Voltage-dependent potassium channels in the amphibian lens membranes: Evidence from radiotracer and electrical conductance measurements

Abstract

The electrical potential and conductance of the perfused frog lens preparation was measured using a two internal microelectrode technique. When the lens potential was depolarized by perfusing with high potassium concentration ringer the increase in conductance could not be fitted by conductance equations where P K was assumed constant. It is suggested therefore that this increase is due to a voltage-dependent component of the conductance, namely voltage-dependent potassium channels. Direct evidence for the existence of a voltage-dependent conductance is presented in the form of conductance-voltage curves which show rectification similar to squid axon. Conductance-voltage curves mapped out in high potassium Ringer show a blocking effect of potassium on the lens conductance. The blocking effect was also observed in lenses where the membrane voltage was clamped to a steady value during exposure to high potassium solutions. The increase in conductance at depolarized lens potentials is demonstrated by changes in potassium permeability as measured from the efflux of 42 K. Both the electrical conductance and 42 K efflux are shown to be sensitive to potassium channel blocking agents, namely, tetraethylammonium (TEA), caesium and rubidium ions. The 42 K efflux rate constant and lens potential were monitored simultaneously whilst depolarizing and hyperpolarizing currents were applied and the changes in 42 K efflux clearly demonstrated the voltage-dependence of the potassium permeability.