The K conductance (gK) kinetics were studied in voltage-clamped frog nodes (Rana ridibunda) in double-pulse experiments. The Cole-Moore translation for gK--t curves associated with different initial potentials (E) was only observed with a small percentage of fibers. The absence of the translation was found to be caused by the involvement of an additional, slow, gK component. This component cannot be attributed to a multiple-state performance of the k channel. It can only be accounted for by a separate, slow K channel, the fast channel being the same as the n4 K channel in R. pipiens. The slow K channel is characterized by weaker sensitivity to TEA, smaller density, weaker potential (E) dependence, and somewhat more negative E range of activation than the fast K channel. According to characteristics of the slow K system, three types of fibers were found. In Type I fibers (most numerous) the slow K channel behaves as and n4 HH channel. In Type II fibers (the second largest group found) the slow K channel obeys the HH kinetics within a certain E range only; beyond this range the exponential decline of the slow gK component is preceded by an E-dependent delay, its kinetics after the delay being the same as those in Type I fibers. In Type III fibers (rare) the slow K channel is lacking, and it is only in these fibers that the Cole-Moore translation of the measured gK--t curves can be observed directly. The physiological role of the fast and slow K channel in amphibian nerves is briefly discussed.

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