Age-related changes in fibre number, fibre size, fibre type composition and adenosine triphosphatase activity in rat soleus muscle.

To study the aging of muscle fibres in red skeletal muscle, fibre number, fibre diameter and fibre type composition in the soleus muscle of male rats of 3, 12 and 24 months old were examined. The total number of muscle fibres remained unchanged, while average diameter increased slightly with increasing age. The staining intensity of myosin adenosine triphosphatase (ATPase) activity in the fibres decreased with advancing age. Therefore, observation on the basis of myosin ATPase histochemistry alone is not adequate to study the aging of muscle fibres. In the muscles of 24 month-old animals, four fibre types were recognized; 1) many (52%) type I-O fibres showing weak ATPase and succinate dehydrogenase (SDH) reactions with slight subsarcolemmal aggregates of diformazan (SAD); 2) some (33%) type M fibres showing weak ATPase and intense SDH reactions with marked SAD; 3) a few (12%) type O fibres showing weak ATPase and intense SDH reactions without SAD; and 4) very few (4%) type IIA fibres. Histochemical and morphometric results suggest that type I-O, type M and type O fibres are derived from type I, type I and type IIA fibres, respectively. Furthermore, no transitional fibres from type IIA to type I were observed. Therefore, age-related changes in fibre type composition in the muscle cannot be explained by the simple idea that most type IIA fibres are transformed into type I fibres.