Exogenous testosterone treatment decreases diaphragm neuromuscular transmission failure in male rats

The effect of chronic exogenous testosterone (T) treatment on neuromuscular transmission in the diaphragm (Dia) muscle of adult male rats Was determin ed. The contribution of neuromuscular transmission failure (NTF) to Dia fat igue was evaluated by superimposing intermittent direct muscle stimulation on repetitive nerve stimulation of isometric contraction in vitro. T treatm ent significantly reduced the contribution of NTF to Dia fatigue by similar to 20% (P < 0.001). Fiber type-specific effects on NTF Were determined by measuring Dia fiber glycogen levels subsequent to repetitive nerve or muscl e stimulation. T treatment had no effect on glycogen depletion in Dia type I and IIa fibers regardless of stimulation route. In the control group, typ e IIx fibers demonstrated significantly less glycogen depletion after nerve stimulation compared with direct muscle stimulation (P < 0.05), suggesting the presence of NTF. In contrast, T treatment increased glycogen depletion of type IIx fibers during nerve stimulation to levels similar to those aft er direct muscle stimulation. These data indicate that testosterone treatme nt substantially improves neuromuscular transmission in the Dia.