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The Journal of Neurophysiology Vol. 84 No. 4 October 2000, pp. 1763-1769
Copyright ©2000 by the American Physiological Society
Division of Neurosurgery, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599
Shi, Riyi,
Tomoko Asano,
Neil C. Vining, and
Andrew R. Blight.
Control of Membrane Sealing in Injured Mammalian Spinal Cord
Axons. J. Neurophysiol. 84: 1763-1769, 2000. The process of sealing of damaged axons was
examined in isolated strips of white matter from guinea pig spinal cord
by recording the "compound membrane potential," using a sucrose-gap
technique, and by examining uptake of horseradish peroxidase (HRP).
Following axonal transection, exponential recovery of membrane
potential occurred with a time constant of 20 ± 5 min, at 37°C,
and extracellular calcium activity
([Ca2+]o) of 2 mM. Most
axons excluded HRP by 30 min following transection. The rate of sealing
was reduced by lowering calcium and was effectively blocked at
[Ca2+]o
0.5 mM, under
which condition most axons continued to take up HRP for more than
1 h. Sealing at higher
[Ca2+]o was blocked by
calpain inhibitors (calpeptin and calpain inhibitor-1) indicating a
requirement for type II (mM) calpain in the sealing process. Following
compression injury, the amplitude of the maximal compound action
potential conducted through the injury site was reduced. The extent of
amplitude reduction was increased when the tract was superfused with
calcium-free Krebs' solution (Ca2+ replaced by
Mg2+). These results suggest that the fall in
[Ca2+]o seen following
injury in vivo is sufficient to prevent membrane sealing and may
paradoxically contribute to axonal dieback, retrograde cell death, and
"secondary" axonal disruption.
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