NUTRITIONAL MODULATION OF THE SOMATOTROPIN INSULIN-LIKE GROWTH-FACTORSYSTEM - RESPONSE TO FEED DEPRIVATION IN LACTATING COWS

Mid-lactation Holstein cows (n = 4) were used to evaluate the effects of complete feed deprivation on circulating insulin-like growth factor s (IGF) and their binding proteins (IGFBP). Comparisons were made betw een cows in fed and unfed conditions and included response to a single injection of bovine somatotropin (bST). Two days of feed deprivation decreased milk yield 66%. Concentrations of bST nearly doubled during feed deprivation, whereas glucose and insulin steadily decreased. Circ ulating concentrations of IGF-I decreased nearly 50%, IGF-II decreased 28% and IGFBP-2 increased 49% during the 2 d of feed deprivation. In contrast, IGFBP-3 was not affected. Eighteen to 24 h after the bST cha llenge, circulating concentrations of IGF-I rose 100% during the fed p eriod, but there was no response during the unfed period. Blood concen trations of IGF-II decreased 17% after bST challenge in the fed period but were unchanged in the unfed period. More moderate undernutrition (80% of requirements) did not affect circulating IGFBP-2 but did alter the response of IGFBP-2 to bST. A conceptual model was developed that illustrates that nutritional regulation of the ST/IGF system represen ts a key component signaling the appropriate use of nutrients. Overall , results demonstrate that nutritional modulation of the ST/IGF system plays a key role in animal performance and well-being over a wide ran ge of physiological situations.