Myonuclear domain and myosin phenotype in human soleus after bed rest withor without loading

After 2 or 4 mo of bed rest (6 degrees head-down tilt) and 1 mo of ambulati on, there was a tendency toward a higher percentage of fibers expressing fa st myosin heavy chain (MHC) isoforms and a de novo appearance of fibers coe xpressing type I+IIa+IIx and IIa+IIx MHC in human soleus fibers. After 2 an d 4 mo of bed rest, the mean size of type I fibers decreased by 12 (P > 0.0 5) and 39%, respectively. Because myonuclear number/mm of fiber length was unchanged, myonuclear domain was smaller after bed rest than before. The me an size and myonuclear domain of type I fibers were largest after 1 mo of r ecovery. The effects of wearing an antigravity device (Penguin suit), which had a modest but continuous resistance at the knee and ankle (Penguin-1) o r knee resistance without loading on the ankle (Penguin-2), for 10 consecut ive h/day were determined during 2 mo of bed rest. Mean fiber sizes in Peng uin-1, but not Penguin-2, group were maintained at or above pre-bed-rest le vels, whereas neither group showed phenotype changes. Myonuclear domain in type I fibers was larger in Penguin-l and smaller in Penguin-2 group post-c ompared with pre-bed rest, indicating that a single daily 10-h bout of mode st muscle loading can prevent bed-rest-induced soleus fiber atrophy but has minimal effect on myosin phenotype. The specific adaptive cellular strateg ies involved may be a function of the duration and magnitude of the adaptiv e stimulus as well as the immediate activity history of the fiber before th e newly changed functional demands.