Noise and relaxation measurements of the Ca²⁺ activated K⁺ current in Helix neurones

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.