Fig. 2. Fentanyl and antipyrine uptake by human lung microvascular endothelial cells. The symbols represent the ratio (K^EQ) of total cell-associated drug concentrations (nm/cell) to supernatant drug concentrations plotted as a function of supernatant drug concentration (C^S, μm). The symbols (circles = fentanyl; triangles = antipyrine) represent the mean value from three experiments; the error bars represent the SD; the solid line represents the best fit of the model to the data for fentanyl. For antipyrine, the solid line represents the mean value for K^EQfor the concentration range studied.