Mechanisms of injury-induced calcium entry into peripheral nerve myelinated axons: in vitro anoxia and ouabain exposure

In the present investigation, electron probe X-ray microanalysis was used to characterize the effects of in vitro ouabain (2 mM) or anoxia on elemental composition (e.g. Na, K, Ca) and water content of rat peripheral (tibial) nerve myelinated axons and Schwann cells. Results showed that independent of axon size, both ouabain and anoxia markedly increased axoplasmic Na and decreased K concentrations. However, only anoxia was associated with significant elevation of axonal Ca content. Mitochondrial areas from ouabain- or anoxia-exposed fibers exhibited changes in element and water contents that were similar to axoplasmic alterations. Schwann cells and myelin displayed small increases in Na and substantial losses of K in response to ouabain exposure. In contrast, these glial compartments were relatively resistant to anoxia as indicated by the modest and delayed nature of the elemental changes. Nonetheless, neither treatment significantly affected glial Ca concentrations. Our results suggest that Ca²⁺ accumulation in peripheral nerve axons is complex and involves not only deregulation of Na⁺ and K⁺ but other fundamental pathogenic changes as well. In addition to providing baseline information, we have identified an in vitro model (anoxia) which features Ca²⁺ build-up in PNS myelinated axons. Thus, the present study offers a foundation for investigation into mechanisms of Ca²⁺ entry following peripheral nerve injury.