Biochemical and physiological changes in overloaded rat fast- and slow-twitch ankle extensors.

The rat soleus (SOL) or medial gastrocnemius (MG) were chronically overloaded by removing their major synergists bilaterally. After 12-14 wks the overloaded SOL (OS) and overloaded MG (OMG) muscles had approximately 50% greater cross-sectional areas (CSA) than the controls. Maximum twitch (Pt) and tetanic (Po) tensions were approximately 46% larger in the OS compared with the normal SOL. The OMG produced 10 and 37% higher Pt and Po, respectively. Specific tension (Po/CSA) was not altered in either group (P greater than 0.05). Contraction times and half-relaxation times were unchanged. Myofibrillar and myosin ATPase specific activities indicated a shift toward that resembling a slower muscle in both the OS and the red portion but not the white portion of the OMG. Generally, markers of glycogen metabolism were reduced (P less than 0.05) in the same muscle areas that showed reduced ATPase activity. These biochemical results were consistent with the apparent histochemical conversion of fibers from fast-twitch, glycolytic----fast-twitch, oxidative-glycolytic----slow-twitch, oxidative types in these muscle areas. These results suggest that overloading either a fast- or slow-twitch plantarflexor results in an increase in muscle mass and maximum tension and in metabolic shifts that generally resemble those observed in a slower muscle. Further, the degree of adaptation appears to be related to the initial fiber type composition of the muscle and/or of the muscle region.