Effect of oxidative stress on sympathetic neurotransmission in the bovine iris

In a previous study, we showed that hydrogen peroxide (H₂O₂) enhanced adrenergic neurosecretion in irides from several mammalian species presumably via a prejunctional action (Ohia et al., Exp. Eye Res. 59:Abs. No. 173, 1994). Purpose: To examine the effect of stress caused by various oxidants on norepinephrine (NE) release and membrane lipid peroxidation in the bovine isolated iris. Methods: Hemi-rides were isolated from bovine eyes and prepared for [³H]NE release assays using the superfusion method. For biochemical assays, lipid peroxidation was measured in iris membranes by determining their content of thiobarbituric acid reactive substances in the absence and presence of oxidants. Results: H₂O₂, cumene hydroperoxide (cuOOH) and t-butyl hydroperoxide (buOOH) caused a concentration-related enhancement of field stimulation-induced [³H]NE overflow with the following rank of potency: cuOOH>H₂O₂>buOOH. Potentiation of evoked [³H]NE release produced by submaximal concentrations of H₂O₂ (0.3 mM) and cuOOH (0.03 mM) was abolished by the antioxidant, melatonin (2 mM). Both H₂O₂ and cuOOH caused a concentration-dependent increase in lipid peroxidation over basal levels in isolated iris membranes. For instance, cuOOH (0.3 mM) increased lipid peroxidation (nmoles MDA/mg protein) from 2.99 ± 0.11 (n=10) to 8.16 ± 0.37 (n=7). Conclusion: We conclude that oxidants produce a specific enhancement of evoked NE release from sympathetic nerves in the bovine iris probably via an effect on membrane lipid peroxidation.