Phenobarbital in the genetically obese Zucker rat. II. In vivo and in vitro assessments of microsomal enzyme induction.

In vivo and in vitro alterations in drug metabolism and the extent of enzyme induction of the hepatic microsomal cytochrome P-450 system were evaluated in obese and lean Zucker and lean Sprague-Dawley rats. Phenobarbital enzyme-inducing regimens were administered p.o. to achieve similar steady-state phenobarbital plasma concentrations. Control rats received p.o. placebo solution. No significant intra- or inter-strain differences in antipyrine clearance (milliliters per hour) or apparent volume of distribution (liters) were observed between the placebo-treated lean Sprague-Dawley, lean Zucker and obese Zucker rats. Intra- and inter-strain differences in hepatic microsomal protein and cytochrome P-450 content were observed. Compared to placebo, antipyrine clearance (milliliters per hour) after chronic phenobarbital pretreatment was increased in the Sprague-Dawley (198%) and lean Zucker rats (131%), but not significantly altered in the obese Zucker rats. Similarly, increases in hepatic weight, whole liver microsomal protein and cytochrome P-450 content were also observed in the Sprague-Dawley (34, 124 and 352%, respectively) and the lean Zucker rats (24, 96 and 249%, respectively). However, no significant alterations in these parameters were observed in the obese Zucker rats after phenobarbital treatment. Results from these in vivo and in vitro studies implicate alterations in drug metabolism and genetic differences in cytochrome P-450 content in Zucker rats relative to the Sprague-Dawley strain. Obese Zucker rats failed to exhibit a significant induction response after phenobarbital pretreatment.