Objective—To determine proglycogen (PG) and
macroglycogen (MG) content in equine skeletal muscle
and to compare 2 analytical methods (acid hydrolysis
[AC] and PG plus MG determination) for measurement
of total muscle glycogen content (Glytot) in
Sample Population—Muscle biopsy specimens
obtained from 41 clinically normal horses.
Procedure—Forty-five muscle biopsy specimens
obtained from the middle gluteal (n = 31) or triceps
(14) muscle were analyzed, using AC and MG plus
PG determination for Glytot. Variability within muscle
biopsy specimens for each method was calculated
from duplicate analyses of muscle specimens.
In a second experiment, variation in MG and
PG content between muscle biopsy specimens and
the effect of sample collection depth on the concentration
of MG and PG in the middle gluteal muscle
Results—There was a strong correlation ( r = 0.99)
between Glytot values obtained by use of AC and MG
plus PG determination. Coefficients of variation for
within- and between-specimen variability of Glytot
were approximately 4% for each method. The PG
fraction was always in excess of the MG fraction.
Biopsy specimens obtained from the superficial part
of the middle gluteal muscle contained significantly
more Glytot and PG than specimens obtained from
Conclusions and Clinical Relevance—This study
confirms that MG and PG exist in equine skeletal
muscle and can be measured reliably in biopsy samples.
This technique could be applied in future studies
to investigate glycogen metabolism in exercising
horses and horses with glycogen-storage diseases.
(Am J Vet Res 2002;63:570–575)
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 quantify insulin sensitivity and monitor glucose, insulin, and lipid concentrations in a group of moderately insulin-resistant horses during induction of obesity by use of a forage diet supplemented with fat and during subsequent turnout to pasture.
ANIMALS 9 adult Standardbred mares (11 to 20 years old).
PROCEDURES Weight gain of horses was induced during 22 weeks by use of a forage diet supplemented with fat fed in gradually increasing amounts, followed by feeding of that fat-supplemented diet at 2.5 times the daily maintenance requirements. Horses were then turned out to pasture. Insulin sensitivity was measured with the euglycemic hyperinsulinemic clamp method before and after weight gain and after 4 weeks at pasture. Body weight, body condition score, and cresty neck score as well as fasting and postprandial concentrations of plasma insulin, plasma glucose, serum triglyceride, and serum nonesterified fatty acids were measured during the study.
RESULTS Body weight typically increased by 10%, and body condition score (scale, 1 to 9) increased by > 1.5 from the start to the end of the weight-gain period. There was no difference in insulin sensitivity or metabolic clearance rate of insulin during the weight-gain period. Four weeks at pasture generally improved insulin sensitivity and metabolic clearance rate of insulin by 54% and 32%, respectively, but there was no change in body weight or body condition score.
CONCLUSIONS AND CLINICAL RELEVANCE Findings indicated that dietary composition played a more important role than did short-term weight gain on alterations in insulin sensitivity of horses.