Transport number effects in the transverse tubular system and their implications for low frequency impedance measurement of capacitance of skeletal muscle fibers

Summary It has been shown in an earlier paper that the slow transient decrease in conductance, somtimes referred to as “creep”, obtained with small-to-medium hyperpolarizing current or voltage pulses is due to K+ transport number differences across the walls of the transverse tubular system. Using the same basic numerical analysis and the parameters already obtained experimentally in the previous paper for frog skeletal muscle in a sulphate Ringer's solution, this paper predicts the equivalent membrane capacitance and dynamic resistance due to transport number effects for very low amplitude and low frequency sinusoidal currents from the phase lag of the voltage response behind the current. Such sinusoidal currentper se give rise to an equivalent capacitance which increased from less than 1µF·cm-2 at 10 Hz to about 16µF·cm-2 at 0.01 Hz and to an equivalent dynamic membrane resistance which increases from its instantaneous slope resistance value of 11.7k?cm2 at 10 Hz to about 16k?cm2 at 0.01 Hz. Similar small sinusoidal components of current superimposed on depolarizing and hyperpolarizing pulses (25–45 mV) give rise to even greater “capacitances” at low frequencies (e.g., 24–28µF·cm-2 at 0.01 Hz). The response due to large sinusoidal currents was also investigated. These transport number effects help to explain the small discrepancies obtained by some workers between experimental and predicted values of skeletal muscle fiber impedances measured in the 1–10 Hz range and would seem to be critical for the interpretation of any skeletal muscle fiber impedance studies done at frequencies less than 1 Hz.