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The single K+-channel conductance was calculated from the variance of the spontaneous potassium noise currents in voltage clamped frog node. Essential for this calculation is the mean potassium conductance during the noise measurement. So far this quantity has been underestimated, apparently due to K+-ion accumulation. With the proper values, the single K+-channel conductance is an increasing function of membrane voltage.

The kinetics of potassium conductance were analyzed in response to voltage-clamp steps with holding potential (-75 mV) as initial condition and after a positive prepulse towards +45 mV of 10-msec duration. As the potassium reversal potential EK altered during potassium current flow, a method to obtain the conductance independent of EK was used. Conductance kinetics at 15 degrees C were analyzed according to the Hodgkin-Huxley (HH) model. The time constant of potassium activation, with holding potential as initial condition, is a monotonous decreasing function of membrane potential. Its value of ca. 9 msec at -50 mV decreases to 1 msec at +30 mV. Changes in EK did not affect the voltage dependency of this time constant. The time constant of potassium deactivation, i.e. the off-response following a 10-msec prepulse towards +45 mV, shows a completely different voltage dependency. At a membrane potential of -90 mV it is approximately 2 msec and gradually increases for more positive voltages towards a maximum value of about 6 msec, that is reached between -5 and 0 mV. At still larger values of membrane voltage this time constant starts to fall again. It is concluded that a HH-model, as applied for a single population of potassium channels, has to be rejected. Computer simulations indicate that an extension to two populations of independent potassium channels, each with HH-kinetics, is also inconsistent with the observed results.