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

Uptake of L-[1-C-14]ascorbic acid (Asc) of 12.5-200 mu M for 1 h into bovine aortic endothelial BAE-2 cells grown to confluence was as low a s 43-64% (per cell) of uptake into the cells grown to nearly one-fourt h confluence. [C-14]Asc undergoing transmembrane uptake was concentrat ed and accumulated in the cell less efficiently ([Asc](in/ex) = 8-13) at confluence than at subconfluence ([Asc](in/ex) = 15-24). The declin ed Asc uptake at confluence is attributable to slowdown of the cell cy cle, because a similar decrease in [Asc](in/ex) was shown by subconflu ent cells precultured in serum-insufficient medium, resulting in an in crease in G1 phase and concurrent decreases in S and G2 + M phase dist ributions as determined by flow cytometry. [1-C-14]Dehydroascorbic aci d (DehAsc) was taken up and accumulated as Asc, after metabolic reduct ion, 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 as 62-75% for Asc, suggesting the moderate control of Asc uptake against slowdown of the cell cycle. At either c onfluence or subconfluence, dose-dependence for DehAsc uptake was more marked than for Asc uptake as shown by an uphill slope in a curve of doses versus [Asc](in/ex) for DehAsc in contrast to a downhill slope f or Asc, suggesting the moderate control for Asc uptake against fluctua tion of the dose. Increasing of coexistent glucose of 5 mM to 20-40 mM , plasma concentrations in diabetic patients, declined DehAsc uptake t o 46-48%, which was less moderately controlled than Asc uptake retaine d to 59-73%. Asc uptake did not compete with DehAsc uptake, suggesting different transporter proteins for Asc and DehAsc. Thus, Asc uptake i nto the aortic endothelial cells is mole moderately controlled against slowdown of the cell cycle, decreasing of the extracellular concentra tions or increasing of coexistent glucose than DehAsc uptake, suggesti ng a homeostatic advantage of Asc over DehAsc in terms of retention of intracellular Asc contents within a definite range.