Serum bilirubin constituents in different experimental models of conjugated hyperbilirubinemia

The object of the study was to explain the differences in bilirubin level in various experimental jaundice models. The Bil constituents in conjugated hyperbilirubinemic dog models were identified. Total Bil was measured using the Jendrassik and Grof method and direct spectrophotometry. Conjugated and unconjugated Bil were measured using the Weber-Schalm extraction method. Bilirubin covalently bound to albumin was measured indirectly from the total Bil and the non-CBBA fraction. The non-CBBA was estimated either as the sum of the CB and UCB concentrations determined by the W-S method or as the nonprecipitated fraction after deproteinization with ammonium sulfate-saturated ethanol when using DS.

The TB, CB, UCB, and CBBA levels were compared in two hyperbilirubinemic dog models: (a) chronic bile duct ligation (CBDL) and (b) internal choledochocaval anastomosis (CDCA). The mean TB in internal CDCA (16.5 ± 3.67 mg%) was significantly higher than in CBDL (3.4 ± 1.75 mg%). Most of the serum Bil in these two models was conjugated, 13.4 ± 2.24 and 3.2 ± 1.7 mg%, respectively. No CBBA was found in the CBDL or in the partially obstructed internal-CDCA dogs. The TB in an external-CDCA model was essentially similar to the internal-CDCA model. The indirect Bil level in the external-CDCA model was six to seven times higher than the UCB level, and the CBBA level varied between 30 and 80% of the TB. Up to 50% CBBA was found also in patients with intra- or extrahepatic cholestasis.

The findings indicate that, unlike the commonly assumed hypothesis, the serum TB level in the CDCA models, which was higher than in the CBCL one, is not due to high UCB levels. Rather, hyperbilirubinemia in the external-CDCA model is due to increased levels of CBBA or, perhaps, to variations in the amount and/or composition of CB entering the blood and cleared by the kidney.