In vitro spinal cord conduction block during exposure to a xanthine oxidase/hypoxanthine system: noninvolvement of superoxide and hydrogen peroxide

The effects of a reactive oxygen system on axonal conduction were assessed in an in vitro rat spinal cord preparation. An enzyme system, containing hypoxanthine and xanthine oxidase as a source of superoxide and hydrogen peroxide, was used in combination with ADP and FeCl3 as catalysts for peroxidative activity. The reactants were mixed as they entered a temperature-controlled Plexiglas chamber containing a longitudinal hemisection of adult rat spinal cord. Extracellular action potentials were recorded with a glass microelectrode before, during, and after the exposure. A significant conduction block developed during the 30 min exposure. Action potential amplitude decreased to less than 45% of pre-exposure level while absolute refractory period to paired stimuli increased 160%. Following reintroduction of normal bathing medium, amplitude and absolute refractory period exhibited recovery toward pre-exposure control levels, but did not fully recover. Isolated spinal cord membranes exposed to the same xanthine oxidase system produced significant levels of malondialdehyde (MDA). Superoxide dismutase (SOD), but not catalase, effectively inhibited MDA production. Hypoxanthine, xanthine oxidase, and ADP-Fe3+ were all required to induce conduction block in the spinal cord and peroxidation in the isolated membranes. However, addition of intermediate scavengers, SOD and catalase, alone or in tandem, did not prevent the conduction block. Mechanisms other than radical-induced lipid peroxidation may be working to alter the membrane ionic equilibrium in the cord preparation.