Succinate dehydrogenase (SDH) activities and cross-sectional areas (CSAs) of different types of fibers in the superficial (EDLs) and deep (EDLd) regions of the extensor digitorum longus and soleus (SOL) muscles and the left ventricular muscle of the heart (HEART) of 10-week-old male rats were determined using quantitative histochemistry and a computer-assisted image processing system. The fibers were classified as type I, type IIA, type IIB, or type IIC according to their histochemically assessed adenosine triphosphatase activities. The mean SDH activity was higher and the mean CSA was smaller in type IIA fibers than in type IIB fibers in both the EDLs and EDLd. The mean SDH activity of type IIA fibers in the SOL was higher than that of type I fibers. Fibers in the HEART showed the highest mean SDH activity and the smallest mean CSA among all fiber types in the muscles examined. There was an inverse correlation between CSA and SDH activity for the different fiber types in different muscles. These data suggest that the SDH activity of fibers in muscle is fiber type- and size-specific, and that the highest SDH activity of fibers in the left ventricular muscle of the heart contributes to their functional properties, i.e., high fatigue resistance.

Skeletal muscle fibers are classified into slow-twitch type I and fast-twitch type IIA and type IIB according to their myosin-based enzyme histochemical profiles [1,2]. Previous studies [3,4] have reported relationships among fiber types, and morphological (cell size) and metabolic (oxidative and glycolytic enzyme activities) features in the muscle. Muscle fibers have wide variations in their cell sizes [5], and large muscles (i.e., the gastrocnemius, tibialis anterior, and vastus lateralis muscles) show an increasing gradient of fibers having a small cell size proceeding from the superficial to deep region [6,7]. Therefore, it would be expected that smaller-sized fibers have higher oxidative enzyme activity than larger-sized fibers in the muscle even within the same fiber type because supplies of oxygen and substrates for oxidative energy metabolism from capillaries which are located close to the membrane are more plentiful in smaller-sized fibers. In the present study, we examined the cell size and oxidative enzyme activity of different types of fibers in different regions of the fast plantaris and tibialis anterior muscles in rats. MATERIALS AND METHODSExperimental animals. Five ten-week-old Wistar male rats (body weight, 293-322 g) were used in the present study. All experiments were approved by the Institutional Animal Care Committee at the university and conducted under the Guide for the Care and Use of Laboratory Animals published by the Office of Science and Health Reports of the USA National Institutes of Health, Bethesda, Maryland, USA.Tissue processing. The right plantaris and tibialis anterior muscles were removed under sodium pentobarbital anesthesia (50 mg/kg body weight, i.p.). Japanese Journal of Physiology Vol. 50, No. 4, 2000 413Japanese Journal of Physiology, 50, 413-418, 2000 Key words: cross-sectional area, muscle fiber, plantaris muscle, succinate dehydrogenase activity, tibialis anterior muscle. Abstract:The cross-sectional areas and succinate dehydrogenase activities of different types of fibers in different regions of the plantaris and tibialis anterior muscles in 10-week-old male rats were determined using quantitative histochemistry. The muscle fibers were classified as type I, type IIA, or type IIB according to their adenosine triphosphatase activities. There were no regional differences in either the mean cross-sectional area or the mean succinate dehydrogenase activity of type IIA fibers in both muscles. In contrast, type IIB fibers in the deep region of both muscles had smaller cross-sectional areas and higher succinate dehydrogenase activities than those in the superficial and middle regions. These data suggest the presence of regional differences in the cross-sectional area and succinate dehydrogenase activity of type IIB fibers in the muscle.

Twelve male Sprague-Dawley rats (21 days old) were randomly assigned into two experimental groups: sea level control (CONT) and hypobaric hypoxia (HYPO). The HYPO rats were kept in an hypobaric chamber maintaining a simulated altitude of 4000 m (61.1 kPa). After 10 weeks of treatment, the rat hindlimb muscles [soleus (SOL) and extensor digitorum longus (EDL)] were subjected to histochemical and electro-mechanical analyses. Results indicated that compared to CONT the HYPO SOL muscle had a significantly greater relative distribution of fast-twitch-oxidative-glycolytic (FOG) fibres (28.9% SEM 2.0 vs 18.3% SEM 1.8, P less than 0.01) with a significant decrease in slow twitch oxidative fibre distribution (69.5% SEM 2.4 vs 82.9% SEM 3.1, P less than 0.01). Compared to CONT the HYPO EDL muscle also manifested a significant increase in FOG fibre distribution (51.6% SEM 0.8 vs 46.6% SEM 1.1, P less than 0.01), but this was accompanied by a significant decrease in fast twitch glucolytic fibres (44.3% SEM 0.9 vs 49.2% SEM 1.7, P less than 0.05). These histochemical fibre type transformations accompanied significant and expected changes in the electro-mechanical parameters tested in situ, e.g. maximal twitch force, maximal rate of force development, contraction time, half relaxation time, force: frequency curve, and fatigability. It was concluded that chronic hypobaric hypoxia could have a potent influence upon the phenotype expression of muscle fibres.

Histochemical fibre-type composition and myosin heavy chain isoform component in the soleus muscle were studied in normoxic rats at postnatal ages of 5, 10, 15, and 20 weeks and in rats exposed to hypobaric hypoxia (460 torr) for 5 weeks from postnatal ages of 5, 10, and 15 weeks. The increase in the percentage of type I fibres and the concomitant decrease in that of type IIa fibres in the soleus muscle of normoxic rats were observed until 15 weeks of age. On the other hand, no change in the fibre-type composition of the muscle during postnatal development was observed in hypoxic rats, irrespective of the age at which they were exposed to hypoxia. The changes in the myosin heavy chain isoform component (MHC I and MHC IIa) of the muscle during postnatal development and by hypoxia corresponded well with those in the muscle fibre-type composition. It is concluded that hypobaric hypoxia inhibits the growth-related shift of muscle fibre-types from type IIa to type I and of myosin heavy chain isoforms from MHC IIa to MHC I in the rat soleus muscle, and that there are no changes in the muscle fibre-type composition or the myosin heavy chain isoform component caused by hypoxia after the shifts in these parameters which occur during postnatal development are completed.

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