Motor Nerve Terminal and Muscle Membrane Stabilization by Diphenylhydantoin Administration

An in vivo gracilis anticus preparation for microelectrode intracellular study of neuromuscular transmission was developed. The mean resting muscle fiber membrane potential was —73 mV. Resting miniature endplate potential (mepp) frequency was 2 to 4/sec in most fibers. Depolarization induced by external application of 20 millimolar K+ was slow in increasing mepp frequency. Frequency reached 100 to 150/sec rapidly with 35 millimolar K+. The potassium depolarization augmented mepp frequency was reduced up to 60% by increasing intravenous doses of diphenylhydantoin (DPH) up to 40 mg/kg with a median effective dose of 15 mg/kg. Depolarized muscle membrane potential was increased 15% after intravenously administered DPH. The DPH appears to stabilize the motor nerve terminal and muscle membrane. These actions may account for its effect in generalized myokymia and myotonia.