fails to induce proliferation of HUVECs at low extracellular ATP concentrations. (A) HUVECs were cultured in serum-free EBM (C) or serum-free EBM containing VEGF-A165 (20 ng/mL), AP (160 ng/mL) or VEGF-A165 (20 ng/mL) combined with AP at concentrations increasing from 40 ng/mL to 160 ng/mL. After 48 h of incubation CellTiter 96® Aqueous One Solution Cell Proliferation Assay was performed. The absorption of a colored formazan product measured colorimetrically is proportional to the number of viable cells. Values are presented as means ± SD (n = 9). ***p < 0.001 vs. C, ###p < 0.001 and ##p < 0.01 vs. VEGF (ANOVA, Scheffé's test). (B) ATP concentrations corresponding to (a) were analyzed luminometrically (n = 3) in the HUVEC culture medium after 48 h of incubation.