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Effect of exercise, training, circadian rhythm, age, and sex on insulin-like growth factor-1 in the horse¹

G. K. Noble^*,2, E. Houghton, C. J. Roberts, J. Faustino-Kemp, S. S. de Kock, B. C. Swanepoel and M. N. Sillence^*

^* School of Agricultural and Veterinary Sciences, Charles Sturt University, Wagga Wagga, New South Wales 2678, Australia; and HFL Ltd, Fordham, Cambridgeshire CB7 5WW, United Kingdom; and and The Laboratory of the National Horseracing Authority of Southern Africa, Turffontein 2140, South Africa

² Corresponding author: gnoble{at}csu.edu.au

Insulin-like growth factor-1 could be a useful marker in the horse for diagnostic, selection, or forensic purposes, provided its physiological regulation is well understood. The objective of this study was to investigate factors, such as acute exercise, fitness training, time of day, sex, and age, that may influence serum IGF-1 in normal, healthy horses. Throughout a 9-wk training program, 6 geldings maintained a mean ( ± SEM) IGF-1 concentration of 302 ± 29 ng/mL. Moderate or high intensity exercise had no effect on IGF-1 concentrations, when pre- and postexercise values were compared. Over a 24-h period, there was some variation in IGF-1 concentrations but no clear diurnal rhythm. Concentrations of IGF-1 were measured in a large population of thoroughbred horses (1,880) on 3 continents. The population deviated slightly from a normal distribution (P < 0.001) because of large IGF-1 concentrations in 10 horses. The global mean IGF-1 concentration was 310 ± 2.2 ng/mL, with a greater mean value (P < 0.001) in gonad-intact males (336 ± 5.6 ng/mL) than in females (303 ± 3.2 ng/mL) or geldings (302 ± 3.2 ng/mL). However, the greatest IGF-1 concentrations observed for all stallions, mares, and geldings were 627, 676, and 709 ng/mL, respectively. In mares and geldings, IGF-1 concentrations showed a gradual decrease with advancing age (P < 0.001), but the effect was much less marked in stallions. This study confirms that IGF-1 concentrations are stable, compared with GH concentrations, in the horse and that a meaningful measure of IGF-1 status can be obtained from a daily serum sample.

Key Words: circadian rhythm • exercise • horse • insulin-like growth factor-1 • population