Distribution of potassium and chloride permeability over the surface and T-tubule membranes of mammalian skeletal muscle

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Abstract
The distribution of K and Cl permeability,P K andP Cl, over the surface and T-tubule membranes of red rat sternomastoid fibers has been determined. Membrane potential,V m, was recorded with 3-m KCl-filled glass microelectrodes. Changes inV m with changes in [K]o or [Cl]o were used to estimateP Cl/P K in normal and detubulated preparations. The results show that the T-tubule membrane has a highP Cl and is therefore different from the T-tubule membrane of amphibian fibers. Analysis of the time course of depolarization when [K]o was raised (in SO4 solutions) showed thatP K was distributed over the surface and T-tubule membranes. Two observations suggested that T-tubuleP Cl was higher than the surfaceP Cl. Firstly, in normal fibers, the depolarization caused by an increase in [K]o was 3.5 times greater in SO4 solutions than in Cl solutions. In marked contrast, the depolarization in glycerol-treated fibers was independent of [Cl]o. Secondly, the rapid change inV m when [Cl]o was changed was reduced by 80% after glycerol treatment. Both observations suggest thatP Cl was low in glycerol-treated fibers.P Cl/P K was calculated from theV m data using Goldman, Hodgkin and Katz equations for Na and K or for Na, K, and Cl. In normal fibersP Cl/P K=4.5 and in glycerol-treated fibersP Cl/P K=0.28. Since it is unlikely that glycerol treatment would increaseP K, the reduction in the ratio must follow the loss of Cl permeability “channels” in the T-tubule membrane.