Spiegler P A, Vassalle M
Department of Physiology, State University of New York, Brooklyn 11203, USA.
Can J Physiol Pharmacol. 1995 Aug;73(8):1165-80. doi: 10.1139/y95-166.
The role of oscillatory potentials occurring near the threshold for the fast sodium current (ThVos) in the induction of spontaneous and repetitive activity was studied in sheep Purkinje fibers superfused in vitro. In low extracellular potassium concentration, the steepness and amplitude of diastolic depolarization increased and ThVos appeared during quiescence. ThVos amplitude increased progressively until its depolarizing phase reached the threshold potential for the initiation of the action potential. Drive increased the amplitude of diastolic depolarization and of ThVos, and longer drives induced faster and longer-lasting repetitive activity ("overdrive excitation"). In quiescent fibers, barium depolarized the resting membrane and initiated spontaneous discharge through ThVos. Acetylcholine had similar actions. Cesium hyperpolarized the membrane, thereby suppressing ThVos and related spontaneous activity. Tetrodotoxin and lidocaine also suppressed ThVos, but not the driven action potentials. In low extracellular potassium plus high extracellular calcium concentrations, drive induced ThVos as well as the oscillatory potentials related to calcium overload (Vos), but caused overdrive excitation through ThVos, even when caffeine was present. We conclude from our results that in Purkinje "dominant" pacemaker fibers (i) diastolic depolarization initiates spontaneous activity by attaining the threshold for the upstroke of the action potential through the depolarizing phase of a ThVos; (ii) the depolarizing phase of ThVos is caused by a tetrodotoxin-sensitive Na+ component; (iii) ThVos is voltage dependent in that a small depolarization of the resting membrane induces it and a small hyperpolarization suppresses it; (iv) ThVos can induce overdrive excitation; and (v) ThVos occurs in the absence of calcium overload and has distinguishing characteristics from the Vos induced by calcium overload.
在体外灌流的绵羊浦肯野纤维中，研究了快速钠电流阈值附近出现的振荡电位（ThVos）在诱发自发和重复活动中的作用。在低细胞外钾浓度时，舒张期去极化的斜率和幅度增加，并且在静息期出现ThVos。ThVos幅度逐渐增加，直到其去极化阶段达到动作电位起始的阈值电位。驱动增加了舒张期去极化和ThVos的幅度，更长时间的驱动诱导更快且更持久的重复活动（“超速兴奋”）。在静息纤维中，钡使静息膜去极化并通过ThVos引发自发放电。乙酰胆碱有类似作用。铯使膜超极化，从而抑制ThVos和相关的自发活动。河豚毒素和利多卡因也抑制ThVos，但不抑制驱动的动作电位。在低细胞外钾加高细胞外钙浓度时，驱动诱导ThVos以及与钙超载相关的振荡电位（Vos），但即使存在咖啡因，也通过ThVos引起超速兴奋。我们从结果中得出结论，在浦肯野“主导”起搏纤维中：（i）舒张期去极化通过ThVos的去极化阶段达到动作电位上升支的阈值来引发自发活动；（ii）ThVos的去极化阶段由河豚毒素敏感的Na +成分引起；（iii）ThVos是电压依赖性的，即静息膜的小去极化诱导它，小超极化抑制它；（iv）ThVos可诱导超速兴奋；（v）ThVos在没有钙超载的情况下出现，并且与钙超载诱导的Vos具有不同的特征。
