Milk Synthetic Response of the Bovine Mammary Gland to an Increase in the Local Concentration of Amino Acids and Acetate

• Published in: Journal of dairy science 2008-01, Vol.91 (1), p.218-228
• Main Authors: Purdie, N.G., Trout, D.R., Poppi, D.P., Cant, J.P.
• Format: Article
• Language: English
• Subjects: 3-hydroxybutyric acid; 3-Hydroxybutyric Acid - metabolism; acetate; Acetates - administration & dosage; Acetates - blood; Acetates - metabolism; acetic acid; adipose tissue; amino acid; amino acids; Amino Acids - administration & dosage; Amino Acids - blood; Amino Acids - metabolism; Animal Nutritional Physiological Phenomena; Animal productions; Animals; arterial infusion; biochemical pathways; Biological and medical sciences; blood chemistry; Blood Glucose - metabolism; Cattle - metabolism; dairy cows; Eating - physiology; equations; Fatty Acids - metabolism; Female; Food industries; Fundamental and applied biological sciences. Psychology; glucose; Insulin - blood; Kinetics; Lactation; lactose; long chain fatty acids; Mammary Glands, Animal - metabolism; mathematical models; Milk - chemistry; Milk - metabolism; Milk and cheese industries. Ice creams; milk composition; milk fat yield; milk protein yield; Milk Proteins - metabolism; milk yield; Terrestrial animal productions; udders; uptake mechanisms; Vertebrates
• ISSN: 0022-0302; 1525-3198
• DOI: 10.3168/jds.2007-0492

Rates of secretion of components into milk are a function of precursor concentrations and parameters that describe expression of the milk synthetic enzymes and their sensitivity to precursor concentrations. To establish the enzymatic sensitivities of milk fat yield and mammary acetate utilization to circulating acetate concentration, lactating cows were infused for 10h with 0 or 40g of acetate/h in an external iliac artery supplying one udder half. In addition, to investigate the possibility that energy supply influences the milk protein response to an elevated amino acid (AA) concentration, 2 different AA profiles were infused with and without acetate. Six cows, fed a total mixed ration of 21% crude protein ad libitum, were infused with AA at 0g/h, 30g/h in the profile of rumen microbes, or 30g/h in the profile of milk proteins, in a 3×2 factorial arrangement with the 2 acetate treatments of 0 and 40g/h, all in a 6×6 Latin square. Amino acid infusion caused a 60% increase, on average, in plasma concentration of AA entering the infused udder half. From the microbial AA profile, 49% of infused AA were taken up by the udder half, 42% of which occurred during the first pass. From the milk AA profile, 44% of infused AA were taken up by the udder half, 50% of which occurred during the first pass. There was an 8% increase in yield of milk protein with AA infusion, representing 7% capture, but no effect of the infused profile. Acetate infusion caused a decrease in the yields of milk protein and lactose when AA were infused, but not when AA were absent. Milk fat yields were not affected, although acetate concentrations in plasma entering the infused udder half increased by 123% and mammary uptakes increased by 128%. Mammary uptakes of long-chain fatty acids and β-hydroxybutyrate were not affected by acetate infusion, whereas glucose uptakes tended to increase. It was suggested that excess acetate may have been sequestered in adipose tissue in the udder. Yields of both protein and fat in milk showed a low sensitivity to the concentration of their precursors in circulation. It was concluded that the Km in Michaelis-Menten–type equations describing milk synthesis should be assigned a low value, and that the Vmax is regulated to bring about changes in milk yield and composition.