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1 Muscle Function Laboratory, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061; 2 Department of Exercise Science, University of Georgia, Athens, 30602; and 3 Shepherd Center, Atlanta, Georgia 30309
10.1152/japplphysiol.000247.2001.
The effects of
spinal cord injury (SCI) on the profile of sarco(endo) plasmic
reticulum calcium-ATPase (SERCA) and myosin heavy chain (MHC) isoforms
in individual vastus lateralis (VL) muscle fibers were determined. Biopsies from the VL were obtained from SCI subjects 6 and 24 wk
postinjury (n = 6). Biopsies from nondisabled (ND)
subjects were obtained at two time points 18 wk apart
(n = 4). In ND subjects, the proportions of VL fibers
containing MHC I, MHC IIa, and MHC IIx were 46 ± 3, 53 ± 3, and 1 ± 1%, respectively. Most MHC I fibers contained SERCA2.
Most MHC IIa fibers contained SERCA1. All MHC IIx fibers contained
SERCA1 exclusively. SCI resulted in significant increases in fibers
with MHC IIx (14 ± 4% at 6 wk and 16 ± 2% at 24 wk). In
addition, SCI resulted in high proportions of MHC I and MHC IIa fibers
with both SERCA isoforms (29% at 6 wk and 54% at 24 wk for MHC I
fibers and 16% at 6 wk and 38% at 24 wk for MHC IIa fibers). Thus
high proportions of VL fibers were mismatched for SERCA and MHC
isoforms after SCI (19 ± 3% at 6 wk and 36 ± 9% at 24 wk)
compared with only ~5% in ND subjects. These data suggest that, in
the early time period following SCI, fast fiber isoforms of both SERCA
and MHC are elevated disproportionately, resulting in fibers that are
mismatched for SERCA and MHC isoforms. Thus the adaptations in SERCA
and MHC isoforms appear to occur independently.
fatigue; fiber type; myosin heavy chain; sarcoplasmic reticulum; vastus lateralis
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