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Ning Zhang, Ram Kannan, Curtis T. Okamoto, Stephen J. Ryan, Vincent H. L. Lee, David R. Hinton; Characterization of Brimonidine Transport in Retinal Pigment Epithelium. Invest. Ophthalmol. Vis. Sci. 2006;47(1):287-294. doi: 10.1167/iovs.05-0189.
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purpose. To investigate the involvement of carrier-mediated transport mechanisms in brimonidine transport in retinal pigment epithelium (RPE).
methods. The transport of [3H]-brimonidine in bovine RPE-choroid explants was evaluated in a modified Ussing chamber. The uptake of [3H]brimonidine was evaluated in differentiated ARPE-19 cells cultured on permeable transwell filters.
results. The transport of brimonidine into (choroid-to-retina transport [inward]) and out of (retina-to-choroid transport [outward]) the eye in bovine RPE-choroid explants was temperature dependent. Both inward and outward brimonidine transport decreased at 5 μM compared with 10 nM. The melanin pigmentation of RPE did not significantly affect tissue permeability at either brimonidine dose. A saturable component was identified for the inward transport with the apparent Michaelis-Menten constant and a maximum transport rate of 51 μM and 148 pmol/(cm2·h), respectively. Both apical (representing retina-to-choroid transport) and basolateral (representing choroid-to-retina transport) brimonidine uptake in ARPE-19 cells showed temperature dependence. Apical uptake was higher than basolateral uptake at 37°C and was decreased to 70% in the presence of NaN3 or in the absence of extracellular Na+. Besides α2-agonists, apical uptake was inhibited by verapamil, desipramine, and quinidine, but not by MPP+ (1-methyl-4-phenylpyridinium), TEA (tetraethylammonium), decynium-22, carnitine, PHA (p-aminohippurate), alanine, or inosine. Basolateral brimonidine uptake increased by 35% at extracellular pH of 6 and decreased by 50% under cell-depolarized conditions of high medium K+ and 1 μM valinomycin. Temperature-dependent components of basolateral uptake were not saturated at doses up to 2 mM.
conclusions. A carrier-mediated transport process for brimonidine in RPE was demonstrated in bovine RPE-choroid explants and polarized ARPE-19 cells. This transport system may play a significant role in modulating the movement of brimonidine into and out of the eye.
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