METHODS

Adult (> 700 g) male Hartley guinea pigs were used. The muscles selected for study were the soleus, flexor hallucis longus (FHL), flexor digitorum longus (FDL), medial gastrocnemius(MG), and the portion of the vastus lateralis which is grossly red (RV).

Histochemistry. The muscles used for histochemical analysis were rapidly removed from the animals and trimmed free of fat and connective tissue. The belly of each muscle was ablated and placed on a mounting chuck for quick freezing with liquid nitrogen. Serial sections (6, u) were then cut from each muscle and placed on cover glasses for staining. NADH-diaphorase(NADH-D) activity was determined by the method of Novikoff et al.(14) and myosin ATPase activity by the method of Padykula and Herman (16) and a modification of Guth and Samaha (11) method. Photographs were taken from each slide and enlarged to permit classification of individual fibers. Cross sections of the muscles were analyzed from more than 50 soleus muscles, 4 FHL, 4 FDL, 10 vastus lateralis (red portion), and 13 MG. Individual fibers were classified as red, white, or intermediate. Intermediate fibers are characterized by their light staining with myosin ATPase and their distinct pattern of small, uniformly dispersed diformazan granules in the NADH-diaphorase reaction. Both red and white fibers are dark staining with myosin ATPase. The division into red and white is subjectively made from the NADH-D reaction. Red fibers are characterized by an abundance of coarse diformazan granules which are most concentrated in the subsarcolemmal region. White fibers are characterized by few diformazan granules which are also most prominent in the subsarcolemmal region. These histochemical characteristics are shown in Fig. 1, A and B. Further details concerning the histochemical techniques and methods of fiber classification have been published elsewhere (8).