Effect of diabetes, starvation, ethanol and isoniazid on rat liver microsomal 12 alpha-hydroxylase activity involved in bile acid biosynthesis

Dietary rhubarb (Rheum rhaponticum) stalk fibre does not lower plasma cholesterol levels in diabetic rats

Rhubarb (Rheum rhapontiam) stalk fibre was previously shown to be hypolipidaemic under clinical and experimental conditions. The present study was undertaken to investigate whether rhubarb stalk fibre has a hypolipidaemic effect under diabetic conditions. Two models of diabetic rats were used: streptozotocin-induced diabetic rats, and diabetes-prone BB (BBdp) rats. The plasma cholesterol and triacylglycerol concentrations were elevated after the onset of diabetes in BBdp rats, but not in sterptozotocin-induced diabetic rats. The rhubarb-fibre diet had no effect on the plasma cholesterol or triacylglycerol concentrations of diabetic rats. The hypolipidaemic effect of rhubarb stalk fibre has been suggested to be due to the bile-acid-binding capacity of rhubarb fibre, which in turn up regulates cholesterol 7alpha-hydroxylase (cyp7a) activity. cyp7a is the first and the rate-limiting enzyme in the breakdown of cholesterol to bile acids. We measured the cyp7a activity and mRNA levels in control and diabetic rats fed rhubarb- and cellulose-fibre diets. The cyp7a activity and mRNA abundance were increased in both diabetic rat models, indicating that bile acid synthesis is enhanced in diabetes. Feeding a diet enriched with rhubarb fibre caused a slight but significant increase (P<0.05) in cyp7a enzyme activity in BBdp rats, but no change in cyp7a mRNA abundance was detected. These results suggest that although a rhubarb-fibre-enriched diet increased cyp7a activity in BBdp rats, there was no apparent therapeutic benefit in terms of lowering plasma cholesterol concentrations.

Biosynthesis of cholic acid accelerated by diabetes: its mechanism and effect of vanadate administration

[7 beta-3H]3 alpha,7 alpha-Dihydroxy-5 beta-cholestane was infused into the femoral vein of bile-fistula rats. The main metabolites in bile were cholic, chenodeoxycholic, and alpha-muricholic acids. Cholic acid accounted for only 7.5% of the total biliary [3H]bile acids in the normal rats, but 51.8% in the diabetic rats. This increased proportion of cholic acid was partially cancelled by treatment of the diabetic rats with insulin (29.8%) or vanadate (28.8%). These results clearly confirm that an alternative biosynthetic pathway of cholic acid via 3 alpha,7 alpha-dihydroxy-5 beta-cholestane is accelerated by diabetes, and indicate that vanadate has an insulin-like effect on the formation of cholic acid in an insulin-deficient state.