The oxidation and utilization of palmitate, stearate, oleate and acetate by the mammary gland of the fed goat in relation to their overall metabolism, and the role of plasma phospholipids and neutral lipids in milk-fat synthesis

Abstract

1. Measurements were made of milk yield, mammary blood flow and arteriovenous differences of each plasma lipid fraction, and their specific radioactivities, during the infusion of [U-(14)C]stearate, [U-(14)C]oleate, [U-(14)C]palmitate and [1-(14)C]acetate into fed lactating goats. 2. Entry rates of fatty acids into the circulation were 4.2mg./min./kg. body wt. for acetate, and 0.18, 0.28 and 0.42mg./min./kg. for stearate, oleate and palmitate respectively. Acetate accounted for 23% of the total carbon dioxide produced by the whole animal, and contributed to the oxidative metabolism of the mammary gland to about the same extent. Corresponding values for each of the long-chain acids were less than 1%. 3. There were no significant arteriovenous differences of phospholipids, sterols or sterol esters, and their fatty acid composition showed no net changes during passage through the mammary gland. 4. There were large arteriovenous differences of plasma triglycerides, and their fatty acid composition showed marked changes across the gland. The proportions of palmitate and stearate fell, and that of oleate increased. 5. Arteriovenous differences of plasma free fatty acids (FFA) were small and variable, but a large fall in the specific radioactivity of each of the long-chain acids examined indicated substantial uptake of plasma FFA, accompanied by roughly equivalent FFA release from mammary tissue. The uptake of FFA was confirmed by the extensive transfer of radioactivity into milk. The FFA of milk were similar in composition and radioactivity to the milk triglyceride fatty acids, and quite unlike plasma FFA. 6. The formation of large amounts of oleic acid (18-21 mg./min.) from stearic acid was demonstrated. 7. During the terminal stages of the [(14)C]acetate infusion, milk triglyceride fatty acids of chain length C(4)-C(14) showed specific radioactivities that were 75-90% of that of blood acetate, and that of palmitate was roughly one-quarter of this value. Oleate and stearate were unlabelled. 8. The results confirmed that milk fatty acids of chain length C(4)-C(14) arise largely from blood acetate, and palmitate is derived partly from acetate and partly from plasma triglyceride, the latter fraction being almost the sole precursor of oleate and stearate.

Dietary fat and milk fat secretion in the cow

“…At some stage in this process it appears that the triglycerides are partially or completely hydrolyzed and the resulting fatty acids are incorporated into new triglyceride molecules by the mammary gland (e.g. Barry, Bartley, Linzell & Robinson, 1963;Patton & McCarthy, 1963;Annison et al 1967). IJnder normal dietary conditions, the major fatty acids present in the blood triglycerides of the cow are 16:0, 18:o and 18:~ (Duncan & Garton, 1963) and these are the fatty acids that are normally taken up by the mammary gland for incorporation into milk triglycerides.…”

“…This could be due to the complete hydrogenation of I S:2 by rumen micro-organisms, the absorption of the resulting 18:o and its incorporation into blood triglycerides which would then be taken up by the mammary gland where cis rS:r is synthesized by the desaturation of 18:o (e.g. Annison et al 1967;1,inzell et al 1967). However, over one-third of the increase in the yield of I~: I in the milk fat under these dietary conditions is accoun- Thus it would appear that the hydrogenation of 1 8 2 in the rumen is far from complete and that some of the trans isomer of I 8: I is absorbed and incorporated into blood and milk triglycerides.…”

The use of isotopes in the study of milk secretion

“…This was first done by Annison & Linzell (1964) in goats and later in a pig (Linzell, Mepham, Annison & West, 1969) and cows (Bickerstaffe, Annison & Ldnzell, 1974). This approach at once showed that the mammary tissue itself is having a profound effect upon the lactating animal from the fact that it is removing from the circulation a large proportion of the total glucose, fatty acids and amino acids available to the whole animal.…”

“…Tracer techniques in nutrition 21 of glucose and acetate in lactating goats (Annison & Linzell, 1964), we turned our attention to other substrates and used I4C-labelled free palmitate, stearate and oleate. In fed animals, metabolism of these acids was very low and there were no significant net arteriovenous differences across the udder.…”

Toxicity of methylxanthines and local anaesthetics towards muscle

“…Data relevant to this problem include the known mammary uptake of plasma free fatty acids (Annison, Linzell, Fazakerley & Nichols, 1967), the release of lipoprotein lipase by the mammary gland (Barry, Bartley, Linzell & Robinson, 1963) and the change in the 14C/3H ratio that occurs during the transfer of doubly labelled chylomicron triglyceride into milk fat (West, Annison & Linzell, 1967). The reutilization for mammary fat synthesis of the products of triglyceride hydrolysis vitiated quantitative interpretation of the latter data, and largely prevents the direct study of the uptake of partial glycerides.…”