Dose-response relationships for cytochrome P450 induction by phenobarbital in the cotton rat (Sigmodon hispidus).

Abstract The induction of a hepatic pleiotropic response, including increase in liver/body weight ratio, induction of hepatic CYP2B and CYP3A protein and catalytic activity, and hepatic microsomal epoxide hydration activity, was investigated in male cotton rats ( Sigmodon hispidus ) administered graded dietary concentrations (0–1500 ppm) of phenobarbital (PB) for 14 days. A dose-dependent induction of each endpoint was observed, although plateaus in the various dose-response curves were not obtained, and ED 50 values (PB concentrations associated with half-maximal responses) for the various endpoints were not able to be calculated. A maximal 1.31-fold increase, compared to the control value, in liver/body weight ratio was observed, while microsomal epoxide hydration activity was increased as much as 3.6-fold by PB administration. Pentoxy- and benzyloxyresorufin O -dealkylation and testosterone 16 β -hydroxylation activities (considered to be relatively selective for CYP2B in the Norway rat ( Rattus norvegicus )), were induced maximally less than five-fold. Testosterone 6 β -hydroxylation (considered to be relatively selective for CYP3A in R. norvegicus ) was induced maximally less than two-fold. Maximal induction of 7-ethoxy-4-trifluoromethyl-coumarin O -deethylation was 18-fold, compared to the control rate. Western blotting studies indicated that hepatic microsomal proteins immunoreactive with polyclonal antisera to R. norvegicus CYP2B1 or CYP3A1 were induced, in a dose-responsive manner, by PB in the cotton rats. These results indicate that the cotton rat responds to PB treatment with a coordinate pleiotropic response similar to that displayed by R. norvegicus , although the substrate specificity of the induced proteins appears to differ between the two rodent species.