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During intense exercise, lactic-acid accumulates in skeletal muscle and protons build up which causes a decrease in pH and leads to inhibition of muscle function and eventually fatigue. A family of Monocarboxylate Transporters (MCTs) have been identified in equine skeletal muscle but its function during training has not yet been fully elucidated. We hypothesize an important function of MCT1, 2 and 4 in equine muscle. Six Standardbred geldings were trained for a total of 22 weeks in 2 phases (endurance phase and high intensity phase). Exercise intensity during the training sessions were based on fixed percentages of the peak heart frequency (HRpeak) determined during a Standard Exercise Test (SET). At the beginning and end of each phase a SET was performed. Venous blood was drawn from the jugular vein before the test (t=0 minutes), after the warming up (t=9 minutes), and every 5 minutes during the SET (t=14, 19, 24, 29, 34). Samples were kept on ice until whole blood lactic acid had been analysed. Muscle biopsies were taken approximately 60 minutes before each SET using a modified Bergström biopsy needle. Blood and fat tissue were removed from the biopsy and the biopsy was there after immediately frozen in liquid nitrogen for biochemical analysis and Western-blotting for MCT expression. Training (SET1 vs SET2) resulted in an increase in the total plasma lactic acid accumulation during exercise as measured as the area under the curve during the 20-minute endurance run in the SET. Semi quantative densitometric analysis of the blots showed that MCT1 and MCT4 remain unaltered upon training. MCT2 expression was non-significantly increased by more than 60%. We report the expression of MCT1, MCT2 and MCT4 proteins in skeletal muscle of Standardbred trotters during a physical training regimen. The relative high expression of MCT2 was surprising but fits well with the gastrointestinal function of the horse. The high expression of MCT2 suggests an important function in the absorption of short chain fatty acids although such function needs further investigation.

Horses are used for a wide variety of purposes from being used for recreational purposes to competing at an international level. With these different uses, horses have to adapt to numerous challenges and changes in their environment, which can be a challenge itself in continuously safeguarding their welfare. The objective of this study was to assess the prevalence of health disorders with clinical examination and identify possible risk factors of health disorders affecting horse welfare in professional husbandry systems in the Netherlands. With the use of fixed protocols for recording health aspects in horses, 150 horse farms voluntarily participating in the study were assessed by trained assessors. On each farm, 20 horses were clinically examined, in total almost 3,000 animals. This study recorded on the basis of the clinical examinations: the respiratory system (i.e., abnormal breathing [1%], coughing [1%], nasal discharge [1.9%]), body condition (i.e., 18.8% fat body condition and 6.4% poor body condition), locomotion (14.5% exhibited irregularity of locomotion and 4.8% were lame), back palpation (a light response [22.6%] and moderate to severe response [8.4%]), mouth (i.e., irregularities on mouth corners [3.4%] and bars [3.4%]), and ocular discharge (12%). Risk factor analysis, stepwise using mixed model regression, demonstrated several risk factors for health aspects. Horses used for instruction (riding lessons) were almost two times more at risk to develop moderate to severe back pain compared to horses used for recreation (odds ratios [OR] = 0.54) or for competition (OR = 0.61). Horses used for instruction (riding school lessons), breeding, or recreation all had a higher risk for irregular locomotion or lameness compared to competition horses (OR = 0.42, OR = 0.55, OR = 2.14, respectively). Horses used for recreation were more prone to have a higher BCS compared to horses used for breeding (OR = 3.07) and instruction (OR = 2.06). The prevalence of health problems and the identified risk factors are valid for the horses in the present study in which farms voluntarily participated. Furthermore, the results may provide the basis for horse welfare and health programs on farm and horse industry levels. With the development of a valid welfare monitoring system for the horse industry, the welfare of horses can be increased through improving awareness and stimulating changes in management.

BACKGROUND: Insulin resistance in horses is an emerging field of interest as it is thought to be a contributing factor in the pathogenesis of many equine conditions. OBJECTIVES: The objectives of the present study were to determine the effects of long-term oral administration of L-carnitine on insulin sensitivity, glucose disposal, plasma leptin concentrations and acylcarnitine spectrum both in plasma and urine. ANIMALS AND METHODS: Six 3-year-old healthy warmblood geldings were used. In a double blind 2 × 2 Latin square design at a dosage of 100 mg/kg body weight (BW)/day for 28 days the effects of oral supplementation of L-carnitine (as fumarate) were assessed. Glucose disposal and insulin sensitivity were measured by means of the euglycemic-hyperinsulinemic clamp technique. Radioimmunoassays were used to determine plasma leptin and insulin concentrations. Electrospray tandem mass spectrometry was used to assess acylcarnitines both in plasma and urine. Statistical analysis was performed using a linear mixed-effects model and P values

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.

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.

Summary Reasons for performing study: To study the possible long-term effect of improved glucose tolerance in horses after long-term training, as the impact of exercise training on glucose metabolism is still unclear in the equine species. It is not known whether there is a direct long-term effect of training or if the measurable effect on glucose metabolism is the residual effect of the last exercise session. Objectives: To determine the chronic effect on glucose metabolism and peripheral insulin sensitivity of long-term training in horses by use of the euglycaemic hyperinsulinaemic clamp technique. Methods: Eleven Standardbred horses were acclimatised to running on the high-speed treadmill for 4 weeks (Phase 1) followed by training for 18 weeks with an alternating endurance (˜ 60% HRmax) high intensity training programme (˜ 80% HRmax) (Phase 2). Training frequency was 4 days/week. At the end of Phase 1, a euglycaemic hyperinsulinaemic clamp was performed 72 h after the last bout of exercise in all horses. At the end of Phase 2, the horses were clamped 24 h or 72 h after the last bout of exercise. Results: Glucose metabolism rate did not change significantly after 18 weeks of training, measured 72 h after the last exercise bout (0.018 ± 0.009 and 0.022 ± 0.006 mmol/kg bwt/min, respectively). Peripheral insulin sensitivity also did not change significantly following training (7.6 ± 5.7 times 10−6 and 8.0 ± 3.1 times 10−6, respectively). The same measurements 24 h after the last bout of exercise showed no significant differences. Conclusions: Results indicated that long-term training in Standardbreds neither changed glucose metabolism or insulin sensitivity 72 h after the last bout of exercise. Potential relevance: The fact that the beneficial effect of increased insulin sensitivity after acute exercise diminishes quickly in horses and no long-term effects on insulin sensitivity after chronic exercise have as yet been found in horses, implies that exercise should be performed on a regular basis in horses to retain the beneficial effect of improved insulin sensitivity.

Adrenal hormones, ACTH plasma concentrations and the ultrasonographic evaluation of the adrenal glands are considered complementary in clinical evaluations of the hypothalamic–pituitary–adrenal (HPA) axis function in several species. In dogs, age, size and weight have a significant effect on the ultrasonographic size of the adrenal glands. In neonatal foals, ultrasonographic evaluation and measurements of the adrenal glands have been demonstrated to be reliable, however, the effect of individual variables on ultrasonographic features has not been investigated, and the clinical usefulness of adrenal gland ultrasonography is still not known. The aims of this study were: (i) to provide and compare adrenal glands ultrasound measurements in healthy newborn Thoroughbred and Standardbred foals, and assess any effect of age, weight and sex on them, (ii) to assess and compare ACTH and steroid hormone concentrations in healthy neonatal foals of the two breeds. Venous blood samples and ultrasonographic images of the adrenal glands were collected from 10 healthy neonatal Thoroughbred and 10 healthy neonatal Standardbred foals. Ultrasonographic measurements of the adrenal glands were obtained and adrenocorticotropin (ACTH), cortisol, dehydroepiandrosterone (DHEA), progesterone (P4) and aldosterone plasma concentrations were assessed. The ACTH/cortisol, ACTH/progesterone, ACTH/aldosterone, ACTH/DHEA and cortisol/DHEA ratios were calculated. A significant positive correlation was found between the height of the right adrenal gland and the foal’s weight, the width of the right and left adrenal gland and the medulla was significantly lower in Thoroughbreds than in Standardbreds. Cortisol and ACTH plasma concentrations were significantly higher and lower, respectively, in Thoroughbreds compared with Standardbreds. The cortisol/DHEA ratio was significantly higher in Thoroughbreds. This study provides reference ranges for neonatal Thoroughbred and Standardbred foals, and suggests the presence of breed-related differences in ultrasonographic adrenal gland measurements, plasmatic cortisol and ACTH concentrations. The higher cortisol/DHEA ratio detected in Thoroughbred foals could suggest a different response to environmental stimulation in the two breeds.