Hepatic handling of photoirradiated bilirubin. A study in isolated perfused Wistar rat liver

Transepithelial fate of bilirubin in the isolated rat kidney

The renal handling of bilirubin in the rat was studied using an isolated kidney preparation by means of the determination of total pigment concentration decay in the perfusion medium and its renal clearance. Unconjugated bilirubin was incorporated in the perfusate at a concentration of about 4 microg/ml. In order to establish the potential role of secretion in renal handling of the pigment, experiments were also performed incorporating in the perfusate different doses of nicotinic acid (NA) (0.1 and 1.0 mM final concentration), which is considered an alternative substrate for the organic anion transport system, or probenecid (Prob) (0.1 and 1.0 mM final concentration), the classical inhibitor of organic anion transport process. The magnitude of pigment uptake from the perfusion medium, estimated by a first order exponential decay constant, was decreased in a dose-dependent way by NA (40 and 76% decrease for 0.1 and 1.0 mM of NA, respectively) and Prob (57 and 88% decrease for 0.1 and 1.0 mM of Prob, respectively). NA and Prob also induced a diminution in the ratio of pigment renal clearance to glomerular filtration rate (24 and 48% decrease for 0.1 and 1.0 mM of NA and 52 and 55% decrease for 0.1 and 1.0 mM of Prob). Based on these findings, it can be proposed that tubular secretion through the proximal cells contributes significantly to renal pigment depuration. In order to establish the possible contribution of cellular metabolism to the secretory process, a different set of experiments was conducted. The content of bilirubin mono and diconjugates (BMC and BDC) were determined in urine, in arterial and venous samples and in renal cortex. Studies performed using either an open or a closed circulating system, revealed that after conjugation in the renal cell, pigment derivatives can be secreted into both the tubule and the venous compartments. Total bilirubin concentration as well as the relative content of BMC and BDC in urine increased over time, representing the sum of both conjugates about 50% of the total pigment excreted by the end of experiments. Consequently, our results support the existence of a tubular transepithelial transport of bilirubin, playing the metabolism of the pigment an important role in this process.

Large intravenous loads of bilirubin photoconversion products, in contrast to bilirubin, do not cause cholestasis in bile acid-depleted pigs

BACKGROUND

Large intravenous bilirubin infusions in bile acid-depleted pigs (BADP) destroy hepatocyte canalicular membrane microvilli (CMV) and cause cholestasis. This study examines whether bilirubin photoconversion product infusions do the same.

METHODS

The effects of systemic infusion of 135 mumol.kg-1 body weight bilirubin photoconversion products on CMV density and choleretic response to intraportal bile acid infusion were studied in BADP. Furthermore, the effects of 135 mumol.kg-1 b.w. bilirubin infusion, either through an arteriovenous bilirubin photoconversion shunt device (PCD) or intravenously, were measured in PCD-connected BADP.

RESULTS

Intravenous bilirubin photoconversion product infusions affected neither the CMV density nor the choleretic response to cholic acid infusion, and neither did bilirubin infusion through the PCD. In contrast, intravenous bilirubin infusion caused canalicular injury and cholestasis in four of six PCD-connected BADP.

CONCLUSION

Bilirubin photoconversion products do not destroy CMV or cause cholestasis in BADP. A bilirubin photoconversion shunt device can confer cholestasis protection to bilirubin-loaded BADP.

Differential effect of ursodeoxycholate and its taurine conjugate on biliary transport maximum of bilirubin in the rat

The effects of ursodeoxycholate and its taurine conjugate on biliary Tm of bilirubin were evaluated in rats. Ursodeoxycholate was administered at four different doses (4, 8, 12 or 16 mumol per 100 g body wt i.v., followed by an i.v. infusion of 0.3, 0.6, 0.9 or 1.2 mumol/min per 100 g body wt, respectively), whereas tauroursodeoxycholate was administered only at the maximal dose. A dose-dependent diminution of bilirubin Tm was observed during ursodeoxycholate administration, which ranged from no effect at the lowest dose to a virtual excretory blockage at the highest dose. This was associated with an increase in bilirubin concentrations in both plasma and liver as well as in the fractional amount of conjugated pigment in both sites, suggesting an impairment of bilirubin transfer at the canalicular level. Incomplete taurine conjugation of ursodeoxycholate well correlated with these effects. Unlike ursodeoxycholate, tauroursodeoxycholate had no inhibitory effect on bilirubin Tm, although a slight inhibition of bilirubin uptake and bilirubin conjugation became apparent. Taken together, these results suggest that ursodeoxycholate interferes with the hepatobiliary transport of bilirubin by impairing its transfer at the canalicular level and that incomplete taurine conjugation appears to be a key factor determining this effect.