Induction and removal of inward‐going rectification in sheep cardiac Purkinje fibres

Abstract

In sheep cardiac Purkinje fibers superfused with K-free, Na-free medium, the membrane potential can be stable either at a low negative level (-50 mV) or at a high negative level (-100 mV). The mechanism underlying the existence of these 2 stable potential levels was investigated using the 2-microelectrode voltage-clamp technique. By applying a voltage clamp of a certain duration at an appropriate level the membrane potential could be shifted from one stable level to the other. The shift was observed in Cl-free medium, excluding a redistribution of Cl as a possible explanation. Currents during and following a voltage step and their change with amplitude and duration of the voltage step could not be explained on the basis of depletion or accumulation of K ions in the narrow extracellular clefts. Instantaneous currents determined from the high negative resting level showed a high conductance and a pronounced inward rectification, while measurements from the low negative resting level indicated a low conductance and absence of inward rectification. The steady-state current-voltage relation was dependent on the holding potential and showed memory or hysteresis. Estimation of the conductance by superimposed short voltage-clamp pulses showed an increase in conductance during a hyperpolarizing clamp from the low negative level and a decrease in conductance during a depolarizing clamp from the high negative level. The time-dependent current during a hyperpolarizing clamp from the low negative level reversed direction at a potential level corresponding to EK, assuming a cleft K concentration of about 1 mM. In the presence of 0.1 mM Ba the time-dependent current was abolished. The results suggest that the shift between the 2 stable levels is due to a time-dependent conductance change in the K inward rectifier channel iK1. The existence of memory excludes activation or de-activation only depending on the voltage gradient. Interaction of extracellular K ions with a site in the membrane is proposed as the activating mechanism.