COMPLEXITY OF AGE-RELATED CHANGE IN SKELETAL-MUSCLE

Age-related changes in skeletal muscle mass, fiber area, and contracti le function were examined in pathogen-free rats at 6, 12, 28 and 36 mo s of age. The intent of this study was to clarify age-related decline, particularly in contractile force, and to determine if the decline in contractile tension with age is due to alterations at the neuromuscul ar junction. A variable amount of age-associated reduction in muscle m oss was noted for the soleus (18%), extensor digitorum longus (EDL-16% ), plantaris (37%), and gastrocnemius (38%) muscles. The decline in fi ber area for these four muscles was between 5 and 16% greater than the loss in muscle wet weight. A variable amount of change in peak contra ctile force between 6 and 36 mos was observed for the soleus (62%), ED L (48%), and plantaris (34%). For soleus and EDL, the decline in peak tetanic tension exceeded the decline in muscle mass and fiber area. Mo st of the declines for the animals used in this study did not become s ignificant until after the age of 28 mo. The marked reduction in peak tetanic tension, fiber area, and muscle mass between 28 and 36 mos ind icates an accelerated age-related decline in this lime period. The red uced peak twitch and peak tetanic tension in the oldest animals was no t due to likely age-related changes at the neuromuscular junction. Pea k values for tetanic tension were similar, whether tension was elicite d via direct muscle stimulation or through stimulation of the nerve. R esults underscore the complexity of age-related change and suggest tha t multiple mechanisms contribute to the decline of skeletal muscle.