Neuromuscular adaptability of male and female rats to muscle unloading.

Previously, it has been shown that following muscle unloading, males and females experience different maladaptations in neuromuscular function. As a follow-up, the present investigation sought to determine if male and female neuromuscular systems demonstrated similar, or disparate morphological adaptations to muscle unloading. Twenty young adult male, and 20 young adult female rats were randomly assigned to one of two treatment protocols: muscle unloading, or control conditions. Following the 2-week intervention period, immunofluorescent procedures were used to quantify pre- and post-synaptic features of neuromuscular junctions (NMJs), and to assess myofiber profiles (size and fiber type composition) of the soleus, plantaris, and EDL muscles. A 2-way ANOVA with main effects for sex and treatment was then used to identify statistically significant (p ≤ .05) differences among structural parameters. Analysis of NMJs showed a consistent lack of differences between males and females. Overall, NMJs were also found to be resistant to the effects of unloading. When examining myofiber profiles, however, male myofibers were revealed to be significantly larger than female ones in each of the muscles examined. Unloading resulted in significant myofiber atrophy only in the primarily weight-bearing soleus muscle. Only the EDL showed unloading-induced differences in myofiber type distribution (Type II → I). These data indicate that different components of the neuromuscular system (NMJs, myofibers) respond uniquely to unloading, and that sex affects myofiber type profiles, but not NMJs. Moreover, it appears that only muscles that have their habitual activity patterns disturbed by unloading (i.e., the soleus, adapt to that intervention).

Gender-specific neuromuscular adaptations to unloading in isolated rat soleus muscles.

INTRODUCTION: The potential of gender to affect unloading-induced neuromuscular adaptations was investigated. METHODS: Twenty male and 20 female rats were assigned to control (CTL), or unloaded (UL) conditions. After 2 weeks of unloading, soleus muscles were removed, and neuromuscular function was assessed during a train of alternating indirect (neural) and direct (muscle) stimuli. RESULTS: In rested muscle, strength showed significant (P ≤ 0.05) main effects for gender (male > female) and treatment (CTL > UL). By the end of the testing protocol, when muscles showed fatigue, gender-related and treatment-related differences in strength had disappeared. Neuromuscular transmission efficiency and strength suffered a greater decline during the testing protocol in males than females. Unloaded male muscles displayed greater contractile velocity than female muscles both when rested and fatigued. CONCLUSIONS: Gender affected unloading-induced neuromuscular adaptations. The greater strength of rested male muscles was due to greater muscle mass and neuromuscular transmission efficiency. Muscle Nerve 54: 300-307, 2016.