Functional Characterization of Brimonidine Transport in the Retinal Pigment Epithelium

Abstract

Abstract: : Purpose: To characterize brimonidine transport in the retinal pigment epithelium (RPE) Methods: The transport of [³H]brimonidine in bovine RPE–choroid explants was evaluated in a modified Ussing chamber. The basolateral and apical uptake or efflux of brimonidine was also evaluated in differentiated ARPE–19 cells cultured on permeable transwell filters. Results: Bovine RPE–choroid explants produced a trans–epithelial potential difference (PD) of 6.5 ± 0.6mV, and a trans–epithelial electrical resistance (TEER) of 260 ± 40 Ω^.cm². The apparent permeability coefficient for 10nM brimonidine in the choroidal–to–retinal (C to R) direction at 37^oC was 1.7 times higher than that at 4^oC. Brimonidine transport in the C to R direction showed concentration dependency over 0.01µM to 100µM with an apparent Michaelis–Menten constant of 51 ± 17 µM and a maximal flux of 148 ± 22 pmole/(cm^2.h). After one month culture, ARPE–19 cell layer showed a TEER of 33 ± 2 Ω^.cm² with no detectable PD. Apical and basolateral uptake of 20nM brimonidine for 5min at 37^oC in ARPE–19 cell mono–layers was 3.0 and 1.9 times greater than that at 4^oC, respectively. Five–minute apical brimonidine efflux at 37^oC was 5.6 times greater than that at 4^oC, while brimonidine efflux at the basolateral side at 37^oC showed no statistically significant difference from that at 4^oC (p–value>0.1). Conclusion: The transport of brimonidine through the RPE in the choroidal–to–retinal direction is facilitated by a carrier–mediated system. This system might contribute to the fast absorption of brimonidine at the posterior region of the eye after topical application. Acknowledgement: We wish to thank Dr. Diane Tang–Liu(Allergan, CA) for providing brimonidine tartrate. This research is supported by the Arnold and Mabel Beckman Foundation.

Keywords: retinal pigment epithelium