THE ROLE OF INSULIN IN THE REGULATION OF MILK PROTEIN-SYNTHESIS IN DAIRY-COWS

We examined the role of insulin in milk protein synthesis using the hy perinsulinemic-euglycemic clamp approach in combination with abomasal infusion of casein. The two experimental periods consisted of abomasal infusion of water or 0.5 kg/d of casein. An insulin clamp was conduct ed over the last 4 d of each period. During the insulin clamp, circula ting insulin was elevated fourfold, and euglycemia was maintained by t he infusion of exogenous glucose. Casein infusion increased milk yield so that milk protein yield was 10% greater than baseline values. Use of the insulin clamp combined with casein infusion increased milk prot ein yield by 230 g/d (28% greater than baseline values). Milk protein composition was not altered; but content was increased from 3.13% duri ng the baseline period to 3.44% by d 4 of the clamp; calcium concentra tion in milk increased about 10% to 1.2 g/kg. During the clamp, circul ating concentrations of essential amino acids were dramatically reduce d. The most pronounced effects were noted for branched-chain amino aci ds (64% reduction from baseline values). The insulin clamp resulted in alterations in circulating insulin-like growth factor (IGF)-I concent rations (increase) as well as IGF-II and IGF-binding protein-2 concent rations (decreases). Overall, results indicated that the ability of th e mammary gland to synthesize milk protein does not function at maximu m capacity, and there is a previously unrecognized potential to enhanc e milk protein percentage and yield.