Hepatic extraction of endogenous inhibitors of drug binding to serum protein in the pregnant rat.

Significant decreases in the serum protein binding of a fluorescent dye, 1-anilino-8-naphthalenesulfonate (ANS), and salicylic acid (SA) were observed in pregnant rats compared to that in nonpregnant (control) rats. A significant difference in the serum protein binding of ANS and SA between serum samples taken at the hepatic vein and portal vein or femoral artery was also observed in the pregnant rats, while such a sampling site difference in the serum protein binding was not observed in the control rats. In the pregnant rats the affinity of ANS binding to the primary binding site in the serum from the hepatic vein was approximately 70% higher than that in the case of serum from the portal vein. The hepatic extraction of nonesterified fatty acids (NEFA) was also determined, and the extraction ratios in the control and pregnant rats were 0.55 and 0.31 respectively. We concluded from these findings and other evidence that certain endogenous inhibitors of drug binding to serum protein (such as NEFA), which increase during pregnancy, were extracted efficiently by the liver and that the difference in the serum protein binding of ANS and SA between serum samples taken at the hepatic and portal veins or femoral artery in the pregnant rats may be explained by the hepatic extraction of endogenous inhibitors. Our present results support the previous finding by Chou et. al. [Int. J. Pharm. 18, 217 (1984)] that in pregnant rats the serum protein binding of phenytoin is greater in the hepatic vein than that in the femoral vein.