Maeno T, Enomoto K
Department of Physiology, Shimane Medical University, Izumo, Japan.
Jpn J Physiol. 1995;45(5):811-21. doi: 10.2170/jjphysiol.45.811.
A dual action of ouabain on the Mg(2+)-blocked frog neuromuscular transmission was studied in two different experimental procedures by measuring the mean quantal content (m) of endplate potentials evoked during repetitive nerve stimulation. The nontoxic action of low-dose (1 microM) ouabain was observed as a counterclockwise pivoting of the linear stimulation frequency versus the log m relation, termed frequency augmentation-potentiation (FAP), whereas the toxic effect of higher doses (> or = 10 microM) of ouabain was recorded as a slow exponential increase in m. Since neural Na+,K(+)-ATPase consists of two or three isoforms, the nontoxic action of low-dose ouabain was anticipated to be a consequence of the selective inhibition of ouabain-sensitive alpha(+) (alpha 2 and alpha 3 or neural type) Na+,K(+)-ATPase activity. Such a possibility was assessed by applying epsilon-maleimidocaproic acid (MCA), a membrane-impermeant sulfhydryl reagent. MCA 15 microM was found in the present study to suppress the nontoxic effect of ouabain without altering the toxic effect. The results strongly suggest that the toxic and nontoxic actions of ouabain are the result of actions on different entities. The ouabain-sensitive neural type Na+,K(+)-ATPase, which is abundant in neural tissues, seems to play an important role in modulation of transmitter release observable as FAP.
通过测量重复神经刺激期间诱发的终板电位的平均量子含量（m），在两种不同的实验程序中研究了哇巴因对Mg(2+)阻断的青蛙神经肌肉传递的双重作用。低剂量（1 microM）哇巴因的无毒作用表现为线性刺激频率与对数m关系的逆时针旋转，称为频率增强-增效（FAP），而高剂量（≥10 microM）哇巴因的毒性作用记录为m的缓慢指数增加。由于神经Na+,K(+)-ATP酶由两种或三种同工型组成，低剂量哇巴因的无毒作用预计是对哇巴因敏感的α(+)（α2和α3或神经型）Na+,K(+)-ATP酶活性选择性抑制的结果。通过应用ε-马来酰亚胺己酸（MCA），一种膜不透性巯基试剂，评估了这种可能性。在本研究中发现15 microM的MCA可抑制哇巴因的无毒作用而不改变其毒性作用。结果强烈表明，哇巴因的毒性和无毒作用是对不同实体作用的结果。在神经组织中丰富的哇巴因敏感神经型Na+,K(+)-ATP酶似乎在可观察为FAP的递质释放调节中起重要作用。
