Carbonic Anhydrase I (CA–1) Induces Retinal Vascular Permeability in vivo through the Bradykinin (BK)/Kallikrein Pathway

Abstract

Abstract: : Purpose: Extracellular CA and bicarbonate levels regulate transcellular ion and water transport. While CA plays a role in ocular fluid homeostasis, its effect on retinal vascular permeability (RVP) has not been described. This study investigates whether extracellular CA has an effect upon retinal vascular permeability. Methods: RVP was measured by vitreous fluorophotometry in male Sprague–Dawley rats. After baseline measurements, 10 µL intravitreal injections of CA (Sigma) or balanced salt solution (BSS) were performed. Fluorescein was infused into a jugular catheter 15 minutes after injection. Vitreal fluorescence was measured 30 minutes later. Fluorescent spectra were normalized by the incident light intensity. Results: CA increased RVP in a dose dependent manner, with a maximal increase of 120% (20.7+4.4au, p<0.05) at 2ng/µL compared with BSS control (9.8+3.4au). The EC50 of CA–1 was 670 pg/µL and the CA–mediated increase in RVP was sustained for over 24 hrs. The effect of CA–1 alone on RVP was comparable in magnitude to the effect of intravitreal injected VEGF and the two responses were additive. CA–1–induced BRB permeability was inhibited by 68% with co–injection of 10um acetazolamide (13.3±5.1au, p<0.05), 75% with 150 uM of the BK receptor 2 anatgonist HOE–140 (11.1±2.4au, p<0.05), and 70% with 100µM of the carboxypeptidase inhibitor, leupeptin (10.8±2.5au, p< 0.05). Conclusions: CA is a potent retinal permeability factor that exerts its effect through the bradykinin pathway independently of VEGF. The results suggest that CA could be a physiological activator of the kallikrein–kinin system.

Keywords: signal transduction: pharmacology/physiology • vascular cells • enzymes/enzyme inhibitors