Effects of insulin and postruminal supply of protein on use of amino acidsby the mammary gland for milk protein synthesis

We examined the relationships between amino acid supply, net utilization of amino acid by the mammary gland, and milk protein yield, in investigations that utilized a hyperinsulinemic-euglycemic clamp. A two-way crossed facto rial design was employed. There were two 12-d periods involving abomasal in fusions of either water or a mixture of casein (500 g/d) plus branched-chai n amino acids (88 g/d), with a hyperinsulinemic-euglycemic clamp during the last 4 d of each period. During the clamp, insulin was infused at 1.0 mu g . kg BW-1 . h(-1) to increase circulating levels fourfold, and euglycemia was maintained by infusion of glucose. The insulin clamp treatments increas ed milk protein yield by 15 and 25% during abomasal infusion of water or ca sein plus branched-chain amino acids, respectively. Circulating concentrati ons of essential amino acids were reduced (33%) during insulin clamp treatm ents, especially branched-chain amino acids (41%). Arteriovenous difference of essential amino acids across the mammary gland was linearly related to their arterial concentrations. However, milk protein yield was not related to either arterial concentration or arteriovenous difference, for any of th e essential amino acids. During insulin clamp treatments, the mammary gland was able to support the increased milk protein yields by increasing extrac tion efficiency of essential amino acids, mammary blood flow, and glucose u ptake. Furthermore, a positive mammary balance of total amino nitrogen and carbon was maintained for all treatments. These adaptations demonstrate the unique ability of the mammary gland to adjust local conditions to allow fo r an adequate nutrient supply.