Objective—To determine the influence of age on
results of quantitative analysis of electromyographic
(EMG) needle examination in the subclavian, triceps,
and lateral vastus muscles of Dutch Warmblood horses.
Animals—7 healthy young Dutch Warmblood horses
(range, 13 to 18 months old), 7 healthy adult Dutch
Warmblood horses (range, 4 to 10 years old), and 7
healthy elderly Dutch Warmblood horses (range, 18 to
21 years old).
Procedure—An EMG needle examination was performed
to evaluate insertional activity, spontaneous
activity, and motor unit action potential (MUAP) variables.
Although all horses were conscious, young
horses were sedated prior to examination.
Results—Mean insertional activity in young horses
was significantly lower than in elderly horses.
Pathologic spontaneous activity was rarely found in
young and adult horses but was frequently evident in
all muscles in all elderly horses. The MUAP duration
and amplitude were significantly lower in all muscles
of young horses, compared with values for adult and
elderly horses. The MUAP duration and number of
phases and turns were significantly lower in adult
horses than in elderly horses. Group differences for
percentages of polyphasic and complex MUAPs were
also found. The 95% confidence intervals for MUAP
duration, MUAP amplitude, and number of phases
and turns for the subclavian, triceps, and lateral vastus
muscles were significantly lower in young horses
than in adult or elderly horses.
Conclusion and Clinical Relevance—Age of the
horse being examined should be considered when
EMG examination is performed. (Am J Vet Res
Objective—To evaluate the application of analysis of
motor unit action potentials (MUAP) in horses and to
obtain values of MUAP for the subclavian muscle of
Animals—10 healthy adult Dutch Warmblood horses.
Procedure—Electromyographic examination of the
subclavian muscle in conscious nonsedated horses
was performed to evaluate insertional activity, spontaneous
activity, MUAP variables, and recruitment
patterns. Muscle and body temperatures were measured
at the beginning and end of the procedure.
Amplitude, duration, number of phases, and number
of changes in direction (ie, turns) for all representative
MUAP were analyzed to determine values for this
muscle in this group of horses.
Results—Mean ± SD duration of insertional activity
was 471.7 ± 33.45 milliseconds. Mean MUAP amplitude
in the examined horses was 379 µV (95% confidence
interval [CI], 349 to 410 µV). Mean MUAP duration
of the subclavian muscle was 7.27 milliseconds
(95% CI, 6.84 to 7.71 milliseconds). Mean number of
phases was 2.9, and mean number of turns was 3.0.
Prevalence of polyphasic MUAP, defined as MUAP
with > 4 phases, was 7.7%. Number of MUAP that
had > 5 turns was 2.4%. Satellite potentials were
found in 1.0% of the MUAP.
Conclusions and Clinical Relevance—This study
revealed that electromyography including MUAP
analysis can be performed in horses, and values for
the subclavian muscle in healthy adult horses can be
obtained. Analysis of MUAP could be a valuable diagnostic
tool for use in discriminating between myogenic
and neurogenic problems in horses. (Am J Vet Res 2002;63:198–203)
Objective—To determine whether electromyographic
abnormalities are evident in skeletal muscles in
horses with induced hypocalcemia and hypomagnesemia.
Animals—7 healthy adult Dutch Warmblood horses.
Procedure—Electromyographic examination was
performed in the lateral vastus, triceps, and subclavian
muscles before and after IV infusion of EDTA. An
initial dose (mean ± SD, 564 ± 48 ml) of a 10% solution
of sodium EDTA was administered IV during a
period of 21 ± 7.3 minutes to establish a blood concentration
of ionized calcium of approximately 0.5
mMol/L. Average rate of EDTA infusion to maintain
ionized calcium at this concentration was 6.6 ml/min.
Results—Mean blood concentrations of ionized calcium
and magnesium were 1.39 ± 0.06 and 0.84 ± 0.09
mM, respectively before EDTA infusion; after EDTA
infusion, concentrations were 0.48 ± 0.05 and 0.44 ±
0.20 mM, respectively. This state induced positive
waves; fibrillation potentials; doublets, triplets, and
multiplets; complex repetitive discharges; and neuromyotonia.
Analysis of motor unit action potentials
(MUAP) after EDTA infusion revealed an increase in
prevalence of polyphasic and complex MUAP in all
Conclusion and Clinical Relevance—None of the
horses had classical signs of hypocalcemia and hypomagnesemia.
In contrast, all horses had spontaneous
activity in the measured muscles indicative of nerve
hyperirritability. Calcium and magnesium deficits
appear to have consequences, which may be subclinical,
affecting functions of the neuromuscular system.
This is of interest for equestrian sports in which
hypocalcemia and hypomagnesemia are expected,
such as during endurance rides. (Am J Vet Res
Objective—To evaluate the effect of various head and neck positions on intrathoracic pressure and arterial oxygenation during exercise in horses.
Animals—7 healthy Dutch Warmblood riding horses.
Procedures—The horses were evaluated with the head and neck in the following predefined positions: position 1, free and unrestrained; position 2, neck raised with the bridge of the nose aligned vertically; position 4, neck lowered and extremely flexed with the nose pointing toward the pectoral muscles; position 5, neck raised and extended with the bridge of the nose in front of a vertical line perpendicular to the ground surface; and position 7, neck lowered and flexed with the nose pointing towards the carpus. The standard exercise protocol consisted of trotting for 10 minutes, cantering for 4 minutes, trotting again for 5 minutes, and walking for 5 minutes. An esophageal balloon catheter was used to indirectly measure intrathoracic pressure. Arterial blood samples were obtained for measurement of Pao2, Paco2, and arterial oxygen saturation.
Results—Compared with when horses were in the unrestrained position, inspiratory intrathoracic pressure became more negative during the first trot (all positions), canter and second trot (position 4), and walk (positions 4 and 5). Compared with when horses were in position 1, intrathoracic pressure difference increased in positions 4, 2, 7, and 5; Pao2 increased in position 5; and arterial oxygen saturation increased in positions 4 and 7.
Conclusions and Clinical Relevance—Position 4 was particularly influential on intrathoracic pressure during exercise in horses. The effects detected may have been caused by a dynamic upper airway obstruction and may be more profound in horses with upper airway disease.
Objective—To confirm whether the plasma glucose
concentration curve obtained during oral glucose tolerance
tests (OGTTs) in horses with equine motor
neuron disease (EMND) is decreased, compared with
that obtained in clinically normal horses, and determine
whether that decrease is a result of defective
glucose metabolism or intestinal glucose transport
Animals—8 horses with EMND and 44 matched control
Procedure—Electromyography and OGTTs were performed
in all 8 affected horses and 10 control horses.
Intravenous GTTs (IVGTTs) were performed in 6
affected horses and another 11 control horses. The
activity and levels of jejunal luminal membrane glucose
transporter (Na+/glucose cotransporter isoform 1
[SGLT1]) were measured in 2 affected horses and 23
Results—In horses with EMND, generalized neuropathy
was detected via quantitative electromyography;
the mean increase in plasma glucose concentration
during the OGTT was significantly decreased,
compared with the value in control horses. During the
IVGTT, the mean increase in plasma glucose concentration
was significantly lower than that of control
horses. The activity and levels of SGLT1 in 2 affected
horses were similar to those of control horses.
Diagnosis of EMND was confirmed postmortem in all
Conclusions and Clinical Relevance—Data suggest
that the decreased plasma glucose curve obtained in
horses with EMND during OGTTs (compared with
control horses) is a result of overall enhanced glucose
metabolism or abnormalities in the facilitated glucose
transporters; definitive identification of the underlying
mechanisms could aid in the development of appropriate
treatments of EMND in horses. (Am J Vet Res 2005;66:93–99)
Objective—To determine the influence of intensified training and subsequent reduced training on glucose metabolism rate and peripheral insulin sensitivity in horses and identify potential markers indicative of early overtraining.
Animals—12 Standardbred geldings.
Procedures—Horses underwent 4 phases of treadmill-based training. In phase 1, horses were habituated to the treadmill. In phase 2, endurance training was alternated with high-intensity exercise training. In phase 3, horses were divided into control and intensified training groups. In the intensified training group, training intensity, duration, and frequency were further increased via a protocol to induce overtraining; in the control group, these factors remained unaltered. In phase 4, training intensity was reduced. Standardized exercise tests were performed after each phase and hyperinsulinemic euglycemic clamp (HEC) tests were performed after phases 2, 3, and 4.
Results—10 of 12 horses completed the study. Dissociation between mean glucose metabolism rate and mean glucose metabolism rate-to-plasma insulin concentration ratio (M:I) was evident in the intensified training group during steady state of HEC testing after phases 3 and 4. After phase 4, mean glucose metabolism rate was significantly decreased (from 31.1 ± 6.8 μmol/kg/min to 18.1 ± 3.4 μmol/kg/min), as was M:I (from 1.05 ± 0.31 to 0.62 ± 0.17) during steady state in the intensified training group, compared with phase 3 values for the same horses.
Conclusions and Clinical Relevance—Dissociation between the glucose metabolism rate and M:I in horses that underwent intensified training may reflect non-insulin–dependent increases in glucose metabolism.
Objective—To investigate whether protein kinase C
(PKC) isoforms are expressed in equine skeletal muscle
and determine their distribution in various types of
fibers by use of immunofluorescence microscopy.
Animals—5 healthy adult Dutch Warmblood horses.
Procedure—In each horse, 2 biopsy specimens were
obtained from the vastus lateralis muscle.
Cryosections of equine muscle were stained with
PKC isoform (α, β1, β2, δ, ξ, or ζ)-specific polyclonal
antibodies and examined by use of a fluorescence
microscope. Homogenized muscle samples were
evaluated via western blot analysis.
Results—The PKC α, β1, β2, δ, ξ, and ζ isoforms
were localized within the fibers of equine skeletal
muscle. In addition, PKC α and β2 were detected near
or in the plasma membrane of muscle cells. For some
PKC isoforms, distribution was specific for fiber type.
Staining of cell membranes for PKC α was observed
predominantly in fibers that reacted positively with
myosin heavy chain (MHC)-IIa; PKC δ and ξ staining
were more pronounced in MHC-I-positive fibers. In
contrast, MHC-I negative fibers contained more PKC
ζ than MHC-I-positive fibers. Distribution of PKC β1
was equal among the different fiber types.
Conclusions and Clinical Relevance—Results indicated
that PKC isoforms are expressed in equine
skeletal muscle in a fiber type-specific manner.
Therefore, the involvement of PKC isoforms in signal
transduction in equine skeletal muscle might be
dependent on fiber type. ( Am J Vet Res 2004;
Objective—To investigate the effects of exercise on activation of mitogen-activated protein kinase (MAPK) signaling proteins in horses.
Animals—6 young trained Standardbred geldings.
Procedure—Horses performed a 20-minute bout of exercise on a treadmill at 80% of maximal heart rate. Muscle biopsy specimens were obtained from the vastus lateralis and pectoralis descendens muscles before and after exercise. Amount of expression and intracellular location of phosphospecific MAPK pathway intermediates were determined by use of western blotting and immunofluorescence staining.
Results—Exercise resulted in a significant increase in phosphorylation of p38 pathway intermediates, c-Jun NH2 terminal kinase (JNK), and heat shock protein 27 (HSP27) in the vastus lateralis muscle, whereas no significant changes were found in phosphorylation of extracellular regulated kinase. In the pectoralis descendens muscle, phosphorylation of p38 and HSP27 was significantly increased after exercise. Immunohistochemical analysis revealed fiber-type– specific locations of phosphorylated JNK in type 2a/b intermediate and 2b fibers and phosphorylated p38 in type 1 fibers. Phosphorylated HSP27 was strongly increased after exercise in type 1 and 2a fibers.
Conclusions and Clinical Relevance—The p38 pathway and JNK are activated in the vastus lateralis muscle after a single 20-minute bout of submaximal exercise in trained horses. Phosphorylation of HSP27 as detected in the study reported here is most likely induced through the p38 signaling pathway.
Procedures—Percutaneous biopsy specimens were
obtained from the vastus lateralis, pectoralis descendens,
and triceps brachii muscles. Cryosections were
stained with combinations of GLUT4 and myosin
heavy chain (MHC) specific antibodies or FAT/CD36
and MHC antibodies to assess the fiber specific
expression of GLUT4 and FAT/CD36 in equine skeletal
muscle via indirect immunofluorescent
Results—Immunofluorescent staining revealed that
GLUT4 was predominantly expressed in the cytosol
of fast type 2B fibers of equine skeletal muscle,
although several type 1 fibers in the vastus lateralis
muscle were positive for GLUT4. In all muscle fibers
examined microscopically, FAT/CD36 was strongly
expressed in the sarcolemma and capillaries. Type 1
muscle fibers also expressed small intracellular
amounts of FAT/CD36, but no intracellular FAT/CD36
expression was detected in type 2 fibers.
Conclusions and Clinical Relevance—In equine
skeletal muscle, GLUT4 and FAT/CD36 are expressed
in a fiber type selective manner. ( Am J Vet Res 2004;65:951–956)
Objectives—To determine whether increased glucose
metabolism is the potential cause of the
decreased plasma glucose curve determined after
oral glucose tolerance testing in horses with lower
motor neuron degeneration.
Animals—3 horses with signs suggestive of lower
motor neuron degeneration, 1 horse with malignant
melanoma with multiple metastases, and an obese
but otherwise healthy horse.
Procedures—Glucose metabolism was assessed by
use of the hyperglycemic clamp and euglycemic
hyperinsulinemic clamp techniques.
Results—Mean rate of glucose metabolism of
horses with lower motor neuron degeneration was
significantly greater (mean, 3.7 times greater than
control horses; range, 2.1 to 4.8 times greater)
than that reported in 5 healthy control horses
(41 ± 13 µmol/kg/min vs 11 ± 4.5 µmol/kg/min,
respectively). In addition, one of the affected horses,
an 8-year-old warmblood gelding, had a 5.6-
times increased sensitivity to exogenously administered
insulin, compared with that reported in 5
healthy control horses. Pancreatic insulin secretion
was not insufficient in horses with lower motor
neuron degeneration. Findings in the 2 diseased
control horses were unremarkable.
Conclusions and Clinical Relevance—Increased
glucose metabolism in horses with lower motor neuron
degeneration may be the cause of the decreased
plasma glucose curve detected after oral glucose tolerance
testing. This finding could aid in developing
supportive treatments with respect to adequate glucose
and vitamin E supplementation. (Am J Vet Res 2005;66:271–276)