Mechanisms of Injury‐Induced Calcium Entry into Peripheral Nerve Myelinated Axons: Role of Reverse Sodium‐Calcium Exchange

To investigate the route of axonal Ca 2+ entry during anoxia, electron probe x-ray microanalysis was used to measure elemental composition of anoxic tibial nerve myelinated axons after in vitro experimental procedures that modify transaxolemmal Na + and Ca 2+ movements. Perfusion of nerve segments with zero-Na + /Li + -substituted medium and Na + channel blockade by tetrodotoxin (1 μM) prevented anoxia-induced increases in Na and Ca concentrations of axoplasm and mitochondria. Incubation with a zero-Ca 2+ /EGTA perfusate impeded axonal and mitochondrial Ca accumulation during anoxia but did not affect characteristic Na and K responses. Inhibition of Na + -Ca 2+ exchange with bepridil (50 μM) reduced significantly the Ca content of anoxic axons although mitochondrial Ca remained at anoxic levels. Nifedipine (10 μM), an L-type Ca 2+ channel blocker, did not alter anoxia-induced changes in axonal Na, Ca, and K. Exposure of normoxic control nerves to tetrodotoxin, bepridil, or nifedipine did not affect axonal elemental composition, whereas both zero-Ca 2+ and zero-Na + solutions altered normal elemental content characteristically and significantly. The findings of this study suggest that during anoxia, Na + enters axons via voltage-gated Na + channels and that subsequent increases in axoplasmic Na + are coupled functionally to extraaxonal Ca 2+ import. Intracellular Na + -dependent, extraaxonal Ca 2+ entry is consistent with reverse operation of the axolemmal Na + -Ca 2+ exchanger, and we suggest that this mode of Ca 2+ influx plays a general role in peripheral nerve axon injury.