Changes in lipoprotein composition in horses fed a fat-supplemented diet

This study was designed to compare the effects of a fat-supplemented diet versus a traditional diet on the lipoprotein (LP) content in horses. In the first of two trials, eight two-year old horses were fed a basal diet, in which 80% of the digestible energy was supplied as chopped alfalfa hay. One group of four horses (fat-supplemented group, FS) was fed the remaining 20% of the digestible energy as corn oil, while the other group of four horses (control) was fed rolled corn to complete their diet. Blood samples were collected at the start of the experiment and every 2.5 weeks thereafter for 10 weeks. Total serum lipids were measured in both groups of horses and the lipoproteins were fractionated into very low density LP (VLDL), low density LP (LDL), and high density LP (HDL) using ultracentrifugation and agarose-column chromatography. Each LP fraction was measured for protein, cholesterol (CH) and triglyceride TG) content. Total serum lipids were increased in the FS group above the control (9.16 vs. 4.65 mg/ml, week 5). Serum cholesterol and triglyceride concentrations were elevated in the FS group, but were highly variable. Variations in lipid concentrations may have been due to variation in time of sampling during the day. In the second trial, four of the eight horses were used and divided into the same two groups; FS vs. control. After an initial sample, postprandial serial blood samples (1 hour intervals for 8 hours) were drawn at 2, 4 and 6 weeks. Upon examination of the data, Hour 3 post-feeding was chosen to represent postprandial LP values. In both trials, the horses were able to adapt to the added dietary fat. Maintenance of body weight and increased speed of lipid clearance from the blood by the end of each trial in the FS group support this statement. In the FS group, there was an increase in VLDL TG concentration, but not in LDL. This indicates an increase in VLDL TG clearance from the circulation, presumably by increased lipoprotein lipase (LPL) activity. Cholesterol concentrations were also increased in the FS horses in the LDL and HDL. The rise in cholesterol may be attributed to endogenous recycling of liver products such as bile salts, which aid in digestion and absorption and are cholesterol based. In addition, there were greater quantities of LDL and HOL produced in the FS horses as supported by the increased protein concentrations as well as larger peaks for the eluate from the gel filtration column.     

Digestible energy requirements for work and maintenance of horses fed conventional and fat-supplemented diets

A control and a 10% fat-supplemented diet were fed to exercising horses maintained in two different body conditions, during both temperate and hot weather, to determine the efficacy of fat as dietary aid to reduced energy requirements for thermal regulation in exercising horses. Horses were worked 7.2 km daily, 5 d/w, and in each season were fed sufficient energy to maintain constant body weight and body fat content at each assigned level of body condition. In both seasons and in both body conditions, digestible energy intake was lower (P<.01) when the horses were fed the fat-supplemented diet than when fed the control diet. Digestible energy intake was partitioned into requirements for work and maintenance. Since work levels were similar, digestible energy requirements for work were similar when horses were fed both experimental diets. However, the digestible energy requirements for maintenance were significantly lower (P<.01) when the horses were fed the fat-supplemented diet. Thus, it appears that feeding fat to exercising horses reduces the thermal load and resulting digestible energy requirements for maintenance in both temperate and hot weather.     

Electromyographic activity of the stylopharyngeus muscle in exercising horses

REASONS FOR PERFORMING STUDY: There is a need to understand the process which leads to failure of recruitment of the stylopharyngeus muscle in clinical cases of nasopharygeal collapse. We therefore studied the timing and intensity of stylopharyngeus muscle activity during exercise in horses. OBJECTIVE: To measure the electromyographic (EMG) activity of the stylopharyngeus muscle in exercising horses and correlate it with the breathing pattern. METHODS: Five horses were equipped with a bipolar fine wire electrode placed on the stylopharyngeus muscle and a pharyngeal catheter. The horses exercised on a treadmill at speeds corresponding to 50 (HRmax50), 75 and 100% of maximum heart rate, and EMG activity of the stylopharyngeus muscle and upper airway pressures were recorded. The EMG activity of the stylopharyngeus muscle was then correlated to the breathing pattern and the activity quantified and reported as a percentage of the baseline activity measured at HRmax50. RESULTS: There was ongoing activity of the stylopharyngeus muscle throughout the breathing cycle; however, activity increased towards the end of expiration and peaked early during inspiration. Tonic activity was present during expiration. Peak, mean electrical and tonic EMG activity increased significantly (P<0.05) with exercise intensity. CONCLUSION: The stylopharyngeus muscle has inspiratory-related activity and tonic activity that increases with speed. POTENTIAL RELEVANCE: The stylopharyngeus muscle is one of a group of upper airway muscles that function to support and maintain the patency of the nasopharynx during inspiration. Failure of recruitment of the stylopharyngeus muscle during exercise is a potential explanation for clinical cases of dorsal pharyngeal collapse, but further work investigating the activity of the stylopharyngeus muscle in horses affected by this disease is needed.     

Cricothyroid muscle function and vocal fold stability in exercising horses

OBJECTIVES: To determine (1) if the cricothyroid muscle had respiratory-related electromyographic (EMG) activity that increased with respiratory effort and (2) if bilateral cricothyroid myotomy resulted in vocal fold instability and collapse in exercising horses. STUDY DESIGN: Experimental. ANIMALS: Seven (3 EMG; 4 cricothyroid myotomy) Standardbred horses. METHODS: Three horses exercised on a treadmill at speeds corresponding to the speed that produced maximum heart rate (HR(max)), 75% of maximum heart rate (HR(75%max)), and 50% of maximum heart rate (HR(50%max)) for 60 seconds at each speed while EMG activity of the cricothyroid muscle and nasopharyngeal pressures were measured. Another 4 normal horses were exercised on the treadmill at HR(max) and HR(75%max) for 60 seconds at each speed before and after bilateral cricothyroid myotomy. Upper airway pressures were measured and videoendoscopic examinations were performed and videotaped at each speed. RESULTS: Peak phasic EMBG activity of the cricothyroid muscle was coincident with inspiration and increased with treadmill speed. Bilateral cricothyroid myotomy resulted in vocal fold collapse in all horses. Mean peak inspiratory pressures were significantly more negative compared with control values at both HR(max) and HR(75%max). CONCLUSIONS: Cricothyroid muscle dysfunction may be implicated in vocal fold collapse and likely causes inspiratory airway obstruction in exercising horses. CLINICAL RELEVANCE: Conditions compromising cricothyroid muscle function or motor innervation could result in vocal fold collapse.     

Copper and zinc balance in exercising horses fed 2 forms of mineral supplements

Studies comparing the absorption and retention of various forms of trace minerals in horses have yielded mixed results. The objective of this study was to compare Cu and Zn absorption and retention in exercising horses where the mineral was supplemented in the sulfate or organic chelate form. Nine mature horses were used in a modified switchback design experiment consisting of seven 28-d periods. Horses were fed a diet consisting of 50% concentrate and 50% hay that was balanced to meet the energy, protein, Ca, and P requirements for horses performing moderate-intensity exercise. Horses were subjected to a controlled mineral repletion-depletion diet sequence before feeding the experimental diet to standardize mineral status across horses. The experimental diet was designed to provide 90% of the 1989 NRC for Cu and Zn, with supplemental mineral provided in the inorganic sulfate form (CuSO(4) and ZnSO(4)) or the organic chelate form (Cu-Lys and Zn-Met). Feed, fecal, urine, and water samples collected during a total collection during the last 4 d of the experimental diet periods were analyzed to determine apparent absorption and retention of Cu and Zn from the 2 mineral forms. A formulation error caused horses receiving the organic chelate diet to consume about 3 times the amount of Cu and Zn compared with those fed the sulfate-supplemented diet. Copper and Zn intake and fecal excretion were greater (P < 0.05) for horses consuming the organic chelate-supplemented diet. Apparent absorption values for all horses were negative. Apparent Cu absorption and retention as a percentage of intake were greater for horses fed the organic chelate diet (P < 0.05). It is unknown why excretion of Cu and Zn by the horses during the total collection exceeded the mineral intake. Although Cu-Lys seemed to be better absorbed than CuSO(4) and absorption of Zn-Met and ZnSO(4) were not different, these results are tempered by the observation of abnormally high fecal and urinary excretion values for Cu and Zn in the present study.     

Effects of stylopharyngeus muscle dysfunction on the nasopharynx in exercising horses

REASONS FOR PERFORMING STUDY: Nasopharyngeal collapse has been observed in horses as a potential cause of exercise intolerance and upper respiratory noise. No treatment is currently available and affected horses are often retired from performance. OBJECTIVE: To determine the effect of bilateral glossopharyngeal nerve block and stylopharyngeus muscle dysfunction on nasopharyngeal function and airway pressures in exercising horses. METHODS: Endoscopic examinations were performed on horses at rest and while running on a treadmill at speeds corresponding to HRmax50, HRmax75 and HRmax, with upper airway pressures measured with and without bilateral glossopharyngeal nerve block. RESULTS: Bilateral glossopharyngeal nerve block caused stylopharyngeus muscle dysfunction and dorsal nasopharyngeal collapse in all horses. Peak inspiratory upper airway pressure was significantly (P = 0.0069) more negative at all speeds and respiratory frequency was lower (P = 0.017) in horses with bilateral glossopharyngeal nerve block and stylopharyngeus muscle dysfunction compared to control values. CONCLUSIONS: Bilateral glossopharyngeal nerve anaesthesia produced stylopharyngeus muscle dysfunction, dorsal pharyngeal collapse and airway obstruction in all horses. POTENTIAL RELEVANCE: The stylopharyngeus muscle is probably an important nasopharyngeal dilating muscle in horses and dysfunction of this muscle may be implicated in clinical cases of dorsal nasopharyngeal collapse. Before this information can be clinically useful, further research on the possible aetiology of stylopharyngeus dysfunction and dysfunction of other muscles that dilate the dorsal and lateral walls of the nasopharynx in horses is needed.     

Plasma aldosterone and electrolyte concentrations in exercising thoroughbred horses fed two diets in summer and winter

In a replicated 4 × 4 Latin square experiment conducted in summer, eight mature Thoroughbred horses received two diets, control and fat-supplemented, and were exercised at two fitness levels designed to increase daily digestible energy (DE) requirements to approximately 150% (fitness level I) and 200% (fitness level II) of maintenance. In a second experiment during the winter, horses received the two diets in a switchback design and exercised at fitness level II. After 3 wk adaptation to treatments, feed and fecal samples were collected and horses galloped a standardized exercise test (SET) designed to increase the heart rate above 185 bpm for 1200 m. Vital signs were monitored and blood samples were obtained. Feed intake increased as fitness level increased (P < .05). Horses on the fat-supplemented diet required less feed (P < .05) to meet the energy requirements at a given fitness level. Daily DE intake was higher (P < .05) for the horses exercised at fitness level II to meet the increased metabolic demands. Heart rate, respiration rate and rectal temperature all increased (P < .05) with exercise. No treatment effects were found for heart rate (P >. 10) or rectal temperature (P > .30). Horses exercised in the winter had lower (P < .05) respiration rates, indicative of less problems dissipating excess body heat. Plasma aldosterone concentrations increased (P < .05) with exercise, corresponding to an increase in plasma K concentrations. Horses exercised in the summer exhibited higher (P < .05) plasma aldosterone concentrations than horses worked in the winter. Plasma CI and Na concentrations did not change (P > .30) with any treatment or exercise, indicating that the horses were not sufficiently stressed to induce any significant dehydration.     

Endocrinal responses in exercising horses

The intensity and duration of exercise exert a major influence on energy expenditure and physiological changes in the horse. Stressful environmental conditions, acclimation, and training status may further modify these responses. To maintain functional homeostasis during exercise, changes in autonomic nervous activity and hormone secretion are coupled to both the feedforward and the feedback mechanisms that control substrate mobilisation and utilisation.During exercise, both the sympathetic nervous system and the hypothalamic-pituitary-adrenal axis are activated, which increases the circulating levels of adrenocorticotropin (ACTH), cortisol, adrenaline and noradrenaline. Furthermore, adrenaline inhibits the release of insulin from the pancreas. Catecholamines, adrenaline, and noradrenaline increase glycogen breakdown in the muscles. In the liver, catecholamines, together with cortisol, increase blood glucose by activating glycogen breakdown and gluconeogenesis. Cortisol and catecholamines also enhance the mobilisation of free fatty acids from fat stores.In addition to efficient energy metabolism, the ability to exercise is highly dependent on the well-coordinated neuroendocrine control of cardiovascular function. Catecholamines increase oxygen delivery during exercise by enhancing cardiac output, splenic erythrocyte release, and skeletal muscle flow. Furthermore, cardiovascular homeostasis is maintained by changes in plasma renin activity and in plasma concentrations of atrial natriuretic peptide (ANP), arginine vasopressin, and aldosterone.     

Energy utilization and blood traits of ponies fed fat-supplemented diets

The digestibility and heat production values for three fats of different origin were determined. Four pony geldings (225 kg) were used in a study consisting of four successive digestion trials utilizing a 4 X 4 Latin square design. The four dietary treatments were basal alone and supplemented with 15% corn oil, blended fat or inedible tallow. The blended fat was composed of a mixture of animal and vegetable fats. A 7-d preliminary period preceded a 7-d total fecal collection period for each trial. Heat production values were obtained by indirect calorimetry and calculated from oxygen consumption data. Fat supplementation increased (P less than .05) dietary metabolizable energy from a basal value of 3,224 kcal.kg intake-1.d-1 to a mean value of 3,984 kcal.kg intake-1.d-1 for the three fat diets. No difference in heat production was observed among the diets, averaging 2,883 kcal.kg intake-1.d-1. Fats increased (P less than .05) the energy balance (metabolizable energy-heat production) approximately 88% over the basal. Corn oil and blended fat produced the greatest energy balance of the fats. Utilization of energy in fats, calculated by difference, was not different, but tended to be highest in blended fat and lowest in the corn oil. Apparent fatty acid digestibility increased (P less than .05) with the addition of fat to the basal, partially due to the dilution of endogenous fecal fat, but digestion coefficients were not different (P greater than .40) among the high fat diets.     

Reduction of renal function in exercising horses

Glomerular filtration rate (GFR) and extracellular volume (ECFV) were measured before, during and after treadmill exercise in 5 trained Thoroughbred horses (mean weight 483 kg). GFR/ECFV was determined by plasma disappearance of Tc-DTPA and ECFV was measured independently as thiocyanate space. Resting GFR averaged 1.6 l/min (3.3 ml/kg/min) and fell by over 40% during exercise, moreover the fall was severe even during the first walk, prior to trotting. The results suggest that rather than being protected, GFR is allowed to fall, even with mild exercise and that this is probably an adaptation to allow greater perfusion of muscle and skin. In man, GFR appears to be more resistant to the effects of exercise but it is hard to compare intensity of exercise between such different species.     

Protein synthesis in the liver and breast muscle of chicks fed on a high protein diet

1. The influence of feeding a high protein diet containing 400 g crude protein (CP)/kg on liver and breast muscle protein synthesis in chicks was investigated using a large dose injection of l‐[4 ‐³H]phenylalanine.

2. Liver weight of chicks fed on the high protein diet was significantly heavier than that of chicks fed on a control diet with 200 g CP/kg, and breast muscle weight was not affected by the treatment.

3. Fractional and absolute protein synthesis rates, RNA and protein contents, and RNA:protein ratio of breast muscle were not influenced by feeding the high protein diet.

4. Feeding the high protein diet decreased fractional synthesis rate of liver protein, an observation explained by reduced RNA:protein ratio. Liver protein content was elevated by feeding the high protein diet and this resulted in no change in the absolute synthesis rate of liver protein.     

Technique for continuous stride analysis of horses exercising on a treadmill

A technique for continuous computerised recording and analysis of the stride of horses on a treadmill is described. Advantages of the system are low cost, the use of normal shoes and the calculation of stride parameters in real time.     

Physiological and blood biochemical variables in horses exercising on a treadmill submerged in water

The blood lactate concentration (LA) and heart rate (HR) of 10 horses exercising in water on a treadmill were examined. With the water at 10 and 50% of the withers height (WH), the blood LA increased up to mean values around 1.9 mm during the standardized exercise test (SET) until after step 3 of 5 (each step lasted 5 min, speed increasing step by step). Thereafter, blood LA of horses remained constant, while with the water at 80% of WH, the blood LA decreased from the mean peak of 2.16 ± 0.62 mm after the 4th step. The HR of the horses increased to 132 ± 14 beats/min until the 3rd step of SET with the water at 10% of WH, up to the 2nd step with the water at 50% (134 ± 10 beats/min) and up to the 1st step only with the water at 80% of WH (134 ± 10 beats/min). In another SET, horses were exercised five times for 5 min at the maximal attainable speed of 5.5 m/s in water at 20% of the withers height in step 1, 35% in step 2, 49% in step 3, 63% in step 4 and 77% in step 5. On using this SET, blood LA increased to 1.91 ± 0.25 mm until after the 2nd step and decreased after the 3rd step. The HR increased between before commencing SET and the 1st step (143 ± 13 beats/min) and remained constant thereafter. In conclusion, increasing water height and speed of exercise does not augment continuously blood LA and HR of horses exercising in water treadmills.