Abstract
Endurance exercise relies on transsarcolemmal flux of substrates in order to avoid depletion of intramuscular reserves. Previous studies of endurance trained sled dogs have shown a remarkable capacity of these dogs to adapt rapidly to endurance exercise by decreasing the utilization of intramuscular reserves. The current study tested the hypothesis that the dogs' glycogen-sparing phenotype is due to increased sarcolemmal transport of glucose and fatty acids. Basal and exercise-induced transport of glucose and fatty acids into sarcolemmal vesicles was evaluated in racing sled dogs prior to and after 7 months of exercise conditioning. Sarcolemmal substrate transport capacity was measured using sarcolemmal vesicles and radiolabelled substrates, and transporter abundance was measured using Western blot quantification in whole muscle homogenates and the sarcolemmal vesicle preparations. Conditioning resulted in increased basal and exercise-induced transport of both glucose and palmitate. Neither acute exercise nor conditioning resulted in changes in muscle content of GLUT4 or FAT/CD36, but conditioning did result in decreased abundance of both transporters in the sarcolemmal vesicles used for the basal transport assays, and this decrease was further amplified in the vesicles used for the exercise-induced transport assays. These results demonstrate conditioning-induced increases in sarcolemmal transport of oxidizable substrates, as well as increased gain of exercise-induced sarcolemmal transport of these substrates. These results further indicate that increased sarcolemmal transport of oxidizable substrates may be due to either an increased intrinsic capacity of the existing transporters or to a different population of transporters from those investigated.
Highlights
Metabolizable substrate uptake in skeletal muscle is regulated in order to meet the dual roles of the tissue of energy storage during periods of overall positive energy balance and energy expenditure during muscular work
Dynamic expression of transporters such as GLUT4 and fatty acid translocase (FAT)/CD36, controlled by the appropriate intracellular and extracellular signals, provides a mechanism for a large range of transsarcolemmal flux that is responsive to acute changes in the need for metabolizable substrate uptake
Multiday exercise did not result in a change in contractioninduced sarcolemmal transport of either glucose or palmitate when compared to -conditioned dogs that were rested prior to treadmill exercise (Glucose: 7.35 (SD 5.26) vs 5.41 (SD 4.05) mmol/mg protein/15 sec, p = 0.28; Palmitate: 32.91 (SD 5.31) vs 27.27 (SD 7.20) pmol/mg prot/15 sec, p = 0.10) (Figures 1-2); the data from these groups were combined for further evaluation of the effects of conditioning on sarcolemmal substrate transport
Summary
Metabolizable substrate uptake in skeletal muscle is regulated in order to meet the dual roles of the tissue of energy storage during periods of overall positive energy balance and energy expenditure during muscular work. Dynamic expression of transporters such as GLUT4 and fatty acid translocase (FAT)/CD36, controlled by the appropriate intracellular and extracellular signals, provides a mechanism for a large range of transsarcolemmal flux that is responsive to acute changes in the need for metabolizable substrate uptake Both GLUT4 and FAT/CD36 are stored intracellularly, but will translocate to the sarcolemma during periods of contraction or insulin stimulation. Endurance-trained racing sled dogs have been shown to sustain large caloric expenditures during competition (nearly 12,000 kcal/ day or over 4000 kJ/kg0.75/day) [3], indicating a tremendous capacity to transport oxidizable substrates into working muscle cells This capacity appears to be highly dynamic and capable of rapid increases within the context of a single multiday exercise challenge. We conducted a set of experiments to test the hypothesis that the development of a glycogen-sparing, fatigue-resistant phenotype is the result of increased sensitivity of exercise-induced translocation of substrate transporters to the sarcolemma
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