Voltage- and Use-dependent Effect of 7-chlor-benzylte-trahydropalmatine on Sodium Currents in Guinea Pig Ventricular Myocytes

The whole-cell patch-clamp technique was employed to obtain information about the voltage-dependence and kinetics of interaction of 7-chlor-benzylte-trahydropalmatine (7-Cl-BTHP) with cardiac sodium channels. 7-Cl-BTHP (30 mol/L) significantly decreased the peak sodium current (from 7. 8±1. 8 nA to 5. 3±1. 4 nA, P<0. 01, n=5), without producing a shift of the current-voltage curve. It shifted the inactivation curves of sodium current to hyperpolarized potentials, and the V(0.5) was shifted from - (82. 5±2. 5) mV to - (95±2.4) mV (P <0. 05, n=4). 7-Cl-BTHP produced a significant use-dependent effect that was proportional to the duration of the voltage step. In addition, 7-Cl-BTHP slowed the recovery of sodium channel from inactivation, which could explain its use-dependent effects on sodium current. The characteristics of 7-Cl-BTHP blockage suggest that this agent binds preferentially to inactivated sodium channels.

EFFECTS OF ADENOSINE IN SIMULATED ISCHEMIA ANDREPERFUSION ON ISOLATED GUINEA-PIG VENTRICULAR MYOCYTES

Objective To investigate the effects of adenosine （Ado） on myocardiac electrophysiology in simu- lated ischemla and reperfusion in guinea-pig ventricular myocytes. Methods Electrical activity was recorded using standard intracellular microelectrode technique. Right ventricle was superfused with simulated ischemic Tyrode’s so- lution for 15 min, and reperfued with normal Tyrode’s solution for 30 min. Results The results showed Ado had no measurable effects on guinea-pig ventricular myocytes in normal Tyrode’s solution. In the presence of Ado, maximal diastolic potential tended to be more depolarized during ischemia, and action potential （AP） parameters were abbrevi- ated greatly in a concentration-dependent manner. Especially, the concentration of Ado 100 μmol·L-1 had significant effects on AP parameters in ischemic phase [APD30, APD50, and APD90 reduced by （86±8）% versus （65±6）%, （70 ±7）% versus （50±6）%, and （60±6）% versus （42±4）% for control after 15 min, P<0.O5]. During reperfu- sion, AP parameters did not completely return to initial values in presence of Ado. This study illustrated that Ado significantly decreased incidence of arrhythmias induced by ischemia and reperfusion (in presence of Ado 100 μmol· L-1, the incidence of DAD decreased by 17% versus 82% for control during reperfusion). Conclusion Ado has no significant effects on guinea-pig ventricle in normal conditions, abbreviates greatly AP parameters during ischemia with a concentration-dependent manner, and has marked antiarrhythmic effects in ischemia and reperfusion.

Effects ofβ_2-Adrenergic Antagonist on Cytosolic Ca^(2+) in Ventricular Myocytes from Infarcted Rat Heart

Objectives To investigate the effects of β2-adrenergic antagonist on cytosolic Ca^2 ＋ （[Ca^2＋ ]i） in ventricular myocytes from infarcted rat heart. Methods A ligature was placed around left anterior descending coronary artery of rat hearts. Rats in the control group were sham-operated. Cardiomyocytes were dissociated at two, four, eight weeks after myocardial infarction （MI） and [Ca^2＋]i was measured via fura-2 fluorescence. The response of cardiomyocytes to isoproterenol in presence or absence of betal-adrenergic antagonist atenolol, beta2-adrenergic antagonist ICI118, 551 or non-selective β1, 2- adrenergic antagonists propranolol was examined. Results The followings were found that ICI 118, 551 had no significant effects on the rise of [Ca^2＋]i induced by isoproterenol in normal ventricular myocytes （P 〉 0.05）, ICI118, 551 only significantly attenuated the rise of [Ca^2＋]i induced by isoproterenol at four weeks and eight weeks after MI （24.5%±5.7% vs 57.8% ± 13.2%, P〈 0.01; 12.2%±7.9% vs 44.6%±11.3%, P〈 0.01）. Atenolol had suppressive effects only in the control group and the post-MI group of two weeks （P 〈 0.05）, and propranolol had suppressive effects in the control and all the three post-MI groups （P 〈 0.01）. Conclusions Beta2-adrenergic antagonist ICI118, 551 may exert negative effects on Ca^2＋ overload initiated by sympathetic stimulation after MI.

The Effect of Levobunolol Hydrochloride on the Calcium andPotassium Channels in Isolated Ventricular Myocytes of Guinea Pig

The effects of levobunolol hydrochlorid (Bun) on the type L calciumchannel currents (Ica) and delayed rectifier potassium channel currents (Ik) in isolated ventricular myocytes of guinea pig were studied by using patch clamp wholecell recording techniques. The results were showed that: 1) Bun caused a dosedependent decrease in Ica and a dose-dependent increase in Ik of the ventricular myocytes.The threshold concentrations of Bun for Ica and Ik were 10-8 mol/L and10-7 mol/L respectively. The maximum effective concentration of Bun for both Ica and Ik was 3 × 10-5 mol/L, and half-maximal concentration was 3 × 10-6 mol/L;2 ) Ik was blocked by 2× 10-6mol/L tetraethylammonium (TEA). A concentration of 3 × 10-6 mol/L Bun showed a decreasing effect on the Ica as revealed by the current-voltage relationship curve, i. e., Bun caused an elevation of the curve; 3)When Ica was blocked by 2 × 10-6 mol/L Isoptin (Verapamil), at a concentrationof 3 × 106- mol/L Bun showed an increasing effect on Ik and the effect could be blocked by TEA. The above-mentioned results indicated that Bun had an inhibito-ry effect on Ica and a fascilitatory effect on Ik The results suggested that themolecular mechanisms of antihypertensive, heart rate slowing and β-receptorblocking effects of Bun might be due to decrease of Ica and increase of Ik.

Differences of promethazine and terfenadine on ion channels in guinea pig ventricular myocytes

Promethazine, a first generation antihistamine,has an antiarrhythmic effect on ischemia-reperfusion inducing arrhythmias^1 and experimental arrhythmias.^2 However, terfenadine as a second generation of antihistamine, has been reported to elicit hypotension, bradycardia, prolongation of the QTc interval and torsades de pointes （TdP） like ventricular arrhythmia.^3 This may be due to the blockage on rectifier postassium current （Ik） of terfenadine, resulting in the prolongation of the action potential duration （APD） and dispersion of the repolarization duration, which might provoke a specific form of polymorphic ventricular tachydysrhythmia, i.e. TdP.^4 In clinical practice,however, the class Ⅲ antiarrhythmic agents, which target on the lk and prolong the action potential duration and QTc interval, rarely lead to arrhythmias.Other actions must be considered to underlie the arrhythmogenic tendency of terfenadine besides its inhibition on Ik. Though both promethazine and terfenadine block the H1 receptor, there must be a different pharmacology profile between the two compounds on ion channels of cardiac myocytes.Whole-cell patch clamp technique was used to investigate the effects of these two antagonists of the H1 receptor on the main ion currents in cardiac electrical activities.

THE ELECTROPHYSIOLOGIC EFFECTS OF ENDOTHELIN A PATCH CLAMP STUDY IN GUINEA PIG VENTRICULAR MYOCYTES

To study the electrophysiologic effects of endothelin-1 (ET-1), we used patch clamp and glass microelectrode techniques to investigate the effects of ET-1 on cardiac L-Ica, Ik and Ik2 in guinea pig ventricular myocytes. The prolongation of APD50, was induced and EADs was triggered by 50 nM ET-1 perfusion. L-lea and Ik were enhanced by various ET-1 concentration from 1 to 50 nM with dose-dependence. Their steady-state activations of L-Iea and Ik shifted left with ET-1 concentration increments. ET-1 elicited a kind of GTP- dependent inward rectifier K current having a mean conductance of 82.36±1.27 pS. The open time and close time ( both interburst intervals and burst durations ) abbreviated with ET-1 concentration increase. The results suggested that EADs -ET evoked was ascribed to the prolongation on the plateau level, which resulted from L-Ica inhancement. The ET- evoked inward rectifier K+ current should be further studied.

CLOFILIUM, A POTENT BLOCKER OF SODIUM AND CALCIUM CHANNELS IN GUINEA-PIG VENTRICULAR MYOCYTES

The unsatisfactory results of clinical trials with class-Ⅰ antiarrhythmic drugs have prompted a renaissance of interest in compounds with class-Ⅲ antiarrhythmic action. In the present, we have reevaluated the class-Ⅲ antiarrhythmic ef-

Characteristics of the Nonselective Cation Current (NSCCs) in Rabbit Left Ventricular Epicardial, Midmyocardial and Endocardial Myocytes

Objectives Recent studies have described regional differences in the electrophysiology and pharmacology of ventric- ular myocardium in canine, feline, rat, guinea pig, and human hearts. This has been shown to be due to a smaller IKs and a lager sodium-calcium exchange current （INa-Ca） and late INa in M region （ deep subepicardial to midmyocardial）. Studies from our laboratory have found a new repolarization current-nonselective cation current （NSCCs） existing in rabbit fight ventricular myocytes. Methods We examined the characteristics of NSCCs in epicardial, M region, and endocardial cells isolated from the rabbit left ventricle with standard microelectrode and whole-cell patch-clamp tech- niques. The permeability to Na^＋ , K^＋ , Li^＋ , Cs^＋ but not to Cl^- indicating that it was a nonselective cation current. Gd^＋3 （0. 1 mmol/1） and La^3＋ （0. 1 retool/1 ） can block the current markedly. Results Further characterization of NSCCs was significantly smaller in M cells than in epicardial and endocardial cells. NSCCs current density was significantly smaller in M cells than in epicardial and endocardial cells. With repolarization to - 80 mV, INa current density was （ -0. 44 ±0. 05） PA/PF in endocardial cells, （ -0. 12 ±0. 05） PA/PF in M cells and （ - 0. 28 ±0. 07） PA/PF in epicardial cells ; and with repolarization to ＋ 30 mV, INa, current density was （ 1.09 ± 0. 29） PA/PF in endocardial cells, （0. 38±0. 09） PA/PF in M cells and （0. 91 ± 0. 32） PA/PF in epicardial cells. Conclusions Transmural dispersion of repolarization was due to the heterogeneity of NSCCs in rabbit left ventricle epicardial, endocardial myocytes and M cells. These findings may advance our understanding of the ionic basis for our understanding of factors contributing to the development of cardiac arrhythmias.

Carvedilol Inhibits Na^+ Current in Rat Ventricular Myocytes

Objectives The effects of carvedilol on sodium current （INa） were investigated in isolated adult rat ventricular myocytes. Methods Single ventricular myocytes were enzymatically dissociated. INa was recorded by whole-cell patch- clamp recording technique. Results an IC50of （6. 35 - 0.40） mol . L^- 1. Carvedilol reversibly inhibited INa in a concentration-dependent manner, with 2. This inhibition was voltage- and frequency-dependent. 3. Carvedilol decreased the peak of the I-V relationship curve at -35 mV from （17.31± 1.68） pA/pF to （6. 58 ± 1.35） pA/pF, but did not change active potential, peak potential and the reverse potential significantly. 4. The steady-state inactivation curve of INa was shifted to more negative potentials. Conclusions Carvedilol inhibits INa in adult rat ventricular myocytes by mechanisms involving preferential interaction with the inactivated state of sodium channel.

Effects of remifentanil on intracellular Ca^2＋ and its transients induced by electrical stimulation and caffeine in rat ventricular myocytes

Background Preconditioning with remifentanil confers cardioprotection. Since Ca^2＋ overload is a precipitating factor of injury, we determined the effects of remefentanil on intracellular Ca^2＋ （[Ca^2＋]i） and its transients induced by electrical stimulation and caffeine, which reflects Ca^2＋ handling by Ca^2＋ handling proteins, in rat ventricular myocytes. Methods Freshly isolated adult male Sprague-Dawley rat myocytes were loaded with Fura-2/AM and [Ca]i was determined by spectrofluorometry. Remifentanil at 0.1-1000 μg/L was administered. Ten minutes after administration, either 0.2 Hz electrical stimulation was applied or 10 mmol/L caffeine was added. The [Ca^2＋]i, and the amplitude, time resting and 50% decay （t50） of both transients induced by electrical stimulation （E[Ca^2＋]i） and caffeine （C[Ca^2＋]i） were determined. Results Remifentanil （0.1-1000.0 μg/L） decreased the [Ca^2＋]i in a dose-dependent manner. It also decreased the amplitude of both transients dose-dependently. Furthermore, it increased the time to peak and tso of both transients dose-dependently. Conclusion Remifentanil reduced the [Ca^2＋]i and suppressed the transients induced by electrical stimulation and caffeine in rat ventricular myocytes.

Sex Difference in the Repolarization Currents of Rabbit Ventricular Cells

The current difference between male and female rabbit ventricular myocytes was investigated for elucidating the mechanism of longer QT interval and higher incidence of drug-associated torsade de pointes in female rabbits than in male rabbits. Whole cell patch clamp technique was used to record APD, I_to, I_K,tail, I_K1 and I_Ca,L of myocytes from left ventricular apex. There was no difference in the membrane capacitance between male and female rabbit myocytes. APD_90 was longer in female rabbits (560.4±26.5 ms, n=15) than in male ones (489.0±20.7 ms, n=14), P<0.05. In female rabbit myocytes, I_K,tail, I_to, I_K1 and I_Ca,L were 0.71±0.05 pA/pF (n=17), 8.28±1.03 pA/pF (n=18), 24.5±3.6 pA/pF (n=12) and 9.0±2.3 pA/pF (n=15) respectively. In male rabbit myocytes, they were 0.84±0.07 pA/pF (n=18), 8.60±1.20 pA/pF (n=18), 25.9±4.5 pA/pF (n=14) and 9.3±2.6 pA/pF (n=16) respectively. I_K,tail in female rabbits was significantly lower than that of male rabbits (P<0.05), but there was no difference in I_to, I_K1 and I_Ca,L between male rabbits and female rabbits (P>0.05). The lower I_K,tail of female rabbit myocytes may contribute to the longer repolarization and the higher incidence of drug-associated torsade de pointes.

Functional Characteristics and Molecular Identification of Swelling-activated Chloride Conductance in Adult Rabbit Heart Ventricles

Outwardly rectifying swelling-activated chloride conductance （ICl, Swell） in rabbit heart plays a critical role in cardioprotection following ischemic preconditioning （IP）. But the functional characterization and molecular basis of this chloride conductance in rabbit heart ventricular myocytes is not clear. Candidate chloride channel clones （e.g. ClC-2, ClC-3, ClC-4 and ClC-5） were determined using RT-PCR and Western blot analysis.Whole cell ICl,Swell was recorded from isolated rabbit ventricular myocytes using patch clamp techniques during hypo-osmotic stress. The inhibitory effects of 4,4＇ isothiocyanato-2,2-disulfonic acid （DIDS）, 5-nitro-2（3-phenylroylamino） benzoic acid （NPPB） and indanyloxyacetic acid 94 （LAA-94） on ICl,Swell were examined. The expected size of PCR products for ClC-2, ClC-3 and ClC-4 but not for ClC-5 was obtained. ClC-2 and ClC-3 expression was confirmed by automated fluorescent DNA sequencing. RT-PCR and Western blot showed that ClC-4 was expressed in abundance and ClC-2 was expressed at somewhat lower levels. The biological and pharmacological properties of ICl,Swell, including outward rectification, activation due to cell volume change, sensitivity to DIDS, LAA-94 and NPPB were identical to those known properties of ICl,Swell in exogenously expressed systems and other mammals hearts. It was concluded that ClC-3 or ClC-4 might be responsible for the outwardly rectifying part of ICl,Wwell and may be the molecular targets of cardioprotection associated with ischemic preconditioning or hypo-osmotic shock.

Panax quinquefolium saponin Optimizes Energy Homeostasis by Modulating AMPK-Activated Metabolic Pathways in Hypoxia-Reperfusion Induced Cardiomyocytes

Objective To investigate the effects and underlying mechanisms of Panax quinquefolium saponin(PQS)on energy deficiency in hypoxia-reperfusion(H/R)induced cardiomyocytes.Methods The H/R injury involved hypoxia for 3 h and then reperfusion for 2 h.Cardiomyocytes recruited from neonatal rat ventricular myocytes(NRVMs)were randomly divided into control,H/R,H/R+compound C(C.C),H/R+PQS,and H/R+C.C+PQS groups.BrdU assay,lactase dehydrogenase(LDH)leakage and early apoptosis rate were evaluated to assess cell damages.Contents of high energy phosphate compounds were conducted to detect the energy production.Protein expression levels of adenosine monophosphate-activated protein kinase a(AMPKα),glucose transporter 4(GLUT4),phosphate fructose kinase 2(PFK2),fatty acid translocase/cluster of differentiation 36(FAT/CD36),and acetyl CoA carboxylase 2(ACC2)in the regulatory pathways were measured by Western blotting.Immunofluorescence staining of GLUT4 and FAT/CD36 was used to observe the mobilization of metabolic transporters.Results PQS(50 mg/L)pretreatment significantly alleviated H/R-induced inhibition of NRVMs viability,up-regulation of LDH leakage,acceleration of early apoptosis,and reduction of energy production(P<0.05).Compared with the H/R group,up-regulated expression of AMPKα,GLUT4,PFK2,FAT/CD36 and ACC2 were observed,and more GLUT4 and FAT/CD36 expressions were detected on the membrane in the H/R+PQS group(P<0.05).These effects of PQS on H/R-induced NRVMs were eliminated in the H/R+C.C+PQS group(P<0.05).Conclusion PQS has prominent advantages in protecting NRVMs from H/R-induced cell damages and energy metabolic disorders,by activation of AMPKα-mediated GLUT4-PFK2 and FAT/CD36-ACC2 pathways.

The Regulation Effects of β_2-AR on I_(NCX) in Myocytes from Infarcted Rat Heart

Objectives This study is designed to investigate the regulation effects of β2-adrenergic receptors （AR） on expression of the Na^± - Ca^2 ± exchanger （ INCX） in myocytes from the infarcted rat heart. Methods Twenty-eight adult Wistar rats were randomly divided into four groups ： the control group, the two weeks, four weeks and eight weeks post-myocardial infarction （post-MI） groups, respectively. The chest of rat was opened and a ligature was placed around the left anterior descending coronary artery. Rats in control group were sham-operated without the coronary artery ligation. After the operation, rats were fed for two, four or eight weeks respectively. Myocytes were enzymatically disassociated by Langendorff perfusion. The whole cell-patch clamp recording technique was used to record INCX in specific pipette solution and superfusion according to the specific holding potential and command potential program. Results The INCX in ventricular myocytes from the border zone of infarcted myocardium increased significantly at eight weeks after MI （0. 51 ± 0. 12 pA/pF vs 1.07± 0. 21 pA/pF, P 〈0.05）. β2-AR agonist increased INcx more strongly in myocytes from post- MI heart than in controls. β2-AR antagonist attenuated the rise of INCX, strongly in myocytes from post-MI heart than in controls, whereas β1-AR onist. Conclusion The regulation effects of β2-AR on INCX in myocytes AR had closer relationship with the genesis of malignant arrhythmia afte