The effect of TYB-3823, a new antiarrhythmic drug, on sodium current in isolated cardiac cells.

1. Sodium current (INa) blockade by TYB-3823, a newly synthesized antiarrhythmic agent, was investigated in isolated single ventricular myocytes by use of the whole cell patch-clamp technique. 2. TYB-3823 blocked INa under steady-state conditions (Kd,rest = 500 microM, Kd,i = 4.9 microM), findings consistent with a shift in the steady state INa availability curve to more negative potentials. 3. TYB-3823 produced use-dependent block at 2 Hz in conjunction with increase in pulse duration (5-300 ms), that was markedly enhanced at less negative holding potentials. 4. The time course of the onset of block was accelerated and the degree of use-dependent block was decreased at more negative holding potential. The time course of the onset of block was accentuated with enhancing block at more positive holding potentials. 5. The time course of recovery from use-dependent block was accelerated at more negative holding potentials but was accentuated at more positive holding potentials. 6. These results suggest that both tonic block and use-dependent block of sodium channels in cardiac tissue might result from an interaction of TYB-3832 with sodium channels mainly in the inactivated channel states and the kinetics of the interaction between drug and receptor may be modulated by the inactivation gate.     

Electrophysiologic effects of an antiarrhythmic agent, bidisomide, on sodium current in isolated rat ventricular myocytes: comparison with mexiletine and disopyramide.

The effects of bidisomide, an antiarrhythmic agent, on sodium current (I(Na)) in isolated rat ventricular myocytes were investigated using a whole cell voltage clamp method. Bidisomide blocked I(Na) with a Ki of 214 microM at a holding potential of -140 mV. The blockade of I(Na) was enhanced at a less negative holding potential of -100 mV with a Ki of 21 microM. Bidisomide shifted the steady state inactivation curve to a negative potential direction by 20 mV without a significant change in the slope factor. Bidisomide slowed the time course of recovery of I(Na) at a holding potential of -140 mV with a slow recovery phase. The time constant of recovery phase for bidisomide, disopyramide and mexiletine were 2703, 1858 and 757 ms, respectively. The development of the block of I(Na) consisted of two phases in the presence of bidisomide. The fast and slow time constants were 11 and 648 ms. Bidisomide produced a use-dependent block of I(Na) when the depolarizing pulse was repeated at 1-3 Hz. Our results indicate that bidisomide binds to rat cardiac sodium channels and that the dissociation kinetics of bidisomide from the inactivated sodium channel is slower than that of disopyramide.     

States and sites of actions of flecainide on guinea-pig cardiac sodium channels.

The states and sites of actions of flecainide on sodium channels were investigated in guinea-pig single cardiac cells, using the whole-cell voltage-clamp technique at 22 degrees C. External application of flecainide caused tonic and use-dependent block of the sodium current (INa). The tonic block and the steady state use-dependent block increased with increasing drug concentrations. The dose-response curve for the use-dependent block was fitted by the equation for 1:1 drug-receptor binding and yielded a KD of 7.0 microM flecainide. At 5 microM flecainide, the use-dependent block of INa with 10 and 200 ms depolarizing pulses at an interpulse interval of 400 ms was 31.1 +/- 2.7 (mean +/- S.E.) and 36.8 +/- 2.7%, respectively. The two values were not significantly different. The block developed as a single exponential function with onset rate of 0.041 +/- 0.005/pulse. Recovery from flecainide block consisted of two components as reported previously. The mean time constant of the initial fast component was 48 +/- 17 ms, which was comparable but significantly longer than that in the absence of the drug. The late slow component was only seen after drug application and the time constant was 26 +/- 7 s at -100 mV. Internal application of 5 and 50 microM flecainide for 30 min after rupture of the cell membrane produced a non-significant block and values of 1.7 +/- 0.8 and 6.9 +/- 2.4%, respectively, for the use-dependent block of INa.(ABSTRACT TRUNCATED AT 250 WORDS)     

Analysis of moricizine block of sodium current in isolated guinea-pig atrial myocytes. Atrioventricular difference of moricizine block

The effects of moricizine on Na+ channel currents (INa) were investigated in guinea-pig atrial myocytes and its effects on INa in ventricular myocytes and on cloned hH1 current were compared using the whole-cell, patch-clamp technique. Moricizine induced the tonic block of INa with the apparent dissociation constant (Kd,app) of 6.3 microM at -100 mV and 99.3 microM at -140 mV. Moricizine at 30 microM shifted the h infinity curve to the hyperpolarizing direction by 8.6 +/- 2.4 mV. Moricizine also produced the phasic block of INa, which was enhanced with the increase in the duration of train pulses, and was more prominent with a holding potential (HP) of -100 mV than with an HP of -140 mV. The onset block of INa induced by moricizine during depolarization to -20 mV was continuously increased with increasing the pulse duration, and was enhanced at the less negative HP. The slower component of recovery of the moricizine-induced INa block was relatively slow, with a time constant of 4.2 +/- 2.0 s at -100 mV and 3.0 +/- 1.2 s at -140 mV. Since moricizine induced the tonic block of ventricular INa with Kd,app of 3.1 +/- 0.8 microM at HP = -100 mV and 30.2 +/- 6.8 microM at HP = -140 mV, and cloned hH1 with Kd,app of 3.0 +/- 0.5 microM at HP = -100 mV and 22.0 +/- 3.2 microM at HP = -140 mV, respectively, either ventricular INa or cloned hH1 had significantly higher sensitivity to moricizine than atrial INa. The h infinity curve of ventricular INa was shifted by 10.5 +/- 3.5 mV by 3 microM moricizine and that of hH1 was shifted by 5.0 +/- 2.3 mV by 30 microM moricizine. From the modulated receptor theory, we have estimated the dissociation constants for the resting and inactivated state to be 99.3 and 1.2 microM in atrial myocytes, 30 and 0.17 microM in ventricular myocytes, and 22 and 0.2 microM in cloned hH1, respectively. We conclude that moricizine has a higher affinity for the inactivated Na+ channel than for the resting state channel in atrial myocytes, and moricizine showed the significant atrioventricular difference of moricizine block on INa. Moricizine would exert an antiarrhythmic action on atrial myocytes, as well as on ventricular myocytes, by blocking Na+ channels with a high affinity to the inactivated state and a slow dissociation kinetics.     

Use-dependent block of atrial sodium current by ethylisopropylamiloride.

Ethylisopropylamiloride (EIPA) is a potent inhibitor of Na(+)-H+ exchange in many tissues and is frequently used to study cellular regulation of pH, but the electrophysiologic effects of EIPA on cardiac cells have not been studied previously. The use-dependent effects of EIPA on the sodium current (INa) of cultured embryonic chick atrial myocytes were investigated using standard whole-cell patch-clamp techniques. With 150-ms depolarizations from -140 to 0 mV, applied at 1-3 Hz in the presence of 10 microM EIPA, a decrement in INa was observed. This use-dependent reduction equaled 31 +/- 6% of control INa at steady state during 1-Hz stimulation. Inhibition increased with stimulation rate and with depolarization of the holding potential to -100 mV, but there was no effect of pulse duration on the EIPA-induced inhibition over the range of 20-500 ms. Moreover, repetitive depolarizations to potentials that did not activate macroscopic current but that did yield pronounced channel inactivation did not result in a decrement in INa. The effect of EIPA increased over the concentration range of 1-30 microM so that with 3-Hz stimuli steady-state inhibition increased from 3 +/- 1 to 85 +/- 5%. Amiloride, which slows repolarization of the cardiac action potential, was at least 100-fold less potent than EIPA in reducing INa. We conclude that EIPA is an "open-channel" blocker of the cardiac sodium current at concentrations comparable to those of many type I antiarrhythmic agents.     

Use- and voltage-dependent depression by ethmozine (moricizine) of the rapid inward sodium current in single rat ventricular muscle cells

Effects of ethmozine (moricizine) on the rapid inward sodium current (INa) were studied in freshly isolated single cells of rat ventricular myocardium. INa was measured by means of a patch clamp method for observing integral ionic currents. Ethmozine was applied extracellularly to a small cell membrane patch at concentrations of 10, 20, and 40 microM. At a stimulation frequency of 0.1 Hz the drug decreased the peak INa without producing a shift of the current-voltage curve, but shifted the V0.5 of the steady-state inactivation curve by -6 mV. At frequencies of 2-5 Hz the ethmozine-induced block exhibited a prominent use dependence, with trains of depolarizing clamp pulses 5-50 ms in duration eliciting maximal INa from holding potentials at which the steady-state inactivation variable h infinity was close to 1. The use-dependent inhibition of INa became more pronounced with an increase in both stimulation rate and pulse duration. In contrast to what has been observed in the node of Ranvier of the frog, the present results indicate that ethmozine binds to both inactivated and open Na+ channels, but that the contribution of the open channel block to the overall block at depolarizing clamp step durations of several hundred milliseconds is small in comparison with the contribution of the block of inactivated channels.     

Modulation of sodium current kinetics by chlorpromazine in freshly-isolated striatal neurones of the adult guinea-pig.

1. The neurones of the striatum were freshly dissociated from the adult guinea-pig brain by enzymatic and mechanical treatments. Sodium channel current kinetics in these neurones were measured using a whole cell variation of the patch-clamp technique. 2. Chlorpromazine, a neuroleptic, in micromolar concentrations reversibly reduced the amplitude of the sodium currents. Activation and inactivation time constants were not affected. The inhibition followed one-to-one binding stoichiometry. 3. The concentration-response curve shifted to the left when the holding potential was less negative. The EC50 shifted from 4.8 microM to 0.9 microM when the holding potential was changed from -120 mV to -70 mV. 4. The steady-state activation curve of the sodium current was not affected by chlorpromazine, whereas the steady-state inactivation curve was shifted in the negative direction. Consequently, the window current which is normally present at a potential range around -50 mV was decreased in the presence of chlorpromazine. 5. Successive sodium currents evoked by a train of depolarizing pulses (30 ms duration) to -10 mV showed a cumulative decrease in size during the application of chlorpromazine. However, such 'use-dependent' block was not observed when the pulse duration was reduced to 1 ms. 6. The recovery from inactivation in the presence of chlorpromazine, was expressed as a second order process. The faster component was similar to the recovery time course of the normal sodium channels. The slower component accounted for the use-dependent effect of chlorpromazine. 7. The results indicate that chlorpromazine binds to the resting sodium channels producing steady-state block at a very negative holding potential. When the membrane is depolarized, chlorpromazine binds to the inactivated form of the sodium channels with much higher affinity and stabilizes them in the inactivated state, slowing their kinetics.     

Study of the interaction of lubeluzole with cardiac sodium channels

The effects of lubeluzole on sodium currents were examined in guinea-pig isolated cardiac myocytes by use of the whole-cell patch clamp technique. Lubeluzole (0.01-100 microM) reduced peak Na+ current (INa) obtained at a holding potential of -80 mV with an IC50 value of 9.5 (3.5-21.9) microM and a Hill coefficient of 1.1. These effects were rapid and reversible. Lubeluzole (10 microM) produced a shift in the inactivation curve to hyperpolarized potentials (by -9.7 mV, P < 0.05), but produced no change in the voltage-dependence of activation. Lubeluzole (10 microM) produced significant tonic block of INa obtained at a holding potential of -120 mV (2.7 +/- 1.4% and 27.5 +/- 5.8% for control and lubeluzole, respectively; n = 6; P < 0.05). Use-dependent block of INa was also observed. Recovery from block was delayed by lubeluzole (10 microM; tau1=4.4 +/- 6.2, tau2=22.7 +/- 1.5 milliseconds for control and tau1=311 +/- 144, tau2 = 672 +/- 23 milliseconds for lubeluzole; n = 6; P < 0.001) confirming use-dependency of block. The results indicate that lubeluzole produces both tonic and use-dependent block of cardiac sodium channels at concentrations similar to those that block neuronal sodium channels, due mainly to interaction of the drug with channels in the inactivated state.     

Block of sodium channels by psychotropic drugs in single guinea-pig cardiac myocytes.

1. Effects of imipramine and haloperidol on voltage-gated sodium channels were investigated in guinea-pig isolated ventricular myocytes by the whole-cell patch clamp technique. Some additional experiments were also performed with chlorpromazine for the purpose of comparison. 2. All test drugs in micromolar concentrations suppressed the amplitude of peak sodium current associated with step depolarization from a holding potential of -140 mV in a reversible manner. The order of potency was chlorpromazine greater than imipramine greater than haloperidol. 3. Dose-response curves obtained with a holding potential of -140 mV were best fitted by 2:1 stoichiometry in all three drugs and were shifted in the direction of lower concentrations when a holding potential of -90 mV was used. 4. The drug-induced block was not associated with any change in the time courses of sodium current activation and inactivation. 5. Steady-state sodium channel inactivation curve was shifted in the direction of more negative potentials by the drugs. 6. All three drugs also produced marked use-dependent block as demonstrated by a cumulative increase in the block during a train of depolarizing pulses. 7. The use dependence was due to a higher affinity of the drugs for the inactivated state of sodium channels than the resting state and to a very slow repriming of the drug-bound sodium channels from inactivation. 8. The steady-state and use-dependent block of voltage-gated sodium channels by psychotropic drugs may contribute to their cardiotoxic and perhaps antiarrhythmic effect.     

葛根素抑制大鼠心室肌细胞的钠电流

AIM: To study the effect of puerarin (Pue) on Na+ channel in rat ventricular myocytes. METHODS: Whole-cell patch-clamp technique was applied on isolated cardiomyocytes from rats. RESULTS: Pue inhibited cardiac INa in a positive rate-dependent and dose-dependent manner, with an IC(50) of 349 micromol/L. The kinetics of blockage of cardiac sodium channel by Pue resembled the ClassIa/Ic of antiarrhythmic agents. Pue 300 micromol/L did not alter the shape of the I-V curve of INa, but markedly shifted the steady-state inactivation curve of INa towards more negative potential by 15.9 mV, and postponed the recovery of INa inactivation state from (21.9+/-1.6) ms to (54.4+/-3.4) ms (P<0.01). It demonstrated that the steady state of inactivation was affected by Pue significantly. CONCLUSION: Pue protected ventricular myocytes against cardiac damage and arrhythmias by inhibiting recovery from inactivation of cardiac Na+ channels.     

双苯氟嗪对豚鼠心室肌细胞膜钠电流的影响(英文)

目的观察双苯氟嗪对豚鼠心室肌细胞膜钠电流的影响。方法用酶解方法分离豚鼠心室肌细胞,全细胞膜片钳技术记录钠电流。结果将细胞钳制在-80mV,给(-80～+50)mV,50ms和步阶10mV的去极化脉冲,记录到的电流被河豚毒素10μmol·L-1完全抑制。在该刺激条件下,该电流最大激活电压在-20mV左右,翻转电压在+30mV左右,提示该电流为钠电流。双苯氟嗪可以浓度依赖性地抑制钠电流。双苯氟嗪对钠电流的抑制作用在冲洗后可部分恢复,表明其对钠通道的抑制作用具有可逆性。双苯氟嗪可使钠电流I-V曲线上移,但对钠电流的电压依赖性特征、最大激活电压和翻转电压无明显影响。在双苯氟嗪40μmol·L-1存在下,最大激活电压下的峰值电流下降约46%;双苯氟嗪可明显使钠电流稳态失活曲线左移,但不影响曲线的斜率因子。双苯氟嗪40μmol·L-1可使钠电流半数失活电压从(-73.0±4.6)mV减少到(-82.8±7.2)mV。但双苯氟嗪对钠电流稳态激活无明显影响,在双苯氟嗪40μmol·L-1存在下,半数激活电压(-33.7±3.6)mV和斜率因子(5.6±2.4)mV与对照组激活电压(-34.9±5.1)mV和斜率因子(6.0±4.8)mV相比无显著性差异。双苯氟嗪可以使钠电流从失活状态下恢复明显减慢,双苯氟嗪40μmo·lL-1可使恢复时间常数延长(79±28)vs(36±11)ms。结论双苯氟嗪可以浓度依赖性、使用依赖性和频率依赖性地抑制心肌钠电流,并且主要作用于钠电流的失活状态。     

Mechanism of inhibition of the sodium current by bepridil in guinea-pig isolated ventricular cells.

1. Effects of bepridil, a sodium-, calcium-, and potassium-antagonistic agent, on the Na+ current were studied by the whole cell voltage clamp technique (tip resistance = 0.5 MOhm, [Na]i and [Na]o 10 mmol l-1 at 20 degrees C). 2. Bepridil produced tonic block (Kdrest = 295.44 mumol l-1, Kdi = 1.41 mumol l-1; n = 4). 3. Bepridil (100 mumol l-1) shifted the inactivation curve in the hyperpolarization direction by 13.4 +/- 2.7 mV (n = 4) without change in the slope factor. 4. In the presence of 50 mumol l-1 bepridil, bepridil showed use-dependent block at 2 Hz, whereas changes in pulse duration did not significantly effect this use-dependent block (81% +/- 2% at 10 ms, 84% +/- 3% at 30 ms, 86% +/- 3% at 100 ms; n = 4). 5. After removal of fast inactivation of the Na+ current by 3 mmol l-1 tosylchloramide sodium, bepridil (50 mumol l-1) still showed use-dependent block which was independent of the holding potential. 6. The recovery time constant from the bepridil-induced use-dependent block was 0.48 s at holding potential of -100 mV and 0.51 s at holding potential of -140 mV. 7. These results indicate that bepridil could bind to the receptor in the sodium channel through the hydrophobic and the hydrophilic pathway and leave the receptor through the hydrophobic pathway in the lipid bilayer. The binding and dissociation kinetics of this drug were shown to be fast, and the accumulation of the drug in the sodium channel appeared to be small.(ABSTRACT TRUNCATED AT 250 WORDS)     

Electrophysiological effects of CD-349, a dihydropyridine-type calcium antagonist, on goat cardiac Purkinje fibers.

We examined the calcium antagonistic action of CD-349, a dihydropyridine derivative, on goat cardiac Purkinje fibers using the two-microelectrode voltage-clamp method. CD-349 at a concentration of 10(-5) M shortened the action potential duration without changing the maximum rise in the action potential (Vmax) in goat Purkinje fibers. CD-349 at 3 x 10(-7) to 3 x 10(-6) M inhibited the slow inward current (Isi) in a concentration-dependent manner. At the holding potential of -55 mV, CD-349 exerted a tonic block of Isi, and, furthermore, it exerted a use-dependent block at a frequency range of 1 Hz, but it did not exert a use-dependent block at 0.5 and 0.2 Hz. This may be because CD-349 delayed the recovery process from the inactivation of Isi. The amplitude of the block of Isi was larger at the holding potential of -45 mV than at -55 mV. The inactivation curve of Isi shifted toward a negative potential in the presence of CD-349. Nifedipine also exerted a tonic block of Isi. The onset of the action of nifedipine was quicker than that of CD-349 or nitrendipine. A use-dependent block at 1 Hz and delay of the recovery process from inactivation was also observed with nifedipine. The inactivation curve shifted toward the negative potential with nifedipine. On washout of the drugs, the effects of CD-349 or nitrendipine were not readily reversed compared with those of nifedipine. CD-349 had no effect on either inward rectifying (IK1) or delayed outward potassium (IK) or hyperpolarization-activated inward (If) currents. These observations suggest that, in cardiac tissues, CD-349 selectively inhibits the calcium current, presumably by acting on the inactivated channel.     

SNX482 selectively blocks P/Q Ca2+ channels and delays the inactivation of Na+ channels of chromaffin cells

The effects of the toxin SXN482 on Ca2+ channel currents (ICa), Na+ currents (INa), and K+ currents (IK) have been studied in bovine adrenal medullary chromaffin cells voltage-clamped at -80 mV. Currents were elicited by depolarising pulses to 0-10 mV (ICa and INa) or to +60 mV (IK). SNX482 blocked ICa in a concentration-dependent manner. The inhibition curve exhibited two phases. The first high-affinity phase comprised 28% of the whole-cell current and exhibited an IC50 of 30.2 nM. The second low-affinity phase comprised over 70% of ICa and had an IC50 of 758.6 nM. Blockade was rapid and fully reversible upon washout of the toxin. Occlusion experiments showed additivity of blockade exerted by nifedipine plus SNX482 (0.3 microM) and by omega-conotoxin GVIA plus SNX482. In contrast, blockade exerted by combined omega-agatoxin IVA plus SNX482 (about 50% of the whole cell) did not show additivity. At 0.3 microM and higher concentrations, SNX482 delayed the inactivation of INa. The time constant (tau) for inactivation of INa in control conditions doubled in the presence of 0.5 microM SNX482. At 0.3 microM, SNX482 did not affect IK. Our data demonstrate that: (i) SNX482 selectively blocks P/Q Ca2+ channels at submicromolar concentrations; (ii) the toxin partially blocks Na+ channels; (iii) SNX482 delays the inactivation of Na+ channels. These results reveal novel properties of SNX482 and cast doubts on the claimed selectivity and specificity of the toxin to block the R-type Ca2+ channel.     

Taurine inhibition of fast Na+ current in embryonic chick ventricular myocytes.

Effects of taurine on the fast Na+ current (INa) in 17-day-old embryonic chick ventricular myocytes were examined using the whole-cell voltage-clamp technique. The cells were spherical (10-15 microns diameter) and had a capacitance of 9.8 +/- 1.3 pF. The experiments were performed at room temperature (22 degrees C), and the holding potential was -90 mV. After the patch membrane was broken, peak INa initially increased, and then decreased and became stable within 3-5 min. The experiments on taurine were started after INa had stabilized. The characteristics of INa were as expected, including sensitivity to tetrodotoxin (10 microM). When added to the bath, taurine inhibited INa and shifted the reversal potential in the hyperpolarizing direction. At 10 mM, taurine inhibited INa by 38.2 +/- 4.3%, and shifted the reversal potential by 10.2 +/- 3.1 mV. The time to peak current was slowed: 0.83 +/- 0.20 ms (n = 11) in control, 1.03 +/- 0.18 ms (n = 9) in 10 mM taurine, and 1.10 +/- 0.19 ms (n = 10) in 20 mM taurine. These effects of taurine were not reversed by 30 min washout. At low concentrations, taurine actually enhanced INa in 3 of 8 cells at 1 mM, and in 4 of 10 cells at 5 mM; the reversal potential was still shifted in the hyperpolarizing direction by 5.7 +/- 1.6 mV. The time course of inactivation (fitted as a single exponential at test potential of -30 mV) was not affected: 1.1 +/- 0.5 ms in control 1.2 +/- 0.4 ms at 10 mM taurine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Isoflurane and propofol inhibit voltage-gated sodium channels in isolated rat neurohypophysial nerve terminals

Mounting electrophysiological evidence indicates that certain general anesthetics, volatile anesthetics in particular, depress excitatory synaptic transmission by presynaptic mechanisms. We studied the effects of representative general anesthetics on voltage-gated Na+ currents (INa) in nerve terminals isolated from rat neurohypophysis using patch-clamp electrophysiological analysis. Both isoflurane and propofol inhibited INa in a dose-dependent and reversible manner. At holding potentials of -70 or -90 mV, isoflurane inhibited peak INa with IC50 values of 0.45 and 0.56 mM, and propofol inhibited peak INa with IC50 values of 4.1 and 6.0 microM, respectively. Isoflurane (0.8 mM) did not significantly alter the V1/2 of activation; propofol caused a small positive shift. Isoflurane (0.8 mM) or propofol (5 microM) produced a negative shift in the voltage dependence of inactivation. Recovery of INa from inactivation was slower from a holding potential of -70 mV than from -90 mV; isoflurane and propofol further delayed recovery from inactivation. In conclusion, the volatile anesthetic isoflurane and the intravenous anesthetic propofol inhibit voltage-gated Na+ currents in isolated neurohypophysial nerve terminals in a concentration- and voltage-dependent manner. Marked effects on the voltage dependence and kinetics of inactivation and minimal effects on activation support preferential anesthetic interactions with the fast inactivated state of the Na+ channel. These results are consistent with direct inhibition of oxytocin and vasopressin release from the neurohypophysis by isoflurane and propofol. Inhibition of voltage-gated Na+ channels may contribute to the presynaptic effects of general anesthetics on nerve terminal excitability and neurotransmitter release.     

Stereoselective interactions of (R)- and (S)-propafenone with the cardiac sodium channel.

The specific interactions of both (R)- and (S)-propafenone with the cardiac sodium channel were studied with patch clamp techniques in the whole-cell recording mode at reduced extracellular Na+ on guinea pig ventricular cells. Both (R)- and (S)-propafenone (10 microM) shifted the membrane potential required for half-maximal steady-state inactivation (E0.5) of the cardiac sodium channel to considerably more negative membrane potentials [E0.5 = -70.8 +/- 2.9 mV for controls vs. -85 +/- 3.1 mV for (R)-propafenone and -91.9 +/- 1.7 mV for (S)-propafenone]. (S)-Propafenone at a concentration of 10 microM is more effective in shifting the h infinity curve of the cardiac sodium channel. Recovery from inactivation of the cardiac sodium current is prolonged by orders of magnitude by both stereoenantiomeric forms [time constants were estimated to be 38 +/- 15 ms at -90 mV vs. 46.5 +/- 14.3 s for (R)-propafenone and 74.2 +/- 37.9 for (S)-propafenone]. Development of block occurs mainly through the inactivated channel conformation for both (R)- and (S)-propafenone. Development of block of inactivated cardiac sodium channels occurs with time constants of 15.9 +/- 3.9 s for (R)-propafenone and 19.7 +/- 7.3 s for (S)-propafenone at 10 microM. Action potential duration and possible stereoselective interaction with ion transport systems other than sodium channels may influence the block developed by either (R)- or (S)-propafenone at a given concentration and beating frequency indirectly through the membrane potential.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of amlodipine on unitary non-L-type high voltage-activated Ca2+ channel currents in differentiated PC12 cells

Effects of amlodipine on unitary non-L-type high voltage-activated Ca2+ channels (Cav2 channels) were investigated in cell-attached patches of nerve growth factor (NGF)-differentiated PC12 cells. Cav2 channels, mainly composed of N-type channel in our experimental condition, were defined in this study as high voltage-activated Ca2+ channels obtained after selection of patches without L-type channel "mode 2" activity in the presence of 1 microM BayK8644, or L-type channel block by 5 microM nifedipine. At a test potential of +20 mV, they had a unitary current amplitude of approximately 0.5 pA and open time constants of approximately 0.4 ms and approximately 1.1 ms when fit assuming double exponential components. As bath application of amlodipine was ineffective to modify Cav2 channels in the sealed patch, we analyzed the channel activity when giga-seal formation was obtained in the presence of amlodipine (10 microM both in the pipette and bath solution). Amlodipine did not modify the unitary current amplitude but suppressed the channel open probability (NPo) when holding potential was depolarized, shifting the voltage-dependent inactivation curve towards negative potentials by 25mV. Amlodipine-induced suppression of NPo was mainly due to the decreased ratio of sweeps with channel openings (availability) and was not associated with changes in open time constants. These results were consistent with the view that amlodipine prevented channel openings through the high-affinity binding to the inactivated state, as often observed when dihydropyridines block L-type Ca2+ channels.     

海葵毒素anthopleurin—Q对豚鼠心室肌细胞钠电流的作用

目的:研究从海葵(Anthopleura xanthogrammica)提取的毒素anthopleurin-Q(AP-Q)对豚鼠心室肌钠电流(I_(Na))的作用。方法:用酶消化法分离豚鼠单个心室肌细胞,用全细胞膜片箝技术记录心室肌细胞钠电流。结果:AP-Q 3-30nmol/L浓度依赖性地增大I_(Na),EC_(50)、为104nmol/L(95％可信范围:78-130nmol/L)。AP-Q 300nmol/L使I-V曲线左移,使半数激活电压从(-36.3±2.3)mV变为(-43±23)mV(n=6,P<0.01),半数失活电压从(-75±6)mV变为(-59±5)mV(n=6,P<0.01)。AP-Q 300nmol/L使I_(Na)半数恢复时间从(114±36)ms缩短为(17±2)ms(n=6,P<0.01),并明显减慢I_(Na)的快速失活时间常数(τ_f)。结论:AP-Q对I_(Na)有促进作用并减慢其失活过程。     

Voltage-dependent and frequency-independent inhibition of recombinant Cav3.2 T-type Ca2+ channel by bepridil

Effects of bepridil on the low voltage-activated T-type Ca2+ channel (CaV3.2) current stably expressed in human embryonic kidney (HEK)-293 cells were examined using patch-clamp techniques. Bepridil potently inhibited ICa,T with a markedly voltage-dependent manner; the IC50 of bepridil was 0.4 micromol/l at the holding potential of -70 mV, which was 26 times as potent as that at -100 mV (10.6 micromol/l). Steady-state inactivation curve (8.4 +/- 1.7 mV) and conductance curve (5.9 +/- 1.9 mV) were shifted to the hyperpolarized potential by 10 micromol/l bepridil. Bepridil exerted the tonic blocking action but not the use-dependent block. Bepridil had no effect on the recovery from inactivation of T-type Ca2+ channels. Thus, high efficacy of bepridil for terminating atrial fibrillation and atrial flutter may be considered to be attributed, at least in a part, to the T-type Ca2+ channel-blocking actions.