Denervation effects on myonuclear domain size of rat diaphragm fibers

Denervation (DNV) of rat diaphragm muscle (DIAm) leads to selective atrophy of type IIx and IIb fibers, whereas the cross-sectional area of type I and IIa fibers remains unchanged or slightly hypertrophied. DIAm DNV also increases satellite cell mitotic activity and myonuclear apoptosis. Similar to other skeletal muscles, DIAm fibers are multinucleated, and each myonucleus regulates the gene products in a finite fiber volume, i.e., myonuclear domain (MND). MND size varies across DIAm fiber types in rank order, I < IIa < IIx < IIb [fiber type based on myosin heavy chain isoform expression]. We hypothesized that, after DNV, the total number of myonuclei per fiber does not change and, accordingly, that MND changes proportionately to the change in fiber size regardless of fiber type. Adult rats underwent unilateral (right side) DIAm DNV, and after 2 wk single fibers were dissected. Fiber cross-sectional area, myonuclear number, and MND were measured by confocal microscopy, and these values in DNV DIAm were compared with those obtained in controls. After DNV, type I fibers hypertrophied, type IIa fiber size was unchanged, and type IIx and IIb fibers atrophied compared with control. The total number of myonuclei per fiber was not affected by DNV. Accordingly, after DNV, type I fiber MND increased by 25%, whereas it decreased in type IIx and IIb fibers by 50 and 70%, respectively. These results suggest that MND is not maintained after DNV-induced DIAm fiber hypertrophy or atrophy. These results are interpreted with respect to consequent effects of DNV on myonuclear transcriptional activity and protein turnover.