Insulin regulates circulating insulin-like growth factors and some of their binding proteins in lactating cows

Four lactating Holstein cows were subjected to a hyperinsulinemic-euglycemic clamp to evaluate the effects of insulin on circulating concentrations of insulin-like growth factors (IGF) and their binding proteins (IGFBP). Baseline blood samples were taken every 4 h for 2 days. For the 4-day clamp, insulin was infused (1 microgram.kg body wt-1.h-1) into the jugular vein and exogenous glucose was infused to maintain euglycemia. Circulating insulin was increased approximately fivefold, while glucose was maintained within 10% of baseline concentrations by infusion of 0.15 g.kg body wt-1.h-1 glucose. Hyperinsulinemia-euglycemia approximately doubled IGF-I (145 vs. 286 ng/ml, SE = 20) while decreasing circulating IGF-II (285 vs. 180 ng/ml, SE = 32). Densitometry of Western blots demonstrated no change in IGFBP-3 or a 30,000 relative molecular weight (M(r)) band during the clamp. However, IGFBP-2 decreased 73% and a 26,000 M(r) band decreased 58% by the end of the clamp. Therefore, insulin, directly or via secondary changes, increased circulating concentrations of IGF-I while decreasing concentrations of IGF-II, IGFBP-2, and a 26,000 M(r) IGFBP in lactating cows.