Intracellular acidification increases adenosine transport in human umbilical vein endothelial cells

Adenosine is taken up via human equilibrative nucleoside transporters 1 (hENT1) and 2 (hENT2) at a physiological extracellular pH (pHo ∼7.4) in human umbilical vein endothelial cells (HUVECs). Acidic pHo increases the uptake of adenosine and 5-hydroxytryptamine (5HT) via hENT4 in this cell type. However, modulation of hENT1 and hENT2 transport activity by the pHi is unknown. We investigated whether hENT1 and hENT2-adenosine transport was regulated by acidic pHi.

HUVECs loaded with a pH sensitive probe were subjected to 0.1-20 mmol/L NH₄Cl pulse assay to generate 6.9-6.2 pHi. Before pHi started to recover, adenosine transport kinetics (0-500 μmol/L, 37 °C) in the absence or presence 1 or 10 μmol/L S-(4-nitrobenzyl)-6-thio-inosine (NBTI), 2 mmol/L hypoxanthine, 2 mmol/L adenine, 100 μmol/L 5HT, or 500 μmol/L adenosine, was measured.

Overall adenosine transport (i.e., hENT1+hENT2) was semisaturable and partially inhibited by 1 μmol/L, but abolished by 10 μmol/L NBTI in cells non-treated or treated with NH₄Cl. The initial velocity and non-saturable, lineal component for overall transport were increased after NH₄Cl pulse. hENT1 and hENT2-mediated adenosine transport maximal capacity was increased by acidic pHi. hENT1 activity was more sensitive than hENT2 activity to acidic pHi.

hENT1 and hENT2-adenosine transport is differentially regulated by acidic pHi in HUVECs. These findings are important in pathologies associated with pHi alterations such as gestational diabetes mellitus.