Posterior Ciliary Artery Contraction by Bradykinin Receptor Subtypes and Implications for Retinal Blood Flow Dysfunction

Abstract

Background: Low retinal blood flow and/or vasospasm represent major risk factors for the development of glaucomatous optic neuropathy (GON), a potentially blinding eye disease. Bradykinin (BK), a nonapeptide, is endogenously produced and released, which can cause smooth muscles to contract and relax in different tissues depending on the physiological/pathological situation and the presence or absence of vascular tone. Several reports have shown the presence of BK receptor mRNAs, and in some cases, B1- and B2-receptor proteins, in ocular tissues, including the retina. However, the function of these receptors remains to be determined, especially in retinal blood vessels.

Methods: We pharmacologically characterized the ability of BK and any related peptide agonists to promote the contraction of isolated bovine posterior ciliary arteries (PCAs) in an organ bath setup using a cumulative compound addition and tension development recording process. Receptor-selective kinin agonists and subtype-selective BK receptor antagonists were utilized to define the possible heterogeneity in the functional BK receptors for PCAs.

Results: All agonist kinin peptides concentration-dependently contracted the PCA rings bi-phasically over a 5-log unit range (0.1 nM-10 μM). The relative potencies (EC₅₀ values; n = 4-5) regarding the high-affinity receptor site were: Lys-BK = 0.9 ± 0.4 nM; Des-Arg⁹-BK = 0.9 ± 0.4 nM; RMP-7 = 1.1 ± 0.6 nM; Met-Lys-BK = 1.3 ± 0.5 nM; Hyp³-BK = 2.7 ± 0.5 nM; BK = 3.0 ± 0.7 nM. The low-affinity receptor site activated by these peptides mostly exhibited EC₅₀ values ranging from 0.3 μM to 3 μM. The concentration-response curves to Des-Arg⁹-BK (B1-selective agonist) were shifted to the left in the presence of increasing concentrations of a B1-receptor antagonist (R715: 1-10 μM; n = 3). Similarly, WIN-64338 (a B2-receptor antagonist: 1-10 μM; n = 3) moved the BK concentration-response curves to the left.

Conclusions: The pharmacological characteristics of BK and analog-induced contractions, and their inhibition by receptor-selective antagonists, indicated the presence of both B1- and B2-receptors, and perhaps another subtype, which mediate the PCA contractions. These results have potential implications for the involvement of heterogeneous kinin receptors, narrowing PCA diameters in vivo, restricting blood flow to the retina, causing GON, and subsequent visual impairment that can eventually cause blindness.

Keywords: blood flow; blood vessel contraction; bradykinin; glaucoma; glaucomatous optic neuropathy; oxidative stress; posterior ciliary artery; retina damage.