The Effects Of Antiglaucomatous Agents On Ocular Blood Flow

Though it is generally accepted that increased intraocular pressure (IOP) is a major risk factor in glaucoma, ocular vascular perfusion is also important in the pathogenesis of optic nerve damage and visual field loss, especially in patients with normal-tension glaucoma. Therefore, a better understanding of the effect of antiglaucomatous agents on IOP and ocular blood flow (OBF) is important for optimizing their clinical use.1. The purpose and significance of this study Based on the body of evidence suggesting the participation of OBF disturbances in the pathogenesis of glaucoma, great interest has ensued in the investigation of the effects of antiglaucoma drugs on the various ocular vascular beds. With the possibility that reduced OBF or a dysfunctional autoregulation may play an important role in the pathogenesis of glaucoma, creating either acute or chronic vasoconstriction in diseased eyes may be harmful, partially counteracting the expected increase in perfusion pressure secondary to an IOP reduction. The purpose and significance of this study, is to better understand the effects of antiglaucomatous agents on OBF for optimazing their clinical use to protecting and maintaining the visual function of glaucomatous patients to elevating their existence quality. 2. Myograph systemThe myograph system (JP Trading, Denmark) allowed direct determination of vessel isometric tension while the internal circumference was controlled. To investigate the effect of various agents on ocular circulation, the vessel isometric tension is measured by Myograph system.3. FluorophotometryAquacosmos System (Hamamatsu Photonics, Shizuoka, Japan) was used to determined smooth muscle intracellular fluorescence intensity at 340nm and 380nm representing Fura-2-Ca2+ and Fura-2 respectively. In present study, the fluorescence ratio of 340 nm over 380 nm was used to indicate changes in the intracellular free calcium concentration ([Ca²⁺]_i) of the smooth muscle cells.4. Radioligand binding assaysRadioligand binding assays is the classic method for studing the character of receptor. Radioligand binding studies to H1 and H2 receptors were carried out with commercially obtained 3H-pyrilamine and 3H-tiotidine respectively. We have studied the effect of histamine receptor on the mechanism of antiglaucomatous agents inducing relaxation of rabbit ciliaty artery, and determined the type of histamine receptor.5. Effect of antiglaucomatous agents on isolated rabbit ciliary artery Timolol induced a relaxation in isolated rabbit ciliary arteries precontracted by K-Krebs solution, which didn't depend on the L-NAME or the denudation of vascular endothelium. Timolol decreased [Ca²⁺]_i, as did that of diltiazem.Betaxolol induced a relaxation in isolated rabbit ciliary arteries precontracted by K-Krebs solution. At the concentration of 1mM, betaxolol induced nearly complete relaxation, while timolol induced approximately 70% relaxation. Thus, the relaxing potency of betaxolol was significantly stronger than that of timolol. Betaxolol induced relaxation didn't depend on the L-NAME or the denudation of vascular endothelium. Betaxolol decreased [Ca²⁺]_i, as did that of diltiazem.Levobunolol relaxed ciliary artery rings that were pre-contracted with either K-Krebs solution, histamine, phenylephrine or endothelin-1. The relaxing pattern of levobunolol was not like that of diltiazem. Levobunolol does not inhibit Ca²⁺ release from intracellular stores. In Ca²⁺-free media, levobunolol have inhibited histamine induced contraction of rabbtit ciliary artery.Tafluprost induced a relaxation in isolated rabbit ciliary arteries precontracted by K-Krebs solution. The concentration-dependent relaxing curve of tafluprost is different from exogenous PGE2 or PGI2. Tafluprost induced relaxation didn't depend on the L-NAME, indomethacin or the denudation of vascular endothelium. On thapsigargin pretreated preparations, after superfusion of Ca²⁺-free solution, the [Ca²⁺]_i increased upon extracellular Ca²⁺ reintroduction. Tafluprost decreased the Ca²⁺ reintroduction-induced amplitude of [Ca²⁺]_i. 6. DiscussionTimolol inducing relaxation of rabbit cillary artery was acting as a Ca²⁺ channel blocker to inhibit extracellular Ca²⁺ entry. Betaxolol is a particularly selectiveβ1-adrenoceptor antagonist, that inducing relaxation of rabbit cillary artery was acting as a Ca²⁺ channel blocker to inhibit extracellular Ca²⁺ entry.Levobunolol relax rabbit ciliary artery by two different mechanisms. First, the relaxation may be due to the blockade of Ca²⁺ entry through non voltage-dependent Ca²⁺ channels. Second, levobunolol may induce relaxation by intracellular Ca²⁺-independent mechanisms. Relaxant effect of tafluprost may cause at least in part by inhibiting capacitative-Ca²⁺ entry from extracellular space.7. SummaryIn present study, we have investigated the relaxing effects of timolol, betaxolol, levobunolol and tafluprost on isolated rabbit ciliary artery. Further, we incorporated spectrofluorometry and radioligand binding assay to examine the underlying mechanism of relaxing effects of antiglaucomatous agents. This study may is a strong support for explaning the effects of antiglaucomatous agents on ocular blood flow in vivo.