Effects of somatotropin and training on indices of exercise capacity in Standardbreds

The recent availability of recombinant equine somatotropin (eST) has led to concern about its use as an ergogenic aid in racehorses. This study was undertaken to investigate the changes in exercise capacity in maturing horses in a training programme, and to assess whether eST is an ergogenic aid to this group. We tested the hypothesis that the combination of training and eST, compared to training alone, would further improve exercise capacity in maturing Standardbreds, by virtue of ST's anabolic effects and potential to enhance cardiac function, circulating fluid volume and red cell mass. Twelve, untrained Standardbreds (mean +/- s.d. 20.7 +/- 1.1 months) were paired according to similar bodyweight and then assigned randomly to treatment or control group. The horses underwent a 12 week treadmill training programme. Methionyl eST (10 microg/kg for the first 7 days, then 20 microg/kg) was administered once daily, i.m., for 42 consecutive days (Weeks 4 to 9 inclusive) to 6 horses in the treatment group. All horses performed a standardised maximal exercise test to fatigue at Weeks 0, 3, 6, 9, and 12. During each exercise test VO2, VCO2, plasma lactate concentrations ([La]), heart rates, blood volume and total run time were measured. There was no significant effect of eST administration on VO2max, V200, V(LA4), LA9, red cell volume (RCV), plasma volume (PV), or run time to fatigue. Combining the data for all horses, training significantly altered the VO2max (mean +/- s.d. 98.2 +/- 11.1 ml/kg/min in Week 0 to 117.6 +/- 4.8 ml/kg/min in Week 12), V(LA4) (5.1 +/- 0.8 m/s to 7.4 +/- 1.0 m/s), LA9 (12.7 +/- 3.9 mmol/l to 7.1 +/- 1.9 mmol/l), RCV (46.3 +/- 4.7 ml/kg to 63.5 +/- 5.0 ml/kg), PV (46.0 +/- 4.8 ml/kg to 57.0 +/- 6.3 ml/kg), and run time to fatigue (431.8 +/- 30.9 to 490.2 +/- 30.5 s), but not V200 (5.0 +/- 0.5 m/s to 5.2 +/- 1.1 m/s). The administration of eST to young Standardbred horses in training did not significantly improve their exercise capacity or indices of fitness. However, these maturing horses demonstrated a rapid physiological response to training exercise. Further research is required to determine the relationship between exercise capacity and ST in the horse.