Effects of thyroxine on transmembrane resting potentials of skeletal muscle cells in culture

The effect of thyroxine (T₄) was studied on the transmembrane resting potential (Em) of rat skeletal muscle cells in culture. In agreement with previous studies, we found the Em of developing muscle cells to increase with age from Day 3 to Day 7 or 8, at which time a plateau is reached. The values of Em we obtained (−65 to −75 mV) were 15–20 mV higher than those (−50 to −55 mV) reported by others. Treatment of mature myotubes with T₄ resulted in a significant increase in resting Em. There was only a slight difference between T₄‐treated and control cells with regard to the relation between extracellular K⁺ concentration and resting Em. Within 20–30 min of exposure to ouabain (10⁻⁴–10⁻³M) resting Em of both T₄‐treated and control cells had fallen to about the same value. This effect was reversible. Thus, T₄ stimulates the activity of an electrogenic Na/K‐ATPase in the membrane of cultured skeletal muscle cells.