Effect of growth hormone-releasing factor on the response to insulin of cows during early and late lactation

The euglycemic clamp technique was used to determine the effect of bovine growth hormone-releasing factor on glucose kinetics and on the response to insulin of 4 dairy cows during early lactation (mean, 35 d postpartum) and of 6 dairy cows during late lactation (mean, 194 d postpartum). Beginning 3 d prior to experiments, cows were injected twice daily with either saline or 2.5 micrograms/kg of growth hormone-releasing factor. On the day of the experiment, saline or the releasing factor (0.0119 microgram/kg per min) was infused into each cow for 5 h. After a basal period, insulin was infused at 1 and then at 6 mU/kg per min; plasma glucose concentrations were maintained at basal concentrations by the infusion of normal glucose. Growth hormone-releasing factor reduced the glucose infusion rate that was required to maintain euglycemia during the insulin infusions during late lactation but had no effect during early lactation. During the insulin infusions of the late lactation experiment only, the rates of glucose appearance, disappearance, and metabolic clearance were lower when plasma growth hormone was elevated. The results demonstrated that elevated concentrations of growth hormone decreased the responsiveness of peripheral tissues to high concentrations of insulin during late lactation but apparently had little effect during early lactation.