Distribution and functional characterization of equilibrative nucleoside transporter-4, a novel cardiac adenosine transporter activated at acidic pH.
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.
Barnes
Kay
K
Institute of Membrane and Systems Biology, University of Leeds, UK. K.Barnes@leeds.ac.uk
Dobrzynski
Halina
H
Foppolo
Sophie
S
Beal
Paul R
PR
Ismat
Fouzia
F
Scullion
Elspeth R
ER
Sun
Lijie
L
Tellez
James
J
Ritzel
Mabel W L
MW
Claycomb
William C
WC
Cass
Carol E
CE
Young
James D
JD
Billeter-Clark
Rudi
R
Boyett
Mark R
MR
Baldwin
Stephen A
SA
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
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