MODULATION OF DELAYED RECTIFIER K+ CHANNEL ACTIVITY BY EXTERNAL K+ IONS IN XENOPUS AXON

The effect of external K+ ions upon the activation of delayed rectifie r K+ channels was studied in demyelinated amphibian nerve fibres by me ans of the patch-clamp technique. In external 105 mM K+ solution (high -K-o) macroscopic K+ currents activated at more negative potentials (a pproximate to -15 mV) than in external Ringer (2.5 mM K+). Since the r apid substitution of external Ringer with high-K-o solution at holding potentials of -70 mV and -60 mV directly activated K+ currents, it is concluded that K+ ions interact with a binding site outside the chann el that facilitates channel activation. At - 70 mV an increase in exte rnal K+ concentration from 5 mM to 10, 20, 50 and 105 mM gradually inc reased the open probability of the channels. Although Rb+ ions were le ss permeant through the channels, they were more potent in their inter action with the binding site and shifted K+ channel activation to more negative potentials. In contrast, external Cs+ ions had only a weak e ffect on the binding site. Thus, external K+ ions at physiological con centrations modulate the activation of delayed rectifier K+ channels a t potentials between -90 mV and -60 mV.