ADAPTIVE RESPONSES OF HYPERTROPHYING SKELETAL-MUSCLE TO ENDURANCE TRAINING

We determined the effects of 6 wk of endurance running on citrate synt hase (CS) activity and myosin heavy chain (MHC) expression in bilatera l surgical-overloaded rodent fast-twitch plantaris and slow-twitch sol eus muscles. The overload induced significant hypertrophy in both musc le types, and this response was enhanced by endurance training. The ov erload-induced compensatory hypertrophy was accompanied by a proportio nal increase in muscle CS content. Although endurance training produce d significant increases in CS concentration in either muscle type of t he normal-trained groups (P < 0.05), it was not effective in causing s imilar changes in the overloaded trained muscles. Also, overload of ei ther the sedentary or trained groups produced an increase in slower MH C isoforms (i.e., type I in the soleus and types I and IIa in the plan taris) and a concomitant decrease in the faster MHC isoforms (type IIa in the soleus and IIb in the plantaris; P < 0.05), whereas endurance training alone produced the opposite effect, especially in the plantar is. Collectively, these data suggest that 1) increments in muscle oxid ative enzyme content due to endurance training are compromised when a hypertrophying process is occurring concomitantly; and 2) the relative loading state imposed on the muscle during repetitive locomotor activ ity is critical in regulating the pattern of MHC plasticity.