The influence of exercise on serum enzyme levels in the horse.

A group of clinically normal horses was subjected to controlled strenuous exercise. Elevated serum concentrations of lactic dehydrogenase, aldolase and creatine kinase were observed after exercise but no significant change in serum glutamic-oxalacetic transaminase was noted. These changes were reduced by repeated exposure to exercise suggesting that measurement of serum enzyme elevations, particularly creatine kinase, might be a useful index of fitness in the horse. Administration of prednisolone prior to exercise also reduced these changes. Since the serum enzyme concentrations had returned to normal within 72 hours of exercise, and since the cytoplasmic enzyme glutamic oxalacetic transaminase was not released from the tissues this supported the hypothesis that efflux of intracellular enzymes into the circulation was due to a temporary selective change in cell membrane permeability rather than to tissue necrosis.     

Tissue distribution and blood levels of gamma-glutamyl transferase in the horse.

In the horse, gamma-glutamyl transferase (GGT) was found to be mainly located in the kidneys, liver and pancreas. As renal lesions are followed by a urinary escape of enzyme, it can be assumed that if there are raised serum enzyme levels then the source will be chiefly from the liver and pancreas. In the blood, GGT can be measured either in plasma or serum. Its mean level in 58 horses was 12 U/L. This level was not affected by moderate dilution or slight haemolysis and its activity was only slightly decreased by storage at--30 degrees C. The relative hepatic specificity of this enzyme and its easy measurement make it a potentially very useful measure of liver dysfunction in the horse.     

The effect of exercise on the lactic dehydrogenase and creatine kinase isoenzyme composition of horse serum.

The distribution of lactic dehydrogenase, aldolase and creatine kinase in various horse tissues was determined. Using polyacrylamide gel electrophoresis the lactic dehydrogenase and creatine kinase isoenzyme composition of horse serum, taken before and after exercise, was studied. Horse tissue isoenzyme patterns were also obtained. By comparing tissue and serum patterns, skeletal muscle was found to be the tissue of origin of the increase in serum lactic dehydrogenase and creatine kinase observed after exercise.     

The influence of dietary selenium levels on blood levels of selenium and glutathione peroxidase activity in the horse

Twenty mature geldings, averaging 535 kg, were used to determine the influence of dietary selenium (Se) on the blood levels of Se and Se-dependent glutathione peroxidase (SeGSH-Px) activity in the horse. Horses were randomly assigned within breed to four treatments consisting of five horses each and fed a basal diet containing .06 ppm of naturally occurring Se. Diets were supplemented with .05, .10 and .20 ppm Se, as sodium selenite. Blood was drawn for 2 wk before, and for 12 wk following, the inclusion of supplement Se in the diets. Whole blood and plasma Se concentrations and plasma SeGSH-Px activities were determined from all blood samples. Selenium concentrations in plasma and whole blood increased linearly from wk 1 to wk 5 and 6, respectively, in Se-supplemented horses. After these times, no significant changes in Se concentration were observed in Se-supplemented or in unsupplemented horses throughout the remainder of the 12-wk trial. Plasma Se reached plateaus of .10 to .11, .12 to .14, and .13 to .14 micrograms/ml in horses supplemented with .05, .10 and .20 ppm Se, respectively. Whole blood Se reached plateaus of .16 to .18, .19 to .21, and .17 to .18 micrograms/ml in horses supplemented with .05, .10 and .20 ppm Se, respectively. Plasma SeGSH-Px activity was not significantly affected by dietary treatment. Therefore, this enzyme was not a good indicator of dietary Se in these mature horses.     

The effect of laryngoplasty on pharyngeal function in the horse.

Using fluoroscopic techniques and videotape recordings, a study of normal deglutition was made in 2 ponies. Paryngeal function was studied at intervals after laryngoplasty on 1 animal and after a sham technique on the other. Two clinical cases of laryngeal paralysis, which had been treated by laryngoplasty, were also examined. In the 2 experimental ponies, liquid food passed into the lower respiratory tract post-operatively. One of the clinical cases appeared to swallow solid food normally, but some food material entered the larynx of the other horse. It is suggested that pharyngeal dysfunction, as well as over-abduction of the arytenoid cartilage, might be involved in causing the chronic post-operative cough which the experiment was designed to investigate.     

Early exercise in the horse

Across all equestrian disciplines, the single largest reason for wastage is musculoskeletal injury. It is, therefore, of importance that management and competition structures are in place to optimize the development of the equine musculoskeletal system to minimize wastage.Data from other species and, in particular, humans have demonstrated the benefit of early exercise and the dire consequences of inactivity. The horse has evolved as a cursorial animal capable of covering up to 10 km/d within 9 days of birth. Yet, modern equine management systems restrict, rather than promote, early exercise. Foals were shown to have a positive response to early preweaning paddock exercise (greater cartilage health), and more recent work has demonstrated that exercise over and above that normally occurring with pasture-reared foals, introduced as early as 3 weeks old, had positive effects on the equine musculoskeletal system. The response of juvenile horses to additional exercise is because of the tissue being responsive to priming. Epidemiological data indicate that the window for tissue modification may still be open when the horse is a yearling and even as a 2 year old.However, the method in which the exercise is applied may be of as much importance as the timing of the stimuli. A recent prospective study of both Thoroughbred and Standardbred horses demonstrated that the horses that entered training as 2 year olds had longer and more successful racing careers than those that entered training later in life. It would appear that even the initial stages of training are enough to provide a positive stimulus, as horses first registered with a trainer at 2 years old had the same advantages as those that raced as 2 year olds.The physiological, clinical, and epidemiological data indicate that, rather than restrict exercise and the use of horses at a young age, we should realign expectations with the capability of the horses’ musculoskeletal system and evolutionary template to maximize orthopedic health.     

Effects of high-intensity exercise on plasma catecholamines in the thoroughbred horse.

In Study 1, a single speed test of 6 to 12 m/sec was performed for 2 mins at an incline of 5 degrees on a high-speed treadmill (single-step test). Only one speed was performed per session and blood samples were taken before and after the test. In Study 2 horses cantered for 1 min at increasing speeds of 6 to 13 m/sec on an incline of 3 degrees (multiple-step test). Blood samples were taken before exercise, throughout the test and during recovery. In the single-step test plasma concentrations of adrenaline and noradrenaline both increased at speeds of 9 m/sec, as did blood lactate. Mean concentrations of adrenaline and noradrenaline at the end of the 12 m/sec test were 153 and 148 nmol/litre, respectively. Plasma concentrations were similar over all speeds although there was a tendency for the increase in noradrenaline to be greater than that of adrenaline at the lower speeds. The multiple-step test resulted in smaller increases in both adrenaline and noradrenaline. Although again closely correlated, increases in adrenaline were 20-30% greater than those for noradrenaline. In both exercise models, changes in plasma adrenaline and noradrenaline values with exercise showed an exponential relationship to plasma lactate. A plasma half-life of less than 30 secs was indicated during recovery from the multiple-step test. Changes in adrenaline and noradrenaline were much greater than previously recorded in man and emphasise the importance of catecholamines in mediating the physiological response of the horse to exercise.     

The effect of exercise on blood parameters in standardbred and Finnish-bred horses

Serum enzyme activities, albumin, protein, urea, cholesterol, triglyceride, free fatty acid, glucose and lactate concentrations as well as hematocrit values were measured in standardbred and Finnish-bred horses at rest and after (i) a short controlled exercise and (ii) a trotting competition. There were no breed differences in the enzyme activities at rest and the 2 breeds responded in the same manner to the exercise. Only after the race proper significant increases in the enzyme activities were found. The activities rose more in the standardbred horses than in the Finnish-bred horses. Urea and cholesterol concentrations did not change after either exercise. Protein and albumin concentrations as well as hematocrit values increased significantly after the exercise. At rest hematocrit values were significantly higher in the standardbred horses and the difference persisted throughout the exercise. After the race proper also albumin and protein concentrations were higher in the standard-bred than in the Finnish-bred horses. Free fatty acid and triglyceride concentrations increased significantly during the exercise. Although glucose and lactate concentrations increased in both breeds, the behaviour of these parameters differed. Glucose concentrations remained increased for a longer period and the recovery from the increased lactate level was faster in the standardbred than in the Finnish-bred horses. The observed differences suggest that the standardbred horses have higher anaerobic capacity than the Finnish-bred horses.     

The effect of submaximal exercise on the peripheral blood pressure of untrained ponies

A significant increase in systolic and diastolic peripheral blood pressure was recorded after 8 minutes submaximal exercise in untrained ponies. Blood pressure was recorded by a modified auscultatory method from the coccygeal artery.     

Caffeine-induced hyperactivity in the horse: comparisons of drug and metabolite concentrations in blood and cerebrospinal fluid

The goals of this study were to elucidate the temporal and quantitative relationships between caffeine and its major bioactive metabolites in blood and cerebrospinal fluid (CSF) and to characterize the pharmacokinetic-pharmacodynamic relationship for caffeine-induced changes in spontaneous locomotor activity in the horse. We hypothesized that caffeine and its metabolites distribute efficiently into the CSF to antagonize adenosine A1 and A2a receptors and that spontaneous locomotor activity correlates well with caffeine and/or metabolite concentrations in CSF and blood. A microdialysis system was developed to allow simultaneous monitoring of locomotor activity and collection of CSF and blood samples for pharmacokinetic analysis. CSF concentrations of caffeine and its metabolites were evaluated to determine the percentage of central adenosine receptor blockade by the established standard inhibition curves. Caffeine increased the spontaneous locomotor activity for up to 4 h in a dose-dependent manner. After 3 mg/kg caffeine administration, blood caffeine concentration as well as locomotor activity increased sharply to near peak level while CSF caffeine concentrations exhibited a slow rise to a steady-state 75 min later. High correlation coefficient was found between locomotor activity and caffeine concentrations in blood (R(2 )=0.95) and in CSF (R(2) = 0.93). At 3 mg/kg dosage, theophylline was the only detectable caffeine metabolite in the CSF. The concentrations reached in the CSF were sufficient to partially block central adenosine A1 (14% blockade) and A2a (11% blockade) receptors. There were no statistically significant differences between the pharmacokinetics of caffeine in the blood and CSF. This study provides novel evidence that locomotor stimulation in horses is closely correlated with caffeine concentrations in the blood and CSF and, furthermore, is consistent with blockade of central adenosine receptors.     

Respiratory responses to exercise in the horse

Horses are elite athletes when compared with other mammalian species. In the latter, performance is limited by cardiovascular or musculoskeletal performance whereas in athletic horses it is the respiratory system that appears to be rate limiting and virtually all horses exercising at high intensities become hypoxaemic and hypercapnoeic. This is due to both diffusion limitation and a level of ventilation inadequate for the metabolic level that enables horses to exercise at these intensities. In conjunction with these blood gas changes, total pulmonary resistance increases and the work of breathing rises exponentially and airflow eventually plateaus despite increases in inspiratory and expiratory intrapleural pressures. Horses breathe at comparatively high frequencies when galloping due to the tight 1:1 coupling of strides to breathing. Whether this effects gas exchange and, if so, to what extent, has not been fully elucidated.     

The effect of exercise and conditioning on equine red blood cell characteristics

The effect of exercise and conditioning on 2,3-diphosphoglycerate levels was studied in nine mature horses. During a 12 minute exercise bout producing heart rates of 165 bpm, 2,3-DPG was significantly increased (p<.05). In addition, exercising levels of 2,3-DPG were increased (p<.05) approximately 8% after a six-week submaximal conditioning program. These increases could not be entirely attributed to changes in erythrocyte number. Mean corpuscular volume was also increased during exercise (p<.05) but was not altered by conditioning.     

Acetylsalicylic acid and blood coagulation in the horse.

Equine blood may contain salicylic acid (SA) taken up as free acid or represents the metabolite of acetylsalicylic acid (ASA). To obtain information of SA in race horses we screened blood samples of trotting-horses routinely drawn to be analyzed for doping substances. The individual values determined followed a Gaussian distribution displaying a geometric mean of 19 ng SA per ml serum. A probit analysis revealed linear relationship (r = 0.995). Additional studies examined the antithrombotic efficacy of ASA in the horse. An oral dose of 300 mg ASA considerably elevated the bleeding time for more than 2 hours with concomitant SA serum levels between 800 and 1000 ng/ml. It is concluded that salicylate levels even below 1 microgram/ml serum bring about considerable pharmacologic effects such as prolongation of bleeding time, decrease in blood viscosity and possibly dilatation of blood vessels. These effects may improve the tissue supply with blood including oxygen.