Cholesterol and bile-acid metabolism in normal and cholesterol-fed rabbits after partial ileal bypass

Reduction of LDL production rate in ileal bypassed rabbits treated with lovastatin

The effect of lovastatin on the low density lipoprotein metabolism in bypassed rabbits was investigated. Partial ileal bypass effectively reduced total and low density lipoprotein (LDL) cholesterol by 44 and 48% respectively. The LDL drop was due to an increased fractional catabolic rate (FCR) of apolipoprotein B (apo B) from 0.74 to 1.27 pools per day with no effect on the apo B absolute catabolic rate and an increased expression of liver LDL receptors (+71%). Association of lovastatin with PIB resulted in a further decrease of total and LDL cholesterol (56 and 75% respectively) as compared to bypassed animals, without effects on the LDL FCR (1.27 +/- 0.11 versus 1.42 +/- 0.13 pools/day) or the expression of LDL receptors by the liver. The reduction of LDL was due to a decrease of the apolipoprotein B absolute synthetic rate (8.5 +/- 1.7 versus 13.6 +/- 1.7 mg/day). From these data we conclude that in bypassed rabbits lovastatin lowers total and LDL cholesterol mainly by reducing apolipoprotein B production rate.

Effect of dietary restriction on cholesterol biosynthesis in the liver and the intestine of cholesterol-fed rabbits

The effect of dietary restriction (half of the control ration) on cholesterol biosynthesis was investigated in rabbits fed either standard or cholesterol-rich diets. Accompanying the amplification of hypercholesterolemia, additional disturbances of cholesterol metabolism were observed when cholesterol feeding was associated with dietary restriction. In the intestine, underfed rabbits showed a more marked inhibition of duodenal cholesterol biosynthesis from [14C]acetate following cholesterol feeding than rabbits on normal caloric ration. In contrast liver cholesterogenesis was equally suppressed in both groups receiving cholesterol-rich diets. Cholesterol biosynthesis from [14C]mevalonate was also inhibited by cholesterol feeding particularly in the duodenum of underfed rabbits. In addition cholesterol feeding induced a marked increase of the labeled esterified: free cholesterol ratio in the liver, demonstrating intensive esterification, this was enhanced by dietary restriction. The additional cholesterol which accumulates in the plasma and in various tissues in underfed rabbits is of dietary origin since the feedback control of cholesterogenesis by exogenous cholesterol was shown to be very effective in these animals.

The effect of cholestyramine on liver HMG-CoA reductase and cholesterol 7 alpha-hydroxylase in various laboratory animals

The activity of HMG-CoA reductase and cholesterol 7 alpha-hydroxylase was assayed in the liver of rats, rabbits, hamsters and guinea pigs at the minimum of the day cycle and after one night fasting. The amount of HMG-CoA reductase, as determined after its complete dephosphorylation in vitro was of the same order of magnitude in the tested species. The dephosphorylated active form of the enzyme was detectable only in the rat. Cholesterol 7 alpha-hydroxylase activity was also much higher in the rat. Cholestyramine treatment stimulated the activity of both enzymes. In particular, the ratio between active and inactive HMG-CoA reductase in rabbits, hamsters and guinea pigs became of the same order of magnitude of that found in rats.