Short-Term Mild Hypoxia Maintains Endplate Integrity and Lipid Membrane Stability in Rat Skeletal Muscle under Simulated Gravitational Unloading

An important factor in the reliability and plasticity of neuromuscular transmission is the structural and functional organization of the motor endplate, which is a highly specialized area of the sarcolemma. The structural features of the endplate largely depend on motor activity and change with motor dysfunction. In this study, rats were exposed to simulated high-altitude (3000 m above sea level) hypobaric hypoxia for 3 h using a hyperbaric pressure chamber. Then, 18 h after control or hypoxic conditions, rats underwent 6 h of hindlimb suspension (HS), a model of gravitational motor unloading. Isolated soleus and diaphragm muscles were tested. Labeled α-bungarotoxin was used to analyze the localization of nicotinic acetylcholine receptors (nAChRs). To assess the plasma membrane properties, 22-NBD-cholesterol (NBD), a fluorescent sterol, was used. HS itself increased the fragmentation of nAChR distribution without changing the total endplate area. These changes were accompanied by an increase in the intensity of NBD fluorescence throughout the sarcolemma, indicating destabilization of its lipid-ordered phase. Hypoxia per se did not affect endplate integrity and NBD fluorescence intensity. However, hypoxic pretreatment protected against HS-induced impairments. The relative phosphorylation level of AMP-activated protein kinase, a key metabolic regulator, remained unchanged. Similar effects were observed in both the soleus and diaphragm muscles. We suggest that these results may have potential therapeutic implications in the search for countermeasures in cases of neuromuscular disorders.