Relaxation of Precontracted Ciliary Artery and Ciliary Muscle <i>In Vitro</i> by Kinin Peptides: Relevance to Retinal Blood Flow and Intraocular Pressure Regulation

<b><i>Purpose:</i></b> To study the ability of five kinin peptides to relax phenylephrine-contracted bovine posterior ciliary artery (PCA; major retinal blood supplier) and carbachol-contracted ciliary muscle (CM) (involved in accommodation and aqueous humor drainage) <i>in vitro</i>. <b><i>Methods:</i></b> Isolated bovine CM strips and PCA rings were mounted in small organ baths and perfused with oxygenated Krebs' solution containing 3 µM flurbiprofen. The tissues were then contracted with 10 µM carbachol (for CM) and 10 µM phenylephrine (for PCA), and the relaxant effects of kinins (0.3 nM to 10 µM) were determined. <b><i>Results:</i></b> All tested kinin peptides concentration-dependently relaxed precontracted CM and PCA in a biphasic manner. The concentrations of the peptides (BK [bradykinin], Hyp³-BK, Lys-BK, Met-Lys-BK, and Des-Arg⁹-BK) yielding half-maximal relaxation (EC₅₀) of PCA via the high potency receptor site ranged between 0.1 nM and 9.0 nM (<i>n</i> = 5-6). Precontracted PCA relaxation by the same kinins via the low potency receptor site occurred with EC₅₀ values of 39.5-399 nM (<i>n</i> = 5-6). In contrast, EC₅₀ values for these peptides relaxing the precontracted CM strips were 0.25-30 nM (high potency receptor site) and 100-3,000 nM (low potency receptor site) (<i>n</i> = 3-6). Precontracted PCA rings were significantly (generally <i>P</i> < 0.005-0.05) more sensitive to the relaxant activity of the kinin peptides than precontracted CM strips. <b><i>Conclusions:</i></b> The tissue relaxant agonist profile of the kinins in both precontracted CM strips and PCA rings, as judged by the 2-site-fit data, indicated the involvement of both B1- and B2-receptor subtypes.