Bidirectional membrane transport: simulations of transport inhibition in uptake studies explain data obtained with flavonoids.

The purpose of the simulations was to obtain an estimate of concentration-dependent uptake curves when two counteracting transporters are present. On the basis of this experimental data obtained with a pair of ovarian carcinoma cell lines, one of which was not expressing the exsorptive transporter P-glycoprotein and one of which was an MDR1-transfected, P-glycoprotein expressing variant, the kinetics of cellular uptake of the radiolabel (3)H-talinolol were calculated and the inhibitory constants at P-gp were determined for different flavonoids. With respect to the inhibition of P-gp function, among others, naringenin and isoquercitrin were identified as inhibitors, yet estimation of the inhibitory constant was only possible for uptake values corrected for non-P-glycoprotein-mediated processes. It was assumed that an additional inside-directed transporting protein (Carrier B), which is inhibited by the presence of test compounds, uptake of radiolabel was simulated as a function of the concentration of test-compound, with exemplary parameters for the rate constant (k(B)) of the additional Carrier B and the inhibition constants (K(I)-values) for both transporting proteins. The obtained uncorrected experimental data, which showed either inhibition or enhancement of radiolabel uptake as a function of the inhibitor concentration, were appropriately explained by the respective model. The respective model included an exsorptive transporter as well as carrier-mediating facilitated diffusion. It is concluded that flavonoids, such as naringenin and isoquercitrin, inhibit an inside-directed process in addition to their inhibition of P-glycoprotein-mediated exsorption.