MECHANISM OF PEROXIDE-INDUCED POTENTIATION OF SYMPATHETIC NEUROTRANSMISSION IN BOVINE IRIDES - ROLE OF EXTRACELLULAR CALCIUM

Hydrogen peroxide (H2O2) and enzymes that regulate its metabolism are present in tissues of the anterior segment of the eye. We have previou sly shown that in vitro, H2O2 can enhance sympathetic neurotransmissio n in irides from several mammalian species. In the present study, we i nvestigated the role of extracellular calcium in H2O2-induced potentia tion of sympathetic neurotransmission in the bovine isolated iris. Iso lated bovine hemiirides were incubated in a bicarbonate-buffered, carb ogen-gassed Krebs buffer solution containing [H-3]-norepinephrine ([H- 3]NE) for 60 min. After incubation, tissues were prepared for studies of [H-3]NE release using the superfusion method. Release of [H-3]NE wa s elicited by consecutive trains of electrical field stimulation. Remo val of calcium from the buffer solution attenuated field-stimulated [H -3]NE overflow in isolated, superfused bovine hides without affecting basal tritium efflux. H2O2 (1 mM) enhanced evoked [H-3]NE release to t he same extent in tissues exposed to buffer solutions containing norma l calcium (1.3 mM) as in those containing low calcium (0.13 mM) or zer o calcium. However, in the presence of zero-calcium buffer solution co ntaining the chelator, EDTA (1 mM), H2O2 (1 mM) caused a gradual and s ustained increase in basal tritium efflux. In buffer solutions contain ing high calcium (1.95 mM), the magnitude of H2O2-induced increase in field-stimulated [H-3]NE release was significantly (P < 0.05) attenuat ed. Although the neuronal calcium channel antagonist omega-conotoxin ( 20 nM) inhibited [H-3]NE by 25%, it had no effect on H2O2 (1 mM)-induc ed potentiation of evoked [H-3]NE overflow. We conclude that while tra ce amounts of extracellular calcium are necessary for H2O2-induced enh ancement of sympathetic neurotransmission, increasing extracellular (b uffer) calcium concentration impaired peroxide-induced enhancement of [H-3]NE release. Furthermore, voltage-activated calcium channels may n ot be directly involved in peroxide-induced alteration of adrenergic n eurosecretion in bovine isolated irides.