A Quantitative Balance Between Substrates and Metabolic Products of the Mammary Gland

Abstract

Summary 1. An attempt has been made to assess from published experiments, and largely for the goat, the principal compounds that are absorbed from the blood by the lactating mammary gland, the quantities of each that are absorbed during the formation of 100 ml. of milk, and the proportion of each of the major components of milk, and of the carbon dioxide expired by the gland, which is derived from each of the components of blood. 2. An assessment has first been made of which compounds of blood could, and which could not, be quantitatively significant metabolites of the gland. This has been done by calculating the maximum amount of each compound which would be available to the gland during the formation of 100 ml. of milk, from its concentration in blood together with the rate of blood flow through the gland. 3. For components of blood which could be quantitatively significant metabolites of the gland, it should be possible to discover whether or not they are absorbed in significant amounts by making arterio‐venous measurements. Those measurements which have been made are considered, and rough calculations have been made of the quantity of each compound which is absorbed from the blood of the goat during the formation of 100 ml. of milk. 4. From this information, and from the results of experiments with isotopic tracers, an attempt has then been made to assess the proportion of each of the major components of milk, and of the carbon dioxide expired by the lactating gland, which is derived from each of the components of blood. 5. It is realized that the calculations made in this article are approximate. In conclusion, possible improvements in technique are briefly considered which would enable a balance of this kind to be made accurately.

The Effect of Starvation on the Uptake of the Precursors of Milk Fat by the Bovine Mammary Gland

“…The fatty acids of the milk fat of cows are synthesized within the mammary gland from acetate and J3-hydroxybutyrate or derived directly from the plasma triglycerides (Barry 1964) and possibly from the plasma free fatty acids (Hartmann and Lascelles 1964). In addition, the glycerol moiety of the triglycerides of milk fat appears to come partly from the plasma triglycerides (Barry 1964) and is partly synthesized from blood glucose (Luick and Kleiber 1961).…”

Amino Acid and Glucose Uptake by the Isolated Perfused Guinea‐pig Mammary Gland

“…The advantages of the AV difference technique over the serial single sample method for the measurement of substrate uptake can be great in terms of ease of computation and accuracy of results. The accuracy of the AV difference method when applied in vivo has been estimated to be ±10% [Barry, 1964]. During isolated perfusion, problems of in vivo experiments, such as obtaining representative blood samples and measurement of blood flow [see Zierler, 1961], are simplified and in the latter case, at least, determined with greater accuracy.…”

“…The comparison of AV differences between animals without concomitant measurement of blood flow, as recently reported by Hawkins and Williamson [1972] for the rat, would seem to be of questionable validity. Barry [1964] considered that for the mammary uptake of any amino acid to be convincingly demonstrated by the AV difference method its extraction should be at least 20%. In these experiments amino acid extraction declined as either arterial amino acid concentrations or plasma flow rate increased.…”

Dietary fat and milk fat secretion in the cow

“…These fatty acids are synthesized in mammary tissue by way of the malonyl CoA pathway and are then incorporated into triglycerides by the metabolic route that involves u-glycerophosphate, phosphatidic acid and a,p-diglyccride as intermediates (cf. Ganguly, 1960;Peeters & Lauryssens, 1964;Barry, 1964;Hibbitt, 1966;Annison, Linzell, Fazakerley & Nichols, 1967 ;Linzell, Annison, Fazakerley & Leng, 1967). I n the second process, triglycerides circulating in the blood as chylomicra and lowdensity (d< 1.019) lipoproteins are taken up by the mammary gland.…”