Moderately controlled transport of ascorbate into aortic endothelial cells against slowdown of the cell cycle, decreasing of the concentration or increasing of coexistent glucose as compared with dehydroascorbate

Abstract

Uptake of L-[1-14C]ascorbic acid (Asc) of 12.5-200 µM for 1 h intobovine aortic endothelial BAE-2 cells grown to confluence was as low as43-64% (per cell) of uptake into the cells grown to nearly one-fourthconfluence. [14C]Asc undergoing transmembrane uptake was concentrated andaccumulated in the cell less efficiently ([Asc]in/ex = 8-13) at confluencethan at subconfluence ([Asc]in/ex = 15-24). The declined Asc uptake atconfluence is attributable to slowdown of the cell cycle, because a similardecrease in [Asc]in/ex was shown by subconfluent cells precultured inserum-insufficient medium, resulting in an increase in G1 phase andconcurrent decreases in S and G2 + M phase distributions as determined byflow cytometry. [1-14C]Dehydroascorbic acid (DehAsc) was taken up andaccumulated as Asc, after metabolic reduction, without detectable DehAsc.The [Asc]in/ex values for DehAsc at confluence were as low as 15-69%of those at subconfluence in contrast to the values as retentive as62-75% for Asc, suggesting the moderate control of Asc uptake againstslowdown of the cell cycle. At either confluence or subconfluence,dose-dependence for DehAsc uptake was more marked than for Asc uptake asshown by an uphill slope in a curve of doses versus [Asc]in/ex for DehAsc incontrast to a downhill slope for Asc, suggesting the moderate control forAsc uptake against fluctuation of the dose. Increasing of coexistent glucoseof 5 mM to 20-40 mM, plasma concentrations in diabetic patients, declinedDehAsc uptake to 46-48%, which was less moderately controlled thanAsc uptake retained to 59-73%. Asc uptake did not compete with DehAscuptake, suggesting different transporter proteins for Asc and DehAsc. Thus,Asc uptake into the aortic endothelial cells is more moderately controlledagainst slowdown of the cell cycle, decreasing of the extracellularconcentrations or increasing of coexistent glucose than DehAsc uptake,suggesting a homeostatic advantage of Asc over DehAsc in terms of retentionof intracellular Asc contents within a definite range.