Calpains are ubiquitous Ca(2+)-activated neutral
proteases that have been implicated in ischemic and traumatic CNS injury.
Ischemia and
trauma of central white matter are dependent on Ca2+ accumulation, and
calpain overactivation likely plays a significant role in the pathogenesis. Adult rat optic nerves, representative central white matter tracts, were studied in an in vitro anoxic model. Functional recovery following 60 min of
anoxia and reoxygenation was measured electrophysiologically.
Calpain activation was assessed using western blots with
antibodies against
calpain-cleaved
spectrin breakdown products. Sixty minutes of in vitro
anoxia increased the amount of
spectrin breakdown approximately 20-fold over control, with a further increase after reoxygenation to >70 times control, almost as much
as 2 h of continuous
anoxia. Blocking voltage-gated Na+ channels with
tetrodotoxin or removing bath Ca2+ was highly neuroprotective electrophysiologically and resulted in a marked reduction of
spectrin degradation. The membrane-permeable
calpain inhibitors MDL 28,170 and
calpain inhibitor-I (10-100 microM) were effective at reducing
spectrin breakdown in anoxic and reoxygenated optic nerves, but no electrophysiological improvement was observed. We conclude that
calpain activation is an important step in anoxic white matter injury, but inhibition of this Ca(2+)-dependent process in isolation does not improve functional outcome, probably because other deleterious Ca(2+)-activated pathways proceed unchecked.