Electrophysiology of human muscle in culture

Human muscle obtained from biopsy specimens was grown in a primary tissue culture system and studied electrophysiologically. Myotubes with well-developed striations were formed after 3 to 5 weeks in culture. The myotubes had stable resting potentials (-50 mV), and generated all-or-none action potentials (92 mV amplitude) when stimulated. The resting permeability of the myotubes was found to be primarily dependent on K⁺ and to a lesser extent Na⁺ ( P_Na P_K = 0.14 ± 0.06). Changes in external Cl^- did not affect the resting potential or resting conductance indicating that the Cl^- permeability is low compared with the permeabilities of K⁺ and Na⁺. A small but significant electrogenic NaK pump component to the resting potential was identified at low K⁺ concentrations (< 10 mm). The passive membrane parameters of myotubes were determined by analyzing the voltage decay transients of uniform fibers in response to the injection of a short hyperpolarizing pulse of current. The resting membrane resistance was 7780 ohm-cm²; membrane time constant, 22.9 ms; membrane capacitance, 3.8 μF; and electrotonic length, 1.1. The identification of the electrical characteristics of normal human muscle grown in primary tissue culture will serve as the basis for detailed comparative studies on diseased human muscle.