The kallikrein/kinin system in ocular function.

The kallikrein/kinin system uses two distinct serine proteases, plasma kallikrein and tissue kallikrein, to yield bradykinin and Lys-bradykinin (kallidin) from specific substrate kininogens. The kallikrein/kinin system is known to have a role in contact-activated coagulation mechanisms and in inflammatory responses, and recently has been shown to contribute to homeostatic and protective mechanisms in the cardiovascular and renal systems. This article reviews current knowledge of the ocular kallikrein/kinin system within the context of proposed roles for this system in other important organs and tissues. All components of the kallikrein/kinin system are present in the eye and are positioned to participate in key ocular functions. Plasma kallikrein binds to vascular endothelium and generates bradykinin, which may contribute to regulation of ocular blood flow, and, in excess, has been implicated in the pathogenesis of retinal edema in patients with proliferative diabetic retinopathy. Tissue kallikrein is expressed in retina, ciliary muscle, and trabecular meshwork cells and could be a significant factor in the protective mechanism of ischemic preconditioning, and in the modulation of aqueous dynamics. Improved understanding of the role of plasma and tissue kallikreins and kinins in such processes has the potential to identify significant new targets for the therapy of ocular dysfunction.