Regulation of uveal sympathetic neurotransmission by peroxides

Purpose. To investigate the effect of naturally occurring and synthetic peroxides on norepinephrine release from isolated iris-ciliary bodies of several mammalian species. Methods. Hemiirides (bovine) and iris-ciliary bodies (human, rabbit, and rat) were incubated in Krebs solution containing [³H]-norepinephrine ([³H]NE) for 60 minutes. After incubation, tissues were set up for studies of [³H]NE release using the superfusion method. Release of [³H]NE was elicited through electrical field stimulation. Results. In bovine irides, hydrogen peroxide (H₂O₂), cumene hydroperoxide (cuOOH), and tert-butyl hydroperoxide (buOOH) caused a concentration- dependent potentiation of field-stimulated [³H]NE release with the following rank order of potency: cuOOH > H₂O₂ > buOOH. Furthermore, the free radical scavenger, melatonin (2 mM), prevented the enhancement of evoked [³H]NE overflow elicited by H₂O₂ and cuOOH. At equimolar concentrations, H₂O₂ (1 mM) increased stimulated [³H]NE release from rabbit, human (mean age, 29.7; range, 15 to 48 years), and Fischer 344 rat (4 months old) iris-ciliary bodies by 98%, 50%, and 40%, respectively. However, H₂O₂ (1 mM) caused a 9% increase in evoked [³H]NE release in tissues from aged Fischer 344 rats (30 months old) and a 5% decrease in neurotransmitter release in tissues from old human donors (mean age, 72.3 years; range, 69 to 74 years). Conclusions. Peroxides such as H₂O₂ can potentiate sympathetic neurotransmission in the anterior uvea of several mammalian species. In bovine irides, H₂O₂ induced enhancement of neurotransmitter release can be mimicked by synthetic peroxides and may involve the generation of reactive oxygen species.