ACTIVITY-INDUCED REGULATION OF MYOSIN ISOFORM DISTRIBUTION - COMPARISON OF 2 CONTRACTILE ACTIVITY PROGRAMS

This study examined the role of specific types of contractile activity in regulating myosin heavy chain (MHC) isoform expression in rodent s oleus. A combination of hindlimb suspension (SN) and two programmed co ntractile training activity paradigms, either isometric contractile ac tivity (ST-IM) or high-load slowly shortening isovelocity activity, we re utilized. Both training paradigms increased muscle mass compared wi th SN alone. However, only ST-IM resulted in a partial prevention of t he suspension-induced decrease in type I MHC. With the use of a fluore scently labeled antibody to type IIa MHC, the distribution of MHCs amo ng fibers was examined immunohistochemically. In SN, the percentage of cells staining positive for type IIa MHC was increased but the staini ng intensity of the positively staining cells was unchanged compared w ith control cells. In the ST-IM soleus, the percentage of positively s taining fibers was unchanged but the intensity of the positively stain ing cells was decreased compared with SN values. These results suggest that 1) isometric contractile activity is more effective than isovelo city activity in preventing suspension-induced shifts in soleus MHC di stribution and 2) changes associated with both suspension and training occur in only a small number of fibers, with the majority of fibers a pparently unresponsive to these interventions.