Insulin regulates milk production and mammary gland and hind-leg amino acid fluxes and blood flow in lactating goats

We investigated the roles of insulin and amino acid (AA) in regulating milk production and the uptake of AA and blood flow (BF) by the mammary gland and hind-leg of goats (n = 4). During two periods, either saline or AA (65 g/d) was infused i.v. for 7.5 d, and, beginning on d 5, goats were subjected to a hyperinsulinemic-euglycemic clamp. The insulin clamp elevated plasma insulin levels threefold and insulin-like growth factor-1 by 27%, and euglycemia was maintained by the infusion of glucose. Arterial, mammary, and tarsal vein blood samples were obtained on d 4 and 8 of each period, and blood flow was monitored continuously by probes. Insulin and insulin plus AA infusions increased the yields of milk by 13 to 18% and protein by 10 to 21%, but AA infusion alone had no effect. The insulin clamp reduced milk fat content by 21 to 31% and yield by 8 to 19%, and reduced the yields of milk fatty acids >C16. The insulin clamp increased mammary blood flow by 42%, but insulin and AA infusions both increased hind-leg BF by 29 to 52% and by 25%, respectively. Net uptakes of most plasma AA by the udder were reduced by insulin, whereas AA infusion had no effect. For the leg, the uptake of His and Thr were decreased by insulin, whereas the infusion of AA stimulated the uptake of total essential AA. Insulin increased the uptake of glucose by the udder but not by the leg. This study suggests that the udder and leg tissues respond differently to infusions of insulin and AA; the udder was more responsive to insulin, while the leg was more responsive to AA concentration (supply), at least in terms of AA uptake and net anabolism (protein gain or secretion).