title: Ion conductance and ion selectivity of potassium channels in snail neurones.
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 (gamma 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 (gk) of the membrane. Voltage dependent time constants of activation of gK, 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: Tl+ greater than K+ greater than Rb+ greater Cs+ greater than NH4+ greater Li+ greater than 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.
mesh_terms: D000818:Animals; D002414:Cations, Monovalent / Q000378:metabolism; D004558:Electric Stimulation; D005071:Evoked Potentials; D006372:Helix, Snails; D007473:Ion Channels / Q000502:physiology*; D008433:Mathematics; D009431:Neural Conduction; D009474:Neurons / Q000502:physiology*; D011188:Potassium / Q000502:physiology*
chemical_list: D002414:Cations, Monovalent; D007473:Ion Channels; D011188:Potassium