The economy of the enterohepatic circulation of bile acids in the baboon. 1. Studies of controlled enterohepatic circulation of bile acids

Adaptive changes of hepatic bile salt transport in a model of reversible interruption of the enterohepatic circulation in the rat

The reversibility and time course of the adaptive changes in hepatic bile salt transport related to modifications of the bile salt enterohepatic circulation and bile salt pool size have not been previously studied. For this reason a model of reversible interruption of entero-hepatic circulation was characterized in unrestrained rats, which allowed the study of changes in hepatic bile salt transport following bile salt pool depletion and subsequent restoration by either the de novo synthesis of bile acids or i.v. administration of exogenous taurocholate. Rats subjected to biliary drainage for 24 h through a transduodenal common bile duct cannula, followed by removal of the cannula and restoration of the enterohepatic circulation were studied at 24, 48 and 72 h. Neither light and electron microscopy examination nor plasma biochemical parameters showed evidence of necrosis, fibrosis, cholestasis or inflammatory changes. Maximum taurocholate secretory rate decreased to 50% following 24-h bile salt depletion. After restoration of the enterohepatic circulation maximum taurocholate secretory rate progressively increased to normal values at 72 h, following the normalization of the bile salt pool size, which had a similar composition compared with controls. The same effect was obtained when the native bile salt pool was substituted with exogenous taurocholate. Thus, adaptive down-regulation of hepatic bile salt transport capacity is a reversible process, related to restoration of entero-hepatic circulation and normalization of bile salt pool size.

Negative feedback regulation of the ileal bile acid transport system in rodents

BACKGROUND

Active transport of conjugated bile acids by ileal enterocytes is a key mechanism for conservation of the bile acid pool. Experiments were performed to determine whether such transport is regulated by substrate load.

METHODS

Using anesthetized biliary fistula guinea pigs or rats, the ileum was perfused with ursodeoxycholyltaurine at a concentration causing maximal ileal transport of this bile acid; absorption was assessed by biliary recovery. Before ileal perfusion, animals ingested one of three diets: chow, chow with added conjugated bile acid, or chow with added cholestyramine.

RESULTS

In the guinea pig, ingestion of a taurocholate-enriched diet resulted in a 75% decrease in the absorption rate of ursodeoxycholyltaurine. Similar results were obtained with cholylsarcosine (a deconjugation-dehydroxylation resistant analogue) or with chenodeoxycholylglycine, the endogenous bile acid of the guinea pig. In contrast, cholestyramine ingestion caused an increase in ursodeoxycholyltaurine absorption. In the rat, cholyltaurine or cholylsarcosine ingestion also caused decreased ileal transport. In the guinea pig, maximal down-regulation of active ileal bile acid transport occurred after 2-3 days of bile acid feeding; up-regulation required 3-4 days.

CONCLUSIONS

Bile acid metabolism is regulated by feedback inhibition of active ileal transport in addition to the well-established feedback inhibition of bile acid biosynthesis in the liver. Together, these two regulatory mechanisms ensure constancy of bile acid secretion.

Substitution of mixed amino acids resembling soy protein for mixed amino acids resembling casein in the diet reduces plasma cholesterol in slowly, but not rapidly fed nor fasted baboons

Studies of cholesterol and triglyceride metabolism were conducted under steady-state conditions in conscious restrained female baboons to ascertain whether the cholesterol- and triglyceride-lowering effects of soy protein v casein in nonrodents can be explained by differences in amino acid composition between the two proteins and whether the lipid-lowering effects of soy protein are influenced by fasting or the rate of feeding, ie, the nutritional state. The metabolic changes underlying the changes in serum cholesterol and triglyceride concentration were also investigated. Isocaloric diets consisting of 45% mixed amino acids (soy or casein type), 50% glucose and 5% corn oil, including all nutritional requirements, were infused intraduodenally for five days. There were no differences in lipid levels between the soy and casein diets in fasting baboons (days 4 or 5) or when the daily diet (calculated to equal 24 hours of energy utilization) was given rapidly by constant intraduodenal infusion over seven hours (day 4). During rapid feeding there was, unexpectedly, no significant change in plasma cholesterol concentration, but the estimated rate of cholesterol oxidation to bile acids fell significantly by 38 +/- 6% (from 39 +/- 4 to 24 +/- 3 mg/d). Significant differences between the soy and casein diets were observed only in the fed state produced by the slow constant isocaloric intraduodenal infusions of the diets [5.8 mg mixed amino acids (soy or casein type) plus 7.2 mg glucose/min/kg body wt0.75] on day 5. These diets were calculated to equal the simultaneous rate of energy utilization.(ABSTRACT TRUNCATED AT 250 WORDS)