Efflux of acetylcholine in dimorphic skeletal muscle from castrated male rats.

The effect of testosterone on motor neurons of dimorphic muscles is demonstrable by the increased frequency of miniature end-plate potentials (mepp) and decreased end-plate acetylcholinesterase activity observed in castrated rats. No change occurs in induced acetylcholine (ACh) release. Although these muscles atrophy after castration there is no loss of muscle fibers. In the present study we reinvestigate the neuromuscular transmission in levator ani (LA) muscles from normal (N) adult (120 days) male rats and from rats castrated (C) 30 days before. The measurement of radioactive [3H]-choline was used to evaluate ACh release since it permits simultaneous estimation of quantal and non-quantal ACh release. The LA muscle was incubated with [3H]-choline (1 microCi/ml) for 30 min and ACh efflux was measured after washout. The basal release of [3H]-choline (dpm total tissue radioactivity-1 number of fibers-1) was 296 +/- 33 and 156 +/- 24 in N and C, respectively. Induced ACh release (25 Hz, 5 min) was the same in N and C (653.19 +/- 66.46 and 496.62 +/- 68.67, respectively). These results indicate that castration increased mepp frequency but reduced the total spontaneous release of ACh.