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Skeletal Muscle Changes Following Six-Month Spaceflight.    DA Riley¹, JLW Bain¹, RH Fitts², JG Romatowski² and SW Trappe³.  ¹Dept. Cell Biol., Neurobiol. & Anatomy, Medical College of Wisconsin and ²Dept. Biology, Marquette University, Milwaukee, WI and ³Human Performance Lab, Ball State University, Muncie, IN.

   Skeletal muscle wasting raises concern for the health and performance of humans undertaking long term spaceflight.  The present study examined soleus and gastrocnemius muscle biopsies from 5 astronauts and 5 cosmonauts who participated in 6-month, Expedition missions on the International Space Station during 2000-2005.  Tissues were analyzed by light and electron microscopy.  The intersubject variation in the percentage of muscle fiber atrophy was striking; atrophy ranged from 3 to 64% in soleus and 5 to 46% in gastrocnemius.  Mitochondrial content and cytochrome oxidase activity and oil red O-positive intramyofiber lipids were retained in the muscles exhibiting the least atrophy.  PAS-positive glycogen and putative glycogen granules were increased in all muscles, most dramatically in fibers showing the greatest atrophy.  Immunostaining for a battery of metabolic markers, including carnitine palmityol transferase, phosphofructokinase, glycogen transporter 4, and uncoupling protein 3, revealed no major changes during spaceflight.  For soleus and gastrocnemius, on the average only a small percentage (5-7%) of slow fibers transitioned into slow/fast (I/IIA) and fast (IIA) fibers.  In normal skeletal muscle fibers, myonuclei are peripherally located, and central myonuclei indicate muscle fiber degeneration/regeneration.  Following spaceflight, the percentages of fibers with central myonuclei (soleus 4.62±.02%, gastrocnemius  2.01±.01%) were not significantly different from preflight values (soleus 3.76±.01%, gastrocnemius 1.75±.01%).  The large increase in glycogen after long term spaceflight indicates shifts in muscle fibers toward utilizing lipids and reduced glycogen breakdown.  The 5% or less atrophy is an acceptable risk for long term spaceflight.  However, atrophy of 46% or greater is of concern because countermeasure exercise was performed.  (Supported by NASA grant EC400 NCC 9-116.)