Effect of rat soleus muscle overload on neuromuscular transmission efficacy during continuous and intermittent activation.

Increased neuromuscular activity is known to provoke morphological and functional adaptations at the neuromuscular synapse. Most of these changes have been documented following endurance exercise training programmes. In this study, the effect of rat soleus muscle overload produced by tenotomy plus voluntary wheel-cage activity on neuromuscular transmission efficacy was investigated. The overload protocol increased miniature endplate potential (MEPP) and endplate potential (EPP) amplitudes by 17 and 19%, respectively (both P < 0.01), and increased MEPP frequency by 86% (P < 0.01). EPP amplitude rundown during continuous trains of activation was attenuated by approximately 10% in the overloaded group (P < 0.01). Also, during intermittent activation, the overload protocol attenuated EPP amplitude rundown, mainly by enhancing EPP amplitude recovery by approximately 10% during the quiescent periods (P < 0.01). Although the present results show that both the degree and direction of adaptation are similar to what has been observed at rat soleus neuromuscular junctions following an endurance training protocol, there are important nuances between the results, suggesting different mechanisms through which these changes may occur.

Incomplete recovery of endplate potential amplitude while intermittently activating rat soleus neuromuscular junctions in situ.

Studies dealing with neuromuscular transmission efficacy typically employ continuous patterns of activation to demonstrate decrements in endplate potential (epp) amplitude. Recent evidence from rat diaphragm muscle has shown that including periods of quiescence to the stimulation protocol allows epp amplitude to recover between series of contractions. Whether similar recovery occurs in rat hindlimb muscle is unknown. In this study, we have measured declines in epp amplitude in rat soleus muscle during trains of stimulation evoked either continuously (10 s) or intermittently (400 ms repeated every second), using an in situ approach. As in diaphragm, we found that rest periods within the intermittent trains significantly improved neuromuscular transmission efficacy. However, unlike the diaphragm, epp amplitude recovery was incomplete even by the second train in the intermittent protocols, recovery being frequency-dependent and ranging from 40 to 50%. The results suggest that the kinetics of epp amplitude rundown and recovery may be muscle-specific, and should be considered when evaluating situations in which neuromuscular transmission efficacy may be altered.