Histochemical, biochemical, and contractile properties of red, white, and intermediate fibers.

BARNARD, R. JAMES, V. REGGIE EDGERTON, TETSUO FURUKAWA, AND J. B. PETER. Histochemical, biochemical, and contractile properties of red, white, and intermediate jibers. Am. J. Physiol. 220 (2) : 41 O-41 4. 197 1 .-Individual fibers from guinea pig muscles (soleus, flexor hallucis longus (FHL), flexor digitorium longus (FDL), medial gastrocnemius (MG) and the portion of the vastus lateralis (RV) which is grossly red) were classified as “red,” “white,” or “intermediate” according to their histochemical activity of NADHdiaphorase and myosin ATPase. Actomyosin and myosin ATPase activities of these muscles were determined biochemically and contraction times were measured in situ. The data demonstrate that in guinea pigs the soleus, which is composed entirely of intermediate fibers, is a slow-twitch muscle and has the lowest myosin and actomyosin ATPase activities. Conversely, muscles composed predominately of white fibers (FHL and FDL) or predominately of red fibers (MG and RV) are both fast-twitch muscles and have higher ATPase activities. Since both the soleus and RV are red in appearance the generalization that red muscles have slow contraction times is not valid. We suggest that these fibers be referred to as fast-twitch red, fast-twitch white, and slow-twitch intermediate.