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

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 "alpha" and "beta". In 0.1 M K+ the single-channel conductances are 50 and 150 pS for alpha and beta, respectively. The probabilities of appearance of the three states are voltage-dependent, and transitions between the closed and beta states proceed through the alpha state. Both open states follow a quantitatively identical selectivity sequence in channel conductance: K+ greater than NH4+ greater than Rb+ greater than Na+ greater than Li+ greater than 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 beta than in the alpha state.