Abstract
1. The conductance and kinetics of the Ca2+ activated K+ channels were studied in voltage clampedHelix neurones by using noise and relaxation techniques. 2. The increase of outward current activated by the injection of Ca2+ ions into the cells is associated with an increase of membrane current fluctuations. The spectral density of the K+ current fluctuations decays at high frequency quency with an overall slope of aboutf−1.5. Most of the spectra can be fitted by double Lorentzian curves. The single channel conductance derived from integrated powder spectra is 12–16 pS. 3. Voltage jump experiments show that the gating of the Ca2+ activated K+ current follows first order kinetics provided that the currents are small. The time constant is voltage dependent and increases about e-fold per 85 mV membrane depolarization. Its magnitude isindependent of the previous membrane potential. 4. The instantaneous current-voltage relation of the Ca2+ activated K+ current is non-linear and can be fitted by the constant field relation. The steady-state current-voltage relation exhibits stronger rectification than the instantaneous current-voltage relation and follows the constant field relation with voltage dependent permeability coefficient. The voltage dependence of the permeability coefficient is the same as that of the relaxation time constant. 5. The voltage dependent increase of the K+ conductance together with the voltage dependent increase of the time constant suggests that the effective open time of the ion channel is prolonged by a decrease in the backward rateconstants determining the transition to the closed state, i.e. either the channel life-time or the unbinding rate-constant of Ca2+. The opening rate-constant appears to be voltage independent.
Published Version
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