Ion conductance and ion selectivity of potassium channels in snail neurones

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Abstract
Delayed potassium channels were studied in internally perfused neurone somata from land snails. Relaxation and fluctuation analysis of this class of ion channels revealed Hodgkin-Huxley type K channels with an average single channel conductance (γ K) of 2.40±0.15 pS. The conductance of open channels is independent of voltage and virtually all K channels seem to be open at maximum K conductance (g K) of the membrane. Voltage dependent time constants of activation ofg K, calculated from K current relaxation and from cut-off frequencies of power spectra, are very similar indicating dominant first-order kinetics. Ion selectivity of K channels was studied by ion substitution in the external medium and exhibited the following sequence: T1+>K+>Rb+>Cs+>NH 4 + >Li+>Na+. The sequence of the alkali cations does not conform to any of the sequences predicted by Eisenman's theory. However, the data are well accommodated by a new theory assuming a single rate-limiting barrier that governs ion movement through the channel.

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