Conditioning prepulses and kinetics of potassium conductance in the frog node

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.