A K+-selective, three-state channel from fragmented sarcoplasmic reticulum of frog leg muscle

Abstract

Sarcoplasmic reticulum (SR) vesicles from frog leg muscle were fused with a planar phospholipid bilayer by a method described previously for rabbit SR. As a result of the fusion, K⁺-selective conduction channels are inserted into the bilayer. Unlike the two-state rabbit channel, the frog channel displays three states: a nonconducting (“closed”) state and two conducting states “α” and “β”. In 0.1m K⁺ the single-channel conductances are 50 and 150 pS for α and β, respectively. The probabilities of appearearance of the three states are voltage-dependent, and transitions between the closed and β states proceed through the α state. Both open states follow a quantitatively identical selectivity sequence in channel conductance: K⁺>NH ₄ ⁺ >Rb⁺>Na⁺>Li⁺>Cs⁺. Both open states are blocked by Cs⁺ asymmetrically in a voltage-dependent manner. The zero-voltage dissociation constant for blocking is the same for both open states, but the voltage-dependences of the Cs⁺ block for the two states differ in a way suggesting that the Cs⁺ blocking site is located more deeply inside the membrane in the β than in the α state.