牛磺酸镁配合物对获得性长QT综合征模型的抗心律失常作用研究

目的 研究牛磺酸镁配合物（TMCC）对获得性长QT综合征（LQTS）模型的抗心律失常作用及作用机制。方法 采用Langendorff逆行主动脉灌流酶解法,急性分离获得豚鼠单个心室肌细胞;建立表达KCNQ1/KCNE1基因的HEK293细胞模型。色原烷醇293B （5 μmol/L）用来建立LQTS模型,采用全细胞膜片钳技术记录TMCC （0.01、0.10、1.00 mmol/L）对正常和LQTS模型豚鼠心室肌细胞动作电位和缓慢激活延迟整流外向钾通道（I_Ks）电流的影响。结果 色原烷醇293B可以显著延长50%和90%复极化动作电位持续时间（APD₅₀和APD₉₀）,0.01、0.10、1.00 mmol/L TMCC可以显著减弱色原烷醇293B延长APD₅₀和APD₉₀的作用（P<0.05、0.01）。TMCC （0.01、0.10、1.00 mmol/L）对抗色原烷醇293B对I_Ks电流的抑制作用,减弱色原烷醇293B对I-V曲线的下移,0.1、1.0 mmol/L浓度组显著减弱色原烷醇293B对I_Ks电流的抑制作用（P<0.01）,呈现对抗LQTS的作用。结论 TMCC通过缩短动作电位复极时程,增大被抑制的I_Ks电流,发挥一定的抗LQTS的作用。     

3,6-[二甲氨基]-二苯骈碘因甲酸盐对豚鼠单—心室肌细胞动作电位和慢内向钙电流的影响

采用全细胞电压钳技术,观察了3,6-[二甲氨基]-二苯骈碘因甲酸盐(IHC-64)对豚鼠单一心室肌细胞动作电位和慢内向钙电流(I_si)的影响。结果表明,IHC-64 20和200 μmol/L可明显缩短APD₂₀和APD₉₀。予先给予钙通道阻滞剂尼索地平可取消IHC-64缩短APD₂₀的作用。在电压钳条件下,上述浓度的IHC-64对I_si有明显的抑制作用,I_si由给药前的1.92±0.41 nA分别降低至给药后1.73±0.50 nA(P<0.05)和0.62±0.38 nA(P<0.01).提示IHC-64可能对心肌细胞钙通道有阻滞作用。     

用β-escin穿孔膜片技术研究粉防己碱对豚鼠心室肌钙电流的作用

目的用β-escin穿孔膜片(PPR)技术研究粉防己碱(tetrandrine,Tet)对I_Ca,L和I_Ca,T的作用。方法用PPR和全细胞记录(WCR)模式记录心室肌细胞I_Ca,L和I_Ca,T。结果25 μmol·L^-1 β-escin可在心室肌细胞形成稳定的PPR模式,用此模式记录的I_Ca,L衰减明显减慢。在PPR模式下1～300 μmol·L^-1 Tet浓度依赖性地减小I_Ca,L幅值。3,30和300 μmol·L^-1 Tet对I_Ca,T的抑制率分别为(16±5)%,(40±7)%和(75±11)%。结论用25 μmol·L^-1 β-escin在豚鼠心室肌细胞能得到较稳定的PPR模式,在此模式下Tet浓度依赖性地抑制I_Ca,L和I_Ca,T。     

苄基四氢巴马汀细胞内用药对豚鼠乳头状肌动作电位和心室肌细胞延迟整流钾电流的作用

目的　研究将苄基四氢巴马汀(BTHP)导入细胞内对豚鼠乳头状肌动作电位及单个心室肌细胞延迟整流钾电流的影响。方法　利用外加电压脉冲将药物导入乳头状肌细胞内，并用标准微电极方法测定动作电位；利用浓度差扩散方式使药物进入单个心室肌细胞内，采用全细胞膜片钳技术记录延迟整流钾电流（I_K）。结果　100 μmol.L^-1 BTHP使APD₂₀和APD₉₀分别延长13.5%和20.5%。30 μmol.L^-1 BTHP使I_K和I_K,tail分别从（14.1±2.2) pA.pF^-1和(4.0±0.6) pA.pF^-1降至(9.4±1.3) pA.pF^-1和(2.1±1.0) pA.pF^-1，下降率分别为33.2%和35.3%。 该药使I_K和I_K,tail的I-V曲线幅度降低，对曲线形状影响不明显。结论　BTHP入细胞内后可阻滞延迟整流钾电流和延长动作电位时程。     

荭草苷对大鼠心室肌细胞钠通道的影响

目的 研究荭草苷（orientin,Ori）对大鼠心室肌细胞钠通道电流（I_Na）的影响,探讨Ori在离子通道层面的抗心律失常机制。方法 使用Langendorff恒温恒压灌流装置、单酶解消化法分离大鼠心室肌细胞,应用全细胞膜片钳技术记录、观察不同浓度Ori作用前后大鼠心室肌细胞钠通道电流（I_Na）的变化。结果 低于2 μmol·L^-1的Ori对I_Na无明显影响,3,10,30 μmol·L^-1的Ori使I_Na的I-V曲线显著上移,峰值钠电流（I_Na-Peak）由给药前的（-81.49±3.9）pA/pF依次变为（-74.38±4.1）pA/pF,（-63.05±2.8）pA/pF和（-55.35±3.2）pA/pF;Ori使激活曲线右移、失活曲线左移,失活后恢复曲线显著右移,最大半数激活电位（V_1/2-ac）由给药前的（-53.66±4.12）mV分别变为（-44.64±1.9）mV,（-38.95±1.7）mV和（-30.21±1.5）mV;最大半数失活电位（V_1/2-in）由给药前的（-51.68±0.76）mV分别变为（-60.17±1.5）mV,（-68.51±1.4）mV和（-75.22±1.37）mV,恢复时间（τ）由给药前的（18.38±0.84）ms分别变为（24.53±1.4）ms,（35.25±1.3）ms和（68.75±1.58）ms。结论 Ori能浓度依赖性抑制大鼠心室肌细胞的I_Na,并能显著影响其激活、失活及失活后恢复的动力学特征。     

苄基四氢巴马汀对豚鼠心室肌细胞钾电流及钙、钠电流的作用

目的：研究苄基四氢巴马汀（BTHP）对心肌细胞的作用特点,以探讨其抗心律失常机制。方法：用全细胞膜片钳技术考察BTHP对心室肌细胞钾电流及钙、钠电流的作用。结果：BTHP 30 μmol.L^-1明显阻滞延迟整流钾电流（I_K包括：I_Kr及I_Ks）。可使I_Kr及I_Kr,tail的幅值下降,且对I_Kr阻滞作用呈频率依赖性;对I_Ks及I_Ks,tail幅值也有明显的抑制作用。BTHP 200　μmol.L^-1可明显阻滞I_Ca,L,使其电流幅值降低,但对I_K1,I_Ca,T,I_Na电流均无影响。结论：BTHP可明显阻滞心室肌细胞I_Kr,I_Ks,I_Ca,L电流,且对I_Kr阻滞作用呈频率依赖性。     

4-氨基吡啶对豚鼠心室肌钙和钠电流的影响

目的研究4-氨基吡啶(4-AP)对心肌细胞L型钙通道和钠通道的影响。方法用全细胞膜片钳技术考察4-AP对豚鼠心室肌细胞L型钙电流和钠电流的作用。结果4-AP0.1,0.5,1.0mmol·L^-1浓度依赖性地抑制L型钙电流(I_Ca,L)和钠电流(I_Na),抑制率分别为(11.6±1.7)%,(37.5±8.3)%和(54.5±6.9)%以及(22.1±14.3)%,(39.4±8.8)%和(62.3±6.8)%。0.5mmol·L^-14-AP使I_Ca,L和I_NaI-V曲线均上移。结论4-AP可浓度依赖性地阻滞豚鼠心室肌细胞L型钙通道和钠通道。     

三七皂苷Fc对SD大鼠心室肌细胞钠离子通道电流的影响

目的 探究三七皂苷Fc (notoginsenoside Fc，N-Fc)对SD大鼠心室肌细胞钠离子通道电流的影响。方法 使用Langendorff恒温恒压灌流装置通过酶解法急性分离SD大鼠心室肌细胞，采用标准全细胞膜片钳技术观察并记录不同浓度的N-Fc对大鼠心室肌细胞钠通道电流(I_Na)及其动力学特征的影响。结果 5µmol·L^–1的N-Fc对I_Na无明显影响，随着药物浓度的增加，10，20，50µmol·L^–1的N-Fc可使钠峰值电流由给药前的(–87.49±3.40) pA/pF依次下降为(–62.91±2.37)，(–49.30±1.27)，(–34.68±3.21) pA/pF (P<0.01)；I_Na的I-V曲线上移，但形态轨迹、激活电位及峰电位基本不变；I_Na的激活曲线向去极化方向迁移，半数激活电压由给药前的(–52.37 ±1.32) mV变为(–42.74 ±0.37)，(–38.92 ±2.77)，(–33.78 ±0.96) mV (P<0.05)；失活曲线显著左移，半数失活电压(V_1/2-in)由(–55.95±4.88) mV依次变为(–64.29 ±1.77)，(–69.57±3.47)，(–73.32±3.79) mV (P<0.05)；此外，N-Fc能够显著延长I_Na失活后恢复时间，时间常数τ值(给药前，10，20，50µmol·L^–1N-Fc处理后)分别为(12.50±1.17)，(21.36±2.23)，(25.35±1.25)，(34.35±1.24) ms (P<0.05)。结论 N-Fc能浓度依赖性地抑制SD大鼠心室肌细胞的I_Na，并能显著影响其激活、失活及失活后恢复的动力学特征。     

苄基四氢巴马汀对豚鼠心室肌细胞快激活延迟整流钾电流的作用

目的研究苄基四氢巴马汀(BTHP)对心室肌细胞快激活(I_kr)延迟整流钾电流的作用。方法 用全细胞膜片钳技术记录豚鼠心室肌细胞钾离子通道电流。结果BTHP在1～100 μmol·L^-1以浓度依赖性方式阻滞I_kr,其IC₅₀为13.5 μmol·L^-1(95%可信范围:11.2～15.8 μmol·L^-1)。30 μmol·L^-1 BTHP可使I_kr及I_kr,tail分别降低(31±4)%和(36±5)% (N=6,P<0.01)。与多数III类抗心律失常药物不同,BTHP可频率依赖性地抑制I_kr。该药主要改变I_kr的失活过程,可使I_kr的失活时间常数(τ)从(238±16) ms降至(196±14) ms,而对I_kr的激活动力学影响不大。结论BTHP对I_kr有明显的抑制作用,且其阻滞作用呈现频率依赖性特征。     

肉毒碱对兔在体缺血心室肌细胞动作电位的影响

本实验观察了人工合成肉毒碱d,1-carnitine盐酸盐(VBt)对在位兔心急性心肌缺血早期左室肌细胞动作电位的影响。阻断冠脉引起动作电位振幅(APA)和零期最大除极速度(dv/dt)明显降低以及复极50%时程(APD₅₀)和复极90%时程(APD₉₀)明显缩短。VBt有延长心室肌细胞动作电位时程的作用(P<0.01),并能显著减轻缺血心肌细胞的APD₅₀和APD₉₀的缩短程度,但对APA和dv/dt的缺血性变化则无明显改善。     

Self-augmentation of the lengthening of repolarization is related to the shape of the cardiac action potential: implications for reverse rate dependency

Background and purpose:

The aims of the present work were to study the mechanism of the reverse rate dependency of different interventions prolonging cardiac action potential duration (APD).

Experimental approach:

The reverse rate-dependent lengthening effect of APD-prolonging interventions and the possible involvement of I_Kr (rapid component of the delayed rectifier potassium current) and I_K1 (inward rectifier potassium current) were studied by using the standard microelectrode and the whole-cell patch-clamp techniques in dog multicellular ventricular preparations and in myocytes isolated from undiseased human and dog hearts.

Key results:

All applied drugs – dofetilide (1 µmol·L⁻¹), BaCl₂ (10 µmol·L⁻¹), BAY-K-8644 (1 µmol·L⁻¹), veratrine (1 µg·mL⁻¹) – lengthened APD in a reverse rate-dependent manner regardless of their mode of action, suggesting that reverse rate dependency may not represent a specific mechanism of APD prolongation. The E-4031-sensitive current (I_Kr) and the Ba²⁺-sensitive current (I_K1) were recorded during repolarizing voltage ramps having various steepness and also during action potential waveforms with progressively prolonged APD. Gradually delaying repolarization results in smaller magnitude of I_Kr and I_K1 currents at an isochronal phase of the pulses. This represents a positive feedback mechanism, which appears to contribute to the reverse rate-dependent prolongation of action potentials.

Conclusions and implications:

Action potential configuration may influence the reverse rate-dependent APD prolongation due to the intrinsic properties of I_Kr and I_K1 currents. Drugs lengthening repolarization by decreasing repolarizing outward, or increasing depolarizing inward, currents are expected to cause reverse rate-dependent APD lengthening with high probability, regardless of which current they modify.     

牛磺酸镁配合物对乌头碱诱发心律失常大鼠心肌细胞钙通道的影响

目的研究牛磺酸镁配合物(TMCC)对乌头碱诱发心律失常大鼠心肌细胞L型钙电流(I_(Ca,L))的影响。方法酶解法分离大鼠单个心室肌细胞,乌头碱诱导大鼠心律失常模型,胺碘酮作为阳性对照,100、200和400μmol·L~(-1)TMCC作用于心肌细胞。应用全细胞膜片钳技术记录心室肌细胞I_(Ca,L)的变化。结果 400μmol·L~(-1)TMCC显著增加大鼠正常心肌细胞I_(Ca,L),胺碘酮则使之减小。1μmol·L~(-1)乌头碱使I_(Ca,L)电流密度由(4.3±1.6)pA·pF~(-1)增加到(6.4±1.9)pA·pF~(-1)(p<0.01),400μmol·L~(-1)TMCC能显著降低乌头碱诱导的I_(Ca,L)增加。胺碘酮则能使之恢复为(3.7±1.4)pA·pF~(-1)。结论 400μmol·L~(-1)TMCC能增加正常细胞的I_(Ca,L),对钙内流有促进作用,并可减少乌头碱诱导的心律失常大鼠心肌细胞异常增加的电流。     

Inhibition of L-type calcium current by propafenone in single myocytes isolated from the rabbit atrioventricular node

1. The atrioventricular node (AVN) is an important part of the conduction system in the heart and is a significant site of antiarrhythmic drug action. The class 1 antiarrhythmic propafenone is effective in treating a variety of arrhythmias, including those involving the AVN. In this study, we have investigated the effects of propafenone on ionic currents in single rabbit AVN cells, focusing in particular on those on L-type calcium current (I_Ca,L).
2. With a standard K-based internal dialysis solution, exposure to 5 μM propafenone reduced significantly the amplitude of I_Ca,L. In spontaneously active AVN myocytes, action potential upstroke velocity was decreased by propafenone exposure, consistent with the observed change in I_Ca,L.
3. By use of a Cs-based internal dialysis solution to record I_Ca,L selectively, voltage clamp test pulses were applied from a holding potential of −40 mV to +10 mV (stimulation frequency 0.33 Hz). Propafenone 5 μM reduced mean I_Ca,L density at +10 mV from −9.58±1.05 pA/pF to −4.19±0.60 pA/pF (P<0.002). A range of propafenone concentrations were applied which reduced I_Ca,L in a dose-dependent manner (IC₅₀ 1.7 μM). When test pulses were applied to a range of potentials, propafenone reduced I_Ca,L at each potential without significantly affecting the activation curve for this current. Thus, propafenone reduced I_Ca,L conductance, without affecting the voltage-dependent activation properties of the current.
4. I_Ca,L block by propafenone exhibited tonic-, use- and frequency-dependent characteristics.
5. In the presence of propafenone, the voltage-dependence of inactivation of I_Ca,L was shifted 8 mV in the hyperpolarizing direction. Also, the recovery of I_Ca,L from inactivation was slowed by propafenone.
6. The I_Ca,L blocking properties of propafenone may mediate some of the antiarrhythmic properties of this agent, particularly in regions of the heart such as the AVN in which I_Ca,L contributes significantly to the action potential upstroke.

     

L‐type Ca2+ channel opener BayK 8644‐induced Ca2+ influx and Ca2+ release in human oral cancer cells (OC2)

The effect of BayK 8644, a chemical widely used to activate L‐type Ca²⁺ channels, on cytosolic free Ca²⁺ concentrations ([Ca²⁺]_i) in human oral cancer cells (OC2) has not been explored to date. The present study examined whether BayK 8644 altered basal [Ca²⁺]_i levels in suspended OC2 cells by using fura‐2. BayK 8644 (10 pM–10 µM) increased [Ca²⁺]_i in a concentration‐dependent manner. The Ca²⁺ signal was reduced partly by removing extracellular Ca²⁺. BayK 8644‐induced Ca²⁺ influx was blocked by nifedipine, but was not altered by the store‐operated Ca²⁺ entry inhibitors, econazole and SKF96365; protein kinase C modulators phorbol 12‐myristate 13‐acetate (PMA) and GF109203X; the protein kinase A inhibitor H89; and the phospholipase A₂ inhibitor, aristolochic acid. In Ca²⁺‐free medium, after pretreatment with 1 µM BayK 8644, 1 µM thapsigargin (an endoplasmic reticulum Ca²⁺ pump inhibitor)‐induced [Ca²⁺]_i rises were abolished; and conversely, thapsigargin pretreatment abolished BayK 8644‐induced [Ca²⁺]_i rises. Inhibition of phospholipase C with U73122 did not change BayK 8644‐induced [Ca²⁺]_i rises. Collectively, in OC2 cells, BayK 8644 induced [Ca²⁺]_i rises by causing phospholipase C‐independent Ca²⁺ release from the endoplasmic reticulum; and Ca²⁺ influx via L‐type Ca²⁺ channels. Drug Dev Res 69: 2008. © 2008 Wiley‐Liss, Inc.     

Risperidone-induced action potential prolongation is attenuated by increased repolarization reserve due to concomitant block of <Emphasis Type="Italic">I</Emphasis><Subscript>Ca,L</Subscript>

The neuroleptic risperidone is an effective blocker of the rapidly activating component of the delayed rectifier current (I_Kr) and hence is expected to prolong cardiac action potential duration (APD). However, unlike with other typical I_Kr blockers we failed to demonstrate a marked prolongation of late repolarization with risperidone. It is hypothesized that the APD-prolonging effect of risperidone is masked by the high repolarization reserve due to the prominent delayed rectifier currents I_Kr and I_Ks in guinea pig papillary muscle. Action potentials and force of contraction were recorded in isolated guinea pig papillary muscles. L-type calcium current I_Ca,L and I_Kr were measured using the standard patch clamp technique in single ventricular cardiomyocytes. Reduction of the repolarization reserve by the blocking of I_Ks with chromanol 239B augmented the effect of the selective I_Kr blocker E-4031, but not of risperidone, although both drugs completely blocked I_Kr. In contrast to E-4031 risperidone markedly reduced the force of contraction due to the partial blocking of I_Ca,L in the same concentration range as required for block of I_Kr. Reduction of the repolarization reserve by the blocking of I_Ks cannot exacerbate the APD-prolonging effect of risperidone. However, even incomplete concomitant blocking of I_Ca,L attenuates the APD-prolonging effect of the complete blocking of I_Kr. This behaviour may explain the small APD-prolonging effect of risperidone despite the drugs robust blocking of I_Kr.T. Christ and E. Wettwer contributed equally to this work     

Hyposmotic challenge modulates function of L-type calcium channel in rat ventricular myocytes through protein kinase C

Aim:

To study the effects and mechanisms by which hyposmotic challenge modulate function of L-type calcium current (I_Ca,L) in rat ventricular myocytes.

Methods:

The whole-cell patch-clamp techniques were used to record I_Ca,L in rat ventricular myocytes.

Results:

Hyposmotic challenge(∼220 mosmol/L) induced biphasic changes of I_Ca,L, a transient increase followed by a sustained decrease. I_Ca,L increased by 19.1%±6.1% after short exposure (within 3 min) to hyposmotic solution. On the contrary, long hyposmotic challenge (10 min) decreased I_Ca,L to 78.1%±11.0% of control, caused the inactivation of I_Ca,L, and shifted the steady-state inactivation curve of I_Ca,L to the right. The decreased I_Ca,L induced by hyposmotic swelling was reversed by isoproterenol or protein kinase A (PKA) activator foskolin. Hyposmotic swelling also reduced the stimulated I_Ca,L by isoproterenol or foskolin. PKA inhibitor H-89 abolished swelling-induced transient increase of I_Ca,L, but did not affect the swelling-induced sustained decrease of I_Ca,L. NO donor SNAP and protein kinase G (PKG) inhibitor Rp-8-Br-PET-cGMPS did not interfere with swelling-induced biphasic changes of I_Ca,L. Protein kinase C (PKC) activator PMA decreased I_Ca,L and hyposmotic solution with PMA reverted the decreased I_Ca,L by PMA. PKC inhibitor BIM prevented the swelling-induced biphasic changes of I_Ca,L.

Conclusion:

Hyposmotic challenge induced biphasic changes of I_Ca,L, a transient increase followed by a sustained decrease, in rat ventricular myocytes through PKC pathway, but not PKG pathway. PKA system could be responsible for the transient increase of I_Ca,L during short exposure to hyposmotic solution.     

Calcium-antagonizing activity of S-petasin, a hypotensive sesquiterpene from Petasites formosanus, on inotropic and chronotropic responses in isolated rat atria and cardiac myocytes

Petasites formosanus, an indigenous species of Petasites, has been used to treat cardiovascular diseases such as hypertension for years. We have suggested recently that S-petasin, a major sesquiterpene from P. formosanus, inhibits vascular smooth muscle contraction through inhibition of voltage-dependent Ca²⁺ channels, a phenomenon possibly responsible for the hypotensive effect of P. formosanus. This study was designed to examine the chronotropic and inotropic actions of S-petasin in the heart in vivo and in vitro. Administration of S-petasin (0.1–1.5 mg/kg i.v.) in anesthetized rats reduced heart rate dose-dependently. This response was consistent with significant suppression of both contractile amplitude and spontaneous firing rate of isolated atria, responses that were not antagonized by atropine (1 µM). Mechanical evaluation in isolated ventricular myocytes showed that S-petasin (0.1 to 100 µM) depressed peak myocyte contraction and intracellular Ca²⁺ transients concentration-dependently. The duration of myocyte contraction was not affected. Whole-cell voltage clamp analysis revealed that S-petasin inhibited the L-type Ca²⁺ current (I_Ca,L) concentration-dependently and shifted the steady-state inactivation curve of I_Ca,L to more negative potentials. However, a receptor-binding assay failed to identify any significant interaction between S-petasin (0.1–300 µM) and the dihydropyridine binding sites of L-type voltage-dependent Ca²⁺ channels. Taken together, these data show that the negative chronotropic and inotropic properties of S-petasin that can be ascribed mainly to I_Ca,L inhibition, but not to blockade of dihydropyridine binding sites of L-type Ca²⁺ channel or to muscarinic receptor activation.     

Role of action potential shortening in the prevention of arrhythmias in canine cardiac tissue

1. The effects of the K+ channel opener diazoxide and the oxime-containing Ca2+ and K+ channel blocker salicylaldoxime were tested in canine cardiac Purkinje tissue. 2. Both drugs shortened action potential duration (APD). For salicylaldoxime (0.1-1.0 mmol/L), the reductions in APD were statistically significant at the 25% level of repolarization (APD25) for 0.1 mmol/L (P < 0.05, n = 14) and 0.5 and 1.0 mmol/L (P < 0.01, n = 6), at the 50% level of repolarization (APD50) for 0.1 mmol/L (P < 0.05, n = 14) and 0.5 and 1.0 mmol/L (P < 0.01, n = 6) and at the 90% level of repolarization (APD90) for 0.5 and 1.0 mmol/L (P < 0.01, n = 6). In contrast, diazoxide (0.05-0.1 mmol/L) significantly shortened APD at all levels of repolarizations, with the APD50 and APD90 reduced most significantly (P < 0.01, n = 6) for higher concentrations of the drug (0.07-0.1 mmol/L). Both drugs significantly reduced the force of contraction. 3. Diazoxide (10 experiments) was more potent in suppressing strophanthidin-induced arrhythmias than salicylaldoxime (three of seven experiments). Salicylaldoxime reduced APD even further in the presence of diazoxide. 4. Although salicylaldoxime and diazoxide modulate different ion channels, it appears APD shortening may be a necessary, but insufficient, factor for the suppression of strophanthidin-induced arrhythmias.     

Characteristics of L-type calcium channel blockade by lacidipine in guinea-pig ventricular myocytes

1. The Ca²⁺-antagonistic properties of lacidipine were investigated in patch-clamp guinea-pig ventricular myocytes.
2. In basal conditions, 0.1 μM lacidipine reduced the action potential duration, associated with a decrease in the L-type calcium current (I_Ca,L) to 66±4% of the control value, without a change in the current-voltage relationship. Sodium current and background potassium currents were not affected. All the effects reached a steady state within 2 min.
3. The Ca²⁺-antagonistic effect of lacidipine was voltage-dependent: a marked negative shift (about 20 mV) of the steady-state inactivation curve was observed with long (10 s) conditioning prepulses, but not with short (350 ms) prepulses.
4. The onset of and recovery from the voltage-dependent effect caused by 0.1 μM lacidipine were significantly slower when compared to those of equiactive concentrations of nimodipine (0.5 μM) and nisoldipine (0.1 μM). I_Ca,L measured after prepulses at −40 mV lasting 500 ms or less was unchanged (95±5% of maximum current value) while it was reduced to 49±10% by nimodipine and 43±9% by nisoldipine (P<0.05 vs lacidipine for both).
5. Similarly, the recovery from block in the presence of lacidipine was slower than with nimodipine and nisoldipine. After a prepulse of 1 s at −80 mV, I_Ca,L recovered up to 54±2% of the maximum current value in the presence of lacidipine, and up to 91±3% and 93±5% in the presence of nimodipine and nisoldipine, respectively (P<0.05 vs lacidipine).
6. Blockade of I_Ca,L by lacidipine was use-dependent. After ten 200 ms long pulses (1 Hz) from −80 mV, I_Ca,L was reduced to 55±7% of the current measured at the first pulse. In the presence of nimodipine and nisoldipine, I_Ca,L elicited by the tenth pulse amounted to 93±3% and 80±6% of the first pulse value, respectively (P<0.05 vs lacidipine). Lacidipine did not cause use-dependent blockade of I_Ca,L in cells stimulated with 10 ms long pulses.
7. These results demonstrate that lacidipine selectively inhibits I_Ca,L in isolated cardiomyocytes and suggest that this effect occurs mainly through binding to the inactivated Ca²⁺ channels.