Objective—To determine the effects of initial handling
and training on autonomic nervous functions in
Animals—63 healthy Thoroughbreds.
Procedure—All horses were trained to be handled
and initially ridden in September of the yearling year
and then trained until the following April by conventional
training regimens. To obtain the heart rate (HR),
electrocardiograms were recorded in the stable
before initial handling and training and following 7
months of training; variations in HR were then evaluated
from the power spectrum in terms of the low frequency
(LF; 0.01 to 0.07 Hz) power and high frequency
(HF; 0.07 to 0.6 Hz) power as indices of autonomic
nervous activity. To evaluate the fitness, the V200
(velocity at HR of 200 beat/min), which is reflective of
the aerobic capacity of the horse, was measured.
Results—Mean (± SE) resting HR decreased significantly
from 41.5 ± 0.8 to 38.7 ± 0.4 beat/min following
7 months of training. The LF power of horses
increased significantly from 1,037 ± 128 milliseconds2
in September of the yearling year to 2,944 ± 223 milliseconds2
in the following April. Similarly, the HF
power increased significantly from 326 ± 30 milliseconds2
to 576 ± 39 milliseconds2 at the corresponding
time points. The V200 increased significantly following
Conclusions and Clinical Relevance—Increases in
LF and HF powers indicate that parasympathetic nervous
activity increases in horses by 7 months of training.
The decrease in resting HR may be dependent on
the training-induced increase of parasympathetic nervous
activity in Thoroughbreds. (Am J Vet Res
OBJECTIVE To determine cardiorespiratory responses of Thoroughbreds to uphill and downhill locomotion on a treadmill at identical gradients.
ANIMALS 5 highly trained Thoroughbred geldings.
PROCEDURES Thoroughbreds were exercised for 2-minute intervals on a treadmill at 1.7, 3.5, 6.0, 8.0, and 10.0 m/s at a 4% incline, 0% incline (horizontal plane), and 4% decline in random order on different days. Stride frequency, stride length, and cardiopulmonary and O2-transport variables were measured and analyzed by means of repeated-measures ANOVA and Holm-Šidák pairwise comparisons.
RESULTS Horses completed all treadmill exercises with identical stride frequency and stride length. At identical uphill speeds, they had higher (vs horizontal) mass-specific O2 consumption (mean increase, 49%) and CO2 production (mean increase, 47%), cardiac output (mean increase, 21%), heart rate (mean increase, 11%), and Paco2 (mean increase, 1.7 mm Hg), and lower Pao2 (mean decrease, 5.8 mm Hg) and arterial O2 saturation (mean decrease, 1.0%); tidal volume was not higher. Downhill locomotion (vs horizontal) reduced mass-specific O2 consumption (mean decrease, 24%), CO2 production (mean decrease, 23%), and cardiac output (mean decrease, 9%). Absolute energy cost during uphill locomotion increased linearly with speed at approximately twice the rate at which it decreased during downhill locomotion.
CONCLUSIONS AND CLINICAL RELEVANCE Findings suggested that for Thoroughbreds, downhill locomotion resulted in a lower energy cost than did horizontal or uphill locomotion and that this cost changed with speed. Whether eccentric training induces skeletal muscle changes in horses similar to those in humans remains to be determined.
Objective—To evaluate the effects of a single incremental exercise test (IET) on mRNA expression and protein content of monocarboxylate transporter (MCT) 1 and MCT4 in the gluteus medius muscle of Thoroughbreds.
Animals—12 Thoroughbreds (6 males and 6 females; age, 3 to 4 years).
Procedures—Horses underwent an IET before and after 18 weeks of high-intensity exercise training (HIT). Horses were exercised at 90% of maximal oxygen consumption for 3 minutes during the initial 10 weeks of HIT and 110% of maximal oxygen consumption for 3 minutes during the last 8 weeks of HIT. Gluteus medius muscle biopsy specimens were obtained from horses before (baseline), immediately after, and at 3, 6, and 24 hours after the IET.
Results—Expression of MCT1 and MCT4 mRNA was upregulated at 3 and 6 hours after the IET in muscle specimens obtained from horses prior to HIT (untrained horses) and at 6 hours after the IET in muscle specimens obtained from horses after HIT (trained horses). For both untrained and trained horses, MCT1 and MCT4 protein contents were increased at 6 hours after the IET and did not differ at 24 hours after the IET, compared with those at baseline.
Conclusions and Clinical Relevance—Results indicated that a single IET resulted in transient increases in MCT1 and MCT4 mRNA expression and protein content in untrained and trained horses. These results may be important for the elucidation of exercise-induced alterations in lactate metabolism.
Objective—To determine whether evaluation of heart rate (HR) and HR variability (HRV) during prolonged road transportation in horses provides a sensitive index of autonomic stimulation.
Animals—Five 2-year-old Thoroughbreds.
Procedure—ECGs were recorded as horses were transported for 21 hours in a 9-horse van. Heart rate, high-frequency (HF) power, low-frequency (LF) power, and LF-to-HF ratio from Fourier spectral analyses of ECGs were calculated and compared with values recorded during a 24-hour period of stall rest preceding transportation.
Results—HR, HF power, and LF power had diurnal rhythms during stall rest but not during road transportation. Heart rate was higher and HF power and LF power lower during road transportation than stall rest, and HR, HF power, LF power, and LF-to-HF ratio all decreased with time during road transportation. Heart rate during stall rest was weakly and inversely associated with LF power, but during road transportation was strongly associated with LF power, HF power, and LF-to-HF ratio. Neither LF power nor HF power was correlated with LF-to-HF ratio during stall rest, but LF power was strongly and HF power weakly correlated with LF-to-HF ratio during road transportation. High-frequency power and LF power were significantly correlated with each other during stall rest and road transportation. Heart rate was significantly influenced by LF power and LF-to-HF ratio during stall rest (R2 = 0.40) and by HF power and LF-to-HF ratio during road transportation (R2 = 0.86).
Conclusions and Clinical Relevance—HR is influenced by different sympathovagal mechanisms during stall rest, compared with during road transportation; HRV may be a sensitive indicator of stress in transported horses.
Objective—To investigate the effects of high-intensity training (HIT) on carbohydrate and fat metabolism in Thoroughbreds.
Animals—12 Thoroughbreds (3 to 4 years old; 6 males and 6 females).
Procedures—Horses performed HIT for 18 weeks. They ran at 90% or 110% of maximal oxygen consumption (
o2max) for 3 minutes (5 d/wk) and were subjected to incremental exercise testing (IET) before and after training. Blood samples were collected during IET, and muscle samples were obtained from the gluteus medius muscle immediately after IET. Phosphofructokinase, citrate synthase, and β-3-hydroxyacyl CoA dehydrogenase (β-HAD) activities were measured to determine glycolytic and oxidative capacities. Peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) and fatty acid translocase (FAT/CD36) protein contents were detected via western blotting. Metabolome analysis was performed via capillary electrophoresis–electrospray ionization mass spectrometry to measure substrate concentrations related to carbohydrate metabolism.
Results—Peak speed during IET and
o2max increased after HIT. Activities of citrate synthase and β-HAD increased after HIT, whereas phosphofructokinase activity remained unchanged. The PGC-1α and FAT/CD36 protein contents increased after HIT, but plasma lactate concentration and the respiratory exchange ratio decreased after HIT. The plasma free fatty acid concentration increased after HIT, whereas the glucose concentration was not altered. Fructose 1,6-diphosphate, phosphoenolpyruvate, and pyruvate concentrations decreased after HIT.
Conclusions and Clinical Relevance—HIT caused an increase in oxidative capacity in equine muscle, which suggested that there was a decreased reliance on carbohydrate utilization and a concomitant shift toward fatty acid utilization during intensive exercise.
OBJECTIVE To determine whether racehorses undergoing regular exercise at 2 intensities or stall rest during a period of reduced training (detraining) would differentially maintain their cardiopulmonary and oxygen-transport capacities.
ANIMALS 27 Thoroughbreds.
PROCEDURES Horses trained on a treadmill for 18 weeks underwent a period of detraining for 12 weeks according to 1 of 3 protocols: cantering at 70% of maximal rate of oxygen consumption (
o2max) for 3 min/d for 5 d/wk (canter group); walking for 1 h/d for 5 d/wk (walk group); or stall rest (stall group). Standardized treadmill exercise protocols (during which cardiopulmonary and oxygen-transport variables were measured) were performed before and after detraining.
o2max, maximal cardiac output, and maximal cardiac stroke volume of all groups decreased after 12 weeks of detraining with no differences among groups. After detraining, arterial-mixed-venous oxygen concentration difference did not decrease in any group, and maximal heart rate decreased in the walk and stall groups. Run time to exhaustion and speeds eliciting
o2max and maximal heart rate and at which plasma lactate concentration reached 4mM did not change in the canter group but decreased in the walk and stall groups.
CONCLUSIONS AND CLINICAL RELEVANCE Horses following the cantering detraining protocol maintained higher values of several performance variables compared with horses following the walking or stall rest protocols. These results suggested that it may be possible to identify a minimal threshold exercise intensity or protocol during detraining that would promote maintenance of important performance-related variables and minimize reductions in oxygen-transport capacity in horses.
Objective—To evaluate sevoflurane as an inhalation
anesthetic for thoracotomy in horses.
Animals—18 horses between 2 and 15 years old.
Procedure—4 horses were used to develop surgical
techniques and were euthanatized at the end of the
procedure. The remaining 14 horses were selected,
because they had an episode of bleeding from their
lungs during strenuous exercise. General anesthesia
was induced with xylazine (1.0 mg/kg of body weight,
IV) followed by ketamine (2.0 mg/kg, IV). Anesthesia
was maintained with sevoflurane in oxygen delivered
via a circle anesthetic breathing circuit. Ventilation
was controlled to maintain PaCO2 at approximately 45
mm Hg. Neuromuscular blocking drugs (succinylcholine
or atracurium) were administered to eliminate
spontaneous breathing efforts and to facilitate
surgery. Cardiovascular performance was monitored
and supported as indicated.
Results—2 of the 14 horses not euthanatized died as
a result of ventricular fibrillation. Mean (± SD) duration
of anesthesia was 304.9 ± 64.1 minutes for horses
that survived and 216.7 ± 85.5 minutes for horses
that were euthanatized or died. Our subjective opinion
was that sevoflurane afforded good control of
anesthetic depth during induction, maintenance, and
Conclusions and Clinical Relevance—Administration
of sevoflurane together with neuromuscular
blocking drugs provides stable and easily controllable
anesthetic management of horses for elective thoracotomy
and cardiac manipulation. (Am J Vet Res