Effect of phenobarbital on the expression of bile salt and organic anion transporters of rat liver

Background/Aims: The hepatic clearance of drugs and cholephilic organic anions is stimulated by phenobarbital (PB). Our aim was to analyze the effects of PB on the expression of hepatocellular bile salt and organic anion transporters. Methods: Male Sprague–Dawley rats were treated intraperitoneally with PB (80 mg/kg/d) or saline for 5 days. Transporter expression was quantified by northern and western blot analysis and initial uptake rates of bromosulphophthalein (BSP) and digoxin were measured in isolated hepatocytes. Results: Compared to control rats, PB treatment increased expression of the organic anion transporting polypeptide 2 (Oatp2; Slc21a5) more than 2-fold on the RNA (P<0.05) and protein (P<0.001) levels. Expression of Oatp1 (Slc21a1), Oatp4 (Slc21a6) and the Na+-taurocholate cotransporting polypeptide (Ntcp; Slc10a1) was unaltered. At the canalicular pole, expression of the bile salt export pump (Bsep; ABCB11) and of the multidrug resistance proteins 2 (Mrp2; ABCC2) and 6 (Mrp6; ABCC6) was not significantly changed. Whereas hepatocellular BSP uptake was unaffected by PB, digoxin uptake was stimulated 4-fold. Conclusions: The induction of digoxin uptake by PB correlates with Oatp2 expression. In contrast, the lack of increase of Oatp1 and Oatp4 expression is in accordance with unchanged BSP uptake. These data challenge the previously held view that PB induces hepatocellular BSP uptake systems.