Post-ruminal supplies of glucose and casein, but not acetate, stimulate milk protein synthesis in dairy cows through differential effects on mammary metabolism

摘要

Amino acids and glucose have been shown to regulateprotein synthesis in the mammary gland through theireffects on cellular signaling pathways. Acetate mightalso have an effect on protein synthesis via the AMPactivatedkinase signaling pathway, because it is themain energy source for the mammary secretory cell.Thus, the objective of this experiment was to evaluatethe effects of casein and energy-yielding nutrients(acetate and glucose), and their combination, on performanceand mammary metabolism. Six multiparousHolstein cows, averaging 49 kg of milk/d, were used ina 6 × 6 Latin square design with 14-d periods. Cowswere fed to 100% National Research Council requirementsfor metabolizable protein (MP) and energy (ME)for 9 d, after which they were feed-restricted for 5 dto 85% of their individual ad libitum intake and thenabomasally infused with 1 of 6 treatments. Treatmentswere acetate (A), glucose (G), each at 5% of ad libitumME intake, casein (C) at 15% of ad libitum MP intake,A + C, G + C, or a saline solution (negative control).Casein infused alone increased milk protein yield numerically,with 25% recovery of the infused casein inmilk protein. Glucose infused alone increased milk andmilk protein yield and promoted the highest efficiencyof nitrogen utilization (37%), with an efficiency of MPuse for milk protein of 58%. We discovered no effect oftreatment on mammary plasma flow, and the increasein milk protein yield with glucose infusion was broughtabout by greater mammary AA clearance rate. Infusionof casein and glucose together further increased milkprotein yield in an additive fashion, and 47% of theinfused casein was recovered in milk protein. Acetateinfused alone had no effect on milk protein yield butincreased milk fat yield numerically, suggesting thatthe greater amount of acetate taken up by the mammarygland was used for milk fat synthesis. Infusion ofacetate and casein together yielded responses similarto those of casein alone. In conclusion, glucose has amajor effect on stimulating milk protein synthesis, andthe mammary gland has the ability to increase its supplyof nutrients to match its synthetic capacity.