VEGF receptors Flk1 and NRP1 were decreased by KOR activation in Flk1⁺ vascular progenitors. (A) Quantitative RT-PCR showing mRNA expression at Flk-d1. Control mRNA expression was set as 1.0. Flk1⁺ cells were stimulated with control, VEGF (50 ng/mL), DAMGO (10μM), SNC80 (10μM), U50,488H (10μM), TRK820 (10μM), 8br-cAMP (0.5mM), and 6-Bnz-cAMP (0.1mM) as indicated (**P < .01 vs VEGF). (B) Fluorescent staining for Flk1 (green) at Flk-d1. Nuclei are stained with DAPI (blue). Scale bars, 200 μm. (C) Flow cytometry at Flk-d1. X-axis: Flk1; y-axis: SSC. Percentages of Flk1⁺ cells among vascular progenitor-derived cells are indicated. (D) Fluorescent staining for NRP1 (green) at Flk-d1. Nuclei are stained with DAPI (blue). Scale bars, 200 μm.