Three pharmacologically distinct potassium channels in molluscan neurones.

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Abstract
K+-currents were studied under voltage-clamp conditions in nerve cell bodies of the nudibranch Tritonia diomedia. K+-currents could be separated into 3 distinct components on the basis of their sensitivity to 4-aminopyridine (4-AP), tetraethylammonium (TEA) and to Co2+ and Mn2+. A transient K-current, similar to the fast outward current described by Connor and Stevens (1971) and Neher (1971), was blocked by externally applied 4-AP but was much less sensitive to TEA or to Co2+ of Mn2+. A single 4-AP ion binds each receptor with an apparent dissociation constant of 1.5 .times. 10-3 M. 4-AP decreased the activation and inactivation rates and reduced the maximum conductance of transient current channels. Delayed outward current was not effected by 4-AP at concentrations which blocked the transient current, but it could be divided into 2 components by external application of TEA and Co2+ or Mn2+. A voltage-dependent component of delayed current, termed K-current, was blocked by TEA. Each K-current receptor bound a single TEA+ with an apparent dissociation constant of 8 .times. 10-3 M. Co2+ and Mn2+ had little or no effect on K-current. A 2nd component of delayed outward current, termed C-current, depended on Ca2+ entry for its activation. It was similar to the Ca2+ dependent K+-current reported by Meech and Standen (1975) in Helix cells. C-current was essentially blocked by 30 mM external Co2+ or Mn2+. It was little affected by TEA, being reduced by .apprx. 20% at a TEA concentration of 100 mM. Three sets of K+ selective channels contribute to the outward current and these channels can be separated pharmacologically.