Qualitative differences between actomyosin ATPase of slow and fast mammalian muscle

Cross-reinnervation of slow and fast muscles increases the contraction velocity of slow muscles and decreases that of fast ones. It, therefore, appears that the innervation regulates this physiological property of muscle. If actomyosin ATPase activity is, indeed, rate-limiting in the sequence of chemical events mediating contraction velocity, a comparison of the properties of this enzyme from slow and fast muscles should help elucidate the neural regulatory mechanism. Actomyosin, isolated from fast muscle of the cat, had threefold greater ATPase activity and was relatively more alkali-stabile and acid-labile than was actomyosin from slow muscle. These differences were demonstrable histochemically. Fibers having high ATPase activity predominate in fast muscles and fibers having low ATPase activity predominate in slow ones. By exposing frozen sections to acid or alkali before staining for ATPase, it was shown that the high-ATPase fibers are alkali-stabile and acid-labile, whereas as low-ATPase fibers are acid-stabile and alkali-labile. In addition, the fibers having high-ATPase activity could be further subdivided. The large-diameter fibers that predominate in the superficial parts of fast muscle are inhibited by formaldehyde, whereas the small-diameter fibers found mainly in the deeper parts of fast muscle (and in slow muscle of some species) are not. Preliminary observations indicate that under the influence of a foreign nerve, some acid-stabile fibers are converted to alkali-stabile ones. We conclude, on the basis of pH stability, that there are at least two qualitatively distinct actomyosin ATPases, and suggest that the nerve regulates the type of enzyme found in the muscle fiber.