Various mechanisms, including high levels of cytokines and nitric oxide (NO), have been proposed as mediators for inflammation-induced cytochrome 450 down-regulation. However, the contribution of each of these mediators to the observed effects is controversial. We used an isolated perfused rat liver (IPRL) model to test the direct effects of NO donors on CYP450 down-regulation in the absence of cytokines or other confounding in vivo factors. Our hypothesis was that NO rapidly and concentration-dependently decreases CYP450 activities in IPRL. Livers were perfused (60 min) with 50 to 500 microM sodium nitroprusside (SNP) or 100 to 500 microM isosorbide dinitrate (ISDN) as NO donors, and the perfusate and biliary disposition of SNP, ISDN, and generated nitrate/nitrite (NO(x)) were determined. Additionally, at the end of perfusion, catalytic activities and protein levels of various cytochrome isoenzymes were measured. Both SNP and ISDN exhibited linear hepatic disposition with extraction ratios of approximately 0.30 and 0.50, respectively. Furthermore, although in small amounts, both NO donors and NO(x) were found in the bile. Except for CYP2D1, the catalytic activities of all the studied isoenzymes were substantially (up to 85%) decreased by both NO donors. However, the apoprotein levels of isoenzymes remained largely unchanged. Additionally, the inhibitory effects of NO donors were concentration-dependent, with the concentrations of SNP producing one-half of maximum inhibition being in the order of 2C11 > 2B1/2 > 2E1 = 3A2 > 1A1/2. These studies indicate that the effects of NO on the down-regulation of cytochrome 450 catalytic activity are rapid, concentration-dependent, and isoenzyme-selective.