Slow-twitch Oxidative Research Articles

Ultrastructural alterations in skeletal muscle fibers of streptozotocin-diabetic rats.

The ultrastructure of fast-twitch-oxidative-glycolytic (FOG), fast-twitch-glycolytic (FG) and slow-twitch-oxidative (SO) fibers in plantaris and soleus muscles of normal and streptozotocin-diabetic rats was studied. In the diabetic animals, the mitochondria of FOG and SO fibers showed a loss of cristae and an increase in electron-dense granules. There was also an increased number of lipid droplets in close proximity to the mitochondria and the nuclei, and a separation of individual muscle nuclei to form satellite cells. Higher incidences of surface projections and sarcoplasmic splittings at the nuclear region were noticed in SO fibers. The FG fibers showed some disorientation of the T-tubular system. It is concluded that streptozotocin-diabetes has differential effects on the fine structure of the three fiber types of rat skeletal muscle.

Human muscle fiber types in power lifters, distance runners and untrained subjects

Muscle biopsies were taken from the vastus lateralis of 12 males: 5 control subjects, 4 power lifters and 3 distance runners. Three fiber "types" were distinguished by comparing serial sections for alkaline myofibrillar adenosine triphosphatase (ATPase) and succinic dehydrogenase (SDH) activities: 1. high ATPase and low SDH; fast-twitch-glycolytic (FG). 2. High ATPase and high SDH; fast-twitch-oxidative-glycolytic (FOG). 3. Low ATPase and high SDH; slow-twitch-oxidative (SO). In some cases the distinction between the FOG and FG classess was not clear and a group termed "transitional" was employed. A variation in percentage of fiber types and fiber area was found among individuals. The percentage of SO fibers varied from 19.6-60.1% within all 3 groups, with a mean of 40.5%. In the control group approximately 75% of the fibers were oxidative (FOG + SO). The major characteristics of the lifters were a decrease in the percentage of FOG fibers and a hypertrophy of FOG and FG fibers. The distance runners had a high percentage of oxidative fibers with few FG fibers. It is suggested that the fast-twitch fibers are mainly involved in the adaptation of muscle to exercise since the percentage of SO fibers varies greatly among individuals within and between the 3 groups studied.

Histochemical localization of lactate dehydrogenase isozymes in human skeletal muscle fibers

Lactate dehydrogenase (LDH) activity was histochemically localized in fibers of the vastus lateralis muscle of men and for comparative purpose in the soleus and plantaris muscleo of rats. Human muscle fibers were identified as fast twitch (FT) or slow twitch (ST) from the histochemical stain for myofibrillar adenosine triphosphatase activity. Rat skeletal muscle fibers were classified as fast-twitch-oxidative-glycolytic (FOG), fast-twitch-glycolytic (FG), or slow-twitch-oxidative (SO) on the basis of NADH-diaphorase and myofibrillar adenosine triphosphatase activities. Heart-type (H) LDH was identified by inhibition of the muscle-type (M) isozyme with 4 M urea. Total LDH as estimated histochemically was highest in the human FT and rat FG fibers. This was predominantly the M-LDH isozyme. ST fibers of human and SO fibers of rat skeletal muscle had the least total LDH but the most H-LDH activity. The FOG fibers of rat muscle contained a total LDH activity intermediate to that of the FG and SO fibers and a combination of H- and M-LDH. There were no fibers in the human muscle samples studied that had LDH activities similar to the FOG fibers of rat muscle.

Glycogen depletion in rat skeletal muscle fibers at different intensities and durations of exercise.

Total muscle glycogen depletion, the glycogen depletion pattern (PAS staining) in the different fiber types of skeletal muscle, and several other measures of carbohydrate metabolism were studied in rats that ran 162.2 m at varying speeds (22.5–80.5 m/min) or swam from 0.5–4 hrs. Muscle glycogen declined as an increasing function of exercise intensity during running whereas during swimming there was a near linear decline in muscle glycogen throughout the 4 hrs of exercise. Blood lactate did not increase until running speed exceeded a load that would required a VO2 of about 60% of aerobic capacity. a peak lactate of 21.15 mM occurred after the rats ran at 67m/min. Liver glycogen declined steadily at a rate of about 0.6 mg×g−1×min−1 during the first 2 hrs of swimming. During this time blood glucose was maintained at or above resting levels. During the final 2 hrs of swimming glycogenolysis in the liver declined to about 0.09 mg×g−1×min−1 and there was then a sharp decrease in blood glucose to a final value of 68.7 mg/100 ml. At low running speeds and during the first hour of swimming the greatest loss in PAS staining occured in fast-twitch-oxidative-glycolytic (FOG) and slow-twitch-oxidative (SO) fibers. At running speeds above about 50 m/min a rapid loss of PAS staining was observed in the fast-twitch-glycolytic (FG) fibers. These higher speeds are above those that should elicit VO2 max for the rat. In the late stages of swimming FG fibers also demonstrated a loss of glycogen. These data suggest that at low work intensities there is a primary reliance upon oxidative fibers for contractile activity and that a major use of anaerobic fibers only occurs at high work levels or when the aerobic fibers are depleted of glycogen during prolonged-low intensity work.