Origin and Formation of Lymph Lipids in the Sheep

Abstract

3H and 14C-labelled palmitic, stearic, oleic, linoleic and linolenic acids introduced into the duodenum of sheep were recovered in thoracic duct lymph, and the specific radioactivities of the fatty acids in the various lipid fractions were determined. Lymph fatty acids were transported mainly as triglycerides (77 %) and phospholipids ( 18%), probably mostly in the form of very low density lipoproteins rather than as chylomicrons. Lymph triglycerides transported most of the palmitic, stearic and oleic acids (74-79%) whereas phospholipids were important in the transport of the essential fatty acids, linoleic and linolenic acids (54% and 38% respectively). Specific radioactivity measurements indicated that part of the lymph cholesteryl ester and phospholipid fatty acids were of a non-radioactive origin. In lymph phospholipids 52% of the linoleic acid, 81% of the linolenic acid and more than 88% of the palmitic, stearic and oleic acids were derived from endogenous esterified sources. Evidence is presented to indicate that the major part of lymph phospholipids could be derived from biliary phospholipid. The possible roles of bile and pancreatic juice in fat absorption in the ruminant are discussed.The major fatty acids ingested by ruminant animals are linolenic acid (C18: 3) when the animal is grazing pasture, and linoleic acid (C18: 2) when concentrates are the main component of the diet. These fatty acids are present in the diet in esterified forms, but in the rumen extensive hydrolysis occurs followed by hydrogenation of the C18: 2 and C18: 3 acids to monoenoic acids and stearic acid [see Garton, 1967]. The small amounts of C18: 2 and C18: 3 acids that escape hydrogenation account for less than 1% of the total energy of the diet [Leat and . As a result of this microbial action in the rumen, the major lipid in digesta reaching the small intestine is free fatty acid consisting mainly of palmitic (C 16: 0) stearic (C18: 0) and monoenoic (C18: 1) acids. The free fatty acids are rendered soluble by the action of bile and pancreatic juice and absorbed into the lymphatics where they appear as esterified lipids. Harrison and Leat [1972] showed that radioactively labelled C16: 0, C18: 0 and C18: 1 acids introduced into the duodenum of the sheep are absorbed rapidly into the lymphatics and most of the radioactivity (90%) was associated with the triglyceride fraction. Few quantitative data are available on the synthesis and formation of lymph lipids in the ruminant, and to obtain more information on the origin of lymph lipids the distribution by weight and by radioactivity (i.e. specific activity) of C16: 0, C18: 0 and C18: 1 acids between the various esterified lipids has been determined. Additional observations were made on the absorption from the intestine of the small amounts of essential fatty acids (C 18: 2 and C18: 3 acids) which have escaped hydrogenation in the rumen; and the specific activities of these acids in lymph lipids have been compared with the values obtained for C16: 0, C18: 0 and C18: 1 acids. 131

Digestion, Absorption and Transport of Lipids in Ruminant Animals

“…115 When abnormal dietary or other experimental conditions have been imposed upon ruminant animals, changes in the flow and lipid concentration of the lymph within the intestinal and thoracic ducts have been demonstrated. 175 This feature has, indeed, been confirmed in sheep where analysis of thoracic lymph separated into fractions by ultracentrifugation showed that 73% of the lipid present was associated with the VLDL fraction and only 27% with the chylomicrons. 115 ' 130 ' 132 As in non-ruminant animals, lactation and other changes in the physiological status of the animal also influence lymphatic circulation.…”

“…26 ' 54 ' 254 Such enzymic activity appeared to be maximal within the mucosal cells of the duodenum and upper jejunum 26 and was accompanied by much higher glycerokinase activity than has been found in other animal species. 175 Very low specific activities were found for the fatty acids in the phosphohpids compared to the triglycérides of lymph indicating that there was very little synthesis of phosphohpids de novo by the a-glycerophosphate pathway. Although initial work 26 failed to provide unequivocal evidence for this biosynthetic pathway, subsequent investigations 54 ' 254 demonstrated the ability of both ovine and bovine jejunal cells to synthesize triglycérides by direct acylation of both 1-and 2-monoacylglycerol substrates.…”

“…130 ' 131 ' 175 When lipids were infused directly into the abomasum or duodenum, 131 ' 204 absorption was rapid with the appearance of the lipid metabolites in the lymph and plasma within about 1-2 hr; this compared with about 3-4 hr for the appearance of the metabolites in lymph follow ing intraruminal infusions. This has been demon strated by a variety of methods ranging from direct digestibility trials 140 ' 281 and indirect observations of lymph, plasma and tissue lipid compositions following administration of particular lipid emulsions through gastrointestinal fistulae 149 ' 204 ' 215 ' 219 to more sophisticated techniques involving radiolabeled substrates.…”

Size of Lipoproteins in Intestinal Lymph of Sheep and Suckling Lambs

“…Discussion Leat and Harrison (1974) observed that ruminant lymph contained a high content of phospholipids relative to triacylglycerols. as indicated by a triacylglycerol .to phospholipid ratio of 3 .…”

“…In ruminants, however, Leat and Harrison (1974), after observing a high level of phospholipid in lymph, suggested that in these animals lymph lipids are transported as very low density lipoproteins (VLDL) rather than chylomicrons.…”