The effects of hepatic microsomal enzyme inducers on the pharmacokinetics of ouabain after portal and systemic administration to rats.

The microsomal enzyme inducers 3-methylcholanthrene, phenobarbitone and pregnenolone-16alpha-carbonitrile (PCN) are known to affect other aspects of hepato-biliary disposition in addition to metabolism. This study was designed to determine if presystemic elimination of the non-metabolized xenobiotic ouabain could be altered by these inducers. Male Sprague-Dawley rats were pretreated with inducers or saline for four days. A day later, ouabain (0.5 mg kg-1) was administered into either the ileocolic vein (portal administration) or the femoral vein (systemic administration). Blood and bile samples were collected for up to 90 min after ouabain administration. Biliary excretion rate and cumulative biliary excretion of ouabain were increased by pretreatment with PCN (75 mg kg-1 day-1) relative to controls. Phenobarbitone pretreatment (75 mg kg-1 day-1) also increased these parameters, but to a lesser extent than PCN. In contrast, 3-methylcholanthrene pretreatment (20 mg kg-1 day-1) had no effect on biliary excretion. Plasma concentrations of ouabain were much lower after PCN pretreatment relative to controls, whereas neither phenobarbitone nor 3-methylcholanthrene had any effect. Similarly, clearance (both biliary and total) and volume of distribution were increased by PCN, but not by phenobarbitone or 3-methylcholanthrene pretreatment. Interestingly, the magnitude of biliary and plasma effects induced by PCN appeared to be comparable whether ouabain was administered portally or systemically. Pretreatment of rats with PCN, but not phenobarbitone or 3-methylcholanthrene was shown to increase total clearance of ouabain, mainly via an increase in biliary clearance. Furthermore, because the enhanced clearance occurs after systemic as well as after portal administration of ouabain, a significant change in hepatic presystemic elimination was not detected.