Fig. 2. Transepithelial transport across L-MDR1 (top ) and LLC-PK1 (bottom ) cell culture monolayers: (A  and B ) [³H]-loperamide (5 μm), (C  and D ) [³H]-morphine (5 μm), (E  and F ) [³H]-sufentanil (5 μm), (G  and H ) [³H]-fentanil (5 μm), and (I  and J ) [³H]-alfentanil (5 μm). Translocation from basal to apical compartments is indicated by triangles and solid line; translocation from apical to basal compartments is indicated by squares and dotted line. Data are mean ± SD from three or more experiments.