Harrison S M
Department of Physiology, University of Leeds, UK.
Exp Physiol. 1995 Nov;80(6):941-58. doi: 10.1113/expphysiol.1995.sp003906.
Ventricular myocytes, isolated from the guinea-pig, were stimulated to contract by 100 ms long voltage clamp pulses from -80 to 0 mV at 0.5 and 3 Hz. An increase in frequency from 0.5 to 3 Hz led to a positive inotropic effect. Contraction-voltage relationships (CVR) were determined at each frequency. The CVR at 0.5 Hz was bell shaped and peaked between 0 and +20 mV, displaying a voltage dependence similar to the L-type Ca2+ current (ICa). At 3 Hz, contractions continued to increase at positive voltages, giving a more sigmoidal CVR. At 0.5 Hz, TTX reduced the size of steady-state contractions to 91 +/- 2% of control values, but had no effect on the shape of the CVR. At 3 Hz, TTX significantly reduced (P < 0.05) the magnitude of contractions at positive voltages (> or = +20 mV) but had no significant effect on contractions at voltages negative to 0 mV. These data illustrate that intracellular sodium activity (aNa(i)) and, in particular, Na+ entry due to the sodium current (INa) are important in determining the voltage dependence of contraction at positive voltages. Thapsigargin (2.5 microM), a blocker of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase, reduced the size of steady-state contractions at 0 mV to 65 +/- 7% at 0.5 Hz. Increasing frequency to 3 Hz abolished the positive inotropy seen under control conditions. With thapsigargin present, contractions at 0.5 Hz were reduced at all potentials and the CVR was bell shaped. At 3 Hz the CVR was sigmoidal in shape. Contractions were significantly inhibited by thapsigargin at all potentials, but most significantly at more positive potentials (> or = +20 mV). These data show that, at normal body temperature, the shape of the CVR of guinea-pig ventricular myocytes changes with stimulation rate. Due to the voltage dependence of ICa, contractions evoked at positive voltages at 3 Hz must be supported by other mechanisms. The sensitivity of such contractions to TTX and thapsigargin suggests the involvement of both a Na(+)-dependent process and the SR. One possibility is that when aiNa and the Ca2+ content of the SR are raised at higher stimulation rates, enhanced Ca2+ entry via reverse Na(+)-Ca2+ exchange leads to a direct activation of the myofilaments and, to a lesser extent, the release of Ca2+ from the SR.
从豚鼠分离出的心室肌细胞，通过在0.5和3 Hz频率下从-80 mV至0 mV的100 ms长电压钳脉冲刺激使其收缩。频率从0.5 Hz增加到3 Hz会导致正性肌力作用增强。在每个频率下测定收缩-电压关系（CVR）。0.5 Hz时的CVR呈钟形，在0至+20 mV之间达到峰值，显示出与L型钙电流（ICa）相似的电压依赖性。在3 Hz时，正电压下的收缩持续增加，产生更呈S形的CVR。在0.5 Hz时，河豚毒素（TTX）将稳态收缩幅度降低至对照值的91±2%，但对CVR的形状没有影响。在3 Hz时，TTX显著降低（P<0.05）正电压（≥+20 mV）时的收缩幅度，但对负于0 mV电压时的收缩没有显著影响。这些数据表明，细胞内钠活性（aNa(i)），尤其是由于钠电流（INa）引起的Na+内流，在确定正电压下收缩的电压依赖性方面很重要。毒胡萝卜素（2.5 microM），一种肌浆网（SR）Ca(2+)-ATP酶的阻滞剂，在0.5 Hz时将0 mV处的稳态收缩幅度降低至65±7%。将频率增加到3 Hz消除了对照条件下观察到的正性肌力作用。存在毒胡萝卜素时，0.5 Hz时所有电位下的收缩均降低，CVR呈钟形。在3 Hz时，CVR呈S形。毒胡萝卜素在所有电位下均显著抑制收缩，但在更正电位（≥+20 mV）时最为显著。这些数据表明，在正常体温下，豚鼠心室肌细胞的CVR形状随刺激频率而变化。由于ICa的电压依赖性，3 Hz时正电压诱发的收缩必须由其他机制支持。这种收缩对TTX和毒胡萝卜素的敏感性表明涉及一个依赖Na+的过程和SR。一种可能性是，当在较高刺激频率下aNa和SR中的Ca2+含量升高时，通过反向Na(+)-Ca2+交换增强的Ca2+内流导致肌丝的直接激活，并在较小程度上导致Ca2+从SR释放。
