Objective—To determine whether disruption of adenine
triphosphate (ATP) regeneration and subsequent
adenine nucleotide degradation are potential mechanisms
for rhabdomyolysis in horses with polysaccharide
storage myopathy (PSSM) performing submaximal
Animals—7 horses with PSSM and 4 control horses.
Procedures—Horses with PSSM performed 2-minute
intervals of a walk and trot exercise on a treadmill until
muscle cramping developed. Control horses exercised
similarly for 20 minutes. Serum creatine kinase (CK)
activity was measured 4 hours after exercise. Citrate
synthase (CS), 3-OH-acylCoA dehydrogenase, and lactate
dehydrogenase activities prior to exercise and glucose-
6-phosphate (G-6-P) and lactate concentrations
before and after exercise were measured in gluteal
muscle specimens. Adenine triphosphate, diphosphate
(ADP), monophosphate (AMP), and inosine
monophosphate (IMP) concentrations were measured
before and after exercise in whole muscle, single muscle
fibers, and pooled single muscle fibers.
Results—Serum CK activity ranged from 255 to
22,265 U/L in horses with PSSM and 133 to 278 U/L
in control horses. Muscle CS activity was lower in
horses with PSSM, compared with control horses.
Muscle G-6-P, lactate, ATP, ADP, and AMP concentrations
in whole muscle did not change with exercise in
any horses. Concentration of IMP increased with
exercise in whole muscle, pooled muscle fibers, and
single muscle fibers in horses with PSSM. Large variations
in ATP and IMP concentrations were observed
within single muscle fibers.
Conclusions and Clinical Relevance—Increased IMP
concentration without depletion of ATP in individual
muscle fibers of horses with PSSM during submaximal
exercise indicates an energy imbalance that may contribute
to the development of exercise intolerance and
rhabdomyolysis. (Am J Vet Res 2005;66:839–845)
Objective—To investigate influence of the
Rendement Napole (RN–) mutation on proglycogen
(PG) and macroglycogen (MG) content in skeletal
muscles before and after exercise and evaluate glycogen
concentrations within various muscle fiber types.
Animals—5 pigs with the RN– mutation and 3 noncarrier
Procedure—Pigs performed 2 exercise tests on a
treadmill. In the first, pigs (mean body weight, 27 kg)
ran a distance of approximately 800 m. In the second,
pigs (mean body weight, 63 kg) ran until fatigued.
Biopsy specimens (biceps femoris muscle) for determination
of PG and MG contents were obtained
before and after exercise, 24 hours after the first test,
and 3 hours after the second test. Histochemical
analysis was performed on specimens obtained
before and after the second test.
Results—Before exercise, PG stores did not differ
markedly between groups, but MG stores were
twice as high in pigs with the RN– mutation, compared
with noncarrier pigs. The MG content
decreased to a similar extent in both groups after
exercise. Resynthesis of MG was greater in pigs with
the RN– mutation than in noncarrier pigs by 3 hours
after exercise. A low glycogen content after exercise
was observed in many type I and type IIA fibers and
in some type IIB fibers.
Conclusions and Clinical Relevance—The RN– mutation
was associated with high MG stores in skeletal
muscle that did not influence exercise performance.
The RN– mutation did not impair glycogenolysis during
exercise but may induce faster resynthesis of MG
after exercise. (Am J Vet Res 2005;66:1197–1201)
Objective—To determine whether repeated oral administration of glucose and leucine during the period immediately after intense exercise would increase the release of insulin and thereby enhance glycogen synthesis in horses.
Animals—12 Standardbred horses.
Procedures—In a crossover study design, after glycogen-depleting exercise, horses received oral boluses of glucose (1 g/kg at 0, 2, and 4 hours) and leucine (0.1 g/kg at 0 and 4 hours) or boluses of water (10 mL/kg at 0, 2, and 4 hours; control treatment). Blood samples for determination of glucose, insulin, and leucine concentrations were collected prior to and during a 6-hour period immediately after exercise. Biopsy specimens of a gluteus muscle were obtained before and immediately after exercise and at 3, 6, and 24 hours after exercise for measurement of glycogen concentration.
Results—When glucose and leucine were administered to the horses, plasma insulin concentration was significantly higher during the 6 hours immediately after exercise than it was when water was administered to the horses. Serum glucose concentration during the 4 hours immediately after exercise was significantly higher when glucose and leucine were administered than the serum glucose concentration when water was administered. Muscle glycogen concentrations did not differ between the 2 treatments during the 24 hours after exercise.
Conclusions and Clinical Relevance—Synthesis of muscle glycogen after intense intermittent exercise was not enhanced by oral boluses of glucose and leucine after exercise despite pronounced increases in plasma insulin and serum glucose concentrations.
Objective—To determine concentrations of proglycogen (PG), macroglycogen (MG), glucose, and glucose-6-phosphate (G-6-P) in skeletal muscle of horses with polysaccharide storage myopathy (PSSM) before and after performing light submaximal exercise.
Animals—6 horses with PSSM and 4 control horses.
Procedures—Horses with PSSM completed repeated intervals of 2 minutes of walking followed by 2 minutes of trotting on a treadmill until muscle cramping developed. Four untrained control horses performed a similar exercise test for up to 20 minutes. Serum creatine kinase (CK) activity was measured before and 4 hours after exercise. Concentrations of total glycogen (Gt), PG, MG, G-6-P, free glucose, and lactate were measured in biopsy specimens of gluteal muscle obtained before and after exercise.
Results—Mean serum CK activity was 26 times higher in PSSM horses than in control horses after exercise. Before exercise, muscle glycogen concentrations were 1.5, 2.2, and 1.7 times higher for PG, MG, and Gt, respectively, in PSSM horses, compared with concentrations in control horses. No significant changes in Gt, PG, MG, G-6-P, and lactate concentrations were detected after exercise. However, free glucose concentrations in skeletal muscle increased significantly in PSSM horses after exercise.
Conclusions and Clinical Relevance—Analysis of the results suggests that glucose uptake in skeletal muscle is augmented in horses with PSSM after light exercise. There is excessive storage of PG and MG in horses with PSSM, and high concentrations of the 2 glycogen fractions may affect functional interactions between glycogenolytic and glycogen synthetic enzymes and glycosomes.
Objective—To compare effects of corn oil or a 7-carbon fat (triheptanoin) on acylcarnitine, lipid, and carbohydrate metabolism in plasma or muscle of exercising horses.
Animals—8 Thoroughbred geldings.
Procedures—Horses received isocaloric diets containing 650 mL of oil (triheptanoin or corn oil)/d for 18 or 25 days in a crossover design with a 26-day washout period. On day 17 or 24 of each feeding period, the respective oil (217 mL) was nasogastrically administered; 120 minutes later, horses performed a 90-minute submaximal exercise test (SET). Blood and muscle samples were obtained before oil administration and immediately before (blood only), during (blood only), immediately after, and 24 hours after SETs.
Results—Compared with values before oil administration, triheptanoin administration increased plasma insulin and C7:0-, C5:0- and C3:0-acylcarnitine concentrations, whereas corn oil administration increased plasma NEFA concentrations. During SETs, plasma C7:0-, C5:0-, and C3:0-acylcarnitine concentrations were higher when triheptanoin, rather than corn oil, was administered to horses. Plasma glucose, NEFA, and C2:0-, C18:1-, and C18:2-acylcarnitine concentrations increased during SETs similarly for both oils. Respiratory quotient and muscle lactate, citrate, malate, glycogen, and ATP concentrations changed similarly from before to after SETs for both oils. Compared with muscle concentrations immediately after SETs, those for glucose-6-phosphate and citrate 24 hours after SETs were lower and for glycogen were similar to values before SETs.
Conclusions and Clinical Relevance—Fatigue was not associated with depletion of citric acid cycle intermediates for either oil. Triheptanoin induced a significantly higher insulin secretion and did not appear to enhance muscle glycogen repletion.