Endothelial cells express functional P-gp protein. Panel a and b: Verapamil blocks Rho efflux in endothelial cells. Dox- or noninduced HMEC and HUVEC cells were incubated in the absence or presence of Rho for 1 hour at 37°C. Rho accumulation was inhibited by the addition of Verapamil (Vrp) at 30 μM to the cell incubation mixture. The cells were analyzed in the flow cytometer. Panel c and d: Activity of endothelial P-gp is blocked by Verapamil and MRK16. Parental HMEC, HMECd1, and HMECd2 cells were incubated with 1 μM (c) or 2 μM (d) Rho for 1 hour at 37°C in the absence or presence of 30 μM Verapamil or 10 μg/ml MRK16 (MRK). Rho accumulation was measured by flow cytometry and quantified as the MFI. The background fluorescence level, determined using cells not exposed to Rho, was subtracted from the data. Results are expressed as the mean±SEM of 3 separate experiments. *p < 0.025, **p < 0.01.