THE EFFECTS OF INACTIVITY AND EXERCISE ON SINGLE SKELETAL MUSCLE FIBERS FROM AGED RATS 1650

Abstract

The cellular basis of disuse atrophy in muscle tissue has been studied primarily in adult animals. Our study examined the effects of disuse and intermittent exercise on single muscle fibers of the deep region of the lateral head of the gastrocnemius muscle (RG) from aged animals. Thirty month old male rats were randomly assigned to one of three groups for 7 days: control (C), hindlimb unweighted (HU), or HU plus weight bearing exercise for 4 daily sessions of 15 min (HU-X). Following 7 days, fiber-type specific alterations were investigated using the single glycerinated fiber preparation 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 technique) were tested and fiber type was determined by myosin heavy-chain composition (MHC) using SDS-PAGE. Following HU, the MHC type I fibers showed a decrease in dia (83 ± 2 μm vs 72 ± 2μm) whereas the MHC type IIa fibers showed no change (84 ± 3 μm vs 79 ± 2 μm). The peak force of MHC type I and MHC type IIa fibers declined with HU (36% and 23%, respectively). Vo increased in MHC type I fibers in both HU and HU-X groups. The HU-X resulted in an attenuation of dia loss in the MHC type I fibers but had no effect on the dia of MHC type IIa fibers. Peak force was not attenuated in MHC type IIa fibers, however, there was a trend toward attenuation with exercise in MHC type I fibers. In conclusion, the MHC type I and MHC type IIa fibers were found to be responsive to HU and HU-X in a fiber-type specific manner.