Three pharmacologically distinct potassium channels in molluscan neurones.

Abstract

1. Potassium currents were studied under voltage‐clamp conditions in nerve cell bodies of the nudibranch Tritonia diomedia. 2. Potassium currents could be separated into three distinct components on the basis of their sensitivity to 4‐aminopyridine (4‐AP), tetraethyl‐ammonium (TEA) and to Co2+ and Mn2+ ions. 3. A transient potassium current, similar to the fast outward current described by Connor & Stevens (1971b) and Neher (1971), was blocked by externally applied 4‐AP but was much less sensitive to TEA or to Co2+ or Mn2+. A single 4‐AP ion binds each receptor with an apparent dissociation constant of 1‐5 X 10(‐3) M. 4‐AP decreases the rates of activation and inactivation and reduces the maximum conductance of transient current channels. 4. Delayed outward current was not effected by 4‐AP at concentrations which blocked the transient current, but it could be divided into two components by external application of TEA and Co2+ or Mn2+. 5. A voltage‐dependent component of delayed current, termed K‐current, was blocked by TEA. Each K‐current receptor binds a single TEA ion with an apparent dissociation constant of 8 X 10(‐3) M. Co2+ and Mn2+ have little or no effect on K‐current. 6. A second component of delayed outward current, termed C‐current, depends on Ca2+ entry for its activation. It is similar to the Ca2+ dependent potassium current reported by Meech & Stranden (1975) in Helix cells. C‐current is essentially blocked by 30 mM external Co2+ or Mn2+. It is little affected by TEA, however, being reduced by about 20% at a TEA concentration of 100 mM. 7. It is concluded that three sets of potassium selective channels contribute to the outward current and that these channels can be separated pharmacologically.