Habitual exercise enhances neuromuscular transmission efficacy of rat soleus muscle in situ

Rat motor nerve terminals and the endplates they interact with exhibit chan ges to varying patterns of use, as when exposed to increased activation in the form of endurance exercise training. The extent to which these changes affect neuromuscular transmission efficacy is uncertain. In this study, the effects of habitual exercise on the electrophysiological properties of neu romuscular transmission in rat soleus muscle were investigated using a nove l in situ approach. Consistent with previous reports, miniature endplate po tential frequency was enhanced by habitual exercise. Other passive properti es, such as resting membrane potential, miniature endplate potential amplit ude, and "giant" miniature endplate potential characteristics were unaltere d by the training program. Full-size endplate potentials were obtained by b locking soleus muscle action potentials with mu -conotoxin GIIIb. Quantal c ontent values were 91.5 and 119.9 for control and active groups, respective ly (P < 0.01). We also measured the rate and extent of endplate potential a mplitude rundown during 3-s trains of continuous stimulation at 25, 50, and 75 Hz; at 50 and 75 Hz, we found both the rate and extent of rundown to be significantly attenuated (10-20%) in a specific population of cells from a ctive rats (P < 0.05). The results establish the degree of activity-depende nt plasticity as it pertains to neuromuscular transmission in a mammalian s low-twitch muscle.