Changes in plasma and milk concentrations of glucose and IGF-1 in response to exogenous growth hormone in lactating goats

Abstract

Exogenous growth hormone was administered subcutaneously to five lactating goats during the post-peak period of lactation. Milk yields increased significantly by ∼20% in response to growth hormone. Blood and milk samples were taken in the periods before, during and after growth hormone treatment. The concentrations of glucose in milk increased significantly by ∼50% in the period following growth hormone treatment at a time corresponding to the increase in milk yield. There was a transient increase in plasma glucose concentrations immediately following growth hormone treatment before either milk glucose concentrations or milk yields were increased. Both free and total IGF-1 concentrations in plasma increased slowly following growth hormone treatment. The increase in plasma IGF-1 corresponded to the increase in milk yields and milk glucose concentrations. Concentrations of IGF-1 in milk increased more rapidly than those in plasma, rising by ∼150% following growth hormone treatment, and were starting to decline by the time that milk yield and milk glucose concentrations were at their maximum. As milk glucose concentrations have been shown previously to reflect changes in the intracellular concentration of glucose, the results indicate that part of the mechanism by which growth hormone stimulates milk production is by increasing the intracellular availability of glucose for lactose synthesis. The results also suggest that changes occur in the concentrations of IGF-1 in the environment of the mammary gland before changes are observed in the general circulation, and that these are reflected in the changed concentrations in milk.