Differentiation of fiber types in skeletal muscle from the sequential inactivation of myofibrillar actomyosin ATPase during acid preincubation.

A method is described for identifying fiber types of skeletal muscle from several mammalian species on the basis of the sequential inactivation of myofibrillar actomyosin ATPase during acid preincubation. When this method is used in combination with the standard alkaline preincubation at least 5 types of fibers can be identified. Of these, 2 are type I fibers with those of the slow twitch soleus muscle being different from those that exist in mixed muscles. The 3 subtypes of type II fibers exist independent of their metabolic properties. The need for careful standardization of histochemical methods for the visualization of myofibrillar actomyosin ATPase and the implication of the existence of different fiber types in apparently homogeneous muscle for the preparation of antibodies used for immunocytochemical methods of fiber identification are discussed.