Fig. 4. Inhibitory effects of varying concentrations of loperamide (open squares), sufentanil (filled circles), fentanyl (open circles), and alfentanil (filled triangles) on P-glycoprotein–mediated [³H]-digoxin (5 μm) transport across a Caco-2 cell culture monolayer. Data are mean ± SD from three or more experiments. The units on the x-axis for inhibitor concentration are micromolars.