MECHANISMS OF CALCIUM AND SODIUM FLUXES IN ANOXIC MYELINATED CENTRAL-NERVOUS-SYSTEM AXONS

Electron probe X-ray microanalysis was used to measure water content a nd concentrations of elements (i.e. Na, K, Cl and Ca) in selected morp hological compartments of rat optic nerve myelinated axons. Transaxole mmal movements of Na+ and Ca2+ were modified experimentally and corres ponding effects on axon element and water compositions were determined under control conditions and following in vitro anoxic challenge. Als o characterized were effects of modified ion transport on axon respons es to postanoxia reoxygenation. Blockade of Na+ entry by tetrodotoxin (1 mu M) or zero Na+/Li+-substituted perfusion reduced anoxic increase s in axonal Na and Ca concentrations. Incubation with zero-Ca2+/EGTA p erfusate prevented axoplasmic and mitochondrial Ca accumulation during anoxia but did not affect Na increases or K losses in these compartme nts. Inhibition of Na+-Ca2+ exchange with bepridil (30 mu M) selective ly prevented increases in intra-axonal Ca, whereas neither nifedipine (5 mu M) nor nimodipine (5 mu M) influenced the effects of anoxia on a xonal Na, K or Ca. X-ray microanalysis also showed that prevention of Na and Ca influx during anoxia obtunded severe elemental deregulation normally associated with reoxygenation. Results of the present study s uggest that during anoxia, Na+ enters axons mainly through voltage-gat ed Na+ channels and that subsequent increases in axoplasmic Na+ are fu nctionally coupled to extra-axonal Ca2+ import. Na-i(+)-dependent, Ca- o(2+) entry is consistent with reverse operation of the axolemmal Na+- Ca2+ exchanger and we suggest this route represents a primary mechanis m of Ca2+ influx. Our findings also implicate a minor route of Ca2+ en try directly through Na+ channels. (C) 1997 IBRO. Published by Elsevie r Science Ltd.