Succinate dehydrogenase activity in fibres classified by myosin ATPase in three hind limb muscles of rat.

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Abstract
Succinate dehydrogenase (SDH) activity was assessed in situ in single fibers of cross-sectioned extensor hallucis longus, extensor digitorum longus and soleus muscles of rat by means of microphotometric recording of initial maximum reaction rates. Each fiber assessed for SDH activity was subjectively classified into myosin subgroups by its histochemical reaction for myofibrillar actomyosin ATPase (myosin ATPase) following preincubation at pH 4.6. The majority of fibers classified into myosin types I and II a were highly reactive for SDH, such that those myosin groups could be interchangeable with the metabolic subgroups; myosin I = slow-twitch oxidative, myosin IIa = fast-twitch oxidative glycolytic. The myosin type IIb fibers demonstrated marked variability in activity levels of SDH. Over 40% of those fibers had high SDH activity and thus could not be equated with the metabolic subgroup fast-twitch glycolytic. The histochemical reaction for myosin ATPase in muscle fibers therefore cannot be used as a reliable means to predict the fibers'' metabolic characteristics.

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