THE EFFECTS OF REMOVAL OF WEIGHT-BEARING ON MHC TYPE IIa SKELETAL MUSCLE FIBERS

Abstract

218 The contractile properties of the myosin heavy chain type I fibers (MHC) of the soleus muscle are adaptable to the removal of weight-bearing. The preferential adaptability observed with the removal of weight-bearing is hypothesized to be associated with the MHC type I fiber activity level with weight-bearing. Since the activity level with weight-bearing is different between the plantaris (PL) and the extensor digitorum longus (EDL) muscles, the present study hypothesized that the MHC type IIa fibers of the two muscles would respond to removal of weight-bearing in a muscle-specific manner. Male rats (30 month old) were randomly assigned to control or hindlimb unweighted groups for 7 days. Following 7 days, the PL and EDL muscles were removed and prepared for single glycerinated fiber experiments. 100 individual fiber were mounted between a force transducer and position motor. Single fiber diameter(dia), peak force (mg), maximal unloaded shortening velocity (Vo, determined by the slack test) were evaluated and the fiber type was determined by SDS-PAGE. Following removal of weight-bearing, significant declines in fiber dia (97 ± 4 μm vs 77 ± 4 μm), fiber peak force (54± 7 mg vs 35 ± 4 mg) and fiber Vo (6.69 ± 0.78 fl/s vs 5.41 ± 0.44 fl/s) were observed in MHC type IIa fibers from the PL muscle. In contrast, the single fiber contractile properties of the MHC type IIa fibers from the EDL did not change. In conclusion, the specific muscle source appears to have an influence on the extent of adaptability of single muscle fibers with MHC type IIa isoform following removal of weight-bearing.