Figure 1.
Spinal cord motoneuron counts.
A: Photomicrograph of choline acetyl transferase labeled cross-sections of the spinal cord from a young adult rat and B: senescent rat (scale bar = 400 µm). Insets for both panels are higher power images showing motoneuron soma (arrows) (scale bar = 40 µm). C: mean motoneuron counts for young adult (YA) and senescent (SEN) rats. *P<0.05 versus YA.
Figure 2.
Fiber type changes with aging.
Photomicrographs of serial sections labeled for myosin heavy chain (MHC) slow (A,C) and MHC fast (B,D) within the red region of gastrocnemius muscle from a young adult (A,B) and senescent (C,D) rat. In these images 1 denotes a MHCs fiber, 2 denotes a MHCf fiber, and 3 denotes a fiber co-expressing both MHCs and MHCf. Scale bar is 50 µm.
Figure 3.
Fiber type proportion and fiber size in young adult (YA) and senescent (SEN) muscle.
Panel A depicts the percentage of fibers in each MHC class examined (*P<0.05 vs YA; too few MHC co-expressing fibers in YA to permit statistical comparison to SEN). Panel B depicts differences in fiber size according to each MHC class examined (*P<0.05 vs YA).
Figure 4.
Alterations in myofiber Nav1.5 expression with aging.
Photomicrographs of serial sections of the red region of gastrocnemius muscle labeled for Nav1.5 (A,C) and MHC slow (B,D) in young adult (A,B) and senescent (C,D) rats. In senescent muscle we saw three classes of Nav1.5 labelling; negative (N), ringed (displaying circumferential labelling, open arrowhead) and cytoplasm (showing cytoplasmic staining, shaded arrowhead). Scale bar is 50 µm.
Figure 5.
Fiber size in denervated versus innervated myofibers.
A: the mean fiber size of young adult (YA) myofibers where no Nav1.5 labeling was seen, versus Nav1.5 negative senescent (SEN) myofibers, Nav1.5 positive SEN ringed fibers, and Nav1.5 positive SEN cytoplasm fibers (*P<0.05 vs YA, **P<0.05 vs SEN negative, ***P<0.05 vs SEN ringed). B: the size of Nav1.5 positive fibers versus negative fibers according to the MHC labeling category in SEN muscle (*P<0.05 vs Nav1.5 negative, **P<0.05 vs ringed). C: the proportion of fibers within a given MHC labeling category that are Nav1.5 negative, Nav1.5 positive ringed, and Nav1.5 positive cytoplasm in SEN muscle (*P<0.05 vs MHC slow, **P<0.05 vs MHC slow and MHC fast).
Figure 6.
In Situ myofiber ubiquitin ligase expression with aging.
Serial images of the red region of gastrocnemius muscle labeled for MHC slow (A,B), MAFbx (C,D) and MuRF1 (E,F) in young adult (YA) (A,C,E) and senescent (SEN) (B,D,F) rats. Fibers sharing the same number are the same fiber in serial sections. Scale bar is 100 µm.
Figure 7.
Mean alterations in myofiber ubiquitin ligase expression with aging.
In situ expression levels of the ubiquitin ligases, MAFbx (A) and MuRF1 (B), in the red region of senescent gastrocnemius muscle in fibers that are MHC slow, MHC fast, and MHC fast/slow co-expressors, in relation to their Nav1.5 labeling status (*P<0.05 versus Nav1.5 negative, **P<0.05 versus Nav1.5 negative and versus Nav1.5 ringed).
Figure 8.
Schematic representation of atrophy in MHC slow versus MHC fast myofibers following denervation in aging muscle.
Thickness of arrow indicates relative proportion of fibers taking a given path. Briefly, upon denervation, MHC slow fibers exhibit increased MAFbx expression, followed by the majority of denervated fibers beginning to co-express MHC fast, coincident with initiation of atrophy. Upon denervation of MHC fast fibers, MAFbx is unchanged but atrophy is already seen, followed by a majority of denervated fibers beginning to co-express MHC slow. MuRF1 increases with prolonged duration of denervation in all MHC categories and is associated with further atrophy in MHC fast and MHC co-expressing myofibers.