K⁺ channels close more slowly in the presence of external K⁺ and Rb⁺

Abstract

The voltage-sensitive gates regulating the permeability of ionic channels in nerve membrane were originally thought to be insensitive to the concentration¹ and species^2,3 of readily permeant cations. In contrast, blocking cations—tetraethyl-ammonium⁴, N-methylstrychnine⁵ or pancuronium⁶—when present in the inner end of potassium or sodium channels, hinder closing of the gates by a ‘foot in the door’ effect. Recent work on acetylcholine channels has clearly shown that the lifetime of the conducting state is prolonged by certain species of permeant cations^7,8. In this case, the presence of a permeant ion near the inner end of the channel is thought to prevent closing. Somewhat similar effects have recently been reported for the K⁺ channels of nerve membrane^9–11. We report here unequivocal evidence that the permeant cations K⁺ and Rb⁺ when present externally, slow the closing of K⁺ channels in squid axon, supporting the view that gating is indeed sensitive to the presence and species of permeant cations in the channels.