Calpain inhibitors confer biochemical, but not electrophysiological, protection against anoxia in rat optic nerves

Calpains are ubiquitous Ca2+-activated neutral proteases that have been imp licated in ischemic and traumatic CNS injury. Ischemia and trauma of centra l white matter are dependent on Ca2+ accumulation, and calpain overactivati on likely plays a significant role in the pathogenesis. Adult rat optic ner ves, representative central white matter tracts, were studied in an in vitr o anoxic model. Functional recovery following 60 min of anoxia and reoxygen ation was measured electrophysiologically. Calpain activation was assessed using western blots with antibodies against calpain-cleaved spectrin breakd own products. Sixty minutes of in vitro anoxia increased the amount of spec trin breakdown approximate to 20-fold over control, with a further increase after reoxygenation to >70 times control, almost as much as 2 h of continu ous anoxia. Blocking voltage-gated Na+ channels with tetrodotoxin or removi ng bath Ca2+ was highly neuroprotective electrophysiologically and resulted in a marked reduction of spectrin degradation. The membrane-permeable calp ain inhibitors MDL 28,170 and calpain inhibitor-1 (10-100 mu M) were effect ive at reducing spectrin breakdown in anoxic and reoxygenated optic nerves, but no electrophysiological improvement was observed. We conclude that cal pain activation is an important step in anoxic white matter injury, but inh ibition of this Ca2+-dependent process in isolation does not improve functi onal outcome, probably because other deleterious Ca2+-activated pathways pr oceed unchecked.