THE FUNCTIONAL PROPERTIES OF VERY SMALLER MYOFIGERS IN REGENERATED SKELETAL MUSCLE AFTER EXERCISE-INDUCED MUSCLE INJURY / 体力科学

Markedly smaller myofibers including branched myofibers (SF) were contained in regenerated skeletal muscles. However, its functional properties are not yet sufficiently understood. The present study examined the metabolic, contractile properties and hypertrophy of SF included in rat skeletal plantaris muscles regenerated from eccentric contraction-induced muscle injury. Succinate dehydrogenase and alpha-glycerophosphate dehydrogenase activity of SF were respectively similar to that of other normal size myofibers (NF) within regenerated muscles. The shortening velocity of skinned fibers prepared from branched myofibers isolated from regenerated muscles was slightly slower than that of control muscles. In addition, the extent of glycogen depletion in SF was similar to that of NF after exhaustive running. Therefore, SF were innervated by motoneurons and recruited during the contractile activity of the skeletal muscle. No matter when the regenerated muscle was loaded by synergistic ablation, the continued existence of SF was observed. Therefore, the ability of hypertrophy in SF may be distinct from that in NF. The physiological properties of SF were similar to that of normal myofiber, except for the ability to regulate hypertrophy.

Time Course of Rat Soleus Muscle Fiber Composition and Motoneurons During Three Weeks Hindlimb Suspension / 体力科学

We examined the time course of soleus muscle fiber type composition. Soma area and succinate dehydrogenase (SDH) activity of soleus motoneurons during three weeks of hindlimb suspension (HS) in rats. Adult female Wistar rats (n=34, 252-288g BW) were divided into four groups: control (n=8), hindlimb suspended for one week (HS 1 wk, n=8), two weeks (HS 2 wk, n=9), and three weeks (HS 3 wk, n=9) . Soleus muscle fiber composition was calculated from transverse sections stained for myosin ATPase (preincubation pH 10.3, 4.3) . The fiber type composition did not change in the HS 1 wk, but in the HS 2 wk and HS 3 wk, the proportion of type I fibers decreased and that of type IIc and ha fibers increased. Using a fluorescent neuronal tracer nuclear yellow, motoneurons innervating the soleus muscle were identified, and the soma area and SDH activity were measured. The soma area did not change for up to two weeks of HS, but decreased in the HS 3 wk. Compared with control, SDH activity of soleus motoneurons decreased in the HS 1 wk. However, in the HS 2 wk, the activity increased to the level of control. In the HS 3 wk, the activity tended to increase further. Generally, muscle fibers and their motoneurons have unitary characteristics. However, these results suggest that change in soleus muscle fiber composition are not accompanied by changes in soma area and SDH activity in soleus motoneurons during three weeks of HS.

Relationship between muscle fiber composition, capillary supply, oxidative enzyme activity and aerobic work capacity in middle- and long-distance runners / 体力科学

It was the purpose of this study to examine the property of skeletal muscle fiber in middle- and long-distance runners, and to evaluate the relationship between the property of skeletal muscle fiber and aerobic work capacity.<BR>Biopsy samples were taken from M, vastus lateralis of 23 middle- and long-distance runners, and analyzed for muscle fiber composition, capillary supply and SDH activity. Aerobic work capacity was measured on a treadmill with continuous method.<BR>The mean value for % ST in M. vastus lateralis was 61.3%, and the muscle fiber distribution showed the tendency of slow type. The crosssectional area of ST and FT fibers ranged approximately from 3, 000 μm<SUP>2</SUP> to 13, 000 μm<SUP>2</SUP>, and the cross-sectional area of ST fiber tended to be larger than that of FT fiber. The mean values for CD, C/F ratio, and CC (/ST) as capillary supply were 279/mm<SUP>2</SUP>, 1.62 and 5.7, respectively. The mean value for SDH activity was 5.86 μmoles/g/min, and was lower than reported previously for elite runners. In addition, SDH activity correlated with % ST and CC (/ST) (r=0.480, p<0.05, r=0.640, p<0.01) . On the other hand, the mean value for VOG<SUB>2</SUB>max was 68.5 ml/ kg/min (4, 066 ml/min), while VO<SUB>2</SUB>@AT averaged 45.9 ml/kg/min (2, 756 ml/min) . And it was found that VO<SUB>2</SUB>max and VO<SUB>2</SUB>@AT correlated significantly with % ST, CC (/ST) and SDH activity. Particular, VO<SUB>2</SUB> max was most strongly related to SDH activity (r=0.612, p<0.01) .<BR>The results of this study suggest that in regard to middle- and long-distance runners, structural characteristics of the ST fiber and capillary are some of the indispensable factors to regulate such a functional property as SDH activity and that the distinctive characteristics of the skeletal muscle fiber might possibly influence aerobic work capacity to a great extent.