VEGF-mediated endocytosis and relative receptor levels impacts VEGF binding to VEGFR2. A. Simulations were performed for Model 1 (VEGFf can bind to VEGFR1 and NP-1) and Model 2 (VEGFf cannot bind to VEGFR1 and NP-1) with VEGF (0.023 nM) and variable VEGFf for 3 h using base parameter values except for the internalization rate for VEGF-bound receptors. Total VEGFR2 bound to VEGF (surface and internal) is shown. B. Similar simulations were performed using all base parameter values but the initial conditions were altered to have 12,000 VEGFR1, VEGFR2, and NP-1 (VEGFR1 = VEGFR2 = NP-1), or 12,000 VEGFR1 and NP-1 but 120,000 VEGFR2 (10*VEGFR2), or 12,000 VEGFR2 and NP-1 but 120,000 VEGFR1 (10*VEGFR1), or 12,000 VEGFR1 and VEGFR2 but 120,000 NP-1 (10*NP-1). Model 1 is open circles and Model 2 is the filled circles in both A and B.