Enhancement of maximal bilirubin excretion by bile salts in the anaesthetized rabbit

Bile acids protect the liver against the cholestatic effect of large bilirubin loads

BACKGROUND

This study was undertaken to elucidate why large bilirubin loads cause canalicular cholestasis and whether bile acid infusions protect against bilirubin-induced cholestasis.

METHODS

The effects of bilirubin infusion on canalicular bile secretion and canalicular membrane morphology were studied in bile acid-depleted pigs (BADP), bile acid-primed pigs (BAPP), and pigs co-infused with bile acids during bilirubin loading (BACIP).

RESULTS

Bilirubin caused complete cholestasis in BADP, 38% bile flow reduction in BAPP, and no effect on bile flow in BACIP. Scanning electron micrographs showed loss of 70% of canalicular microvilli in BADP, 13% loss and pathologic changes in the remaining 75% of microvilli in BAPP, and no canalicular changes in BACIP. Cholestasis was not due to hydromechanical obstruction of bile ductules or bile Ca2+ depletion.

CONCLUSION

Bilirubin causes cholestasis in BADP by injuring canalicular microvilli. Intravenous glycocholate infusions fully protect the liver against bilirubin-induced cholestasis and canalicular microvillar injury.

Hepatic transport of bilirubin in rats with streptozotocin-induced diabetes

This study was undertaken to evaluate the effects of streptozotocin-induced diabetes on the hepatic transport of bilirubin in male Wistar rats. Rats were pretreated with streptozotocin (60 mg/kg i.p.) to induce uncontrolled diabetes. Six days later endogenous biliary excretion and plasma bilirubin concentration were significantly enhanced compared to control animals (+36% and +46%, respectively), while the blood levels of free hemoglobin remained unchanged. Following a bilirubin load, the maximal biliary excretion of the pigment (Tm) in diabetic animals was significantly enhanced compared to control animals (+49%). Liver and plasma bilirubin concentrations at the end of bilirubin administration were significantly reduced (-28% and -30%, respectively). Bilirubin UDP-glucuronosyltransferase activity and UDP-glucose concentration in liver were significantly enhanced (+31% and +81%, respectively), as was the biliary excretion of unconjugated bilirubin (+37%) and bilirubin mono- (+38%) and diconjugates (+53%). When streptozotocin-diabetic rats were treated with insulin, the parameters of bilirubin transport and metabolism were significantly reduced compared to diabetic animals receiving no hormone replacement. In summary, our data indicate that in short-term streptozotocin-diabetic rats there is increased bilirubin production as well as enhanced hepatic conjugation and subsequent biliary excretion of the pigment. These effects appear to be a direct consequence of diabetes.

Effects of ursodeoxycholate on maximal biliary secretion of bilirubin in the rat

The effect of sodium ursodeoxycholate (0.5 and 1.0 mumol/min/100 g) on the maximal biliary secretion (Tm) of bilirubin and on the concentration of bilirubin in liver and plasma at the end of a bilirubin load was studied in Wistar rats. Administration of ursodeoxycholate at 0.5 mumol/min/100 g caused a 0.8-fold increase in bile flow and a significant increase in the bilirubin Tm (+24%). This was associated with a significant reduction of liver and plasma bilirubin concentrations (-16% and -17%, respectively). Bilirubin UDP-glucuronosyltransferase activity was not significantly enhanced. There was a significant increase in the biliary excretion of bilirubin conjugates (+30%) and in the diconjugates/monoconjugates ratio in bile (+31%). When ursodeoxycholate was given at 1.0 mumol/min/100 g, it produced a 1.7-fold increase in bile flow, but the bilirubin Tm was significantly reduced (-21%). Liver bilirubin concentrations were decreased (-20%) and there was a significant enhancement in total pigment concentration in plasma (+19%). Both the excretion of unconjugated bilirubin and that of bilirubin conjugates were significantly reduced (-60% and -18%, respectively). There was a significant decrease in the bilirubin-UDP glucuronosyltransferase activity and the diconjugates/monoconjugates ratio in bile (-27% and -27%, respectively). These results indicate that ursodeoxycholate is able to increase maximal bilirubin secretion only when administered at low doses and that infusion at higher rates can significantly interfere with different steps in the hepatobiliary transport of the pigment.

Influence of bile acids on the biliary transport maximum of phenolsulfonphthalein in the rat

1. The effect of changes in bile acid secretion induced by cholestyramine treatment or taurocholate infusion on the biliary transport maximum (Tm) of phenolsulfonphthalein (PSP) was studied in Wistar rats. 2. Five hours after oral administration of cholestyramine (1.5 g/kg bodyweight) the biliary output of bile acids decreased to 51% and bile flow to 76% of control values. The percentage of conjugated and unconjugated PSP excreted into bile and the Tm of the dye were not significantly modified by cholestyramine pretreatment. 3. Administration of sodium taurocholate at increasing rates (60-480 nmol/100 g bodyweight per min) enhanced bile flow and the biliary output of bile acids in a linear dose-related fashion. The Tm of PSP increased progressively until a maximum of 29% above the control values was reached at a taurocholate dose of 240 nmol/100 g bodyweight per min). The enhancement corresponded mainly to the unconjugated dye, the excretion of conjugated PSP not being significantly modified by the infusion of the bile acid. 4. The results indicate that bile acids can influence to some extent biliary excretion of PSP in the rat, although this component is of minor importance at low bile acid secretory rates.

Effect of bilirubin and bile acids on biliary phospholipid secretion during bromobenzene-induced hepatic injury in the rabbit

The interrelationship between the biliary secretion of phospholipids, bilirubin and bile acids was studied in New Zealand rabbits after acinar zone 3 hepatocyte damage induced by bromobenzene (4 mmol/kg body wt; i.p.). Treatment with the toxin did not significantly modify biliary phospholipid secretion, suggesting that the transport of phospholipids is mainly a zone 1 function. Bilirubin infusion at 1 mumol/min kg body wt induced a significant inhibition in biliary phospholipid secretion both in control and treated animals. This effect was overcome by the additional infusion of sodium glycodeoxycholate at 1.6 mumol/min kg body wt, although higher increases in phospholipid output were found in the control than in the treated rabbits. This would be related to the lowered recruitment of zone 3 cells for secretion in bromobenzene-damaged livers.

Bile pigment formation and excretion in the rabbit

Bile and plasma levels of biliverdin and bilirubin, together with the hepatic biliverdin reductase and bilirubin UDP-glucuronosyl transferase activities, were studied in the rabbit. No biliverdin could be detected in the blood plasma. The bilirubin concentration in blood was 7.81 +/- 0.79 mumol/l. Biliverdin was the predominant pigment in bile (63%). Hepatic biliverdin reductase activity was 0.086 +/- 0.016 nmol/mg protein/hr. The synthesis of bilirubin was apparently limited by the enzyme activity. Most of the bilirubin in bile was conjugated (90%) with monoconjugates predominating (75%). Hepatic UDP-glucuronosyl transferase activity was 2.65 +/- 0.18 and 1.14 +/- 0.16 mumol/mg protein/hr with and without activation, respectively.

Heterogeneity of rabbit hepatocytes for bile secretion after acinar zone 3 damage induced by bromobenzene. Effect of bilirubin and bile salt infusions

Anaesthetized rabbits were used to study the effect of bromobenzene-induced hepatic damage to the acinar zone 3 on bile flow, bile salt, sodium secretion as well as bilirubin transport in basal conditions or with infusion of sodium glycodeoxycholate. The bromobenzene-pretreated animals exhibited in basal conditions a lower bile flow (44%) than that of the controls, with a smaller decrease in bile salt output (27%) and sodium output (29%), whereas no modification in endogenous bilirubin excretion was observed. The bile salt independent fraction of secretion (BSIF) was reduced significantly after the toxic lesion both in terms of absolute and relative values. The hepatocytes of the periportal zone were capable of excreting the totality of bilirubin presented to the liver, regardless of the extent of bile flow or the input of bile salts. The infusion of bilirubin at 1.0 mumole/kg/min led to a fall in bile flow which was attributed to the interference of the pigment with the BSIF. The maximal bilirubin excretion was significantly smaller in bromobenzene-pretreated animals than in the controls, which could be due to the incapacity of the intoxicated rabbits to recruit quiescent hepatocytes. When glycodeoxycholate was administered under conditions of maximal bilirubin transport, bile flow increased as did bile salt secretion in both controls and animals with damaged livers. However, clear differences persisted between the two, which could be attributed not only to the volume fraction of necrosis but also to an interference by bilirubin with the hepatic handling of bile salts. Maximal bilirubin excretion increased in a similar way in both groups after glycodeoxycholate administration. It is proposed that glycodeoxycholate infusion facilitates the hepatic depletion of bilirubin, probably by stimulating transport processes.