Ionic interactions in the potassium channel of squid giant axons

Abstract

ONE of the simplest questions that can be asked about how ions cross biological membranes is whether or not they do so independently of one another. That is, is the probability of movement of an ion unaffected by the presence of other ions? In particular, with independent movement, the efflux of an ion should not be a function of the external concentration of that ion. Using long-lasting depolarisations, Hodgkin and Keynes demonstrated¹ non-independent movement of K⁺ ions across the membrane of Sepia axons. Since the Na channel responsible for initiation of the action potential inactivates very rapidly on depolarisation, potassium ions were probably not moving through this pathway—even though the channel is permeable to them as well as to Na⁺ ions. But since the K channels (partly responsible for termination of the action potential) also inactivate on prolonged depolarisation², the K efflux measured by Hodgkin and Keynes was presumably mainly through inactivated K channels and the so-called ‘leakage pathway’. We report here on experiments that demonstrate non-independent ion movement through the activated K channel of the giant axon of Loligo pealei, using tetrodotoxin to block the Na channels, and short-step depolarisations to minimise K-channel inactivation.