Abstract
To determine the variations of myonuclear number and cytoplasm-to-nucleus ratio (domain size) relative to the cross-sectional area (CSA) in the expression of myosin heavy chain (MHC) isoforms, and across fiber types in muscle, the nuclear number and CSA were examined in single fibers from rat soleus (Sol) and medial gastrocnemius (MG) muscles. Confocal microscopy was used to measure CSA, count nuclear number, and to estimate domain size.
The nuclear number varies across fiber types in rank order, type I > I+IIa > IIa in Sol, and type IId/x > IIa > IIb in MG. In contrast, the nuclear domain sizes varied across fiber types in rank order, type IIa > I+IIa > I in Sol, and type IIb > IId/x > IIa in MG. These results may imply a higher metabolic protein turnover rate per nucleus in type I in Sol, and in type IId/x in MG.
The nuclei number and domain size seem to vary in relation to CSA in each type of MHC expression and across fiber types in both Sol and MG. We found that higher relationships existed between the number of nuclei and CSA in Sol compared with MG, but, showed that higher relationships existed between the domain size and CSA in MG compared with Sol. These relationships suggest that in fibers from Sol the fiber size is more dependent on nuclear number, however, in the fibers from MG the fiber size is more dependent on domain size per nucleus. Our data suggest that the mechanisms regulating the modulation of the relationship among nuclear number, domain size and fiber size are different in these two muscles.
