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Lack of Secretion of Retinyl Ester by Livers of Normal and Cholesterol-Fed Rabbits¹

Division of Nutritional Sciences, Section of Biochemistry, Molecular and Cell Biology, Cornell University, Ithaca, NY 14853

The use of retinyl ester as a tracer for chylomicrons and chylomicron remnants depends on the observation that newly absorbed dietary retinol is transported as retinyl ester in plasma chylomicrons or chylomicron remnants and also on the assumption that organs other than intestine do not contribute retinyl ester to plasma. To measure the secretion of retinyl ester by rabbit liver, the liver was labeled by injecting labeled retinol intravenously 1) as a colloidal dose, 2) incorporated into liposomes or 3) dispersed in a solution of Tween 20. Depending on the dose, between 63 and 80% of the labeled retinol in the liver was esterified and was found in both parenchymal and nonparenchymal cells. For all types of doses in both normal and cholesterol-fed rabbits, less than 1% of the injected dose was present in the plasma as retinyl ester during the 24-hour time period after injection. The secretion of retinyl ester by liver in response to the uptake of retinyl ester-enriched chylomicrons was also measured. This was done by feeding a diet enriched in retinol followed by a retinol-free diet. Only an insignificant quantity of retinyl ester accumulated in plasma during a 24-hour period after blocking the removal of triglyceride-rich lipoproteins with Triton WR 1339. Apparently, there is little, if any, secretion of retinyl ester by the liver of normal or cholesterol-fed rabbits.

KEY WORDS: • retinyl ester • cholesteryl ester • cholesterolfed rabbits

¹ Supported by HL 10940 from the National Heart, Lung, and Blood Institute.

² Predoctoral trainee supported by National Research Service Award HL 07245 from the National Heart, Lung, and Blood Institute.

³ Current address: College of Osteopathic Medicine, University of New England, Il Hills Beach Road, Biddeford, ME 04005.

⁴ Career Investigator, American Heart Association.

⁵ To whom reprint requests should be addressed.

Manuscript received 25 March 1983.