Non‐stationary fluctuations of the potassium conductance at the node of ranvier of the frog.

Abstract

K currents were recorded from voltage-clamped nodes of isolated, myelinated axons of R. pipiens. Nodes were maintained in a modified Ringer solution containing tetrodotoxin to block Na current and 47.5 mM-K to minimize effects of extracellular K accumulation. Voltage protocols included depolarizing pulses lasting a few milliseconds to several seconds. Fluctuations about the ensemble average of the current were characterized in terms of non-stationary variance and autocovariance. The fluctuations had a Gaussian amplitude distribution and were virtually free of contaminations from systematic variations of the membrane current. Corrections for background noise were based on measurements done while K current was blocked with tetraethylammonium, and on simulations of extrinsic current fluctuations expected to arise from noise in the actual membrane voltage. The fluctuations were attributed to variations of nodal K conductance, since they were absent at the reversal potential of the K current and at membrane voltages that do not activate K current. Covariances indicated that voltage steps that reversed a macroscopic K current also reversed the sign of the fluctuation. Plots of the conductance variance vs. the mean K conductance were generated from both the activation and deactivation (tail) phases of the K currents at various voltages between -80 and +70 mV. When the current was activated by a small depolarization (-50 mV) the trajectories from both phases were indistinguishable and were fitted by the parabola expected for a single population of channels with only 1 open-channel conductance. Apparent single-channel conductance from the early activation phase averaged 24 pS [picoSiemens] and was not significantly voltage dependent. Experiments with large depolarizations (+10 to +70 mV) gave significantly different variance-mean trajectories during activation and deactivation and these trajectories were poorly fitted by parabolae. The fluctuations reflect several populations of channels and/or a population of channels that can have several levels of non-zero conductance. Projections of the fluctuation covariance showed long correlations, as well as the rapidly decaying component expected from the activation gating of channels. A slow fluctuation arose at time slightly later than the rise of K current, spanned the entire length of brief depolarizations, and extended up to 880 ms during long depolarizations. The early onset of this slow fluctuation indicates it is not due to slow turn-on of a population of slow K channels. Rather it is consistent with presence of a slow modulatory process that alters the activity of rapidly activating K channels. Experiments that activated large K conductances revealed a negative correlation between the conductance fluctuations during the onset of the current and those during the stationary tail phases of the current. The negative correlation requires that channels can exist in at least 2 conducting forms and that these forms interconvert slowly during the 2-5 s intervals between successive recordings. A model was conceived incorporating 2 interconverting forms of K channel. One form can be activated to carry a steady K current and has a single-channel conductance of 13 pS. The 2nd form (represented on average by 20 out of 100 channels) is activated at a rate similar to that of the 1st form, but then rapidly inactivates; its open conductance is higher (60 pS).