Role of cyclic AMP in hydrogen peroxide-induced potentiation of sympathetic neurotransmission in the bovine iris

Hydrogen sulfide: role in ion channel and transporter modulation in the eye

Hydrogen sulfide (H₂S), a colorless gas with a characteristic smell of rotten eggs, has been portrayed for decades as a toxic environmental pollutant. Since evidence of its basal production in mammalian tissues a decade ago, H₂S has attracted substantial interest as a potential inorganic gaseous mediator with biological importance in cellular functions. Current research suggests that, next to its counterparts nitric oxide and carbon monoxide, H₂S is an important multifunctional signaling molecule with pivotal regulatory roles in various physiological and pathophysiological processes as diverse as learning and memory, modulation of synaptic activities, cell survival, inflammation, and maintenance of vascular tone in the central nervous and cardiovascular systems. In contrast, there are few reports of a regulatory role of H₂S in the eye. Accumulating reports on the pharmacological role of H₂S in ocular tissues indicate the existence of a functional trans-sulfuration pathway and a potential physiological role for H₂S as a gaseous neuromodulator in the eye. Thus, understanding the role of H₂S in vision-related processes is imperative to our expanding knowledge of this molecule as a gaseous mediator in ocular tissues. This review aims to provide a comprehensive and current understanding of the potential role of H₂S as a signaling molecule in the eye. This objective is achieved by discussing the involvement of H₂S in the regulation of (1) ion channels such as calcium (L-type, T-type, and intracellular stores), potassium (K_ATP and small conductance channels) and chloride channels, (2) glutamate transporters such as EAAT1/GLAST and the L-cystine/glutamate antiporter. The role of H₂S as an important mediator in cellular functions and physiological processes that are triggered by its interaction with ion channels/transporters in the eye will also be discussed.

Pharmacological consequences of oxidative stress in ocular tissues

The eye is a unique organ because of its constant exposure to radiation, atmospheric oxygen, environmental chemicals and physical abrasion. That oxidative stress mechanisms in ocular tissues have been hypothesized to play a role in diseases such as glaucoma, cataract, uveitis, retrolental fibroplasias, age-related macular degeneration and various forms of retinopathy provides an opportunity for new approaches to their prevention and treatment, In the anterior uvea, both H2O2 and synthetic peroxides exert pharmacological/toxicological actions tissues of the anterior uvea especially on the sympathetic nerves and smooth muscles of the iris-ciliary bodies of several mammalian species. Effects produced by peroxides require the presence of trace amounts of extracellular calcium and the functional integrity of mitochondrial calcium stores. Arachidonic acid metabolites appear to be involved in both the excitatory action of peroxides on sympathetic neurotransmission and their inhibitory effect on contractility of the iris smooth muscle to muscarinic receptor activation. In addition to the peroxides, isoprostanes (products of free radical catalyzed peroxidation of arachidonic acid independent of the cyclo-oxygenase enzyme) can also alter sympathetic neurotransmission in anterior uveal tissues. In the retina, both H2O2 and synthetic peroxides produced an inhibitory action on potassium depolarization induced release of [3H] D-aspartate, in vitro and on the endogenous glutamate and glycine concentrations in vivo. Effects caused by peroxides in the retina are mediated, at least in part, by second messengers such as nitric oxide, prostaglandins and isoprostanes. The ability of H2O2 to alter the integrity of neurotransmitter pools from sympathetic nerves in the anterior uvea and glutaminergic nerves in the retina could underlie its role in the etiology of glaucoma.

Potentiation of sympathetic neurotransmission in bovine isolated irides by isoprostanes

Isoprostanes (IsoP) are formed by free radical catalyzed peroxidation of arachidonic acid independent of the cyclooxygenase enzyme. In the present study, we examined the effect of IsoP on norepinephrine (NE) release from the bovine isolated iris. Furthermore, we studied the role of IsoP's in hydrogen peroxide (H2O2)-induced enhancement of NE release from this tissue. Isolated bovine irides were prepared for studies of [3H]NE release using the superfusion method. Release of [3H]NE was induced via electrical field stimulation. Both 8-iso-prostaglandin E2 (E2-IsoP) and 8-iso-prostaglandin F2 alpha (F2-IsoP) produced a concentration-related enhancement of field-stimulated [3H]NE release from isolated bovine irides, an effect that was mimicked by the thromboxane (Tx) receptor agonist, U46619 and by H2O2. The Tx-receptor antagonist, SQ 29548 inhibited responses to E2-IsoP (10 microM) with an IC50 of 370 +/- 50 nM. SQ 29548 (10 microM) also blocked the enhancement of electrically-evoked [3H]NE release induced by U46619 (10 microM) but not that caused by H2O2 (300 microM). The Tx synthetase inhibitor, carboxyheptylimidazole (10 microM) prevented the stimulatory effect of E2-IsoP on evoked [3H]NE release without affecting responses induced by H2O2. We conclude that IsoP's can enhance sympathetic neurotransmission in the bovine isolated iris, an effect that can be blocked by a Tx-receptor antagonist. Furthermore, endogenously produced Tx's mediate the stimulatory effect of IsoP's on NE release. However, endogenously generated IsoP's or Tx's are not involved in H2O2-induced potentiation of sympathetic neurotransmission.

Effect of inhibition of cyclooxygenase on pre- and postjunctional actions of peroxides in the iris-ciliary body

In the present study, we investigated the effect of inhibition of cyclooxygenase (COX) with flurbiprofen (FBF) on peroxide-induced enhancement of field-stimulated [3H]norepinephrine ([3H]NE) release from bovine isolated irides. Furthermore, the effect of FBF was examined on peroxide-induced attenuation of contractions evoked by carbachol on this tissue. Irides were prepared for studies of neurotransmitter release and for measurement of contractile tension in vitro. Pretreatment of tissues with FBF (10 microM) caused significant (P < 0.001) rightward shifts of concentration-response curves to H2O2 and also decreased cumene hydroperoxide (cuOOH)-induced enhancement of evoked [3H]NE release. FBF (10 microM) partially prevented the attenuation of carbachol-induced contractions induced by H2O2 (300 microM) and cuOOH (300 microM). We conclude that inhibition of the biosynthesis of prostanoids reduced both the prejunctional stimulatory effects of H2O2 and cuOOH on sympathetic neurotransmission and inhibitory effects of peroxides on carbachol-induced contractions the in the bovine isolated iris.

Role of intracellular calcium in peroxide-induced potentiation of norepinephrine release from bovine isolated irides

1. BAPTA AM (10 microM), thapsigargin (10 microM), ruthenium red (30 microM) and oligomycin (30 microM) inhibited field-stimulated [3H]NE release from bovine isolated irides by 54%, 30%, 30% and 26%, respectively. 2. Both BAPTA AM and thapsigargin had no significant effect (P>0.05) on H2O2-induced potentiation of evoked [3H]NE release. 3. Ruthenium red prevented (but oligomycin enhanced) H2O2-induced enhancement of evoked [3H]NE release. 4. We conclude that, although intracellular calcium participates in field-stimulation evoked [3H]NE release from bovine isolated irides, only the mitochondrial pool of calcium may be involved in peroxide-induced enhancement of sympathetic neurotransmission.

Prejunctional alpha2-adrenoceptors and peroxide-induced potentiation of norepinephrine release from the bovine iris

Peroxides can enhance field-stimulated [3H]norepinephrine ([3H]NE) release in isolated irides from several mammalian species. In the present study, we investigated the role of prejunctional alpha2-adrenoceptors in peroxide-induced potentiation of sympathetic neurotransmission in bovine isolated irides. Isolated hemi-irides were incubated in a Krebs buffered-solution containing [3H]NE and prepared for studies of neurotransmitter release using the superfusion method. Alpha2-adrenoceptor agonists, oxymetazoline, UK-14304 and clonidine inhibited field-stimulated [3H]NE overflow without affecting basal tritium efflux. Pretreatment of tissues with H2O2 (300 microM) had no effect on inhibition of evoked [3H]NE release caused by the alpha2-adrenergic agonists. However, H2O2 (300 microM) caused significant (P < 0.01) leftward shifts of excitatory concentration-response curves to yohimbine (10 nM-1 microM). In contrast, yohimbine (1 microM) did not prevent the enhancement of evoked [3H]NE overflow induced by H2O2 (300 microM). In conclusion, excitatory effects of peroxides on sympathetic neurotransmission in bovine irides are not mediated by prejunctional alpha2-adrenoceptors.

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 previously shown that in vitro, H2O2 can enhance sympathetic neurotransmission in irides from several mammalian species. In the present study, we investigated the role of extracellular calcium in H2O2-induced potentiation of sympathetic neurotransmission in the bovine isolated iris. Isolated bovine hemiirides were incubated in a bicarbonate-buffered, carbogen-gassed Krebs buffer solution containing [3H]-norepinephrine ([3H]NE) for 60 min. After incubation, tissues were prepared for studies of [3H]NE release using the superfusion method. Release of [3H]NE was elicited by consecutive trains of electrical field stimulation. Removal of calcium from the buffer solution attenuated field-stimulated [3H]NE overflow in isolated, superfused bovine irides without affecting basal tritium efflux. H2O2 (1 mM) enhanced evoked [3H]NE release to the same extent in tissues exposed to buffer solutions containing normal calcium (1.3 mM) as in those containing low calcium (0.13 mM) or zero calcium. However, in the presence of zero-calcium buffer solution containing the chelator, EDTA (1 mM), H2O2 (1 mM) caused a gradual and sustained increase in basal tritium efflux. In buffer solutions containing high calcium (1.95 mM), the magnitude of H2O2-induced increase in field-stimulated [3H]NE release was significantly (P < 0.05) attenuated. Although the neuronal calcium channel antagonist omega-conotoxin (20 nM) inhibited [3H]NE by 25%, it had no effect on H2O2 (1 mM)-induced potentiation of evoked [3H]NE overflow. We conclude that while trace amounts of extracellular calcium are necessary for H2O2-induced enhancement of sympathetic neurotransmission, increasing extracellular (buffer) calcium concentration impaired peroxide-induced enhancement of [3H]NE release. Furthermore, voltage-activated calcium channels may not be directly involved in peroxide-induced alteration of adrenergic neurosecretion in bovine isolated irides.