Intramammary infusion of insulin or long R-3 insulin-like growth factor-I did not increase milk protein yield in dairy cows

Two experiments investigated the regulation of milk protein synthesis in we ll-fed cows (n = 4) using 1) a hyperinsulinemic-euglycemic clamp and 2) int ramammary infusion of insulin or long R-3 insulin-like growth factor-I plus supplementary amino acids. In experiment 1, 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. An insulin clamp increased the yields o f casein and whey protein both with and without supplementary amino acids. Plasma concentrations of insulin-like growth factor-I were increased and in sulin-like growth factor binding protein-a decreased during insulin clamp, while both insulin and insulin-like growth factor-I in milk were elevated b y this treatment. Milk concentrations of insulin peaked on day 4, but insul in-like growth factor-I concentrations in milk peaked on day 1 of the insul in clamp. In experiment 2, intramammary infusion of insulin had no effects on any measured variables, while yields of milk, protein, and fat were slig htly lower following long R-3 insulin-like growth factor-I treatment. This could be associated with an increase in somatic cell count, which occurred following long R-3 insulin-like growth factor-I treatment. Results from exp eriment 1 suggest insulin and insulin-like growth factor-I are likely candi dates responsible for the increased milk protein yields during the insulin clamp. However, in experiment 2 neither hormone enhanced milk protein yield when administered using an intramammary technique.