The Effects of Bovine Recombinant Growth Hormone Administration on Insulin-like Growth Factor-I and the Haemopoietic System in Thoroughbred Geldings

SUMMARY fect of intramuscularly administered recombinant bovine growth hormone (rbGH) on insulin-like growth factor-I (IGF-I) and white and red blood cell indices was studied in Thoroughbred geldings. An insulin-like growth factor binding protein (IGFBP)-blocked radioimmunoassay was modified and validated for the measurement of IGF-I in equine blood plasma. Baseline values of IGF-I and blood indices were determined over a 48 h period and then a single dose of 5 μg/kg, 10 μg/kg or 50 μg/kg of rbGH was administered. Insulin-like growth factor-I levels increased in a dose-dependent manner, with the highest values between 12 h and 24 h. The highest dose (50 μg/kg) yielded the greatest IGF-I response with a 90.2 ± 10.8% increase at 24 h. White blood cell count increased following the three doses of rbGH with the highest white blood cell count at 12 h after the 50 μg/kg dose. Haemoglobin was significantly increased at 24 h (P< 0.05), when values following doses of 10 μg/kg and 50 μg/kg were significantly greater than after the vehicle or the dose of 5 μg/kg. Red blood cell count was not affected by any of the rbGH doses. These results indicated that rbGH is biologically active in the horse and that rbGH at a dose rate of 10 μg/kg or more could be used therapeutically.