The absorption of milk precursors by the mammary gland

Abstract

IN Part IV of this series [Blackwood and Stirling, 1932, 1] some consideration was given to certain aspects of the phosphorus metabolism of the mammary gland. While there were indications in the data obtained that the experimental proof of the phosphatide theory of fat secretion given by Meigs et al. [1919] could not be substantiated, it was felt that in the absence of lipin-phosphorus determinations such a conclusion was not then entirely justified. The present series of experiments was planned to explore more fully the postulated relationship between phosphorus in the blood and the fat metabolism of the mammary gland during lactation. At the same time it was thought desirable to determine the level of the total alcohol-ether-soluble fraction of the whole blood and plasma together with the cholesterol content during the passage of the blood through the active and inactive mammary gland of the cow. EXPERIMENTAL.The technique of blood sampling was that employed in the earlier work of this series but it may be desirable to give our experience in detail. Samples of blood were drawn simultaneously from the radial artery and from the jugular and abdominal subcutaneous (mammary) veins of lactating and non-lactating cows. Arterial puncture in non-aesthetised cows is very frequently attended by disturbance. Where this seemed probable the animal in question was abandoned and a more suitable subject chosen. Finally it was found that by preliminary gauging of the size of the radial artery, it was possible to select animals for experiment such that disturbance was reduced to a minimum. The sample from the jugular vein was obtained with slight constriction either by finger pressure or by momentary application of a "choke" rope. In every case the entire process of jugular sampling was complete in about 30 seconds. No special measures were required for the mammary blood sample. No difficulty was ever experienced with either of the venous samples and no oedema resulted with this technique. Only those cases where the entire procedure of arterial, jugular and mammary sampling was completed within 4 minutes are included in the work on milk secretion. Each sample of 60 ml. volume was drawn into a dry sterile glass bottle containing 0-1 g. of sodium oxalate as anticoagulant. The plasma was separated immediately and the various fractions examined.

The precursors in cows’ blood of milk fat and other milk constituents

“…Where we have covered the same ground, our findings agree, in the main, with those obtained by Lintzel for goats and support Blackwood's (1934) view regarding the part played by phospholipin in fat secretion in the cow.…”

The metabolism of phospholipids in the lactating mammary gland of the rabbit

“…Contrary to the views formerly held (e.g. Meigs, Blatherwick & Cary, 1919) that plasma phospholipids are the precursors of the fatty acids of milk fat, arteriovenous studies subsequently indicated that no uptake of phospholipids into the lactating bovine gland apparently takes place (Blackwood, 1934) and that no fraction of blood lipids except glycerides and possibly cholesteryl esters are absorbed from the blood stream by the ruminant udder (Lintzel, 1934;Graham, Jones & Kay, 1936;Maynard, McCay, Ellis, Hodson & Davis, 1938;Shaw & Petersen, 1940;Voris, Ellis & Maynard, 1940). Aten & Hevesy (1938), in one of the first studies of lactation involving radioisotopes, showed that phospholipids are synthesized in the lactating udder of the goat, but contended that their results were not consistent with the view that plasma phospholipids are degraded in the mammary gland.…”

I. Reproduction and Lactation

“…The study of the formation of milt in the active mammary gland must in the first instance depend on accurate qualitative data regarding the nature of the milk precursors Rapid progress has been made in this field m recent years In a detailed investigation of this subject, Lintzel(4l), working on six goats and one cow, compared blood drawn directly from the left ventricle with that drawn from the mammary vein and in some respects also with that from the jugular vein The criticism of the jugular-mammary comparison made by Blackwood and Stirling (42) has been upheld together with the role of blood sugar and blood ammo acids as the precursors of milk lactose and milk protein The phosphatide theory of milk-fat secretion has been shown to be unfounded by Lintzel and also by Blackwood (43), and evidence is advanced by the former which indicates that milk fat may be derived from the triglyceride fraction of the blood lipoids-a conclusion which seems to confirm the early experiments of Foa(44) on mammary perfusion The phosphorus of milknotably that of caseinogen-appears from the results of these two workers to be derived from the acid-soluble phosphorus compounds of the blood, principally from inorganic phosphate The relative estimates found (43) of the requirements of the active mammary gland for glucose and inorganic phosphate agree closely A high level of all blood lipoids seems to be required during lactation, while a gradual decrease is noted throughout its progress to a level which is markedly low during the non-lactating period The results of Lintzel and of Blackwood, using slightly different methods, are seen to be in substantial agreement Nitzescu (45), also investigating the nature of milk sugar precursors, was able to maintain the milk-sugar level of starving cows by intravenous injections of monosacchandes Glucose was most efficient in this respect, whereas disacchandes such as saccharose or lactose were without effect.…”