Vasodilatory Effects of Nipradilol, an α- and β-adrenergic Blocker with Nitric Oxide Releasing Action, in Rabbit Ciliary Artery

Abstract

Nipradilol is a new antiglaucoma ophthalmic agent used in Japan. Topical application of nipradilol is reported to increase ocular blood flow. To investigate the action of this drug, we studied the effect of nipradilol on the isolated rabbit ciliary artery. Under the dissecting microscope, ciliary arteries were prepared from rabbit eyes and mounted on a myograph system. The effects of nipradilol on the isolated rabbit ciliary artery were investigated using isometric tension recording methods. Nipradilol provoked a dose-dependent (10μM–1m M) relaxation in ciliary arteries that were pre-contracted with high-K solutions (K+: 100.7m M). It also inhibited the amplitude of smooth muscle contraction evoked by field stimulation. Nipradilol was more effective in relaxing phenylephrine-induced contraction (EC50: 21.6±16.3μM) compared to high-K solution-induced contractions (EC50: 230±130μM). Application of Nw-nitro- L -arginine methylester (300μM), a nitric oxide (NO) synthase inhibitor, or denudations of endothelium by rubbing the inner surface with a scalp hair did not affect this relaxation. However, NO scavenger carboxy-PTIO (1m M) or methylene blue (10μM), a guanylate cyclase inhibitor, inhibited the nipradilol-induced relaxation. These results indicate that nipradilol relaxes the rabbit ciliary artery by two different mechanisms. First, the relaxation is due to the NO produced by denitrification of nipradilol itself. Second, nipradilol may act as an α-adrenergic antagonist. These actions of nipradilol may explain the mechanisms of increased ocular blood flow in vivo.