Hepatic microsomal induction and hepatic transport

Hepatic microsomal induction (hexobarbital sleeping time, cytochrome P-450 and microsomal protein concentration, liver weight) and hepatic transport (hepatic uptake, biotransformation, biliary excretion) have been studied in rats. Phenobarbital pretreatment (75 mg/kg i.p. daily for 5 days) produced microsomal induction in the liver and enhanced biliary excretion of bromcresol green, eosine, bromsulphthaleine glutathione conjugate (BSP-GSH), amaranth and iodoxamic acid. However, biliary excretion of indocyanine green was unchanged after phenobarbital pretreatment. Biotransformation of bromsulphthalein (BSP) with glutathione was also increased by phenobarbital. The hepatic concentration of these organic anions was not influenced uniformly after phenobarbital pretreatment: the concentration of indocyanine green, bromcresol green, eosin and BSP-GSH in the liver was unchanged, that of amaranth and iodoxamic acid was enhanced following phenobarbital pretreatment. Investigation of the effect of pretreatment with other barbiturates showed that barbital, butobarbital, pentobarbital and amobarbital produced microsomal induction. Only baribtal and butobarbital stimulated biliary excretion of organic anions, whereas pentobarbital and amobarbital proved to be ineffective in this parameter. The results seem to indicate that the enhanced biliary excretion of exogenous organic anions produced by barbiturates is independent of microsomal enzyme induction.