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Am J Physiol Regul Integr Comp Physiol 297: R403-R411, 2009. First published June 3, 2009; doi:10.1152/ajpregu.90778.2008
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ARTICLES

Overtrained horses alter their resting pulsatile growth hormone secretion

¹Faculty of Veterinary Medicine, Medicine Section, Department of Equine Sciences, Utrecht University, Utrecht; ⁴Department of Clinical Chemistry and ⁵Faculty of Health Sciences, Department of Movement Sciences, University of Maastricht, Maastricht, The Netherlands; ²Department of Pharmacology, University of Virginia Health Sciences Center, Charlottesville, Virginia; and ³Department of Human Physiology and Sportsmedicine, Free University of Brussels, Brussel, Belgium

Submitted 17 September 2008 ; accepted in final form 28 May 2009

The influence of intensified and reduced training on nocturnal growth hormone (GH) secretion and elimination dynamics was studied in young (1.5 yr) Standardbred geldings to detect potential markers indicative for early overtraining. Ten horses trained on a treadmill for 32 wk in age-, breed-, and gender-matched fixed pairs. Training was divided into four phases (4, 18, 6, and 4 wk, respectively): 1) habituation to high-speed treadmill trotting, 2) normal training, in which speed and duration of training sessions were gradually increased, 3) in this phase, the horses were divided into 2 groups: control (C) and intensified trained (IT) group. In IT, training intensity, duration, and frequency were further increased, whereas in control these remained unaltered, and 4) reduced training (RT). At the end of phases 2, 3, and 4, blood was sampled overnight every 5 min for 8 h for assessment of GH secretory dynamics using pulse detection, deconvolution analysis, and approximate entropy (ApEn). Intensified training induced overtraining (performance decreased by 19% compared with C), which was associated with an increase in concentration peaks number (3.6 vs. 2.0, respectively), a smaller peak secretion pattern with a prolonged half-life (15.2 vs. 7.3 min, respectively), and an increased ApEn (0.89 vs. 0.49, respectively). RT did not lead to full recovery for the overtrained horses. The increased irregularity of nocturnal GH pulsatility pattern is indicative of a loss of coordinated control of GH regulation. Longer phases of somatostatin withdrawal are hypothesized to be the underlying mechanism for the observed changes in GH pulsatility pattern.

endocrinology; sports physiology; growth hormone-insulin-like growth factor I axis; hormone pulsatility analysis