Mackiewicz U, Emanuel K, Lewartowski B
Department of Clinical Physiology, Medical Center of Postgraduate Education, Warsaw, Poland.
J Physiol Pharmacol. 2000 Dec;51(4 Pt 2):777-98.
Isolated, superfused with [Ca2+]0 = 2.5-3 mM (T = 37 degrees C) and voltage clamped ventricular myocytes of guinea-pig hearts were stimulated by pulses from a holding potential -40 mV to +5 mV (duration 300 ms). They activated L-type Ca2+ current and a biphasic contractile response: a phasic component of amplitude approximately 7% of resting cell length (duration approximately 150 ms) and a tonic component of amplitude approximately 3% of resting cell length. The phasic component was inhibited by 10(-6) M thapsigargin (Tg). Pulses from -40 mV to +5 mV stimulated a similar bi-phasic contractile response in 74% of cells (n = 126) superfused from the beginning of a 30 s period of rest with 5-10 mM Ni2+ which blocked the Ca2+ current and Na+ -Ca2+ exchanger (Ni2+-contractions). Thus, the Ni2+-contractions could be activated only by intracellular Ca2+ release. The phasic component of control contractions showed the bell-shaped voltage relation at [Ca]0 = 2 mM and sigmoid relation at [Ca]0 = 3 mM. The phasic component o Ni2+-contractions showed a sigmoid relation at voltages from -40mV to +100 mV and could not be activated at [Ca]0 = 2 mM. It was inhibited by 20 microM nifedipine, a blocker of dihydropyridine receptors, even when activated by the pulses to +70 mV, during which the Ca2+ current does not flow. We proved that nifedipine does not affect Na+-Ca2+ exchange. The phasic component of Ni2+-contractions was also inhibited by 2 nM indolizinesulphone SR33557, another dihydropyridine receptor blocker, which halved the phasic component of contractions in sontrol cells without any significant effect on the Ca2+ current. Stimulation did not activate contraction in any of 19 cells in which 20 microM nifedipine was superfused from the beginning of 30 s rest instead of 5 mM Ni2+. These cells were depolarized to +5 mV over the rest period in order to prevent intracellular Ca2+ loss by Na+ -Ca2+ exchange. Residual Ca2+ currents were much stronger in cells superfused with nifedipine than residual currents in cells superfused with Ni2+ (hardly visible in the records). Our results suggest that a vestigial remnant of a voltage-sensing mechanism similar to that in the skeletal muscle may trigger the Ca2+ release from the SR of cardiac myocytes under specific experimental conditions. In normal cells it may be complementary to calcium induced calcium release (CICR).
分离出的豚鼠心脏心室肌细胞，在[Ca2+]0 = 2.5 - 3 mM（T = 37摄氏度）的条件下进行灌流，并通过电压钳制，从-40 mV的钳制电位施加脉冲至+5 mV（持续时间300 ms）进行刺激。它们激活了L型Ca2+电流和双相收缩反应：一个相位成分，幅度约为静息细胞长度的7%（持续时间约150 ms），以及一个张力成分，幅度约为静息细胞长度的3%。相位成分受到10(-6) M毒胡萝卜素（Tg）的抑制。从30秒休息期开始就用5 - 10 mM Ni2+灌流的细胞中，74%（n = 126）的细胞在从-40 mV到+5 mV的脉冲刺激下表现出类似的双相收缩反应，Ni2+阻断了Ca2+电流和Na+-Ca2+交换器（Ni2+收缩）。因此，Ni2+收缩只能由细胞内Ca2+释放激活。对照收缩的相位成分在[Ca]0 = 2 mM时呈现钟形电压关系，在[Ca]0 = 3 mM时呈现S形关系。Ni2+收缩的相位成分在-40mV至+100 mV的电压下呈现S形关系，在[Ca]0 = 2 mM时无法激活。即使在被脉冲刺激至+70 mV（此时Ca2+电流不流动）激活时，它也受到20 microM硝苯地平（一种二氢吡啶受体阻滞剂）的抑制。我们证明硝苯地平不影响Na+-Ca2+交换。Ni2+收缩的相位成分也受到2 nM吲哚嗪磺酸盐SR33557（另一种二氢吡啶受体阻滞剂）的抑制，它使对照细胞中收缩的相位成分减半，而对Ca2+电流没有任何显著影响。在30秒休息开始时用20 microM硝苯地平而非5 mM Ni2+灌流的19个细胞中，任何刺激都未激活收缩。在休息期间，这些细胞被去极化至+5 mV，以防止通过Na+-Ca2+交换导致细胞内Ca2+流失。与用Ni2+灌流的细胞中的残余电流相比（记录中几乎不可见），用硝苯地平灌流的细胞中的残余Ca2+电流要强得多。我们的结果表明，在特定实验条件下，类似于骨骼肌中的电压传感机制的残余部分可能触发心肌细胞肌浆网的Ca2+释放。在正常细胞中，它可能是钙诱导钙释放（CICR）的补充。
