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Block Of Endplate Channels By Permeant Cations In Frog Skeletal Muscle, David J. Adams, W Nonner, T M. Dwyer, B Hille

Illawarra Health and Medical Research Institute

Motor endplates of frog semitendinosus muscles were studied under voltage clamp. Current fluctuations by iontophoretic application of acetylcholine were analyzed to give the elementary conductance, γ, and mean open time, τ, of endplate channels. Total replacement of the external Na+ ion by several other metal ions and by many permanent organic cations changed both γ and τ. Except with NH4 + ions, the γ values with foreign test ions were all smaller than expected from the independence relation and their previously measured permeability ratios. The more hydrophobic ions gave the smallest γ values. Foreign permeant cations also depress γ when …