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The pH stability of the myofibrillar adenosine triphosphatase (ATPase) of muscle fiber types in four different skeletal muscles of the sheep and cattle was histochemically investigated. By incubating frozen sections in acid or alkaline solution before reaction for ATPase, it was found that the myofibrillar ATPase of the type A and B fibers was acid-labile and alkali-stable, whereas the ATPase of the type C, D, and E fibers was acid-stable and alkali-labile in both species. The ATPase activity in the D fibers was more strongly inhibited by preincubation at pH 4.0 than that in the C and E fibers. Few A fibers were acidand alkali-stable. Such A fibers were found in a few muscles from some animals, but not in any muscles from other animals. With the ATPase reaction after pH 4.3 preincubation at room temperatures (16-18°C), slight differences in pH sensitivity between the sheep and cattle were noted in the B fibers of a certain kind of muscles. The temperature during acid and alkaline preincubation was shown to influence the inhibition of the ATPase activity. The ATPase reaction demonstrated by the method of PADYKULA and HERMAN1) is localized in the myofibrils and believed to be due to myosin ATPase in skeletal muscle fibers. With this reaction, the muscle fibers can be distinguished into two kinds: strongly reacting fibers and weakly reacting fibers.2,3) Subsequently, the intensity of reaction in two kinds of fibers was shown to be reversed by immersing cryostat sections in acid solution before incubation for ATPase reaction.4,5) This has been interpreted as indicating that the myofibrillar ATPases in muscle fibers have different pH stability. Skeletal muscle fibers of the sheep6-8) and cattle9) were histochemically classified into five fiber types. The type A and B fibers were strong in the routine ATPase activity demonstrated by the method of PADYKULA and HERMAN,1) the type C and D fibers weak, and the type E fibers moderate. On the basis of the results of GUTH and SAMAHA4) and of BROOKE and KAISER,5) the myofibrillar ATPase of the A and B fibers is assumed to be acid-labile and alkali-stable, and the ATPase of the C and D fibers is acid-stable and alkali-labile, but it is difficult to predict the pH stability of the myofibrillar ATPase of the E fibers. The present study was undertaken to investigate the pH sensitivity of the myofibrillar ATPase and to define the pH stability of it in the five fiber types of the sheep and cattle. Materials and Methods Muscle samples were obtained from two ewes (Corriedale), three wethers (Suffolk), two steers (Holstein) at the age of 17 months, two steers (Holstein) at the age of 20 months, and Jap. J. Zootech. Sci., 47, (2): 95-103. 95 1976. 2.

The pH Stability of Myofibrillar Adenosine Triphosphatase of Five Fiber Types in Skeletal Muscles of the Sheep and Cattle