Posterino G S, Lamb G D
School of Zoology, La Trobe University, Bundoora, Victoria, Australia.
J Muscle Res Cell Motil. 1998 Jan;19(1):53-65. doi: 10.1007/BF03257390.
The effect of the dihydropyridine, nifedipine, on excitation-contraction coupling was compared in toad and rat skeletal muscle, using the mechanically skinned fibre technique, in order to understand better the apparently disparate results of previous studies and to examine recent proposals on the importance of certain intracellular factors in determining the efficacy of dihydropyridines. In twitch fibres from the iliofibularis muscle of the toad, 10 microM nifedipine completely inhibited depolarization-induced force responses within 30 s, without interfering with direct activation of the Ca(2+)-release channels by caffeine application or reduction of myoplasmic [Mg2+]. At low concentrations of nifedipine, inhibition was considerably augmented by repeated depolarizations, with half-maximal inhibition occurring at < 0.1 microM nifedipine. In contrast, in rat extensor digitorum longus (EDL) fibres 1 microM nifedipine had virtually no effect on depolarization-induced force responses, and 10 microM nifedipine caused only approximately 25% reduction in the responses, even upon repeated depolarizations. In rat fibres, 10 microM nifedipine shifted the steady-state force inactivation curve to more negative potentials by < 11 mV, whereas in toad fibres the potent inhibitory effect of nifedipine indicated a much larger shift. The inhibitory effect of nifedipine in rat fibres was little, if at all, increased by the absence of Ca2+ in the transverse tubular (t-) system, provided that the Ca2+ was replaced with sufficient Mg2+. The presence of the reducing agents dithiothreitol (10 mM) or glutathione (10 mM) in the solution bathing a toad skinned fibre did not reduce the inhibitory effect of nifedipine, suggesting that the potency of nifedipine in toad skinned fibres was not due to the washout of intracellular reducing agents. The results are considered in terms of a model that can account for the markedly different effects of nifedipine on the two putative functions of the dihydropyridine receptor, as both t-system calcium channel and a voltage-sensor controlling Ca2+ release.
使用机械去膜纤维技术，比较了二氢吡啶硝苯地平对蟾蜍和大鼠骨骼肌兴奋 - 收缩偶联的影响，以便更好地理解先前研究中明显不同的结果，并检验最近关于某些细胞内因子在决定二氢吡啶类药物疗效方面重要性的提议。在蟾蜍髂腓肌的单收缩纤维中，10微摩尔硝苯地平在30秒内完全抑制了去极化诱导的力反应，而不干扰咖啡因应用对钙释放通道的直接激活或肌浆[Mg2 +]的降低。在低浓度硝苯地平下，重复去极化可显著增强抑制作用，在<0.1微摩尔硝苯地平处出现半数最大抑制。相比之下，在大鼠趾长伸肌（EDL）纤维中，1微摩尔硝苯地平对去极化诱导的力反应几乎没有影响，即使重复去极化，10微摩尔硝苯地平也仅使反应降低约25％。在大鼠纤维中，10微摩尔硝苯地平使稳态力失活曲线向更负的电位移动不到11毫伏，而在蟾蜍纤维中，硝苯地平的强效抑制作用表明移动幅度更大。如果横向管（t -）系统中没有Ca2 +，但用足够的Mg2 +替代，硝苯地平对大鼠纤维的抑制作用几乎没有增加。在浸泡蟾蜍去膜纤维的溶液中存在还原剂二硫苏糖醇（10毫摩尔）或谷胱甘肽（10毫摩尔）不会降低硝苯地平的抑制作用，这表明硝苯地平在蟾蜍去膜纤维中的效力不是由于细胞内还原剂的洗脱。根据一个模型考虑了这些结果，该模型可以解释硝苯地平对二氢吡啶受体的两种假定功能（作为t -系统钙通道和控制Ca2 +释放的电压传感器）的明显不同影响。
