The biophysical and pharmacological properties of the Cs+-carried human ether-a-go-go-related gene (hERG) current were very similar to those of the Cs+-carried I(Kr) in ventricular myocytes. The primary sequence of the selectivity filter in hERG was in part responsible for the Cs+ permeability, which was lost when the sequence was changed from GFG to GYG, characteristic of other, Cs+-impermeable K+ channels. Thus the unique high Cs+ permeability in I(Kr) channels provides an effective way to isolate I(Kr) current. Although the biophysical and pharmacological properties of the Cs+-carried I(Kr) are different from those of the K+-carried I(Kr), such an assay enables I(Kr) current to be recorded at a level that is large enough and sufficiently robust to evaluate any I(Kr) alterations in native tissues in response to physiological or pathological changes. It is particularly useful for exploring the role of reduction of I(Kr) in arrhythmias associated with heart failure and long QT syndrome due to the reduced hERG channel membrane expression.

Overexpression of claudin-7 decreases the paracellular Cl- conductance and increases the paracellular Na+ conductance in LLC-PK1 cells

Tight junctions form the primary barrier regulating the diffusion of fluid, electrolytes and macromolecules through the paracellular pathway. Claudins are the major structural and functional components of tight junction strands and are considered as the best candidates for forming paracellular channels. They are a family of integral membrane proteins with more than 20 members and show distinct tissue distribution patterns. In this study, we found that claudin-7 is expressed in the distal and collecting tubules and the thick ascending limb of Henle of porcine and rat kidneys. To investigate the role of claudin-7 in paracellular transport, we have overexpressed a mouse claudin-7 construct in LLC-PK1 cells. Overexpression of claudin-7 did not affect the expression and localization of endogenous claudin-1, -3, -4, -7, and ZO-1. However, transepithelial electrical resistance in claudin-7-overexpressing cells was greatly increased. In addition, electrophysiological measurements revealed a dramatic reduction of dilution potentials in claudin-7-overexpressing cells compared to that of control cells. To determine which ions are responsible for the effects of claudin-7 overexpression on transepithelial electrical resistance and dilution potentials, we applied an ion substitution strategy. When NaCl was replaced with sodium aspartate, transepithelial electrical resistance was significantly decreased and dilution potentials were increased in claudin-7-overexpressing cells as compared to controls, the opposite effects from that of using NaCl. Furthermore, when NaCl was substituted by arginine-HCl or lysine-HCl, the increase in transepithelial electrical resistance was greater and the reduction in dilution potentials was smaller. Taken together, our studies demonstrated for the first time that the effect of claudin-7 overexpression in LLC-PK1 cells on paracellular transport is mediated through a concurrent decrease in the paracellular conductance to Cl(-) and an increase in the paracellular conductance to Na(+). These results support the model that claudin-7 may form a paracellular barrier to Cl(-) while acting as a paracellular channel to Na(+).

Characterization of the Pharmacology of YM-198313 on Volume-Regulated Anion Channels

Activation of the volume-regulated anion channels (VRAC) is considered to be involved in arrhythmia, but it has not yet been fully elucidated because of the lack of its high affinitive and selective compounds. A newly synthesized compound, YM-198313 (sodium 4-({[2-(methylthio)benzyl]amino}-5-[(1-phenylethyl)thio]isothiazol-3-olate), strongly inhibited VRAC in HeLa cells with an IC50 of 3.03+/-0.05 microM. However, YM-198313 weakly affected both the Ca2+-activated Cl- channels in HTC cells and the cAMP-activated Cl- channels in T84 cells, demonstrating that this compound is selective for VRAC among Cl- channels. At 10 microM, YM-198313 almost completely (100+/-7.8%) inhibited the VRAC current in guinea pig atrial myocytes. However, at the same concentration, YM-198313 showed little inhibitory effect on the cardiac cation currents in ventricular myocytes. We believe that YM-198313 is a potent and selective VRAC inhibitor, therefore, it should be use to clarify the role VRAC plays in arrhythmia.