Abstract
Hydrogen sulfide (H2S) has been identified to be one of the gaseous transmitters in the body. There are studies enumerating its physiological significance in regulation of different processes in various organ systems. Identification of enzymes involved in the endogenous synthesis of the gas in the eye led to the investigation of its role in the eye. It was found that H2S is not only required at physiological concentrations, but its|deficiency can lead to some ocular conditions.|Glaucoma is an ocular disorder that involves progressive damage to the optic nerve. The condition is associated with an increase in the intraocular pressure (IOP). Its levels are maintained with a balance in the inflow and outflow of aqueous humor, a transparent fluid responsible for maintaining IOP. Existing therapies include medications either reducing aqueous humor production or strategies improving its drainage in order to restore the normal IOP. However these therapies are associated with many side effects and there is a need to improve the therapy for glaucoma.|The present study investigated the role of H2S donors in decreasing sympathetic neurotransmission in the anterior uvea in the eye and reducing IOP. There is evidence that decrease in levels of norepinephrine (NE), the major sympathetic neurotransmitter causes relaxation of pre-contracted iris muscles that increases space between the trabecular meshwork. This improves drainage of aqueous humor which can help alleviate IOP.|In this study, the effect of two H2S donors, GYY 4137 and ACS67 and L-cysteine, the substrate for endogenous H2S production on electrically evoked [3H]NE release in isolated bovine iris-ciliary bodies (ICB) in vitro was determined. GYY 4137 (1-30 μM), ACS67 (10 nM-10 μM) as well as L-cysteine (10 nM-10μM) attenuated field-stimulated [3H]NE release in bovine ICB in a concentration-dependent manner. The inhibition was in the order: ACS67 > L-cysteine > GYY 4137. GYY 4137 elicited a maximum inhibition of 20.8% (n=3; p<0.001) at 30 μM while ACS67 achieved a maximum inhibition of 37.26% (n=5; p<0.001) at 10 μM. For L-cysteine, the maximum inhibition achieved was 26.13% (n=5; p< 0.5) at a dose of 10μM. It was also found that at high doses, endogenously produced prostaglandins (PGs) contributed to the effect of the drugs. In order to eliminate this effect, endogenous PG synthesis was blocked using flurbiprofen (FBF) which is a PG synthesis inhibitor.|To determine the mechanism of action of the drugs, antagonists were used. These included aminooxyacetic acid (AOA), a cystathionine-β-synthase (CBS) inhibitor and glibenclamide, a KATP channel blocker. Although AOA (3mM) and glibenclamide (300 μM) had no effect (p>0.05) on [3H]NE release, they both reversed the inhibitory action of GYY 4137 (10-30 μM), ACS67 (0.1-1 μM) and L-cysteine (10 μM) on the neurotransmitter release. This led to the conclusion that the inhibitory action on neurotransmission is partially dependent on in situ release of H2S and possibly on the activation of KATP -channels.|Finally in vivo experiments were performed to study the effect of the drugs on IOP in male normotensive rabbits. GYY 4137 (0.1-2%) and ACS67 (0.005-001%) reduced IOP in both treated and untreated eyes. GYY 4137 (2%) exhibited a maximum inhibition of 27.76% (n=5; p< 0.001) 6h after treatment, ACS67 (0.005%) achieved a maximum effect of 23.27% (n=5; p<0.001) 3h after treatment.|Thus the final conclusion drawn from the studies was that H2S donors attenuate sympathetic neurotransmission in the anterior uvea. This effect could be primarily related to the release of H2S from the drugs which possibly act via activation of KATP channels. This alteration in neurotransmission can be co-related to the ability of these H2S donors in alleviating IOP, however this hypothesis merits further investigation.|One approach to confirm this hypothesis could be investigating the effect of AOA and glibenclamide on the activity of drugs in vivo. Studying the effect of the H2S donors on IOP of glaucomatous rabbits could further confirm the efficacy of these drugs as a potential therapy for glaucoma.