Insulin-like growth factor I responses to recombinant bovine growth hormone during feed restriction in heifers

Insulin-like growth factor I, other hormones and blood metabolites were measured in growing heifers before, during and after a 3-day period of normal feed intake and a corresponding period of reduced feed intake. In addition, 0.1 or 0.5 mg recombinant bovine GH/kg was injected daily for 5 days during normal or during and following reduced feed intake. During reduced feed intake blood concentrations of insulin-like growth factor I, insulin, T4, T3, glucose and urea-nitrogen decreased, whereas those of non-esterified fatty acids, albumin and protein increased (P less than 0.05). GH, insulin-like growth factor I and insulin increased, whereas urea-nitrogen decreased in response to exogenous GH when heifers were adequately fed (P less than 0.05). In contrast, insulin-like growth factor I did not change during GH injections while heifers received reduced amounts of feed. Therefore, during insufficient energy and (or) protein intake, characterized by low glucose, insulin and thyroid hormone levels and increased non-esterified fatty acid concentrations, insulin-like growth factor I concentrations and responses to GH administration were markedly reduced.