Regulation of vascular tone in rabbit ophthalmic artery: Cross talk of endogenous and exogenous gas mediators

doi:10.1016/j.bcp.2014.10.011

Biochemical Pharmacology

Volume 92, Issue 4, 15 December 2014, Pages 661-668

https://doi.org/10.1016/j.bcp.2014.10.011 Get rights and content

Abstract

Nitric oxide (NO), carbon monoxide (CO) and hydrogen sulphide (H₂S) modulate vascular tone. In view of their therapeutic potential for ocular diseases, we examined the effect of exogenous CO and H₂S on tone of isolated rabbit ophthalmic artery and their interaction with endogenous and exogenous NO. Ophthalmic artery segments mounted on a wire myograph were challenged with cumulative concentrations of phenylephrine (PE) in the presence or absence of N^G-nitro-l-arginine (LNNA) to inhibit production of NO, the CO-releasing molecules CORMs or the H₂S-donor GYY4137. The maximal vasoconstriction elicited by PE reached 20–30% of that induced by KCl but was dramatically increased by incubation with LNNA. GYY4137 significantly raised PE-mediated vasoconstriction, but it did not change the response to PE in the presence of LNNA or the relaxation to sodium nitroprusside (SNP). CORMs concentration-dependently inhibited PE-induced constriction, an effect that was synergistic with endogenous NO (reduced by LNNA), but insensitive to blockade of guanylyl cyclase by 1H-[1,2,4]oxadiazolo[4,3,-α]quinoxalin-1-one (ODQ). In vascular tissues cyclic GMP (cGMP) levels seemed reduced by GYY4137 (not significantly), but were not changed by CORM. These data indicate that CO is able per se to relax isolated ophthalmic artery and to synergize with NO, while H₂S counteracts the effect of endogenous NO. CO does not stimulate cGMP production in our system, while H₂S may reduce cGMP production stimulated by endogenous NO. These findings provide new insights into the complexities of gas interactions in the control of ophthalmic vascular tone, highlighting potential pharmacological targets for ocular diseases.

Graphical abstract

Introduction

Nitric oxide (NO), carbon monoxide (CO) and hydrogen sulphide (H₂S) are gaseous molecules that have long been known as environmental pollutants and highly toxic gases [1]. Since the seminal discovery of the endothelium-derived relaxing factor (EDRF) by Furchgott [2], later identified as NO [3], CO [4] and H₂S [5] have also been found to be endogenously produced and to act as signalling molecules in vertebrates. Nowadays these molecules are studied not only as endogenous regulators, but also as potentially exploitable therapeutics. In fact, although NO was initially characterized as the main mediator of endothelium-dependent vasodilatation, CO and H₂S have also been shown to affect vascular tone among other important biological actions. Collectively, these three signalling gases have become known as gasotransmitters [6]. While a large body of studies during the last three decades has focused on the molecular and cellular mechanisms of NO and its functions in different biological systems, including the eye, much less is known about CO and H₂S, for which the mechanisms underlying their pharmacologic effects are still mostly unclear.

Interestingly, CO, NO and H₂S are endogenously produced in vertebrate retinas and have been studied in several non-clinical and clinical ocular paradigms [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19].

A number of chemicals able to deliver CO and H₂S to tissues in a controlled manner have been developed and could potentially be used in human therapy alongside NO-donors, which are approved drugs used to relieve acute cardiac ischemia and hypertension [20].

A potential cross talk between the three gas mediators has been hypothesized but poorly examined. Thus, the aim of the present study was to investigate the effects of exogenous CO and H₂S on tone of isolated rabbit ophthalmic artery and their interaction with endogenous and exogenous NO. Furthermore, in order to gain insight on the mechanism/s whereby these gases exert the regulation of vascular tone, we measured cyclic GMP (cGMP) levels in rabbit ophthalmic arteries.

Section snippets

Preparation of ophthalmic arteries

Animal use was approved by the subcommittee for research and animal care at the University of Catania according to guidelines from Italian Ministry of Health. Male New Zealand rabbits (Charles River, Calco, Italy; 200–250 g) were euthanatized with a single injection of zoletil^® 100 in the peripheral vein of the hear. Ophthalmic arteries from both eyes were dissected, immersed in physiological salt solution (PSS, composition in mM: NaCl, 118; KCl, 4.6; NaHCO₃, 25; MgSO₄, 1.2; KH₂PO₄, 1.2; CaCl₂,

Characterization of NO-dependent responses of isolated ophthalmic artery

Because basal, unstimulated, NO release from endothelium is well known to reduce responses to vasoconstrictor agonists [26], [27], [28], we attempted to functionally assess endothelial NO-production in isolated ophthalmic artery by: (i) comparing vasoconstriction to PE before and after blockade of basal NO synthesis by LNNA; (ii) by measuring vasodilatation of PE-preconstricted preparations following stimulation of NO release with acetylcholine (ACh, Fig. 2, Fig. 3).

In control conditions, PE

Discussion

NO, CO and H₂S have been evaluated for potential clinical applications for ocular diseases, particularly in conditions like glaucoma, where the ocular blood flow regulation is crucial [30], [31], [32], [33].

In this study we used isolated rabbit ophthalmic artery to explore the impact of these gases on ocular circulation. However, because ophthalmic artery supplies also many extra-ocular structures, the significance of our data on retinal or choroidal blood flow regulation may be limited. We

Conflict of interest statement

The authors do not have competing interests to disclose.

Acknowledgements

This work was supported in part by a National grant PON01-00110, the Aremcar Foundation and the International Ph.D. Program in Neuropharmacology, University of Catania, Italy. We thank Prof. Brian Mann for the synthesis of CORM 371.

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Regulation of vascular tone in rabbit ophthalmic artery: Cross talk of endogenous and exogenous gas mediators

Abstract

Graphical abstract

Introduction

Section snippets

Preparation of ophthalmic arteries

Characterization of NO-dependent responses of isolated ophthalmic artery

Discussion

Conflict of interest statement

Acknowledgements

Neurochem Int

Eur J Pharmacol

FEBS Lett

Free Radic Biol Med

Biochem Pharmacol

Anal Biochem

Pharmacol Ther

Neurochem Int

Exp Eye Res

Pharmacol Res

J Biol Chem

Pharmacol Res

Biochem Biophys Res Commun

Brain Res

The obligatory role of endothelial cells in the relaxation of arterial smooth muscle by acetylcholine

Nature

Endothelium-derived relaxing factor produced and released from artery and vein is nitric oxide

Proc Natl Acad Sci USA

Carbon monoxide: a putative neural messenger

Science

The possible role of hydrogen sulfide as an endogenous neuromodulator

J Neurosci

Two's company, three's a crowd: can H2S be the third endogenous gaseous transmitter

FASEB J

Preconditioning with inhalative carbon monoxide protects rat retinal ganglion cells from ischemia/reperfusion injury

Invest Ophthalmol Vis Sci

Interdependence of lipoxin A4 and heme-oxygenase in counter-regulating inflammation during corneal wound healing

FASEB J

Morphine-induced reduction of intraocular pressure and pupil diameter: role of nitric oxide

Pharmacology

Protection against light-induced retinal degeneration by an inhibitor of NO synthase

NeuroReport

Heme oxygenase-1 induction attenuates corneal inflammation and accelerates wound healing after epithelial injury

Invest Ophthalmol Vis Sci

Interactions of the gaseous neuromodulators nitric oxide, carbon monoxide, and hydrogen sulfide in the salamander retina

J Neurosci Res

Two's company, three's a crowd: can H₂S be the third endogenous gaseous transmitter