CYTOPLASM-TO-MYONUCLEUS RATIOS AND SUCCINATE-DEHYDROGENASE ACTIVITIESIN ADULT-RAT SLOW AND FAST MUSCLE-FIBERS

The relationship between myonuclear number, cellular size, succinate d ehydrogenase activity, and myosin type was examined in single fiber se gments (n=54; 9+/-3 mm long) mechanically dissected from soleus and pl antaris muscles of adult rats. One end of each fiber segment was stain ed for DNA before quantitative photometric analysis of succinate dehyd rogenase activity; the other end was double immunolabelled with fast a nd slow myosin heavy chain monoclonal antibodies. Mean +/- S.D. cytopl asmic volume/myonucleus ratio was higher in fast and slow plantaris fi bers (112+/-69 vs. 34+/-21x10(3) mu m(3)) than fast and slow soleus fi bers (40+/-20 vs. 30+/-14x10(3) mu m(3)), respectively. Slow fibers al ways had small volumes/myonucleus, regardless of fiber diameter, succi nate dehydrogenase activity, or muscle of origin. In contrast, smaller diameter (<70 mu m) fast soleus and plantaris fibers with high succin ate dehydrogenase activity appeared to have low volumes/myonucleus whi le larger diameter (>70 mu m) fast fibers with low succinate dehydroge nase activity always had large volume/myonucleus. Slow soleus fibers h ad significantly greater numbers of myonuclei/mm than did either fast soleus or fast plantaris fibers (116+/-51 vs. 55+/-22 and 44+/-23), re spectively. These data suggest that the myonuclear domain is more limi ted in slow than fast fibers and in the fibers with a high, compared t o a low, oxidative metabolic capability.