Inhibitors of P-Glycoprotein-Mediated Daunomycin Transport in Rat Liver Canalicular Membrane Vesicles

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Abstract
P-glycoprotein (P-gp), the multidrug resistance (MDR) gene product, is exclusively located on the canalicular membrane of hepatocytes. Recent studies using isolated rat canalicular liver plasma membrane (cLPM) vesicles indicate that daunomycin (DNM) is a substrate for the ATP-dependent P-gp efflux system in the rat liver. The isoforms of P-gp present in cLPM and in cancer cell lines differ in that the major form present in the liver represents the gene product of mdr2 in mice (MDR3 in humans; class III) while the isoform of P-gp in cancer cells is the gene product of mdr1 in mice (MDR1 in humans, class I). The objective of this study was to examine the inhibitory effects of various organic compounds, most of which have been studied previously in MDR cancer cells, on P-gp-mediated [3H]DNM uptake into cLPM. Also, the stereospecificity of P-gp for its substrates was investigated by comparing the inhibitory effects of the enantiomers and the racemic mixtures of verapamil and propranolol. DNM exhibited ATP-dependent active transport into rat liver cLPM with a Km of 26.8 +/- 13.4 microM and a Vmax of 4.9 +/- 0.8 nmol/45 s/mg of protein (n = 4). ADP, AMP, and a nonhydrolyzable ATP analogue did not increase DNM transport over the control value. Thirty-one potential inhibitors were examined; only acridine orange, doxorubicin, verapamil, propranolol, phosphatidylcholine, beta-estradiol glucuronide, and DNM itself showed statistically significant inhibition of [3H]DNM uptake into cLPM. These results suggest that only a limited number of substrates bind to or are transported across the hepatic canalicular membrane via P-gp. Phosphatidylcholine, a substrate for the gene product of the class III P-gp gene, produced significant inhibition of [3H]DNM transport (30.6% at a 10-fold-higher substrate concentration), suggesting that transport may be mediated, at least in part, by this P-gp gene product. There were no statistically significant differences in the inhibitory effects of the enantiomers and racemate of verapamil on [3H]DNM transport into cLPM, but the enantiomers of propranolol exhibited stereospecific inhibition of DNM transport. (R)-(+)-Propranolol produced a statistically significant inhibition of [3H]DNM transport similar to that observed with the racemic mixture, while (S)(-)-propranolol showed no inhibition. These findings suggest that bile canalicular P-gp may exhibit stereospecificity of binding or transport for its substrates.