Anoxie Injury in the Rat Spinal Cord: Pharmacological Evidence for Multiple Steps in Ca2+-Dependent Injury of the Dorsal Columns

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Abstract
To examine anoxic injury in spinal cord white matter, we studied axonal conduction in the dorsal columns during and following a standard 60 min anoxic insult at 36°C. Perfusion of the spinal cord in 0-Ca2+ Ringer solution resulted in significantly improved recovery of the compound action potential. Similarly, removal of Na+ from the perfusate resulted in significantly improved recovery of conduction in dorsal column axons. Exposure of the anoxic spinal cord to the Na+ channel blocker tetrodotoxin (TTX), the Na–Ca exchange blockers benzamil and bepridil, Na+–H+ exchange blockers amiloride and harmaline, and perfusion in Ringer solution with pH adjusted to 6.4, all resulted in improved recovery. The tertiary anesthetics procaine and lidocaine, as well as phenytoin and carbamazepine, also resulted in improved recovery of compound action potential amplitude after 60 min of anoxia. These results demonstrate that a significant component of irreversible loss of conduction, following anoxic injury of the dorsal columns, is Ca2+-dependent. Moreover, these results demonstrate that TTX-inhibitable Na+ channels participate in the pathophysiology of anoxic injury in spinal cord white matter, and indicate that reverse Na–Ca exchange provides a route for at least part of the damaging influx of Ca2+ into an intracellular compartment in anoxic spinal cord white matter. Our results also suggest that extracellular acidosis may have a protective effect on anoxic spinal cord white matter, and support the hypothesis that anoxic injury of spinal cord white matter may involve the Na+-H+ exchanger. Key words: spinal cord injury; Na+ channels; Na+–Ca2+ exchange; tetrodotoxin; amiloride; harmaline; benzamil; bepridil; acidosis; pH; lidocaine; procaine; phenytoin; carbamazepine