Journal of Lipid Research, Vol. 11, 284-292, July 1970
Copyright © 1970 by Lipid Research, Inc.

An electron microscopic and functional study of very low density lipoproteins in intestinal lymph

Robert K. Ockner and Albert L. Jones

Departments of Medicine and Anatomy, University of California, San Francisco Medical Center, San Francisco, California 94122; and the Veterans Administration Hospital, San Francisco, California 94121

Previous studies with fasting rats showed that the intestine produces endogenous very low density lipoproteins (VLDL) which resemble those in the plasma. Intestinal VLDL also were found to be important in lipid transport during absorption of saturated but not of unsaturated fat. These findings depended upon separations of a chylomicron-rich fraction (S_f > 400) from VLDL (S_f 20-400) by preparative ultracentrifugation methods based on particle flotation rates. The present studies correlate this method with electron microscopic measurement of lipoprotein particle size.

Almost all intestinal lymph lipoprotein particles from fasting rats were less than 750 A in diameter, and could not be distinguished morphologically from plasma VLDL. Cholestyramine administration or bile diversion led to decreased lymph lipid output, correlating with marked reduction in VLDL. This supports the concept that lymph VLDL contain endogenous lipid which is reabsorbed from the intestinal lumen.

During exogenous fatty acid absorption, lymph lipoprotein particle sizes were significantly smaller after administration of palmitate than after administration of linoleate, a finding consistent with ultracentrifugal evidence of the importance of VLDL in lipid transport during palmitate absorption.

These studies fully confirm and extend earlier observations. Together, they show that the intestine is a source of endogenous VLDL in the fasting animal. In addition, significant quantities of exogenous lipid are transported in VLDL during palmitate absorption, whereas with linoleate absorption nearly all lipid is in chylomicrons. These findings indicate that the small intestine plays a role in lipoprotein metabolism which extends beyond the absorption of dietary fat.

Supplementary key words bile diversion • cholesterol • cholestyramine • chylomicrons • intestinal absorption • linoleic acid • palmitic acid • triglyceride

Submitted on August 26, 1969
Revised on December 1, 1969
Accepted on February 13, 1970

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