Poster 46: Immunohistochemical evidence of immature myosin expression in aged rat skeletal muscle

Abstract

Objective: To evaluate 2 hindlimb skeletal muscles in young and aged rats for presence of immature myosin heavy chain (MHC) isoforms. Design: Randomized controlled trial. Setting: Basic science laboratory. Animals: Fisher 344 Brown Norway F1 hybrid rats, ages 12 and 36 months. Interventions: The soleus and extensor digitorum longus (EDL) muscles of young (12mo old) and aged (36mo old) sedentary rats were excised, quick frozen, and stained immunohistochemically for the developmental MHC isoform. Muscle cross-sections were then evaluated for the number of developmental positive fibers per unit area. Main Outcome Measures: Number of muscle fibers per unit cross-sectional area expressing developmental MHC. Results: The soleus muscles of the aged rats showed significantly greater numbers of developmental MHC positive cells when compared with the soleus of the 12-month-old rats. The EDL of the 36-month-old rats exhibited noticeably greater numbers of developmental MHC positive cells than in the young animals in half of the muscles studied. Conclusions: There is evidence of skeletal muscle regeneration occurring in the soleus of the aged rats in the midst of the fiber atrophy that accompanies aging. Such changes in the EDL also occur, but appear to be less common. The muscle specific differences observed in the expression of developmental MHC with age may be influenced by the fiber type and function differences of this muscle as compared with the soleus. Evaluation of the coexpression of the developmental MHC with the mature MHCs (slow, fast) is currently under investigation.