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

Summary

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.