No abstract

Isolated mammary glands of lactating goats were perfused with heparinized and oxygenated blood for 8 to 15 h. Adequate quantities of glucose, acetate, and amino acid were added to the perfusate. After addition of propionate to the perfusion blood, concentrations of odd-numbered and of monomethyl-substituted fatty acids other than those with iso and anteiso configuration increased in the milk fat. These acids seem to be synthesized de novo in the mammary gland. The increase of C17:0 concentration was weak and problematic. We suggest that propionate is acting as a precursor for monomethyl-substituted fatty acids by way of methylmalonyl-CoA. The activating effect of propionate administration upon milk fatty acid production was largest for odd-numbered followed by monomethyl-substituted fatty acids. No increase of iso acids was observed in milk fat in the propionate-infused glands whereas the increase of anteiso acids was extremely small. This agrees with the conception that iso and anteiso fatty acids are synthesized by rumen bacteria.

An electromagnetic method was adapted to quantitatively measure blood flow through an external pudic artery of the udder in lactating cows during several months. Blood flow through the ipsilateral external pudic vein was blocked while blood was taken from a subcutaneous abdominal vein. Blood flow to milk yield ratio amounted to a mean value of 507:1 in three cows. Peak milk yield occurred before the time of our experiments. There was a high correlation (r = .73) between mammary arterial blood flow and milk yield. Arterio-venous differences (AV) across the udder demonstrated that essential amino acids (EAA) were taken up by the mammary gland in sufficient amounts to account for the EAA secreted as milk protein. Uptake of valine, isoleucine, lysine and particularly that of arginine was in excess of that for milk protein synthesis. There was a highly significant correlation (P<.01) between AV differences of several amino acids and their corresponding arterial concentrations. Uptake of glucose was This work was supported by grants from the lnstirut pour l'Encouragement de la Recherche Scientifique dans l'Industrie et l'Agriculture. The authors gratefully acknowledge Professor Dr. W. Welvaert of the Department of Phytopathology and Phytovirology of the University of Ghent, in whose laboratory the ultra-centrifugal analyses were carried out. They also acknowledge Dr. C. Burvenich, Mrs. M. Onghena-Anaf, Mrs. L. De NeveVan Lancker, Mrs. G. Vranken-Verhelst and Mrs. M. Buelens-Van Haute for skilled technical assistance. a Laboratory of Veterinary Physiology. State University Centre of Antwerp. 3 Laboratory of Chemical Analysis of Food from Animal Origin. Veterinary Faculty of the University of Ghent. 4 Department of Surgery. Veterinary faculty of the 19% higher than could be accounted for by secretion as milk lactose. Uptake of plasma triglycerides (TG) accounted for 45% of milk TG. The majority of the TG taken up by the udder was derived from the low density lipoprotein (LDL) fraction.

A lactating mammary gland of a goat was perfused for 9 h in the presence of [U-14 C]-L-threonine and received adequate quantities of glucose, acetate and amino acids. Two lactating sheep udders were likewise perfused in the presence of [U-14 C]-Lphenylalanine: the plasma levels of phenylalanine in the first of these experiments were 4 times higher than in the second.In the P 4 C]threonine experiment, 4 % of the casein and 0-4 % of the expired CO 2 were derived from threonine; 85 % of the threonine and 1-6 % of the glycine residues in casein originated from plasma threonine. Small 14 C levels were found in glutamic acid, aspartic acid and serine of casein. The relative specific activities amongst the casein amino acids and the appearance of appreciable labelling in plasma glycine are consistent with the view that threonine is split by threonine aldolase.In the [ 14 C]phenylalanine experiments virtually no radioactivity was detected in CO 2 , lactose or citric acid, indicating that this substrate is not broken down by mammary tissue. In the second experiment, 96 % of the phenylalanine and 0-3 % of the tyrosine of casein originated from plasma phenylalanine. In the first experiment, a 30-fold higher 14 C incorporation into casein tyrosine relative to phenylalanine was observed. The possible significance of the phenylalanine concentration in the plasma on the degree of conversion of phenylalanine into tyrosine within the mammary gland is discussed.Most milk proteins are synthesized in the mammary gland from free amino acids. Studies on the uptake of free plasma amino acids by the lactating udder have been carried out by the technique of arterio-venous differences across the udder on cows (Verbeke & Peeters, 1965) and goats (Mepham & Linzell, 1966). In both species an important uptake of phenylalanine, tyrosine and threonine by the udder was observed. In the experiments on cows the mammary blood flow was not measured, but calculations were made on the basis of the differences observed, using average values from the literature for the blood flow through the udder. It was calculated that in the cow the uptake of these amino-acids was probably large enough to provide all the corresponding amino-acid residues in milk proteins. Mepham & Linzell (1966) measured arterio-venous differences in lactating goats, simultaneously estimating mammary blood flow by a direct method. These authors calculated that there was a good agreement between the uptake of phenylalanine, tyrosine and threonine and output of these amino acids in the milk.