ELEMENTAL COMPOSITION AND WATER-CONTENT OF RAT OPTIC-NERVE MYELINATEDAXONS AND GLIAL-CELLS - EFFECTS OF IN-VITRO ANOXIA AND REOXYGENATION

Electron probe x-ray microanalysis was used to measure water content a nd concentrations (mmol/kg dry weight) of elements (Na, P, S, Cl, K, C a, and Mg) in myelinated axons and glial cells of rat optic nerve expo sed to in vitro anoxia and reoxygenation. in response to anoxia, large , medium, and small diameter fibers exhibited an early (5 min) and pro gressive loss of Na and K regulation which culminated (60 min) in seve re depletion of respective transmembrane gradients. As axoplasmic Na l evels increased during anoxic exposure, a parallel rise in Ca content was noted. For all axons, mean water content decreased progressively d uring the initial 10 min of anoxia and then returned toward normal val ues as anoxia continued. Analyses of mitochondrial areas revealed a si milar pattern of elemental disruption except that Ca concentrations ro se more rapidly during anoxia. Following 60 min of postanoxia reoxygen ation, the majority of larger fibers displayed little evidence of reco very, whereas a subpopulation of small axons exhibited a trend toward restoration of normal elemental composition. Glial cells and myelin we re only modestly affected by anoxia and subsequent reoxygenation. Thus , anoxic injury of CNS axons is associated with characteristic changes in axoplasmic distributions of Na, K, and Ca. The magnitude and tempo ral patterns of elemental Na and Ca disruption are consistent with rev ersal of Na+-Ca2+ exchange and subsequent Ca entry (Stys et al., 1992) . During reoxygenation, elemental deregulation continues for most CNS fibers, although a subpopulation of small axons appears to be capable of recovery.