Trabecular meshwork bradykinin receptors: mRNA levels, immunohistochemical visualization, signaling processes pharmacology, and linkage to IOP reduction.

Purpose: To localize mRNA and protein of bradykinin (BK) receptors, BK precursor polypeptide (kininogen) mRNA, and to study functional biochemical pharmacology of the signal transduction processes mediated by B2-receptors in isolated human trabecular meshwork (h-TM) cells. Intraocular pressure (IOP) lowering effects of 2 kinins were also investigated.

Methods: Previously documented procedures were utilized throughout these studies.

Results: Kinninogen mRNA was most abundant in TM, ciliary body (CB), and optic nerve head and appeared elevated in glaucomatous h-TM tissue. High levels of B2-receptor mRNA were found in the sclera, iris, TM, and CB. B2-receptor subtype protein was localized in cells of the monkey and h-TM, and the treatment of isolated h-TM cells with transforming growth factor-β2 (5 ng/mL) caused significant (P<0.04) downregulation of B2-receptor mRNA. In isolated primary h-TM cells, BK (EC50=0.8±0.2 nM; n=19) and Met-Lys-BK (EC50=6.5±1.5 nM) mobilized intracellular Ca(2+) and induced the release of prostaglandins (PGs) that was blocked by 2 B2-receptor antagonists [HOE-140; (S)-WIN-64338]. The cyclooxygenase inhibitor, bromfenac, abolished BK-induced PGs production. BK concentration dependently increased cell impedance, and it significantly (P<0.05) decreased h-TM cell volume in vitro. Intravitreal (ivt) administration of BK (50 μg), but not a B1-agonist (Sar-[D-Phe(9)]-Des-Arg(9)-BK; also at 50 μg), efficaciously lowered IOP (22.9% to 37% from baseline) of Dutch-Belted rabbits that naturally have high IOPs (27-28 mmHg).

Conclusions: BK activates multiple signal transduction pathways in h-TM cells via B2-receptors that also mediate IOP reduction as observed in rabbits following ivt administration of BK. These ocular hypotensive effects of BK may be physiologically important and suggest a novel therapeutic potential of BK-related B2-agonists.