Endocrinological and behavioural adaptations to experimentally induced physical stress in horses

The major objective of this thesis is to find parameters to diagnose early overtraining (a stress-related disorder) in horses. Diagnosing overtraining remains still a major challenge. In man, a decrease in performance despite normal training, is indicative for the syndrome. The diagnosis is made by use of the Profile of the Mood (POMS) questionnaire, a valuable but subjective tool. Hormonal disbalance due to prolonged severe stress is considered as a possible cause of the syndrome. In this thesis, the results of an overtraining study in Standardbred horses will be presented. The horses underwent training on a high speed treadmill. The study consisted of four periods, namely an acclimatisation, training, intensified training and recovery period with at the end of each period a test week. After the training period the horses were randomly divided into a control group and an intensified trained (IT) group. Standardized exercise tests were used to monitor performance. Behavioural studies were performed to monitor mood changes. The growth hormone axis as well as glucose metabolism were monitored to detect hormonal disturbances. The performance of the horses increased during the training period, however during the intensified training period the IT group showed a decrease in performance despite the fact that the horses were more intensively trained than the control horses. The horses changed their gait from trot to galop or even stopped during the training. Symptoms are indicative of overtraining. The behavioural study consisted of three different assessments exclusively developed for the current experiment and aimed to distinguish between the mood states as described in the POMS. During the intensified training period the IT horses showed obvious changes in mood state. The horses were less interested in the environment and in other unknown horses as was shown in different behavioural changes. Growth hormone is secreted in a pulsatile fashion which is apparently random in nature and therefore difficult to quantify: concentration time series are needed instead of single blood samples. Deconvolution algorithm is used in this thesis to quantify equine GH secretory and elimination dynamics for the first time. Approximate Entropy was used to quantify orderliness of the GH time series. The growth hormone profile changed significantly for the intensified trained horse during the intensified training period. The profile changed to a more irregular pattern with smaller but more frequent pulses with a longer GH half-life. It was hypothesized that the increase in GH pulsatility might be beneficial to restore homeostatic balance. Glucose metabolism and peripheral insulin sensitivity were measured by use of the euglycaemic hyperinsulinaemic clamp technique. The intensified trained horses showed dissociation of correlation between glucose metabolism and the mean glucose metabolism rate to plasma insulin concentration ratio during the intensified training period indicative of loss of coordinated control of glucose metabolism. In conclusion: after the detraining period the intensified horses were not fully recovered which indicates that an early form of overtraining was induced. This thesis clearly shows that overtraining and possibly other stress related syndromes induce mental problems before physical problems become apparent

Effects of intensified training and subsequent reduced training on glucose metabolism rate and peripheral insulin sensitivity in Standardbreds

Objective-To determine the influence of intensified training and subsequent reduced training on glucose metabolism rate and peripheral insulin sensitivity in horses and identify potential markers indicative of early overtraining. Animals-12 Standardbred geldings. Procedures-Horses underwent 4 phases of treadmill-based training. In phase 1, horses were habituated to the treadmill. In phase 2, endurance training was alternated with high-intensity exercise training. In phase 3, horses were divided into control and intensified training groups. In the intensified training group, training intensity, duration, and frequency were further increased via a protocol to induce overtraining; in the control group, these factors remained unaltered. In phase 4, training intensity was reduced. Standardized exercise tests were performed after each phase and hyperinsulinemic euglycemic clamp (HEC) tests were performed after phases 2, 3, and 4. Results-10 of 12 horses completed the study. Dissociation between mean glucose metabolism rate and mean glucose metabolism rate-to-plasma insulin concentration ratio (M:I) was evident in the intensified training group during steady state of HEC testing after phases 3 and 4. After phase 4, mean glucose metabolism rate was significantly decreased (from 31.1 +/- 6.8 mumol/kg/min to 18.1 +/- 3.4 mumol/kg/min), as was M:I (from 1.05 +/- 0.31 to 0.62 +/- 0.17) during steady state in the intensified training group, compared with phase 3 values for the same horses. Conclusions and Clinical Relevance-Dissociation between the glucose metabolism rate and M:I in horses that underwent intensified training may reflect non-insulin-dependent increases in glucose metabolism

Hormonal responses to acute exercise, training and overtraining. A review with emphasis on the horse.

Overtraining is an imbalance between training and recovery leading to symptoms associated with a neuroendocrine dysbalance called the overtraining syndrome, a disease characterized by behavioral, emotional and physical symptoms similar with depression. Although the prevalence of overtraining is high in human and equine athletes, at present no sensitive and specific test is available to prevent or diagnose overtraining. Nowadays, it is believed that combination of different (hormonal) parameters appear to be the best indicators of overtraining. Therefore, this review provides a summary of previous literature examining the response of the hypothalamic-pituitary-adrenal (HPA) axis and the growth hormone-insulin-like growth factor-I (GH-IGF-I) axis to acute and chronic exercise as well as overtraining in humans and horses. The exercise induced hormonal responses seem to be equal for the equine as well as the human athlete, which makes comparisons possible. Repeated bouts of exercise are suggested to provide a way to detect subtle changes in hormonal responses in the individual athlete, which may make them an important tool in detecting early overtraining. This should be combined with corticotropin releasing hormone (CRH) stimulation tests and basal ACTH and GH pulsatility determination. Further research is needed to establish the correct training intensity and rest period for the exercise test in equines

Normal function of the hypothalamic-pituitary growth axis in three dwarf friesian foals

Serial blood samples were collected from three dwarf Friesian foals to examine their endogenous growth hormone (GH) profiles, and the integrity of the GH-insulin-like growth factor-1 (IGF-1) axis was tested in one of them by examining its responses to the administration of GH-releasing hormone (GHRH) and to 10 days of treatment with recombinant equine GH. The basal serum concentrations of IGF-1 in the three dwarf foals were compared with those in nine age-matched normal foals. All the dwarf foals secreted endogenous GH. Stimulation with 7.0 microg/kg GHRH led to a 1400 per cent increase in plasma GH concentration in the dwarf foal tested, and 10 daily subcutaneous treatments with 20 microg/kg recombinant equine GH led to a 100 per cent increase in its serum IGF-1 concentration. The basal serum concentrations of IGF-1 in the dwarf foals were not significantly different from those of the normal foals

Normal function of the hypothalamic-pituitary growth axis in three dwarf friesian foals

Serial blood samples were collected from three dwarf Friesian foals to examine their endogenous growth hormone (GH) profiles, and the integrity of the GH-insulin-like growth factor-1 (IGF-1) axis was tested in one of them by examining its responses to the administration of GH-releasing hormone (GHRH) and to 10 days of treatment with recombinant equine GH. The basal serum concentrations of IGF-1 in the three dwarf foals were compared with those in nine age-matched normal foals. All the dwarf foals secreted endogenous GH. Stimulation with 7.0 microg/kg GHRH led to a 1400 per cent increase in plasma GH concentration in the dwarf foal tested, and 10 daily subcutaneous treatments with 20 microg/kg recombinant equine GH led to a 100 per cent increase in its serum IGF-1 concentration. The basal serum concentrations of IGF-1 in the dwarf foals were not significantly different from those of the normal foals