MORPHOLOGICAL AND BIOCHEMICAL-EVIDENCE OF MUSCLE HYPERPLASIA FOLLOWING WEIGHT-LIFTING EXERCISE IN RATS

We used a rat model of weight Lifting to examine the serial biochemica l and morphological changes following muscle fiber hyperplasia during 14 days of exercise. [H-3]thymidine and [C-14]leucine labeling were us ed to determine the serial changes in cellular mitotic activity and th e level of amino acid uptake and myosin synthesis. Morphological chang es were assessed with light and transmission electron microscopy, wher eas proliferation of cells was evaluated immunohistochemically with 5- bromo-2'-deoxyuridine (BrdU). The intensity of the exercise and degree of muscle damage were monitored by serum creatine kinase (CK) activit y. Damaged fibers were sparsely distributed, and a significant CK leak age was observed 30-60 min after exercise. Anti-BrdU-positive cells we re observed in damaged fibers and at the periphery of undamaged fibers . Changes typical of muscle regeneration were observed; However the fo rmation of new fibers in the interstitial space was also evident. The mitotic activity also changed and reflected the appearance of anti-Brd U-positive cells and activated satellite cells. Amino acid uptake incr eased during the first week of exercise, probably reflecting muscle hy pertrophy and synthesis of other noncontractile related proteins. The uptake also increased during the second week, probably due to hyperpla sia, a finding also supported by electron microscopy. Our results sugg est that one bout of weight-lifting exercise in untrained rats induced muscle hyperplasia following regeneration. The process of muscle hype rplasia was activated by muscle fiber damage in our model.