Insulin restores glucose inhibition of adenosine transport by increasing the expression and activity of the equilibrative nucleoside transporter 2 in human umbilical vein endothelium

The transport properties of the nucleobase hypoxanthine were examined in the human umbilical vein endothelial cell line ECV 304. Initial rates of hypoxanthine influx were independent of extracellular cations: replacement of Na+ with Li+, Rb+, N-methyl-d-glucamine or choline had no significant effect on hypoxanthine uptake by ECV 304 cells. Kinetic analysis demonstrated the presence of a single saturable system for the transport of hypoxanthine in ECV 304 cells with an apparent Km of 320±10 μM and a Vmax of 5.6±0.9 pmol/106 cells per s. Hypoxanthine uptake was inhibited by the nucleosides adenosine, uridine and thymidine (apparent Ki 41±6, 240±27 and 59±8 μM respectively) and the nucleoside transport inhibitors nitrobenzylthioinosine (NBMPR), dilazep and dipyridamole (apparent Ki 2.5±0.3, 11±3 and 0.16±0.006 μM respectively), whereas the nucleobases adenine, guanine and thymine had little effect (50% inhibition at > 1 mM). ECV 304 cells were also shown to transport adenosine via both the NBMPR-sensitive and -insensitive nucleoside carriers. Hypoxanthine specifically inhibited adenosine transport via the NBMPR-insensitive system in a competitive manner (apparent Ki 290±14 μM). These results indicate that hypoxanthine entry into ECV 304 endothelial cells is mediated by the NBMPR-insensitive nucleoside carrier present in these cells.

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Adenosine transport in cultured human umbilical vein endothelial cells is reduced in diabetes

AJP Cell Physiology ◽

10.1152/ajpcell.1994.267.1.c39 ◽

1994 ◽

Vol 267 (1) ◽

pp. C39-C47 ◽

Cited By ~ 84

Author(s):

L. Sobrevia ◽

S. M. Jarvis ◽

D. L. Yudilevich

Keyword(s):

Endothelial Cells ◽

Umbilical Vein ◽

Diffusion Mechanism ◽

Maximum Velocity ◽

Facilitated Diffusion ◽

Nucleoside Transporter ◽

Apparent Dissociation Constant ◽

Human Umbilical Vein ◽

Adenosine Transport ◽

Umbilical Vein Endothelial Cells

Adenosine transport in cultured human umbilical vein endothelial cells (HUVEC) was characterized and shown to be mediated by a single facilitated diffusion mechanism. Initial rates of adenosine influx at 22 degrees C were saturable [apparent Michaelis constant, 69 +/- 10 microM; maximum velocity (Vmax), 600 +/- 70 pmol.10(6) cells-1.s-1] and inhibited by nitrobenzylthioinosine (NBMPR). Formycin B had an unusually high affinity [inhibitory constant (Ki), 18 +/- 4.3 microM], whereas inosine had a low affinity (Ki, 440 +/- 68 microM) and nucleobases were without effect on adenosine influx. The number of transporters (1.2 x 10(6) sites/cell) was estimated by NBMPR equilibrium binding (apparent dissociation constant, 0.11 +/- 0.01 nM; maximum binding, 2.0 +/- 0.15 pmol/10(6) cells). In addition, we compared these endothelial cells with those obtained from cords from pregnancies complicated by diabetes (HUVEC-D), since embriopathy may occur in these conditions. HUVEC-D exhibited a 2.3-fold reduction in both the Vmax for adenosine influx and the maximum number of NBMPR binding sites (260 +/- 40 pmol.10(6) cells-1.s-1 and 0.86 +/- 0.08 pmol/10(6) cells, respectively). However, the turnover number for each nucleoside transporter in normal and diabetic HUVEC was similar (approximately 300 adenosine molecules/s). Adenosine metabolism at 10 microM in HUVEC-D was modified compared with normal cells. Intracellular phosphorylation (> 90%) was the predominant pathway in normal HUVEC, whereas in HUVEC-D, substantial levels of adenine and adenosine were detected. The present results demonstrate therefore the downregulation of the NBMPR-sensitive nucleoside transporter and changes in adenosine metabolism in HUVEC from diabetic pregnancies.

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