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 dos age 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 ho rse. however, porcine GH caused injection-site reactions that were so serio us that administration had to be terminated. The concentrations of a range of GH-related serum protein markers were determined before, during and afte r the administration period. Because of the short half-life of GH itself, t he objective was to identify GH-related markers that showed changes in conc entration and which could be used as indicators of the abuse of these hormo nes. Among the possible markers identified, serum total insulin-like growth factor (IGF)-I was shown to be the most promising, increasing to 270% of t he basal concentration for equine GH administration. After GH administratio n, IGF-I took longer to attain baseline concentrations than the time requir ed for GH concentrations to recover to normal. The concentration obtained f rom the administration significantly exceeded natural concentrations for IG F-I, as was determined from a population of more than 2000 Thoroughbred hor ses in three continents. The concentrations of serum free IGF-I and IGF bin ding protein 3 (IGFBP-3) were also shown to be significantly affected by eq uine and bovine GH.