title: Saturable absorptive transport of the hydrophilic organic cation ranitidine in Caco-2 cells: role of pH-dependent organic cation uptake system and P-glycoprotein.
abstract: OBJECTIVE
  The purpose of this work was to investigate the involvement of carrier-mediated apical (AP) uptake and efflux mechanisms in the absorptive intestinal transport of the hydrophilic cationic drug ranitidine in Caco-2 cells.
  
  
  METHODS
  Absorptive transport and AP uptake of ranitidine were determined in Caco-2 cells as a function of concentration. Permeability of ranitidine in the absorptive and secretory directions was assessed in the absence or presence of the P-glycoprotein (P-gp) inhibitor, GW918. Characterization of the uptake mechanism was performed with respect to inhibitor specificity, pH, energy, membrane potential, and Na+ dependence. Efflux from preloaded monolayers was evaluated over a range of concentrations and in the absence or presence of high extracellular ranitidine concentrations.
  
  
  RESULTS
  Saturable absorptive transport and AP uptake of ranitidine were observed with Km values of 0.27 and 0.45 mM, respectively. The ranitidine absorptive permeability increased and secretory permeability decreased upon inhibition of P-gp. AP ranitidine uptake was inhibited in a concentration-dependent fashion by a diverse set of organic cations including tetraethylammonium, 1-methyl-4-phenylpyridinium, famotidine, and quinidine. AP ranitidine uptake was pH and membrane potential dependent and reduced under conditions that deplete metabolic energy. Efflux of [3H]ranitidine across the basolateral membrane was neither saturable as a function of concentration nor trans stimulated by unlabeled ranitidine.
  
  
  CONCLUSIONS
  Saturable absorptive transport of ranitidine in Caco-2 cells is partially mediated via a pH-dependent uptake transporter for organic cations and is subject to attenuation by P-gp. Inhibition and driving force studies suggest the uptake carrier exhibits similar properties to cloned human organic cation transporters. The results also imply ranitidine transport is not solely restricted to the paracellular space.
authors.affiliation: Division of Molecular Pharmaceutics, School of Pharmacy, Kerr Hall, CB #7360, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7360, USA.
authors.forename: David L
authors.initials: DL
authors.lastname: Bourdet
authors.forename: Dhiren R
authors.initials: DR
authors.lastname: Thakker
mesh_terms: D019297:2,4-Dinitrophenol / Q000494:pharmacology; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000042:Absorption; D000166:Acridines / Q000494:pharmacology; D018938:Caco-2 Cells; D002412:Cations; D002463:Cell Membrane Permeability; D004305:Dose-Response Relationship, Drug; D006635:Histamine H2 Antagonists / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007413:Intestinal Mucosa / Q000187:drug effects / Q000378:metabolism*; D008564:Membrane Potentials; D027701:Organic Cation Transport Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011802:Quinidine / Q000494:pharmacology; D011899:Ranitidine / Q000737:chemistry / Q000378:metabolism*; D019789:Tetraethylammonium / Q000494:pharmacology; D044005:Tetrahydroisoquinolines / Q000494:pharmacology; D014316:Tritium; D014475:Uncoupling Agents / Q000494:pharmacology
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000166:Acridines; D002412:Cations; D006635:Histamine H2 Antagonists; D027701:Organic Cation Transport Proteins; D044005:Tetrahydroisoquinolines; D014475:Uncoupling Agents; D014316:Tritium; D019789:Tetraethylammonium; D011899:Ranitidine; D011802:Quinidine; C083501:Elacridar; D019297:2,4-Dinitrophenol