Chronic recombinant equine somatotropin (eST) administration does not affect aerobic capacity or exercise performance in geriatric mares.

The purpose of this experiment was to test the hypothesis that chronic (89 days) administration of recombinant equine somatotropin (eST) would increase aerobic capacity and improve exercise performance in old mares. Fifteen, healthy, unfit, aged (20-26 year old) mares were randomly assigned to a treatment (eST, 12.5 mg day-1 in 3 ml glycine/manitol buffer, s.c., n = 7) or control (vehicle, 3 ml day-1, s.c., n = 8) group. Aerobic capacity and exercise performance were measured using a standardized exercise test (SET) performed on a high speed treadmill. Tests were conducted before (-21 days), during (+43 days and +89 days) and after (+127 days) treatment. During the SET, resting data were collected and the horses then ran up a fixed 6% grade, starting at 4 m s-1, with a 1 m s-1 increase every 60 s (omitting 5 m s-1) until fatigue. Oxygen uptake (VO2) was measured using an open flow calorimeter and blood lactate concentration (LA) via a lactate analyser. Venous blood samples (10 ml) were collected at rest, during the last 10 s of each step of the SET, and after exercise and used to measure LA, plasma protein concentration (PP), hematocrit (HCT), and the plasma concentrations of creatine kinase (CK) and aspartamine transferase (AST). There were no differences (P > 0.05) in resting VO2, LA, TPP, or HCT due to treatment or test time. Furthermore, there were no differences (P > 0.05) in maximal oxygen uptake (VO2max), top run velocity, run time, watts at VO2max, velocity to produce a lactate of 4 mmoll-1 (VLA4), watts at VLA4, peak HCT or peak LA. Finally, there were no differences (P > 0.05) in resting or post-exercise CK or AST. These data indicate that chronic eST administration does not affect aerobic capacity or indices of exercise performance in unfit aged mares.