Acute effects of short-term feed deprivation and refeeding on circulating concentrations of metabolites, insulin-like growth factor I, insulin-like growth factor binding proteins, somatotropin, and thyroid hormones in adult geldings.

Two studies were performed with Standardbred geldings 7 to 21 yr of age to determine the sequence of changes in blood plasma concentrations of some hormones and metabolites during feed deprivation for 48 h and for 12 h after refeeding. Plasma hormone and metabolite concentrations were determined with methods validated for horse plasma. Insulin-like growth factor binding proteins (IGFBP) were determined with radioligand analysis following SDS-PAGE electrophoresis. In both experiments, plasma concentrations of triiodothyronine and thyroxine decreased (P < .01) during feed deprivation and increased (P < .01) during refeeding. Plasma glucose and IGF-I either decreased or were not altered during feed deprivation. In contrast, plasma concentrations of NEFA and urea nitrogen increased (P < .01) during feed deprivation and decreased (P < .01) during the refeeding period. Plasma somatotropin (ST) increased (P < .01) approximately 80% at 24 to 36 h of feed deprivation, declined (P < .01) to control values at 48 h of feed deprivation, increased (P < .01) nearly three fold at 3 h after refeeding, and returned to control values by 6 h after refeeding. We identified five IGFBP, and their plasma concentrations were not significantly altered during feed deprivation or following refeeding. We conclude that metabolite availability during feed deprivation and following refeeding alters the secretion of thyroid hormones, ST, and possibly IGF-I, thereby maintaining homeostasis in horses.