Skeletal muscle atrophy in response to 14 days of weightlessness: vastus medialis.

The vastus medialis (VM) from rats after 14 days of microgravity on COSMOS 2044 (F) was compared with VM from tail-suspended hindlimb-unloaded rats (T) and ground controls, including vivarium (V), synchronous (S), and basal (B) animals. The VM is composed chiefly of fast-twitch fibers; however, it contains a deep portion closer to the bone with mixed slow- and fast-twitch fibers. In the mixed-fiber portion, type I and II fiber areas were significantly reduced in F animals. In the homogeneous portion with chiefly fast-twitch fibers, F rats also showed reductions in cross-sectional areas compared with T, V, and B but not S rats. Fiber densities (fibers/mm2) were greatest in VM from F rats. Capillary density changes paralleled fiber density changes. F animals have significantly greater density of capillaries in the mixed-fiber portion. Concentrations of protein, RNA, and DNA were highest in V controls, whereas F rats had the lowest level of total RNA. Lactate dehydrogenase activity, one measure of anaerobic capacity, was greater in F than in S rats. Citrate synthase activity, a measure of oxidative capacity, showed no significant differences between groups. Although triglyceride stores of VM were greater in F than in T rats, there were no significant differences from any of the control groups. It was concluded that VM wet weights may be a less sensitive measure of atrophy than the fiber area measurements. Fiber area decreases and fiber density increases in F animals were quantitatively comparable to those in soleus and extensor digitorum longus after 7 days of weightless flight in Spacelab 3. Our results suggest that VM shows measurable responses to weightlessness.