title: Overtrained horses alter their resting pulsatile growth hormone secretion.
abstract: 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.
authors.affiliation: Dept. of Equine Sciences, Medicine Section, Faculty of Veterinary Medicine, Utrecht Univ., Yalelaan 114, 3584 CM Utrecht, The Netherlands. e.roelfsema@uu.nl
authors.forename: E
authors.initials: E
authors.lastname: de Graaf-Roelfsema
authors.forename: P P
authors.initials: PP
authors.lastname: Veldhuis
authors.forename: H A
authors.initials: HA
authors.lastname: Keizer
authors.forename: M M E
authors.initials: MM
authors.lastname: van Ginneken
authors.forename: K G
authors.initials: KG
authors.lastname: van Dam
authors.forename: M L
authors.initials: ML
authors.lastname: Johnson
authors.forename: A
authors.initials: A
authors.lastname: Barneveld
authors.forename: P P C A
authors.initials: PP
authors.lastname: Menheere
authors.forename: E
authors.initials: E
authors.lastname: van Breda
authors.forename: I D
authors.initials: ID
authors.lastname: Wijnberg
authors.forename: J H
authors.initials: JH
authors.lastname: van der Kolk
mesh_terms: D000818:Animals; D005080:Exercise Test; D013006:Growth Hormone / Q000378:metabolism*; D006207:Half-Life; D006736:Horses / Q000502:physiology*; D007334:Insulin-Like Growth Factor I / Q000378:metabolism; D019344:Lactic Acid / Q000097:blood; D008297:Male; D009919:Orchiectomy; D010805:Physical Conditioning, Animal / Q000502:physiology*; D012146:Rest / Q000502:physiology*; D013997:Time Factors
chemical_list: D019344:Lactic Acid; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone