Influence of Verapamil and Cyclosporin A on bile acid metabolism and transport in rat liver slices

Verapamil (V) is a specific inhibitor of the P-glycoprotein (mdr1) in the hepatocyte canalicular membrane. Cyclosporin A (CsA) as an essential immunosuppressive drug has potentially cholestatic adverse effects on the liver, but increases the expression of mdr1. In precision-cut liver slices from 34- to 40-day-old male Wistar rats 26 individual free and conjugated bile acids (BAs) as markers of hepatic transport and synthesis function were analysed after 4 h incubation with V (100 μM) or CsA (5 μM) in Krebs–Henseleit buffer. Some slices were loaded with cholic acid (CA 5 μM) or tauro-ursodeoxycholic acid (T-UDCA 5 μM) to investigate the V and CsA effects under conditions of BA supplementation. BAs were determined in tissue and medium by HPLC with postcolumn derivatisation and fluorescence detection.V and CsA, influencing different targets in BA transport, enhanced slice concentrations of T- and glyco- (G-) conjugated CA only when exogenous CA was given additionally. This BA accumulation in tissue is more reflected at decreased medium concentrations of these BAs after V and CsA incubations. Both V and CsA also inhibited CA uptake into the slices. The acidic chenodeoxycholic acid (CDCA) synthesis pathway is disturbed: T- and G-CDCA concentrations are diminished in slices and medium after V and CsA incubations. T-UDCA plus V or CsA enhanced not only its own slice concentration but also the concentration of the trihydroxylated tauro-muricholic acid (T-β-MCA), reflecting the conversion of the accumulated dihydroxylated T-UDCA into the T-β-MCA. The similar effects of V and CsA on BA transport and metabolism can be explained by mdr1 mediated disturbances of cellular ATP transport rather than by inhibition of individual BA transporters.