Effects of Spaceflight on Neuromuscular Junctions are Determined by Weight Bearing Function, Not Myofiber Type

Abstract

1033 PURPOSE: The objective of this investigation was twofold: 1) to determine if spaceflight induced remodeling of the neuromuscular junction (NMJ) was systemic in nature, or was confined to weight bearing muscle, and 2) to assess whether NMJ adaptations in the affected muscle(s) were fiber type specific. METHODS: Adult (7 wks old) male Fischer 344 rats were randomly assigned to either a Control (N = 10) or Spaceflight (N = 10) group that participated in a 10 d space shuttle mission. Following euthanization, the Tibialis Anterior (primarily ankle flexion), Gastrocnemius (primarily ankle extension), and Soleus (primarily postural) muscles were obtained and quickly frozen at resting length. Muscles were sectioned and stained with cytofluorescent techniques to visualize pre-synaptic nerve terminal branches and post-synaptic acetylcholine receptors of NMJs on fast- and slow-twitch fibers. Variables quantified included pre-synaptic branch number, total branch length, average length per branch, and post-synaptic stained area and perimeter. RESULTS: In the Tibialis Anterior and Gastrocnemius muscles, neither the NMJs of fast-twitch nor slow-twitch fibers were significantly (P > 0.05) altered by 0 gravity. In contrast, NMJs of soleus muscles demonstrated significant (P < 0.05) morphological adaptation. This was evident with NMJs of the slow-twitch fibers that are predominantly expressed, and those of the fasttwitch fibers which are more difficult to identify in the soleus. CONCLUSION: These data suggest that of paramount importance in determining the sensitivity of NMJs to the effects of space flight is weight bearing status of muscle. Less critical is function of muscle as ankle flexor or extensor, as well as muscle fiber type. Supported by the Jeffress Memorial Trust, and Howard Hughes Medical Institute through a grant to the College of William and Mary