Sjodin R A, Abercrombie R F
J Gen Physiol. 1978 Apr;71(4):453-66. doi: 10.1085/jgp.71.4.453.
In microinjected Myxicola giant axons with elevated [Na]i, Na efflux was sensitive to Cao under some conditions. In Li seawater, sensitivity to Cao was high whereas in Na seawater, sensitivity to Cao was observed only upon elevation of [Ca]o above the normal value. In choline seawater, the sensitivity of Na efflux to Cao was less than that observed in Li seawater whereas Mg seawater failed to support any detectable Cao-sensitive Na efflux. Addition of Na to Li seawater was inhibitory to Cao-sensitive Na efflux, the extent of inhibition increasing with rising values of [Na]o. The presence of 20 mM K in Li seawater resulted in about a threefold increase in the Cao-activated Na efflux. Experiments in which the membrane potential, Vm, was varied or held constant when [K]o was changed showed that the augmentation of Ca-activated Na efflux by Ko was not due to changes in Vm but resulted from a direct action of K on activation by Ca. The same experimental conditions that favored a large component of Cao-activated Na efflux also caused a large increase in Ca influx. Measurements of Ca influx in the presence of 20 mM K and comparison with values of Ca-activated Na efflux suggest that the Na:Ca coupling ratio may be altered by increasing external [K]o. Overall, the results suggest that the Cao-activated Na efflux in Myxicola giant axons requires the presence of an external monovalent cation and that the order of effectiveness at a total monovalent cation concentration of 430 mM is K + Li greater than Li greater than Choline greater than Na.
在微注射的、[Na]i升高的黏液虫巨轴突中，在某些条件下Na外流对Ca₂⁺敏感。在锂海水中，对Ca₂⁺的敏感性较高，而在钠海水中，仅当[Ca]o升高到正常值以上时才观察到对Ca₂⁺的敏感性。在胆碱海水中，Na外流对Ca₂⁺的敏感性低于锂海水中观察到的敏感性，而镁海水则无法支持任何可检测到的对Ca₂⁺敏感的Na外流。向锂海水中添加Na会抑制对Ca₂⁺敏感的Na外流，抑制程度随[Na]o值的升高而增加。锂海水中存在20 mM K会导致Ca₂⁺激活的Na外流增加约三倍。当改变[K]o时改变或保持膜电位Vm的实验表明，K⁺对Ca₂⁺激活的Na外流的增强不是由于Vm的变化，而是由于K⁺对Ca₂⁺激活的直接作用。有利于Ca₂⁺激活的Na外流的大部分的相同实验条件也导致Ca内流大幅增加。在存在20 mM K的情况下测量Ca内流并与Ca₂⁺激活的Na外流值进行比较表明，通过增加外部[K]o可能会改变Na:Ca耦合比。总体而言，结果表明黏液虫巨轴突中Ca₂⁺激活的Na外流需要存在外部单价阳离子，并且在总单价阳离子浓度为430 mM时的有效性顺序为K⁺>Li⁺>胆碱⁺>Na⁺。
