title: Characterization of brimonidine transport in retinal pigment epithelium.
abstract: OBJECTIVE
  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 microM 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 microM and 148 pmol/(cm2 x 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 degrees C and was decreased to 70% in the presence of NaN3 or in the absence of extracellular Na+. Besides alpha2-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 microM 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.
authors.affiliation: Department of Pharmaceutical Sciences, School of Pharmacy, University of Southern California, Los Angeles, CA, USA.
authors.forename: Ning
authors.initials: N
authors.lastname: Zhang
authors.forename: Ram
authors.initials: R
authors.lastname: Kannan
authors.forename: Curtis T
authors.initials: CT
authors.lastname: Okamoto
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Ryan
authors.forename: Vincent H L
authors.initials: VH
authors.lastname: Lee
authors.forename: David R
authors.initials: DR
authors.lastname: Hinton
mesh_terms: D018759:Adrenergic Uptake Inhibitors / Q000494:pharmacology; D000316:Adrenergic alpha-Agonists / Q000378:metabolism*; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology; D001813:Blood-Retinal Barrier / Q000502:physiology; D000068438:Brimonidine Tartrate; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002352:Carrier Proteins / Q000502:physiology*; D002417:Cattle; D002478:Cells, Cultured; D002829:Choroid / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008543:Melanins / Q000378:metabolism; D010857:Pigment Epithelium of Eye / Q000378:metabolism*; D011810:Quinoxalines / Q000378:metabolism*; D012160:Retina / Q000378:metabolism; D013696:Temperature; D013997:Time Factors
chemical_list: D018759:Adrenergic Uptake Inhibitors; D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D002121:Calcium Channel Blockers; D002352:Carrier Proteins; D008543:Melanins; D011810:Quinoxalines; D000068438:Brimonidine Tartrate