THE MEMBRANE CAPACITY OF MAMMALIAN SKELETAL-MUSCLE FIBERS

Membrane capacity was measured as a function of fiber diameter in mammalian skeletal muscle fibers under normal conditions and under conditions designed to reduce the membrane Cl conductance, i.e., in solutions in which Cl ions were replaced by sulfate or methylsulfate ions, or in normal Krebs solutions containing 2,4-dichlorophenoxyacetic acid (2.5 mM). The experiments were done on rat sternomastoid, extensor digitorum longus and soleus muscle fibers. The average membrane capacity of fibers in each muscle was greater than normal when Cl conductance was reduced and the slope of the relationship between membrane capacity and fiber diameter increased. The results were consistent with the hypothesis that the space constant of the transverse tubule system in mammalian fibers is normally short because the transverse tubule membrane has a high Cl conductance. The space constant of the transverse tubule system was < 40 .mu.m for fibers in normal Krebs solution and greater than 100 .mu.m for fibers with low membrane Cl conductance. The space constant was calculated using measured geometrical parameters of the transverse tubule, and measured membrane conductance, and the values were close to 20 .mu.m for fibers in normal Krebs solution and between 50 and 120 .mu.m for fibers with low Cl conductance.