Objective. We evaluated the histamine’s role in regulating the iris vasomotricity in rats, using as a research tool topical olopatadine, a selective H1 blocker, which is indicated for the treatment of allergic conjunctivitis and ranitidine, a selective H2 blocker mainly used for the treatment of peptic ulcer disease. Methods. Two groups of six Wistar rats anesthetized with ketamine 200 mg/kg body weight were used. They received distilled water in conjunctival instillations, initially and after 5 minutes, olopatadine 2.5 mmol/ l for the first group, respectively ranitidine 2.5 mmol/ l for the second group. The changes of the iris arteriolar and venular diameters were recorded. Results. Both olopatadine and ranitidine produced statistically significant iridal arteriolar vasoconstriction and ranitidine determined statistically significant venuloconstriction, while distilled water did not produce any statistically significant effect. Conclusions. There is a vasodilator histaminergic tone exerted through the histaminergic H1 and H2 receptors in the iris arterioles and, respectively, through the H2 receptors in the iridal venules. Olopatadine, a topical H1 antagonist used in the treatment of ocular allergies, may interfere with the humoral regulation of the iris arteriolar tone. Ranitidine, an H2 antagonist, decreased the diameter of the iris arterioles and venules, when administered topically in rats.
Histamine and serotonin, besides known systemic effects, can influence vascular tone at the eye level. The review of the literature from 1997 to 2018 suggests that these ocular effects are very variable -both vasodilation and vasoconstriction. Specifi c agonists or antagonists acting in the histamine and serotonin domains, are probably more useful than endogenous substances as working tools for discovering the functional elements involved in regulating ocular vascular tone. Knowing that intraocular pressure regulation also depends on the vascular tone of the anterior ocular segment, some of the substances under review may be candidates for potential intraocular pressure lowering drugs. RezumatHistamina și serotonina, pe lângă efectele sistemice cunoscute, pot influenţa tonusul vascular de la nivel ocular. Recenzia literaturii de specialitate din perioada 1997-2018 a arătat că aceste efecte oculare sunt foarte variabileatât vasodilataţie, cât și vasoconstricţie. Substanţele care modulează la nivel de receptor aceste efecte (agoniștii și antagoniștii histaminergici și serotoninergici) sunt probabil mai utili decât substanţele endogene ca instrumente de lucru pentru descoperirea elementelor funcţionale implicate în reglarea tonusului vascular ocular. Cunoscând că reglarea presiunii intraoculare depinde și de tonusul vascular al segmentului anterior ocular, unele din substanţele recenzate ar putea prezenta și efecte hipotonizante oculare.
We investigated, in rats, the existence of a cathecolaminergic vasodilator tone in iris vessels, using as a pharmacological tool, timolol, a non-selective β-blocker, devoided of intrinsic sympathomimetic activity. The use of 2.5 mM timolol in conjunctival instillation reduced the diameter of the iris arteries without having an effect on the size of the iris veins diameter. Our results suggest the existence of a catecholaminergic vasodilator tone in rat's iris arteries, not in iris veins, probably achieved via β2-adrenergic receptors.
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