Properties of the slow K+ current of the nodal membrane.

1. In voltage clamped myelinated nerve fibres, some properties of the slow K+ current (Dubois, 1981 b) were analysed under different experimental conditions. 2. The instantaneous slow current-voltage relationships are linear in low and high K+ solutions, and the variation of the reversal potential with external K+ concentration agrees with the predicted Nernst changes for the K+ equilibrium potential. 3. Li+, Na+ and Cs+ are virtually impermeant in the slow channels. Rb+ is permeant and the permeability ratio PRb/PK (calculated from the reversal potential and the Goldman-Hodgkin-Katz equation) is a function of the external K+ and Rb+ concentrations. 4. External and internal Cs+ block respectively the slow inward and outward K+ current and the block is voltage dependent. 5. The closing kinetics of the slow channels are independent of the external K+ concentration but depend on the conditioning pulse duration.