DIETARY-PROTEIN AND(OR) ENERGY RESTRICTION IN MARES - PLASMA GROWTH-HORMONE, IGF-I, PROLACTIN, CORTISOL, AND THYROID-HORMONE RESPONSES TO FEEDING, GLUCOSE, AND EPINEPHRINE

Sixteen light horse mares were fed diets of bermudagrass hay and a cor n/cottonseed hull-based supplement formulated to contain either 100% ( control) or 50% (restricted) of the protein and (or) energy requiremen ts for maintenance in a 2x2 factorial arrangement of treatments. Plasm a IGF-I, prolactin, cortisol, triiodothyronine, and thyroxine were mon itored for 33 d. On the 27th d, frequent blood samples were drawn thro ughout the day for the measurement of growth hormone (GH), and on the 29th d, an epinephrine challenge and an i.v. glucose tolerance test (I VGTT) were performed in the morning and afternoon, respectively. Restr iction of protein and (or) energy reduced (P<.001) plasma IGF-I concen trations within 24 h, and the effect persisted through the 24th d. Ene rgy restriction decreased (P=.01) plasma cortisol concentrations, wher eas thyroid hormones were not influenced (P>.1) by restriction of prot ein and (or) energy. Plasma prolactin concentrations were low througho ut the experiment and after the IVGTT, but they increased (P=.003) aft er feeding. Protein restriction increased (P=.09) the occurrence of GH episodes during the 14-h feeding period on d 27; the greatest effect occurred in the mares restricted in both nutrients. In contrast, energ y restriction reduced (P=.05) the GH response to epinephrine injection . We conclude that 1) protein deficiency in mares increases with GH se cretion, whereas energy restriction alone does not, 2) a deficiency in energy and (or) protein reduces IGF-I secretion, and 3) prolactin con centrations increase after feeding, even at a time of year when secret ion rates are naturally low.