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 ag e 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 radioliga nd analysis following SDS-PAGE electrophoresis. In both experiments, p lasma 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, declin ed (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 co ntrol values by 6 h after refeeding. We identified five IGFBP, and the ir plasma concentrations were not significantly altered during feed de privation or following refeeding. We conclude that metabolite availabi lity during feed deprivation and following refeeding alters the secret ion of thyroid hormones, ST, and possibly IGF-I, thereby maintaining h omeostasis in horses.