THE EFFECT OF SODIUM L-THYROXINE ON THE SLOW MUSCLE FIBER

The effects of 10-5 and 10-4gm/ml sodium 1-thyroxine on the bullfrog's slow muscle fibers innervated by the small motor nerve and on the neuromuscular transmission were investigated by means of the intracellular microelectrodes technique. 1. The slow muscle fibers were affected by more than 10-4 thyroxine-Ringer solution. The resting potential was slightly hyperpolarized but insignificantly. The effective membrane resistance, however, considerably increased to three folds of that in the control solution. The neuromuscular transmission was facilitated, i. e. amplitudes of the s. j. p.'s were augmented by 180%. The s. j. c. under the voltage clamp condition was also augmented by 240%, while their durations somewhat decreased. 2. Ach-potentials produced by the ionophoretic microapplications of acetyl choline are remarkably increased by 10-4 thyroxine. 3. From these evidences, it was concluded that thyroxine accelerated the neuromuscular transmission by both increasing the resting muscle membrane resistance and enhancing the sensitivity of the receptor membrane for the released transmitter substance from the nerve endings. It was also discussed that other factors such as the anticholinesterase-like action of thyroxine and an increase in the amount of the transmitter substance from the nerve endings were not conceivable. 4. The relationships between the thyroid hormones and the neuromuscular disorders in the cases of either the thyrotoxic or the familial form of periodic paralysis were discussed.