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M. Ichikawa, Y. Okada, H. Hara, K. Ishii, M. Araie; Effects of Bunazosin Hydrochloride, an 1-adrenoceptor Antagonist, on the Retinal Artery Diameter in Rabbits . Invest. Ophthalmol. Vis. Sci. 2003;44(13):134.
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Purpose: Bunazosin hydrochloride, an α1-adrenoceptor antagonist, has been launched as an ocular hypotensive drug in Japan. We examined the inhibitory effects of topically instilled bunazosin hydrochloride on the contraction of retinal arteries induced by phenylephrine or endothelin-1 (ET-1) in conscious rabbits. Methods: Phenylephrine or ET-1 was injected into the vitreous of both eyes in pigmented rabbits. A masked observer took and analyzed the photographs as follows. Color fundus photographs were taken 5 min before and 60 min after the injection. The average diameter of the major retinal arteries on the rim of the optic nerve head (ONH) was normalized with respect to ONH diameter, and each value was expressed as a percentage of that obtained 5 min before the intravitreal injection. Bunazosin hydrochloride (0.01%) was instilled into one eye 1 hr before the intravitreal injection. Results: The retinal arteries were contracted by phenylephrine in a dose-dependent manner. Topical bunazosin hydrochloride inhibited the phenylephrine-induced contraction only on the side on which it was instilled, but not on the contralateral side. ET-1 contracted retinal arteries in a dose-dependent manner, and topical bunazosin hydrochloride almost inhibited the contraction, but only on the instilled side. This inhibitory effect of bunazosin hydrochloride on ET-1-induced contraction was diminished by pre-treatment with reserpine intravitreally, which depletes norepinephrine from sympathetic nerve terminals. Furthermore, co-injection of phenylephrine and ET-1 at ineffective doses contacted retinal arteries significantly, and this was almost inhibited by topical bunazosin hydrochloride. Conclusions: This study suggests that the α1-adrenoceptor may play an important role in the regulation of retinal artery diameter in rabbits. Instilled bunazosin hydrochloride may reach the posterior region at pharmacologically active levels by local penetration. The inhibitory effect of bunazosin hydrochloride on ET-1-induced contraction may be at least partly explained by ET-1 interacting with α1 receptors.
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