title: Distribution and functional characterization of equilibrative nucleoside transporter-4, a novel cardiac adenosine transporter activated at acidic pH.
abstract: Adenosine plays multiple roles in the efficient functioning of the heart by regulating coronary blood flow, cardiac pacemaking, and contractility. Previous studies have implicated the equilibrative nucleoside transporter family member equilibrative nucleoside transporter-1 (ENT1) in the regulation of cardiac adenosine levels. We report here that a second member of this family, ENT4, is also abundant in the heart, in particular in the plasma membranes of ventricular myocytes and vascular endothelial cells but, unlike ENT1, is virtually absent from the sinoatrial and atrioventricular nodes. Originally described as a monoamine/organic cation transporter, we found that both human and mouse ENT4 exhibited a novel, pH-dependent adenosine transport activity optimal at acidic pH (apparent K(m) values 0.78 and 0.13 mmol/L, respectively, at pH 5.5) and absent at pH 7.4. In contrast, serotonin transport by ENT4 was relatively insensitive to pH. ENT4-mediated nucleoside transport was adenosine selective, sodium independent and only weakly inhibited by the classical inhibitors of equilibrative nucleoside transport, dipyridamole, dilazep, and nitrobenzylthioinosine. We hypothesize that ENT4, in addition to playing roles in cardiac serotonin transport, contributes to the regulation of extracellular adenosine concentrations, in particular under the acidotic conditions associated with ischemia.
authors.affiliation: Institute of Membrane and Systems Biology, University of Leeds, UK. K.Barnes@leeds.ac.uk
authors.forename: Kay
authors.initials: K
authors.lastname: Barnes
authors.forename: Halina
authors.initials: H
authors.lastname: Dobrzynski
authors.forename: Sophie
authors.initials: S
authors.lastname: Foppolo
authors.forename: Paul R
authors.initials: PR
authors.lastname: Beal
authors.forename: Fouzia
authors.initials: F
authors.lastname: Ismat
authors.forename: Elspeth R
authors.initials: ER
authors.lastname: Scullion
authors.forename: Lijie
authors.initials: L
authors.lastname: Sun
authors.forename: James
authors.initials: J
authors.lastname: Tellez
authors.forename: Mabel W L
authors.initials: MW
authors.lastname: Ritzel
authors.forename: William C
authors.initials: WC
authors.lastname: Claycomb
authors.forename: Carol E
authors.initials: CE
authors.lastname: Cass
authors.forename: James D
authors.initials: JD
authors.lastname: Young
authors.forename: Rudi
authors.initials: R
authors.lastname: Billeter-Clark
authors.forename: Mark R
authors.initials: MR
authors.lastname: Boyett
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Baldwin
mesh_terms: D000143:Acids / Q000378:metabolism*; D000241:Adenosine / Q000378:metabolism; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002478:Cells, Cultured; D033705:Equilibrative Nucleoside Transport Proteins; D006031:Glycosylation; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D026901:Membrane Transport Proteins / Q000378:metabolism*; D051379:Mice; D009206:Myocardium / Q000378:metabolism*; D009419:Nerve Tissue Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D009705:Nucleosides / Q000378:metabolism; D009865:Oocytes; D012701:Serotonin / Q000378:metabolism; D013347:Subcellular Fractions / Q000378:metabolism; D014018:Tissue Distribution; D014981:Xenopus
chemical_list: D000143:Acids; D033705:Equilibrative Nucleoside Transport Proteins; D026901:Membrane Transport Proteins; D009419:Nerve Tissue Proteins; D009705:Nucleosides; C495988:SLC29A4 protein, human; C513828:Slc29a4 protein, mouse; D012701:Serotonin; D000241:Adenosine