Determination of mRNA levels of endogenous influx transporters, canine PepT1 (cPepT1) and canine PepT2 (cPepT2), and exogenous efflux transporters, human P-gp (hMDR1), human MRP2 (hMRP2) and human BCRP (hBCRP) in various transfected MDCKII cell lines with different passage number using real-time PCR: A, MDCKII-MDR1 cells; B, MDCKII-MRP2 cells; C, MDCKII-BCRP cells. Each bar represents mean ± SD (n = 3). *P < 0.05 and **P < 0.01 compared with the lowest cell passage.