Response of the hypothalamic-pituitary-adrenal axis to stimulation tests before and after exercise training in old and young Standardbred mares.

This study tested the hypotheses that age-induced alteration in cortisol, ACTH, and glucose concentrations are due to differences in the response of the hypothalamic-pituitary-adrenal axis and that exercise training would attenuate these differences. Six old (22.0±0.7 yr; mean±SE) and 6 young (7.3±0.6 yr) unfit Standardbred mares ran 3 graded exercise tests (GXT): before (GXT1), after 8 wk of training (GXT2), and at study end at 15 wk (GXT3). Mares trained 3 d/wk at 60% maximum heart rate. Each mare underwent 5 endocrine stimulation tests pre- and posttraining: 1) control (CON), 2) adrenocorticotropin hormone (ACTHtest), 3) combined dexamethasone suppression/ACTH (DEX/ACTH), 4) dexamethasone suppression (DEX), and 5) combined DEX/corticotropin releasing factor (DEX/CRF). For CON, there was no difference in plasma cortisol between age groups pretraining (P=0.19), but young mares had a 102% higher mean (P=0.02) plasma cortisol concentration than old mares posttraining. The pretraining ACTHtest showed young mares had a 72% higher (P=0.05) overall plasma cortisol concentration compared to old. There was no overall age difference in cortisol in the posttraining ACTHtest, but old mares still had lower cortisol concentrations at 30 min during the test, suggesting decreased adrenal response to ACTH stimulation. There was no difference in cortisol response between old and young mares in DEX, DEX/ACTH, or DEX/CRF tests. Young mares had higher (P=0.02) overall plasma cortisol concentration posttraining in response to DEX/ACTH, but old mares showed no change. In CON and DEX/CRF, there were no age differences in plasma ACTH concentration, pre- or posttraining. Pretraining, there was no age difference in glucose response to DEX, but posttraining old mares had a 4% (P=0.04) lower overall plasma glucose concentration compared to young. Posttraining, old mares had lower mean plasma glucose concentrations during DEX compared to pretraining (P=0.02), but there was no change pre- vs. posttraining in young mares (P=0.19). Old and young mares had lower plasma glucose concentrations posttraining during DEX/ACTH (P<0.001 and P=0.05, respectively) and DEX/CRF (P<0.001 and P=0.003, respectively) compared to pretraining. Both the pituitary and adrenal glands experience a decline in function with age although the exact mechanisms behind such changes remain unknown. Exercise training facilitates the counteraction of these deficits.

Metabolic changes in four beat gaited horses after field marcha simulation.

REASONS FOR PERFORMING STUDY: Mangalarga-Marchador is a popular 4-gaited Brazilian horse breed; however, there is little information about their metabolic and physiological response to exercise. OBJECTIVES: To measure physiological and metabolic responses of the Mangalarga-Marchador to a simulated marcha field test and to compare these responses between 2 types of marcha gaits (picada and batida). METHODS: Thirteen horses were used in the study and randomly assigned to either the picada or batida gait for the simulated marcha field test (speed ∼ 3.2 m/s; 30 min; load ∼ 80 kg). MEASUREMENTS: Included body composition, heart rate (HR), respiratory rate (RR), glucose (GLUC), lactate (LACT), packed cell volume (PCV), total plasma protein (TPP), albumin, urea, creatinine, total and HDL cholesterol, triglycerides, creatine kinase, alanine, glutamate and glutamine (GLN). Measurements were obtained pretest (control/fasting), immediately after simulation (T(0)), and 15 (T(15)), 30 (T(30)) and 240 (T(240)) min after the simulation. Lactate (LACT) was measured at T(0), T(15) and T(30). Data were analysed using ANOVA, Tukey's test and t tests with significance set at P < 0.05. RESULTS: Significant acute changes were observed in HR, RR, [GLUC], [LACT], [TPP], PCV and [GLN] (P<0.05) relative to control. Heart rate fell below 60 beats/min at T(15) and RR recovered to pretest values by T(240). Significant increases in [GLUC], [LACT], PCV and [TPP] and a decrease in [GLN] were observed at T(0). Treatment and interaction effects were also observed between marcha types and time of sampling for HR, RF, PCV, and [LACT] (P < 0.05). These parameters were large in picada. CONCLUSION: The simulation of field-test produced changes in some physiological and blood parameters in marcha horses, with some degree of dehydration during recovery period. Also, it was demonstrated that picada horses spend more energy when compared with batida horses at the the same speed. POTENTIAL RELEVANCE: Batida horses spend less energy when compared with picada horses, which will need special attention in their training and nutritional management.

Age related decreases in thermoregulation and cardiovascular function in horses.

REASON FOR PERFORMING STUDY: Older horses have an increased risk of hyperthermia due to impaired cardiovascular function. While many studies have investigated thermoregulation in horses during exercise, none have investigated the effects of ageing. OBJECTIVE: To test the hypothesis that there is a difference in thermoregulation during exercise and plasma volume (PV) in young and old horses. METHODS: Study 1: 6 young (Y, 7.7 ± 0.5 years) and 5 old (O, 26.0 ± 0.8 years) unfit Standardbred mares (507 ± 11 kg, mean ± s.e.) ran on a treadmill (6% grade, velocity calculated to generate a work rate of 1625 watts) until core temperature reached 40 °C. Core (CT), skin (ST), rectal temperature (RT) and heart rate (HR) were measured every min until 10 min post exertion. Packed cell volume (HCT), lactate (LA) and plasma protein (TP) were measured in blood samples collected before, at 40 °C and every 5 min until 10 min post exercise. Sweat loss was estimated using bodyweight. Study 2: Plasma volume was measured in 26 young (8.2 ± 0.7 years) and 8 old (26.6 ± 0.7 years) Standardbred mares (515 ± 12 kg) using Evans Blue dye. Pre-exercise blood (rBV) and red cell (rRCV) volumes were calculated using PV and HCT. Data analysis utilised repeated measures ANOVA and t tests and data are expressed as mean ± s.e. RESULTS: Old horses reached 40 °C faster (998 ± 113 vs. 1925 ± 259 s; P < 0.05) with a greater HR at 40 °C (184 ± 6 vs. 140 ± 5 beats/min; P < 0.05) and greater sweat losses (P < 0.05). Heart rate did not differ (P > 0.05) post exercise. Age did not alter (P > 0.05) CT, ST, RT, LA, HCT or TP. Plasma volume was greater in Y vs. O horses (P < 0.05, 28.5 ± 1.4 vs. 24.1 ± 1.6 l) as was rBV (41.3 ± 2.0 vs. 35.3 ± 2.3 l) and rRCV (13.3 ± 0.6 vs. 11.1 ± 0.8 l). CONCLUSION: Ageing compromises the ability to handle the combined demand of exercise and thermoregulation in part due to decreased absolute pre-exercise PV.

Exercise-induced increases in inflammatory cytokines in muscle and blood of horses.

REASONS FOR PERFORMING STUDY: Studies have demonstrated increases in mRNA expression for inflammatory cytokines following exercise in horses and have suggested those markers of inflammation may play a role in delayed onset muscle soreness. However, measurement of mRNA expression in white blood cells is an indirect method. No studies to date have documented the cytokine response to exercise directly in muscle in horses. HYPOTHESIS: This study tested the hypothesis that exercise increases cytokine markers of inflammation in blood and muscle. METHODS: Blood and muscle biopsies were obtained from 4 healthy, unfit Standardbred mares (∼ 500 kg). The randomised crossover experiment was performed with the investigators performing the analysis blind to the treatment. Each horse underwent either incremental exercise test (GXT) or standing parallel control with the trials performed one month apart. During the GXT horses ran on a treadmill (1 m/s increases each min until fatigue, 6% grade). Blood and muscle biopsies were obtained 30 min before exercise, immediately after exercise and at 0.5, 1, 2, 6 and 24 h post GXT or at matched time points during the parallel control trials. Samples were analysed using real time-PCR for measurement of mRNA expression of interferon-gamma (IFN-gamma), tumour necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6) and interleukin-1 (IL-1). Data were analysed using t tests with the null hypothesis rejected when P < 0.10. RESULTS: There were no changes (P > 0.10) in IL-1, IL-6, IFN-gamma or TNF-alpha during control. Exercise induced significant increases in IFN-gamma, IL1 and TNF-alpha in blood and significant increases in IFN-gamma, IL-6 and TNF-alpha in muscle. There were no significant changes in mRNA expression of IL-1 in muscle or IL-6 in blood following the GXT. These cytokine markers of inflammation all returned to preGXT levels by 24 h post GXT. CONCLUSION: High intensity exercise results in a transient increase in the expression of inflammatory cytokines in muscle and blood.

Effect of sucralfate on total carbon dioxide concentration in horses subjected to a simulated race test.

The purpose of this study was to test the hypothesis that sucralfate, a gastric ulcer medication, would alter plasma concentrations of total carbon dioxide (tCO2), lactate (LA), sodium (Na+), potassium (K+), chloride (Cl-) and total protein (TP), as well as calculated plasma strong ion difference (SID) and packed cell volume (PCV) in horses subjected to a simulated race test (SRT). Six unfit Standardbred mares (approximately 520 kg, 9-18 years) were used in a randomized crossover design with the investigators blinded to the treatment given. The horses were assigned to either a control (40-50 mL apple sauce administered orally (PO)) or a sucralfate (20 mg/kg bodyweight dissolved in 40-50 mL apple sauce administered PO) group. Each horse completed a series of SRTs during which blood samples were taken via jugular venipuncture at five sampling intervals (prior to receiving treatment, prior to SRT, immediately following exercise, and at 60 and 90 min post-SRT). During the SRTs, each horse ran on a treadmill fixed on a 6% grade for 2 min at a warm-up speed (4 m/s) and then for 2 min at a velocity predetermined to produce VO2max. Each horse then walked at 4 m/s for 2 min to complete the SRT. Plasma tCO2, electrolytes, LA, and blood PCV and TP were analysed at all intervals. No differences (P>0.05) were detected between control and sucralfate for any of the measured variables. There were differences (P<0.05) in tCO2, SID, PCV, TP, LA and electrolyte concentrations relative to sampling time. However, these differences were attributable to the physiological pressures associated with acute exercise and were not an effect of the medication. It was concluded that sucralfate did not alter plasma tCO2 concentration in this study.

Developmental changes in the concentrations of glutamine and other amino acids in plasma and skeletal muscle of the Standardbred foal.

Glutamine is concentrated within skeletal muscle, where it has been proposed to play a regulatory role in maintaining protein homeostasis. The work presented here addressed the hypothesis that glutamine would be the most abundant free alpha-AA in plasma and skeletal muscle in the foal during the first year of life. Glycine, however, was the most abundant free alpha-AA in plasma at birth and between 3 and 12 mo of age. The concentration of glutamine, the second most abundant AA at birth, increased through the first 7 d (P < 0.05) and then returned to values similar to those at birth. This resulted in glutamine being the most abundant free alpha-AA in plasma from 1 d through 1 mo of age. The most abundant free alpha-AA in skeletal muscle at birth was glutamine, but the concentration fell by more than 50% by d 15 and continued to decrease, reaching about one-third of the original values by 1 yr of age (P < 0.05). Glutamine synthetase was barely detectable in skeletal muscle at birth, but the abundance increased rapidly within 15 d of birth. The concentration of glycine, the second most abundant alpha AA in muscle at birth, decreased by about 40% by d 15 (P < 0.05) and then stabilized at this value throughout the year. In contrast, glutamate, alanine, and serine concentrations, the third, fourth, and fifth most abundant free alpha-AA in muscle at birth, respectively, increased to new stable concentrations between 3 and 6 mo of age (P < 0.05). This resulted in alanine being the most abundant free alpha-AA in skeletal muscle at 12 mo of age, followed by glutamate, glutamine, and glycine. The decrease in intramuscular glutamine content, particularly during the first 2 wk after birth, is not compatible with a regulatory role for glutamine in muscle protein synthesis because it occurred at the time of maximum growth in these animals. The findings that, at certain times of development, glutamine was not the most abundant free alpha-AA in the foal is novel and signifies that intramuscular glutamine may have functions specific to muscle type and mammalian species.

Changes in glutamine metabolism indicate a mild catabolic state in the transition mare.

Glutamine is the most abundant free alpha-AA in the mammalian body, and large amounts of glutamine are extracted by both the fetus during pregnancy and the mammary gland during lactation. The work presented here addressed the hypothesis that there would be major changes in glutamine metabolism in the mare during the transition period, the time between late gestation, parturition, and early lactation. Eight foals were born to Standardbred mares provided with energy and protein at 10% above NRC recommendations, and foals remained with mares for 6 mo. During lactation, lean body mass decreased by 1.5% (P < 0.05), whereas fat mass was unchanged throughout gestation and lactation. There was a sharp increase in the concentration of most plasma metabolites and hormones after birth, which was due in part to hemoconcentration because of fluid shifts at parturition. Plasma glutamine concentration, however, was maintained at greater concentrations for up to 2 wk postpartum but then began to decrease, reaching a nadir at approximately 6 wk of lactation. Skeletal muscle glutamine content did not change, but glutamine synthetase expression was decreased at the end of lactation (P < 0.05). Free glutamine was highly abundant in milk early in lactation, but the concentration decreased by more than 50% after 3 mo of lactation and paralleled the decrease in plasma glutamine concentration. Thus, lactation represents a mild catabolic state for the mare in which decreased glutamine concentrations may compromise the availability of glutamine to other tissues such as the intestines and the immune system.

Apoptosis and antioxidant status are influenced by age and exercise training in horses.

Eight mature (12 +/- 2 yr; MAT) and 5 older (22 +/- 2 yr; OLD) Standardbred mares were used to test the hypothesis that aging and exercise training would alter apoptosis in white blood cells and antioxidant status. The horses were housed indoors overnight (16 h/d) in 3 m x 3 m stalls and were turned out in a drylot during the day. They were fed a diet consisting of total mixed ration, hay cubes fed ad libitum or an equine senior diet plus grass hay. Horses were trained for 20 to 30 min/d, 3 to 5 d/wk for 8 wk at a submaximal work intensity between 60 to 70% of maximal heart rate. A graded exercise test (GXT; stepwise test until exhaustion) was performed before (GXT1) and after (GXT2) the 8 wk of training. During the GXT, blood samples and heart rate were taken at rest, 6 m/s, fatigue, and at 5 and 60 min postfatigue. Fatigue plasma lactate concentration was greater in MAT (19.3 +/- 1.5 at 10 m/s) compared with the OLD (10.9 +/- 1.2 mmol/L at 9 m/s; P = 0.008) horses. There was no effect of age or training on plasma lipid hydroperoxide (LPO) concentration. However, there was a positive correlation between LPO and plasma lactate concentration (r = 0.27, P = 0.006) during acute exercise. There was a greater concentration of total glutathione after GXT1 than after GXT2 (111.8 +/- 5.0 vs. 98.6 +/- 3.4 microM, respectively; P = 0.0002) for both age groups. Apoptosis was less (P = 0.002) in white blood cells of the MAT vs. the OLD group. These results demonstrate that older horses are under similar amounts of oxidative stress, measured by LPO, and have similar levels of glutathione in their systems compared with mature horses. The observation that more glutathione was needed during GXT1 for both groups of horses indicates that training helps horses adapt their system for the intense post-training exercise tests. The greater level of white blood cell apoptosis also indicates that older horses may be immune-compromised during exercise. However, research still needs to be performed regarding dietary supplementation in the aged horse.

Oral and intravenous carbohydrate challenges decrease active ghrelin concentrations and alter hormones related to control of energy metabolism in horses.

This study tested the hypothesis that grain and intravenous dextrose challenges would alter plasma concentrations of active ghrelin, adiponectin, leptin, glucose, insulin, and cortisol in Standardbred mares. To deliver 0.5 g of glucose (dextrose solution for the intravenous test)/kg of BW, mares received intravenous dextrose (50% solution) or oral grain administration in 2 trials. In response to the oral grain challenge, plasma glucose and insulin concentrations increased (P < 0.001) by 56 and 802%, respectively. Plasma ghrelin concentration initially decreased (P < 0.001) by 40%, then subsequently increased (P < 0.05) from its nadir by 259%. Plasma leptin concentration decreased (P = 0.002) 17% compared with baseline. There was no change (P = 0.34) in plasma adiponectin concentration in response to oral grain challenge; however, plasma cortisol concentrations decreased (P < 0.001) by 24%. In response to the intravenous dextrose challenge, plasma glucose and insulin concentrations increased (P < 0.001) by 432 and 395%, respectively. Plasma active ghrelin concentration initially decreased (P < 0.001) by 56%, then subsequently increased (P < 0.001) from its nadir by 314%. Plasma leptin concentration also increased (P < 0.001) by 33% compared with baseline. There was no change (P = 0.18) in plasma adiponectin concentration throughout the dextrose challenge. Plasma cortisol concentration increased (P = 0.027) by 20%. Hence, oral grain and intravenous nutrient challenges have the ability to alter variables potentially related to energy metabolism in mares, with acute changes in glucose and insulin possibly modulating changes in ghrelin and leptin.

Retrospective study of predictive variables for maximal heart rate (HRmax) in horses undergoing strenuous treadmill exercise.

REASONS FOR PERFORMING STUDY: Heart rate is one of the most commonly measured variables in equine exercise physiology and relative exercise intensity commonly expressed as % of maximal heart rate. A number of influences affect maximal heart rate (HRmax), including age of the horse but other factors have not been described. OBJECTIVES: To determine if fitness, health status, gender, breed, athletic use, body mass, in addition to age, are predictive of HRmax in the horse. METHODS: Maximal heart rate data from 328 horses which underwent treadmill exercise tests at 5 different laboratories were obtained retrospectively. Univariable linear regression analyses were performed on individual variables. Multiple linear regression analysis using a backward elimination modelling procedure was then used to relate the observed HRmax values simultaneously with different predictive variables. Variables were retained in the final regression model if they or any of their categories were significantly predictive of HRmax at P<0.05 and if there was a significant collective contribution to the model from inclusion of each variable, also at P<0.05. RESULTS: Age, fitness status, laboratory, gender and breed/use (combined category) were all statistically significantly predictive of HRmax. Together these variables accounted for 41% of the variance in HRmax. Age alone accounted for only approximately 13% of the variation between horses in HRmax. Neither body mass nor health status were significantly predictive. CONCLUSIONS: HRmax in the horse declines with age but is also influenced by other factors. As the factors investigated accounted for only 41% of the variation between horses, other unidentified variables with a strong influence on HRmax remain to be identified. POTENTIAL RELEVANCE: Factors such as fitness, age, gender, breed and use need to be considered when interpreting estimates or measurements of HRmax.

Low dose exogenous erythropoietin elicits an ergogenic effect in standardbred horses.

REASONS FOR PERFORMING STUDY: Recombinant human erythropoietin (rhuEPO) causes an increase in red blood cell production and aerobic capacity in other species; however, data are lacking on effects in the horse. HYPOTHESIS: This study tested the hypothesis that rhuEPO administration would alter red cell volume (RCV), aerobic capacity (VO2max) and indices of anaerobic power. METHODS: Eight healthy, unfit mares accustomed to the laboratory and experimental protocols were randomly assigned to either a control (CON, n = 4; 3 ml saline 3 times/week for 3 weeks) or EPO group (EPO, n = 4, 50 iu/kg bwt rhuEPO/3 ml saline 3 times/week for 3 weeks). Exercise tests (GXT) were performed on a treadmill (6% incline), 1 week before and 1 week after treatment. The GXT started at 4 m/sec, with a 1 m/sec increase every 60 sec until the horse reached fatigue. Oxygen uptake was measured via an open flow indirect calorimeter. Blood samples were collected before, during (each step) and 2 and 15 min post GXT to measure packed cell volume (PCV), haemoglobin concentration (Hb), blood lactate concentration (LA) and plasma protein concentration (TP). Plasma volume (PV) was measured using Evans Blue dye. Blood volume (BV) and RCV were calculated using PCV from the 8 m/sec step of the GXT. RESULTS: There were no alterations (P>0.05) in any parameters in CON horses. By week 3, EPO produced increases (P<0.05) in resting PCV (37 +/- 2 vs. 51 +/- 2) and Hb (37%). RCV (26%) and VO2max (19%) increased, but BV did not change (P>0.05) due to decreased PV (-11%, P<0.05). There was a significant increase in velocity at VO2max and LApeak for horses treated with rhuEPO and substantial decrease (P<0.05) in VO2 recovery time when the pretreatment GXT was compared to the post treatment GXT. No differences (P<0.05) were detected for TP, VLA4, run time or Vmax. CONCLUSIONS: Low dose rhuEPO administration increases RCV and aerobic capacity without altering anaerobic power. POTENTIAL RELEVANCE: This study demonstrates that rhuEPO enhances aerobic capacity and exercise performance, a question relevant to racing authorities.

Plasma beta-endorphin, cortisol and immune responses to acute exercise are altered by age and exercise training in horses.

REASONS FOR PERFORMING STUDY: Ageing appears to affect immune and neuroendocirne function in horses and response to acute exercise. No studies have examined the combined effects of training and ageing on immune and neuroendocirne function in horses. HYPOTHESIS: To ascertain whether training and age would affect the plasma beta-endorphin (BE) and cortisol (C) as well as immune function responses to acute exercise in Standardbred mares. METHODS: Graded exercise tests (GXT) and simulated race tests (SRT) were performed before and after 12 weeks training at 60 % HRmax. BE and C were measured at rest and at 5, 10, 20, 40, 60 and 120 min post GXT. Leucocyte cell number, CD4+ and CD8+ lymphocyte subsets, and mitogen stimulated lymphoproliferative response (LPR), were measured in jugular blood before and after the SRTs. RESULTS: Cortisol rose by 5 min post GXT in young (Y) and middle-age (MA) mares (P<0.05) and remained elevated until 40 and 60 min post GXT, respectively during both pre- and post training GXT. There was no rise in C in old (0) mares after either GXT (P>0.05). Pretraining BE rose (P<0.05) by 5 min post GXT in all mares. After training, BE was higher in Y and O vs. MA (P<0.05) at 5 min post GXT. Post training BE was higher at 5 min post GXT in Y and O vs. pretraining (P<0.05). After SRT, lymphocyte number rose in all mares (P<0.05); however, lower lymphocyte numbers (P<0.05) were seen in MA vs. Y and O vs. MA (P<0.05). The O had reduced LPR to Con A and PHA stimulation (P<0.05) compared to Y and MA after the SRT after both pre- and post training SRT. LPR to PWM was lower (P<0.05) in O vs. Y and MA after the pretraining SRT. Training caused an increase in resting LPR to PWM in MA only (P<0.05). CONCLUSION: Both age and training altered the plasma beta-endorphin and cortisol responses as well as and immune responses to acute exercise. POTENTIAL RELEVANCE: This study provides important information on the effects of ageing and training that will aid in the management and care of an increasing number of active older horses.

Muscle, tendon, and somatotropin responses to the restriction of muscle blood flow induced by KAATSU-walk training.

OBJECTIVE: The efficacy of KAATSU training has been demonstrated in human athletes, both as a therapeutic method as well as a training aid. The purpose of this study was to investigate the effects of slow walk training combined with restriction of muscle blood flow (KAATSU) on muscle and tendon size. METHODS: Six healthy, unfit Standardbred mares performed walking (240 m/min for 10 min and then 5 min recovery) with KAATSU, and 6 mares performed walking without KAATSU. A specially designed elastic cuff1 was placed at the most proximal position of the forelegs and inflated to a pressure of 200-230 mmHg throughout the walking and recovery sessions. The training was conducted once a day, 6 days/week for 2 weeks. Skeletal muscle thickness and tendon thickness were measured using B-mode ultrasound at baseline and after 2 weeks of training. Venous blood samples were obtained before the first acute exercise and 5, 15 and 60 min afterwards. Serum somatotropin concentration was determined using a commercially available equine-specific ELISA kit. RESULTS: The acute increase in plasma somatotropin was 40% greater (P<0.05) in the KAATSU-walk group than in the Control-walk group 5 min after exercise and remained elevated (P<0.05) at 15 and 60 min post exercise compared with the Control-walk group. After 2 weeks of training, muscle thickness increased (P<0.05) 3.5% in the KAATSU-walk group but did not change in the Control-walk group (0.7%). Tendon thickness did not change (P>0.05) in either group. CONCLUSIONS: These data demonstrate that KAATSU training can induce muscle hypertrophy in horses and suggest that KAATSU training may provide significant therapeutic/ rehabilitative value in horses, as has been shown in man.

Interval exercise alters feed intake as well as leptin and ghrelin concentrations in standardbred mares.

REASONS FOR PERFORMING STUDY: Horses in training tend to become inappetant; however, the mechanism responsible for this training-induced inappetance is not known. HYPOTHESIS: Training and/or ulcers alter the feed intake (FI) and hormonal and/or biochemical (active ghrelin, leptin, glucose, insulin and cortisol) responses to acute high intensity exercise. METHODS: Eight Standardbred mares underwent 3 interval exercise tests (IET) and 3 parallel control tests (CON) before (IET1) and after 8 weeks of training (IET2) and after treatment for gastric ulcers (IET3). Plasma samples were taken before (0 min), during (last 10 sec of velocities eliciting 40, 100 and 20% VO2max), and after (30 min, 60 min, 24 h) exercise (EX) or CON tests for RIA and colorimetric measurement of the concentrations of the above parameters. Samples were also collected before and after feeding. Horses were trained at a work intensity of 70% HRmax for 30 min/day, 5 days per week with FI measured daily. RESULTS: There were no changes (P>0.05) in any variable during the parallel control trials. However, there was a mismatch between FI and digestible energy (DE) requirements (P<0.05) with EX horses not meeting their DE requirements during the post training IETs. During all IETs, ghrelin, glucose and cortisol increased (P<0.05) during EX. Leptin only increased (P<0.05) during EX in the post training IETs. Insulin remained low during EX, but increased (P<0.05) post EX. CONCLUSION: High intensity exercise appeared to be associated with decreases in FI and alterations of leptin and ghrelin. POTENTIAL RELEVANCE: More research is needed to determine if there is a relationship between alterations of these hormones and changes in FI in horses that lose weight while in training.

Effect of omeprazole on markers of performance in gastric ulcer-free standardbred horses.

REASONS FOR PERFORMING STUDY: A large percentage of performance horses develop gastric ulcers and many of those horses are treated with omeprazole. Unfortunately, no data have been published on the effects of the drug on markers of performance in animals without ulcers. HYPOTHESIS: Omeprazole would alter markers of aerobic and anaerobic performance. METHODS: Ten unfit, healthy, ulcer free, Standardbred mares were administered either control (CON; oral apple sauce, 20 ml) or omeprazole (OP; oral paste, 4 mg/kg bwt s.i.d.) in a random crossover fashion with the investigators blind to the treatment. Treatments were administered for 7 days prior to performing an incremental exercise test (GXT) on a high-speed treadmill. Endoscopic examinations were performed just prior to the trial to verify that the mares were ulcer-free. During the GXT, the mares ran on a treadmill up a 6% grade to measure maximal oxygen consumption (VO2max), run time (RT), velocity at VO2max, maximal velocity (Vmax), packed cell volume (PCV), plasma lactate concentration (LA) and plasma protein concentration (TP). Measurements were recorded at rest, at the end of each 1 min step of the GXT and at 2 and 5 min post GXT. Data were analysed using ANOVA for repeated measures and t tests for paired comparisons. RESULTS: There was no effect (P>0.05) of omeprazole on VO2max; velocity at VO2max; RT; Vmax; 2 min recovery plasma LA. Nor were there any changes (P>0.05) in the relationship between treadmill speed and VO2, PCV, TP, or plasma LA. CONCLUSIONS: Omeprazole does not appear to improve physiological markers of performance in healthy, ulcer free horses. POTENTIAL RELEVANCE: These data may benefit various authorities responsible for deciding administration and timing policies of omeprazole as well as clinicians and horse owners.

Diurnal variation of ghrelin, leptin, and adiponectin in Standardbred mares.

Twelve Standardbred mares underwent blood sampling for 24 h to test the hypothesis that there is diurnal variation of humoral mediators of peripheral energy balance including active ghrelin, adiponectin, leptin, glucose, insulin, and cortisol. The experiment was conducted under acclimated conditions. Grass hay and pelleted grain were provided at 0730 and 1530. Plasma concentrations of active ghrelin and leptin concentrations both peaked (47.3 +/- 6.5 pg/ mL and 5.9 +/- 1.1 ng/mL, respectively; P < 0.05) at 1550, 20 min after feeding. Active ghrelin decreased (P < 0.05) to 28.9 +/- 4.5 pg/mL overnight. The nadir of leptin (4.6 +/- 0.9 ng/mL) occurred at 0650. Neither hormone showed variation (P > 0.05) after the morning feeding. Plasma glucose and insulin concentrations increased (P < 0.05) in response to feeding; however, the morning responses (glucose = 96.9 +/- 2.6 mg/dL; insulin = 40.6 +/- 7.3 uIU/mL) were greater (P < 0.05) than the afternoon responses (glucose = 89.9 +/- 1.8 mg/dL; insulin = 23.2 +/- 4.3 uIU/mL at 180 and 60 min after feeding, respectively). Cortisol concentrations increased (P < 0.05) during the morning hours, but did not respond to feeding, whereas adiponectin concentrations remained stable throughout the study. Hence, active ghrelin and leptin may be entrained to meal feeding in horses, whereas adiponectin seems unaffected. We concluded that there seems to be a diurnal variation in glucose and insulin response to a meal in horses. Furthermore, elevated glucose and insulin concentrations resulting from the morning feeding may be responsible for the increase in leptin concentration in the afternoon.

Effect of chronic clenbuterol administration and exercise training on immune function in horses.

Effects of longitudinal exercise training and acute intensive exercise (simulated race test) on immune function have not been reported in horses. Clenbuterol, a beta2-adrenergic agonist, is used to manage inflammatory airway disease in horses. This study investigated the interaction of 8 wk of exercise training with or without 12 wk of clenbuterol administration in horses. Twenty-three untrained standardbred mares (10 +/- 3 yr, Mean +/- SE) were used and divided into four experimental groups. Horses given clenbuterol plus exercise (CLENEX; n = 6) and clenbuterol alone (CLEN; n = 6) received 2.4 microg/kg BW of clenbuterol twice daily (in an average volume of 20 mL) on a schedule of 5 d on and 2 d off for 12 wk. The CLENEX group was also aerobically trained 3 d/wk. Mares given exercise alone (EX; n = 5) were aerobically trained for 3 d/wk, and the control group (CON; n = 6) remained sedentary. Both EX and CON horses were administered similar volumes (approximately 20 mL) of molasses twice daily. A simulated race test (SRT) resulted in an elevation in lymphocyte number postexercise (P < 0.05). There was no significant difference after acute exercise in either monocyte or granulocyte number. Acute exercise resulted in a decrease (P < 0.05) in the percentage of CD4+ and an increase (P < 0.05) in the percentage of CD8+ cells. The SRT resulted in a decreased lymphoproliferative response to pokeweed mitogen (P < 0.05). A SRT had no effect on antibody production in response to equine influenza vaccine. The EX group demonstrated greater cortisol concentrations at rest and at all other time points postexercise after completing the training regimen compared with CLENEX horses (P < 0.05). Preexercise (SRT) peripheral blood monocyte number was lower in CLENEX horses than in other treatment groups (P < 0.05). Clenbuterol and exercise training did not significantly affect post-SRT changes in leukocyte numbers. Exercise training resulted in a decrease (P < 0.05) in the percentage of CD8+ cells post-SRT compared with other groups, but the percentage of CD4+ cells was not altered by either clenbuterol or exercise conditioning. Lymphocyte proliferative response was not affected by clenbuterol or exercise treatment. Horses demonstrated responses to bouts of acute exercise as noted with other species, namely humans and rodents.

Chronic clenbuterol administration alters myosin heavy chain composition in standardbred mares.

The purpose of this study was to examine changes in myosin heavy chain (MHC) composition due to chronic clenbuterol administration with or without exercise in mares. Unfit Standardbred mares (aged 10+/-3 years) were divided into four groups: clenbuterol (2.4 micro/kg BW twice daily) plus exercise (3 days/week for 20 min at 50% VO(2max); CLENEX; n=6), clenbuterol only (CLEN; n=6), exercise only (EX; n=5), and control (CON; n=6). Muscle biopsies were obtained from gluteus medius muscle before and after the eight-week training/administration period. MHC composition was determined via SDS gel electrophoresis and quantified using a scanning and densometric system. CLENEX and CLEN exhibited significant (P<0.05) MHC changes while EX and CON did not. MHC type IIA decreased (29.8+/-6.1 to 19.3+/-4.0%, CLENEX; and 36.8+/-12.4 to 26.4+/-7.9%, CLEN) and MHC type IIX increased (59.4+/-7.2 to 71.8+/-5.8%, CLENEX; and 50.5+/-12.5 to 62.0+/-9.3%, CLEN). Chronic clenbuterol administration with and without exercise resulted in a significant shift in MHC profile in Standardbred mares.

Overview of horse body composition and muscle architecture: implications for performance.

Locomotion requires skeletal muscle to sustain and generate force. A muscle's force potential is proportional to its weight. Since the larger the muscle the larger its potential power output, a better understanding of the proportion of skeletal muscle a horse possesses may lead to a better understanding of horse performance. Several techniques exist to assess body composition, which include dual energy X-ray absorption, underwater (hydrostatic) weighing, derivation from total body water, bio-electric impedance, air displacement, body condition scoring, cadaver dissection and ultrasound. The relevance of each method to the equine industry will be discussed as will the practical information that the existing horse body composition studies have provided. Attention will be given to the data regarding the implications of body composition on the performance horse. The limited number of studies discussing different varieties of muscle architectures and the functional importance of these muscles will also be addressed. These body composition data may provide a better understanding of important issues in horse care that can lead to more optimal horse care techniques and a healthier and safer environment for horses.

Effects of ageing and training on maximal heart rate and VO2max.

The purpose of this study was to test the hypotheses that ageing would result in a decline in maximal heart rate (HRmax) and maximal aerobic capacity (VO2max) and, secondarily, that those effects would be reversible with training. Eighteen, healthy, unfit Standardbred mares representing 3 age groups: young (Y = mean +/- s.e. 6.8 +/- 0.4 years, n = 6); middle-aged (MA = 15.2 +/- 0.4 years, n = 6); and old (O = 27.0 +/- 0.2 years, n = 6) were used. HRmax, VO2max and oxygen pulse at VO2max (OPmax) and the velocities producing HRmax (VHRmax) and VO2max (VVO2max) were measured during pretraining and post-training incremental exercise tests (GXT). During training, mares exercised 3 days/week (Weeks 1-8) and 4 days/week (Weeks 9-12) at a submaximal intensity (approximately 60% HRmax) for approximately 30 min/day. There were no differences (P>0.05) between Y and MA, before (218 +/- 2 vs. 213 +/- 3 beats/min; 116 +/- 3 vs. 109 +/- 3 ml/kg bwt/min; 0.55 +/- 0.01 vs. 0.52 +/- 0.02 ml/kg/beat; 9.0 +/- 0.3 vs. 9.3 +/- 0.2 ms; 8.8 +/- 0.2 vs. 8.8 +/- 0.2 m/s) or after training (224 +/- 2 vs. 218 +/- 2 beats/min; 131 +/- 3 vs. 120 +/- 2 ml/kg bwt/min; 0.58 +/- 0.01 vs. 0.55 +/- 0.01 ml/kg/beat; 10.5 +/- 0.2 vs. 9.5 +/- 0.1 ms; 10.6 +/- 0.2 vs. 9.5 +/- 0.1 m/s) for HRmax, VO2max, OPmax, VHRmax or VVO2max, respectively. Old horses had lower HRmax, VO2max and OPmax and reached them at lower velocities compared to Y and MA (P<0.05), both before (193 +/- 3 beats/min; 83.2 +/- 2.0 ml/kg bwt/min; 0.43 +/- 0.01 ml/kg/beat; 7.8 +/- 0.1 m/s; 7.2 +/- 0.1 m/s) and after training (198 +/- 2 beats/min; 95 +/- 2 ml/kg bwt/min; 0.48 +/- 0.01 ml/kg/beat; 8.2 +/- 0.2 m/s; 8.0 +/-0.2 m/s). Training did not alter HRmax in any age group (P>0.05) but did cause increases in VO2max, OPmax and VVO2max for all groups (P<0.05). Interestingly, training increased VHRmax only in Y (P<0.05). These data demonstrate that there is a reduction in HRmax, VO2max, OPmax, VHRmax and VVO2max in old horses, and that training can partially reverse some effects of ageing.