Dynamic Asymmetries in the Squid Axon Membrane

In 1952 Hodgkin and Huxley were able to separate the m e m b r a n e currents which occur in response to step changes in membrane potential, upon voltage c lamping the squid giant axon, into two major current components : an initial transient component mainly carried under normal conditions by sodium ions and a delayed component mainly carried by potassium ions. Tetrodotoxin ( T T X ) specifically blocks the initial current mechanism when applied externally (Narahashi et al., 1964; N a k a m u r a et al., 1965). However, internal application of T T X at the same concent ra t ion does not block the sodium current. At higher internal concentrations, it becomes effective (Narahashi et al., 1966). O n the other hand, in the squid axon, te t rae thylammonium ion (TEA) blocks the delayed current when applied internally (Tasaki and Hagiwara, 1957; Armstrong and Binstock, 1965; Armstrong, 1966). Whereas cesium ion is able to block the delayed outward steady-state current when applied internally (Chandler and Meres, 1965; Adelman and Senft, 1966), cesium ion has no effect on this current when applied externally (Pickard et al., 1964). T h e object of this paper is to examine in some detail interactions among the five readily available group IA metal cations in contributing to a n d / o r inhibiting membrane conductances when the concentrations of these ions are varied externally.