Spaceflight effects on adult rat muscle protein, nucleic acids, and amino acids.

Exposure to conditions of weightlessness has been associated with decrements in muscle mass and strength. The present studies were undertaken to determine muscle responses at the cellular level. Male Sprague-Dawley rats (360-410 g) were exposed to 7 days of weightlessness during the Spacelab-3 shuttle flight (May 1985). Animals were killed 12 h postflight, and soleus (S), gastrocnemius (G), and extensor digitorum longus (EDL) muscles were excised. Muscle protein, RNA, and DNA were extracted and quantified. Differential muscle atrophy was accompanied by a significant (P less than 0.05) reduction in total protein only in S muscles. There were no significant changes in protein concentration (mg/g) in the muscles examined. In S muscles from flight animals, sarcoplasmic protein accounted for a significantly greater proportion of total protein that in ground controls (37.5 vs. 32.5%). Tissue concentrations (nmol/g) of asparagine-aspartate, glutamine-glutamate, glycine, histidine, and lysine were significantly reduced (from 17 to 63%) in S muscles from flight animals, but only glutamine-glutamate levels were decreased in the G and EDL. Muscle DNA content (microgram) was unchanged in the tissues examined, but S muscle DNA concentration (micrograms/mg) increased 27%. RNA content (micrograms) was significantly (P less than 0.025) reduced in S (-28%) and G(-22%) muscles following spaceflight. These results identify specific alterations in rat skeletal muscle during short term (7-day) exposure to weightlessness and compare favorably with observations previously obtained from ground-based suspension simulations.