Modulation of myonuclear number in functionally overloaded and exercised rat plantaris fibers

The effects of 10 wk of functional overload (FO), with and without daily tr eadmill endurance training, on the cross-sectional area, myonuclear number, and myonuclear domain size of mechanically isolated single fiber segments of the adult rat plantaris were determined. The fibers were typed on the ba sis of high-resolution gel electrophoresis for separation of specific myosi n heavy chain (MHC) isoforms and grouped as type I+ (containing some type I MHC with or without; any combination of fast MHCs), type IIa(+) (containin g some type IIa with or without some type IIx and/or ab but no type I MHC), and type IIx/b (containing only type IIx and/or IIb MHCs). Type I+ fibers had a higher myonuclear number than did both fast types of fibers in the co ntrol and FO, but not in the FO and treadmill trained, rats. All fiber type s in both FO groups had a significantly larger (36-90%) cross-sectional are a and a significantly higher (61-109%) myonuclear number than did control. The average myonuclear domain size of each fiber type was similar among the three groups, except for a smaller domain size in the type IIx/b fibers of the FO compared with control. In general, these data indicate that during hypertrophy the number of myonuclei increase proportionally to the increase in fiber volume. The maintenance of myonuclear domain size near control va lues suggests that regulatory mechanisms exist that ensure a tight coupling between the quantity of genetic machinery and the protein requirements of a fiber.