Snyders D J, Hondeghem L M
Department of Medicine, Vanderbilt University Medical School, Nashville, Tennessee 37232-2171.
Circ Res. 1990 Feb;66(2):565-79. doi: 10.1161/01.res.66.2.565.
In guinea pig cardiac myocytes quinidine (20 microM) caused less than 10% tonic block reduction of the sodium current at -120 mV, but a fast pulse train reduced it more than 90%. Recovery from use-dependent block was time and voltage dependent, and was always slow (tau = 34 +/- 10 seconds at -160 mV; tau = 90 +/- 35 seconds at -120 mV; n = 15, mean +/- SD, p less than 0.001, paired t test). However, in association with repeated activation a fast component of recovery from block was observed: use-dependent unblocking. Availability of sodium channels for use-dependent unblocking was enhanced by hyperpolarization until a plateau was reached near -160 mV. Compared with the availability of drug-free sodium channels (h-curve), the voltage dependence of availability for use-dependent unblocking (h'-curve) was shifted by about 30 mV to more negative potentials, and its slope was reduced 2.5-fold. At -160 mV, the kinetics of development of availability of sodium channels for use-dependent unblocking were rapid (tau less than 10 msec). Depolarization to -120 mV reduced the availability of sodium channels for fast unblocking with a time constant of 191 +/- 46 msec (n = 14). Finally, block established by frequent brief depolarizations (activations) declined during prolonged inactivation. From these results we concluded that the time and voltage dependence of the availability of sodium channels for unblocking are considerably different from the availability for activation of drug-free channels, that rested drug-associated channels do exist, and that drug-associated channels do not conduct (or at least have a greatly reduced conductance) upon activation unless they first unblock. Furthermore, activated and inactivated channels have a different affinity for quinidine, and since quinidine can occupy the channel receptor even when "guarded," our results are incompatible with the guarded receptor hypothesis but can be explained within the framework of the modulated receptor hypothesis.
在豚鼠心肌细胞中，奎尼丁（20微摩尔）在-120毫伏时使钠电流的强直阻滞降低不到10%，但快速脉冲串使其降低超过90%。从使用依赖性阻滞中恢复是时间和电压依赖性的，并且总是很慢（在-160毫伏时τ = 34 ± 10秒；在-120毫伏时τ = 90 ± 35秒；n = 15，平均值 ± 标准差，p < 0.001，配对t检验）。然而，与重复激活相关联时，观察到了从阻滞中恢复的快速成分：使用依赖性解阻。超极化增强了用于使用依赖性解阻的钠通道的可用性，直到在接近-160毫伏时达到平台期。与无药物钠通道的可用性（h曲线）相比，用于使用依赖性解阻的可用性的电压依赖性（h'曲线）向更负的电位偏移了约30毫伏，并且其斜率降低了2.5倍。在-160毫伏时，用于使用依赖性解阻的钠通道可用性发展的动力学很快（τ < 10毫秒）。去极化到-120毫伏以191 ± 46毫秒的时间常数降低了用于快速解阻的钠通道的可用性（n = 14）。最后，由频繁短暂去极化（激活）建立的阻滞在长时间失活期间下降。从这些结果我们得出结论，用于解阻的钠通道可用性的时间和电压依赖性与无药物通道激活的可用性有很大不同，确实存在静止的与药物相关的通道，并且与药物相关的通道在激活时不传导（或至少电导大大降低），除非它们首先解阻。此外，激活的和失活的通道对奎尼丁有不同的亲和力，并且由于即使在“受保护”时奎尼丁也能占据通道受体，我们的结果与受保护受体假说不相符，但可以在调制受体假说的框架内得到解释。
