Administration of bovine, porcine and equine growth hormone to the horse: effect on insulin-like growth factor-1 and selected IGF binding proteins

This study investigated the biochemical effects of administration of three types of recombinant growth hormone (GH; somatotropin) to the Thoroughbred horse. Equine or bovine or porcine GH was administered at a recommended dosage to 3-5-year old Thoroughbred geldings, for up to 21 days. It was shown that, in addition to equine GH, bovine and porcine GH were active in the horse. however, porcine GH caused injection-site reactions that were so serious that administration had to be terminated. The concentrations of a range of GH-related serum protein markers were determined before, during and after the administration period. Because of the short half-life of GH itself, the objective was to identify GH-related markers that showed changes in concentration and which could be used as indicators of the abuse of these hormones. Among the possible markers identified, serum total insulin-like growth factor (IGF)-I was shown to be the most promising, increasing to 270% of the basal concentration for equine GH administration. After GH administration, IGF-I took longer to attain baseline concentrations than the time required for GH concentrations to recover to normal. The concentration obtained from the administration significantly exceeded natural concentrations for IGF-I, as was determined from a population of more than 2000 Thoroughbred horses in three continents. The concentrations of serum free IGF-I and IGF binding protein 3 (IGFBP-3) were also shown to be significantly affected by equine and bovine GH.