SIZE AND MYOSIN HEAVY-CHAIN PROFILES OF RAT HINDLIMB EXTENSOR MUSCLE-FIBERS AFTER 2 WEEKS AT 2G

Method: The effects of 14 d of continuous centrifugation at approximat ely 2G on the hindlimb extensor musculature of male rats were studied. Results: The mean body mass of centrifuged rats was 17% smaller than age-matched controls. In centrifuged rats, the mean absolute masses of the soleus and medial gastrocnemius (MG) were similar to control, whi le the mean relative masses (expressed as milligram muscle mass/gram o f body mass) were larger than control. Based on a battery of monoclona l antibodies for specific myosin heavy chains (MHC), the soleus of cen trifuged rats had a lower percentage (68 vs. 74%) of fibers expressing type I MHC only and a higher percentage (15 vs. 10%) that co-expresse d type I and IIa MHC's. The MHC composition of fibers from the deep po rtion of the MC was unaffected by centrifugation. The MHC compositions based on SDS-PAGE gel electrophoresis for each muscle were similar in the two groups. Mean fiber size of each fiber type in the soleus was unaffected by centrifugation. In the MG, the fibers ex pressing only t ype IIb MHC were smaller in the centrifuge compared to control rats. C onclusion: Although 2 weeks of chronic centrifugation at 2G resulted i n a cessation of body growth, there was essentially no effect on the m uscle masses or fiber size in either a slow or fast extensor muscle. T hese data suggest that periods of centrifugation may be beneficial in maintaining extensor muscle mass in an animal that is not growing at a normal rate, e.g., during chronic unloading.