抄録
Skeletal muscle atrophy occurs in response to unloading condition and is counteracted by mechanical stress to muscle during unloading condition. This study was designed to investigate whether myonuclear number regulates changes in muscle mass induced by altered loading conditions. Adult F344 female rats were assigned to either weight-bearing control or hindlimb-unloaded (HU) group. HU-rats had their hindlimbs suspended for 3 weeks with or without isometric resistance exercise (IRE). IRE (stationary support on a cylindrical wire mesh with 60 or 80 degree incline) was performed three times daily. An additional weight of 50-70% body mass was hung from the rat's tail during IRE. Each training bout lasted 10 min, for a total of 30 min/day. Myonuclear number and fiber size were measured in the medial gastrocnemius muscle using histochemical and immunohistochemical techniques. HU decreased muscle mass and fiber size of all types identified with myofibrillar ATPase staining. IRE ameliorated decrease of muscle mass by 49%. The decreases in fiber size were counteracted by IRE within the range from 33% to 86%. The IRE effectively prevented decrease in fiber size of type IIa (86%) and type IIx (74%) at type I predominant region. Myonuclear number decreased in all types of fibers with HU, but these decreases ameliorated by IRE. These results demonstrate that changes in myonuclear number are associated with changes in myofiber size induced by unloading or intermittent resistance-reloading. [J Physiol Sci. 2006;56 Suppl:S232]
