Objective—To determine the effects of IV administration
of enalaprilat on cardiorespiratory and hematologic
variables as well as inhibition of angiotensin converting
enzyme (ACE) activity in exercising horses.
Animals—6 adult horses.
Procedure—Horses were trained by running on a
treadmill for 5 weeks. Training was continued
throughout the study period, and each horse also ran
2 simulated races at 120% of maximum oxygen consumption.
Three horses were randomly selected to
receive treatment 1 (saline [0.9% NaCl] solution), and
the remaining 3 horses received treatment 2
(enalaprilat; 0.5 mg/kg of body weight, IV) before
each simulated race. Treatment groups were
reversed for the second simulated race.
Cardiorespiratory and hematologic data were
obtained before, during, and throughout the 1-hour
period after each simulated race. Inhibition of ACE
activity was determined during and after each race in
Results—Exercise resulted in significant increases in
all hemodynamic variables and respiratory rate. The
pH and PO2 of arterial blood decreased during simulated
races, whereas PCO2 remained unchanged.
Systemic and pulmonary blood pressure measurements
and arterial pH, PO2, and PCO2 returned to
baseline values by 60 minutes after simulated races.
Enalaprilat inhibited ACE activity to < 25% of baseline
activity without changing cardiorespiratory or blood
gas values, compared with horses administered
Conclusions and Clinical Relevance—Enalaprilat
administration almost completely inhibited ACE activity
in horses without changing the hemodynamic
responses to intense exercise and is unlikely to be of
value in preventing exercise-induced pulmonary hemorrhage.
(Am J Vet Res 2001;62:1008–1013)
Objective—To evaluate the diagnostic value of serum
concentrations of total magnesium (tMg) and ionized
magnesium (iMg), concentrations of magnesium
(Mg) in muscle, intracellular Mg (icMg) concentrations,
urinary Mg excretion (EMg), Mg clearance (CMg),
and fractional clearance of Mg (FCMg) in horses fed
diets with Mg content above and below National
Research Council recommendations.
Animals—9 young female horses.
Procedures—6 horses were fed a reduced-Mg diet
for 29 days followed by an Mg-supplemented diet for
24 days. Control horses (n = 3) were fed grass hay
exclusively. Blood, urine, and tissue samples were
collected, and an Mg retention test was performed
before and after restriction and supplementation of
Mg intake. Serum tMg, serum iMg, muscle Mg,
icMg, and urine Mg concentrations were measured,
and 24-hour EMg, CMg, and FCMg were calculated.
Results—Reductions in urinary 24-hour EMg, CMg, and
FCMg were evident after 13 days of feeding a reduced-Mg diet. Serum tMg and iMg concentrations, muscle
Mg content, and results of the Mg retention test
were not affected by feeding the Mg-deficient diet.
Spot urine sample FCMg accurately reflected FCMg calculated
from 6- and 24-hour pooled urine samples.
Mean ± SD FCtMg of horses eating grass hay was 29
± 8%, whereas mean FCtMg for horses fed a reduced-Mg diet for 29 days was 6 ± 3%.
Conclusions and Clinical Relevance—The 24-hour
EMg was the most sensitive indicator of reduced Mg
intake in horses. Spot sample FCMg can be conveniently
used to identify horses consuming a diet deficient
in Mg. (Am J Vet Res 2004;65:422–430)
Objective—To determine the effect of a single bout
of exercise and increased substrate availability after
exercise on gene expression and content of the glucose
transporter-4 (GLUT-4) protein in equine skeletal
Animals—6 healthy adult Thoroughbreds.
Procedure—The study was designed in a balanced,
randomized, 3-way crossover fashion. During 2 trials,
horses were exercised at 45% of their maximal rate
of oxygen consumption for 60 minutes after which 1
group received water (10 mL/kg), and the other group
received glucose (2 g/kg, 20% solution) by nasogastric
intubation. During 1 trial, horses stood on the
treadmill (sham exercise) and then received water (10
mL/kg) by nasogastric intubation. Muscle glycogen
concentration and muscle GLUT-4 protein and mRNA
content were determined before exercise and at 5
minutes and 4, 8, and 24 hours after exercise.
Results—Although exercise resulted in a 30% reduction
in muscle glycogen concentration, no significant
difference was detected in muscle GLUT-4 protein or
mRNA content before and after exercise. Glycogen
replenishment was similar in both exercised groups
and was not complete at 24 hours after exercise.
Horses that received glucose had significantly higher
plasma glucose and insulin concentrations for 3 hours
after exercise, but no effect of hyperglycemia was
detected on muscle GLUT-4 protein or mRNA content.
Conclusions and Clinical Relevance—Under the conditions of this study, neither
exercise nor the combination of exercise followed
by hyperglycemia induced translation or transcription
of the GLUT-4 protein in horses. (Am J Vet Res 2003;64:1401–1408)
Objective—To determine the anesthetic, cardiorespiratory,
and metabolic effects of 4 IV anesthetic regimens
in Thoroughbred horses recuperating from a
brief period of maximal exercise.
Animals—6 adult Thoroughbreds.
Procedure—Horses were preconditioned by exercising
them on a treadmill. Each horse ran 4 simulated
races, with a minimum of 14 days between races.
Races were run at a treadmill speed that caused horses
to exercise at 120% of their maximal oxygen consumption.
Horses ran until fatigued or for a maximum
of 2 minutes. Two minutes after exercise, horses
received a combination of xylazine hydrochloride (2.2
mg/kg of body weight) and acepromazine maleate
(0.04 mg/kg) IV. Five minutes after exercise, horses
received 1 of the following 4 IV anesthetic regimens:
ketamine hydrochloride (2.2 mg/kg); ketamine (2.2
mg/kg) and diazepam (0.1 mg/kg); tiletamine
hydrochloride-zolazepam hydrochloride (1 mg/kg); and
guaifenesin (50 mg/kg) and thiopental sodium (5
mg/kg). Treatments were randomized. Cardiopulmonary
indices were measured, and samples of
blood were collected before and at specific times for
90 minutes after each race.
Results—Each regimen induced lateral recumbency.
The quality of induction and anesthesia after ketamine
administration was significantly worse than after
other regimens, and the duration of anesthesia was
significantly shorter. Time to lateral recumbency was
significantly longer after ketamine or guaifenesinthiopental
administration than after ketaminediazepam
or tiletamine-zolazepam administration.
Arterial blood pressures after guaifenesin-thiopental
administration were significantly lower than after the
Conclusions and Clinical Relevance—Anesthesia
can be safely induced in sedated horses immediately
after maximal exercise. Ketamine-diazepam and tiletamine-
zolazepam induced good quality anesthesia
with acceptable perturbations in cardiopulmonary and
metabolic indices. Ketamine alone and guaifenesinthiopental
regimens are not recommended. (Am J Vet
Objectives—To determine effects of feeding diets
with various soluble-carbohydrate (CHO) content on
rates of muscle glycogen synthesis after exercise in
Animals—7 fit horses.
Procedures—In a 3-way crossover study, horses
received each of 3 isocaloric diets (a high soluble CHO
[HC] diet, a low soluble CHO [LC] diet, or a mixed soluble
CHO [MC] diet). For each diet, horses were subjected
to glycogen-depleting exercise, followed by feeding
of the HC, LC, or MC diet at 8-hour intervals for 72
Results—Feeding the HC diet resulted in a significantly
higher glycemic response for 72 hours and significantly
greater muscle glycogen concentration at 48 and
72 hours after exercise, compared with results after
feeding the MC and LC diets. Muscle glycogen concentrations
similar to baseline concentrations were
detected in samples obtained 72 hours after exercise in
horses when fed the HC diet. Rate of glycogen synthesis
was significantly higher when horses were fed
the HC diet, compared with values when horses were
fed the MC and LC diets. Glycogen synthase activity
was inversely related to glycogen content. Protein content
of glucose transporter-4 was the lowest at 72
hours after exercise when horses were fed the HC diet.
Conclusions and Clinical Relevance—Muscle glycogen
synthesis was slower after glycogen-depleting
exercise in horses, compared with synthesis in
humans. Feeding HC meals after strenuous exercise
hastened replenishment of muscle glycogen content,
compared with results for feeding of LC and MC
diets, by increasing availability of blood glucose to
skeletal muscles. (Am J Vet Res 2004;65:916–923)
Objective—To determine whether repetitive endurance
exercise in sled dogs was associated with
substantial lipid peroxidation, decreases in antioxidant
capacity of the serum, and skeletal muscle
Animals—24 lightly trained sled dogs.
Procedure—16 dogs completed a 58-km run on each
of 3 consecutive days; the other 8 dogs
(control) did not exercise during the study. Blood samples
were collected before the first exercise run and
after the first and third exercise runs. Plasma isoprostane
and serum vitamin E concentrations, total
antioxidant status of plasma, and serum creatine
kinase activity were measured.
Results—Plasma isoprostane concentrations in dogs
in the exercise group were significantly increased
after the first exercise run and further significantly
increased after the third exercise run. Serum vitamin
E concentration was significantly decreased after the
first exercise run in dogs in the exercise group, and
this change persisted after the third exercise run.
There was a significant linear relationship between
plasma isoprostane concentration and the logarithm
of serum creatine kinase activity (adjusted r2 = 0.84).
Conclusions and Clinical Relevance—Results
demonstrate that repetitive endurance exercise in
dogs is associated with lipid peroxidation and a reduction
in plasma antioxidant concentrations. We interpret
these results as indicating that the antioxidant
mechanisms of minimally trained dogs may, in some
instances, be inadequate to meet the antioxidant
requirements of repetitive endurance exercise. (Am J Vet Res 2000;61:512–517)
Objective—To clone and sequence cDNA for equine
insulin-responsive glucose transporter (glucose transporter
type 4 [GLUT-4]) and determine effects of
glycogen-depleting exercise and meal type after exercise
on GLUT-4 gene expression in skeletal muscle of
Animals—Muscle biopsy specimens
from 7 healthy adult horses.
Procedure—Total RNA was extracted from specimens,
and GLUT-4 cDNA was synthesized and
sequenced. Horses were exercised on 3 consecutive
days. On the third day of exercise, for 8 hours after exercise,
horses were either not fed, fed half of daily energy
requirements as hay, or fed an isocaloric amount of
corn. The GLUT-4 mRNA was determined by use of realtime
reverse transcriptase-polymerase chain reaction in
muscle biopsy specimens obtained before 3 consecutive
days of exercise and within 10 minutes and 4, 8,
and 24 hours after the third exercise bout.
Results—A 1,629-bp segment was sequenced, of
which 1,530 bp corresponded to the coding region
and encoded a protein of 509 amino acids.
Expression of GLUT-4 gene increased by 2.3, 4.3, 3.3,
and 2.6 times 10 minutes and 4, 8, and 24 hours after
exercise, respectively, compared with that prior to
exercise. No differences were observed in GLUT-4
gene expression among conditions of feed withholding,
corn feeding, and hay feeding during the 8 hours
Conclusions and Clinical Relevance—Lack of
increase of GLUT-4 gene expression after grain feeding
and exercise may explain the apparently slower rate of
glycogen synthesis after exercise in horses relative to
that of other species. (Am J Vet Res 2005;66:379–385)
Objective—To assess changes in muscle glycogen (MG) and triglyceride (MT) concentrations in aerobically conditioned sled dogs during prolonged exercise.
Animals—54 Alaskan sled dogs fed a high-fat diet.
Procedures—48 dogs ran 140-km distances on 4 consecutive days (cumulative distance, up to 560 km); 6 dogs remained as nonexercising control animals. Muscle biopsies were performed immediately after running 140, 420, or 560 km (6 dogs each) and subsequently after feeding and 7 hours of rest. Single muscle biopsies were performed during recovery at 28 hours in 7 dogs that completed 560 km and at 50 and 98 hours in 7 and 6 dogs that completed 510 km, respectively. Tissue samples were analyzed for MG and MT concentrations.
Results—In control dogs, mean ± SD MG and MT concentrations were 375 ± 37 mmol/kg of dry weight (kgDW) and 25.9 ± 10.3 mmol/kgDW, respectively. Compared with control values, MG concentration was lower after dogs completed 140 and 420 km (137 ± 36 mmol/kgDW and 203 ± 30 mmol/kgDW, respectively); MT concentration was lower after dogs completed 140, 420, and 560 km (7.4 ± 5.4 mmol/kgDW; 9.6 ± 6.9 mmol/kgDW, and 6.3 ± 4.9 mmol/kgDW, respectively). Depletion rates during the first run exceeded rates during the final run. Replenishment rates during recovery periods were not different, regardless of distance; only MG concentration at 50 hours was significantly greater than the control value.
Conclusions and Clinical Relevance—Concentration of MG progressively increased in sled dogs undergoing prolonged exercise as a result of attenuated depletion.
Objective—To determine the effects of training and sustained submaximal exercise on hematologic values in racing sled dogs.
Animals—39 Alaskan sled dogs bred for endurance racing.
Procedures—Blood samples were collected prior to initiation of a 7-month training regimen (n = 39), after completion of the training regimen (19), and after completion of an 1,100-mile race (9), and a CBC, differential cell count, and flow cytometry for leukocyte surface antigens were performed.
Results—Both training and exercise caused significant decreases in PCV and hemoglobin concentration and significant increases in total WBC count. In contrast, training and exercise were not found to have significant effects on absolute numbers or fractions of CD4+ or CD8+ lymphocytes, other than a significant increase in the fraction of CD8+ lymphocytes associated with training.
Conclusions and Clinical Relevance—Results suggested that training and exercise induced changes in several hematologic values in racing sled dogs. Extracellular fluid volume expansion was the likely explanation for the training-induced decrease in PCV, and acute blood loss secondary to gastrointestinal tract bleeding was likely responsible for the decrease in PCV associated with acute exercise.