SPACEFLIGHT ON STS-48 AND EARTH-BASED UNWEIGHTING PRODUCE SIMILAR EFFECTS ON SKELETAL-MUSCLE OF YOUNG-RATS

Our knowledge of the effects of unweighting on skeletal muscle of juve nile rapidly growing rats has been obtained entirely by using hindlimb -suspension models. No spaceflight data on juvenile animals are availa ble to validate these models of simulated weightlessness. Therefore, e ight 26-day-old female Sprague-Dawley albino rats were exposed to 5.4 days of weightlessness aboard the space shuttle Discovery (mission STS -48, September 1991). An asynchronous ground control experiment mimick ed the flight cage condition, ambient shuttle temperatures, and missio n duration for a second group of rats. A third group of animals underw ent hindlimb suspension for 5.4 days at ambient temperatures. Although all groups consumed food at a similar rate, flight animals gained a g reater percentage of body mass per day (P < 0.05). Mass and protein da ta showed weight-bearing hindlimb muscles were most affected, with atr ophy of the soleus and reduced growth of the plantaris and gastrocnemi us in both the flight and suspended animals. In contrast, the non-weig ht-bearing extensor digitorum longus and tibialis anterior muscles gre w normally. Earlier suspension studies showed that the soleus develops an increased sensitivity to insulin during unweighting atrophy, parti cularly for the uptake of 2-[1,2-H-3]deoxyglucose. Therefore, this cha racteristic was studied in isolated muscles within 2 h after cessation of spaceflight or suspension. Insulin increased uptake 2.5- and 2.7-f old in soleus of flight and suspended animals, respectively, whereas i t increased only 1.6-fold in control animals. In contrast, the effect of insulin was similar among the three groups for the extensor digitor um longus, which provides a control for potential systemic differences in the animals. These results on insulin response and muscle size ind icate that the tail-cast hindlimb suspension model appears suitable fo r mimicking the effects of weightlessness on rapidly growing muscle of juvenile rats.