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 determine the effects of short-term IV
administration of hydrocortisone or equine growth hormone
(eGH) or long-term IM administration of eGH to
horses on tissue sensitivity to exogenous insulin.
Animals—5 Standardbreds and 4 Dutch Warmblood
Procedure—The euglycemic-hyperinsulinemic clamp
technique was used to examine sensitivity of peripheral
tissues to exogenous insulin 24 hours after
administration of a single dose of hydrocortisone
(0.06 mg/kg), eGH (20 µg/kg), or saline (0.9% NaCl)
solution and after long-term administration (11 to 15
days) of eGH to horses. The amounts of metabolized
glucose (M) and plasma insulin concentration (I) were
Results—Values for M and the M-to-I ratio were significantly
higher 24 hours after administration of a single
dose of hydrocortisone than after single-dose administration
of eGH or saline solution. After long-term administration
of eGH, basal I concentration was increased
and the mean M-to-I ratio was 22% lower, compared
with values for horses treated with saline solution.
Conclusions and Clinical Relevance—Increases in
M and the M-to-I ratio after a single dose of hydrocortisone
imply that short-term hydrocortisone treatment
increases glucose use by, and insulin sensitivity
of, peripheral tissues. Assuming a single dose of
hydrocortisone improves sensitivity of peripheral tissues
to insulin, it may be an interesting candidate for
use in reducing insulin resistance in peripheral tissues
of horses with several disease states. In contrast,
long-term administration of eGH decreased tissue
sensitivity to exogenous insulin associated with
hyperinsulinemia. Therefore, increased concentrations
of growth hormone may contribute to insulin
resistance in horses with various disease states.
(Am J Vet Res 2005;66:1907–1913)
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)
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 evaluate alterations in skeletal muscle carnitine metabolism during exercise and training by measuring changes in plasma acylcarnitine concentrations in Standardbreds.
Animals—10 Standardbred geldings with a mean ± SD age of 20 ± 2 months and weight of 384 ± 42 kg.
Procedures—In a 32-week longitudinal study, training on a treadmill was divided into 4 phases as follows: phase 1, acclimatization for 4 weeks; phase 2, 18 weeks with alternating endurance and high-intensity exercise training; phase 3, increased training volume and intensity for another 6 weeks; and phase 4, deconditioning for 4 weeks. In phase 3, horses were randomly assigned to 2 groups as follows: control horses (which continued training at the same level as in phase 2) and high-intensity exercise trained horses. At the end of each phase, a standardized exercise test (SET) was performed. Plasma acylcarnitine, fatty acids, and lactic acid and serum β-hydroxybutyric acid (BHBA) concentrations were assessed before and at different time points after each SET.
Results—Plasma lactic acid, total nonesterified fatty acids, 3-hydroxyisobutyric acid, and acetylcarnitine (C2-carnitine) concentrations significantly increased during SETs, whereas serum BHBA, plasma propionylcarnitine (C3-carnitine), and plasma butyryl- and isobutyrylcarnitine (C4-carnitine) concentrations decreased significantly, compared with those before SETs.
Conclusions and Clinical Relevance—Our findings indicated that the plasma acylcarnitine profile in horses likely reflects skeletal muscle carnitine metabolism following exercise, thereby providing a possible practical method to investigate potential disorders in carnitine metabolism in horses with myopathy.