Intestinal Lymph Lipoproteins in Rats Fed Diets Enriched in Specific Fatty Acids

Abstract

Four groups of rats were fed test diets with fats providing 75% of fatty acids as palmitate, stearate, oleate or linoleate. Absorption of radiolabeled cholesterol and the specific triglyceride into intestinal lymph lipoproteins and the lipid and protein content and composition of intestinal lymph were compared. Cholesterol and triglyceride absorptions were correlated significantly and were less with the saturated fatty acid diets. The fatty acid patterns of triglyceride-rich lymph lipoproteins mirrored the diet. Exogenous cholesterol was recovered primarily in chylomicrons, except with linoleate. In contrast, radiolabeled saturated fatty acids were recovered primarily in very low density lipoproteins and unsaturated fatty acids were recovered in chylomicrons. Lymph chylomicron size and lipid content were greater with unsaturated fat diets. Triglyceride-rich intestinal lipoproteins of rats fed saturated fats were polygonal by electron microscopy, related to the cooling of lymph samples below body temperature. A-I apolipoproteins were increased in relation to C apoproteins as lipid absorption was greater. Plasma triglycerides in all groups increased compared to rats fed the stock diet. A diet enriched in one specific fatty acid has its unique effects on lymph lipoprotein formation presumably affecting some intestinal subcellular mechanisms. Diet-induced changes in plasma lipids and lipoproteins are not directly related to these as yet unknown mechanisms.

Intracellular movement of triacylglycerols in the intestine

“…It is possible, therefore, that activated dietary FA (FA-CoA) are delivered to the TG-synthetase enzyme complex (6) for translocation across the ER membrane as pre-chylomicron-TG separately from FA-CoA preferentially from endogenous sources that are targeted to glycerophosphate acyltransferase for delivery to the storage pool. In support of this hypothesis is the considerable amount of data that shows that the TG-FA exiting the cell in chylomicrons closely reflects that of dietary TG-FA (14,(34)(35)(36) despite the fact that the intestinal mucosa contains large amounts of endogenous lipid during active fat absorption (14,37). The differences in the fate of TG from varying sources is most clearly shown by the fact that during TG-mass steady state conditions in the mucosa induced by ID TO infusions, if the radiolabeled lipid probe enters the intestinal cell from the lumen, the specific activity of mucosal TG is approximately one half that of the infused TO, yet the chylomicron TG specific activity approximates that of the infusate (14).…”

Fatty acid composition of an oral load affects chylomicron size in human subjects

“…Unsaturated fatty acids, which have a lower melting point and a greater affinity for the fatty acid-binding protein (Ockner & Manning, 1976), are transported more rapidly and should thus give rise to larger chylomicrons than SFA. Experiments conducted in the rat have yielded contrasting results, some claiming no impact of fat saturation (Fraser et al 1968;Renner et al 1986), others finding larger particles after unsaturated fatty acids (Boquillon et al 1977;Feldman et al 1983;Levy et al 1991;Kalogeris & Story, I992a,b). Few data were available for human subjects.…”

“…The size of triacylglycerol (TG)-rich lipoproteins (TRL) is largely determined by the rapidity of TG overall absorption, as apolipoprotein (apo) B synthesis appears to be essentially constant (Hayashi et al 1990); large TRL are produced when amounts of TG increase (Boquillon et al \911;Bennett-Clark & Norum, 1978;Hayashi et al 1990). Also, the quality of ingested or infused TG affects TRL size; large chylomicrons are produced after ingestion of polyunsaturated fatty acids (PUFA), compared with VLDL-size particles secreted after ingestion of saturated fatty acids (SFA) (Boquillon et al 1977;Feldman et al 1983;Levy et al 1991;Kalogeris & Story, 1992a) which are characterized by an elevated phospholipid (PL): TG ratio (Kalogeris & Story, 1992a). On the other hand, large size and/ or PUFA-rich chylomicrons are rapidly cleared from serum compared with small and/or SFA-rich chylomicrons (Groot et al 1988;Weintraub et al 1988;Levy et al 1991) due to differences in susceptibility to lipolytic enzymes (Coiffier et al 1987;Weintraub et al 1988).…”

Anatomical Heterogeneity of the Intestinal Mucosa and Cholesterol Homeostasis

“…In restrained animals 2 days after surgery [21,24], cholesterol levels generally reach 0.5-0.8 pmol cholesterol-h-' in bile fistula rats [20], 0.8-1.2 pmol-h-1 in nonfasting rats receiving an intraduodenal infusion of saline-glucose, and 2-3.5pm ol-hr1 when the infusion contains lipid. The use of modelling methods, although more 'undirect', have improved the quantitative ap proach to cholesterol system.…”