INSULIN REGULATES CIRCULATING INSULIN-LIKE GROWTH-FACTORS AND SOME OFTHEIR BINDING-PROTEINS IN LACTATING COWS

Four lactating Holstein cows were subjected to a hyperinsulinemic-eugl ycemic clamp to evaluate the effects of insulin on circulating concent rations of insulin-like growth factors (IGF) and their binding protein s (IGFBP). Baseline blood samples were taken every 4 h for 2 days. For the 4-day clamp, insulin was infused (1 mu g . kg body wt(-1). h(-1)) into the jugular vein and exogenous glucose was infused to maintain e uglycemia. Circulating insulin was increased approximately fivefold, w hile glucose was maintained within 10% of baseline concentrations by i nfusion of 0.15 g . kg body wt(-1). h(-1) glucose. Hyperinsulinemia-eu glycemia approximately doubled IGF-I (145 vs. 286 ng/ml, SE = 20) whil e decreasing circulating IGF-II (285 vs. 180 ng/ml, SE = 32). Densitom etry of Western blots demonstrated no change in IGFBP-3 or a 30,000 re lative 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, increase d circulating concentrations of IGF-I while decreasing concentrations of IGF-II, IGFBP-2, and a 26,000 M(r) IGFBP in lactating cows.