Fig. 1. (A ) Schematic representation of L-MDR1 and Caco-2 cells grown on the Tranwell™ system. Expression of P-glycoprotein (P-gp) is limited to the apical domain, thus enhancing substrate movement in the basal-to-apical direction. (B ) Digoxin transport in L-MDR1 (expressing P-glycoprotein) and LLC-PK1 (lacking P-glycoprotein) reveals movement of the drug is markedly greater in the basal-to-apical direction versus  apical-to-basal in L-MDR1 cells but not in the LLC-PK1 cells.