Overtrained horses alter their resting pulsatile growth hormone secretion.

The influence of intensified- and reduced training on nocturnal GH secretion and elimination dynamics was studied in young (1.5 yr) Standardbred geldings in order to detect potential markers indicative for early overtraining. Ten horses trained on a treadmill for 32 weeks in age, breed and gender matched fixed pairs. Training was divided into four phases (4, 18, 6 and 4 weeks, respectively): (1) habituation to high-speed treadmill trotting, (2) normal training, in which speed and duration of training sessions was gradually increased, (3) in this phase, the horses were divided into 2 groups: control (C) and intensified trained group (IT). In IT, training intensity, duration and frequency was further increased, whereas in C these remained unaltered, (4) reduced training (RT). At the end of phase 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). IT induced overtraining (performance decreased by 19% compared to C), which was associated with an increase in concentration peaks number (3.6 vs. 2.0, resp.), 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. Key words: growth hormone, training, horse, exercise.