TY - JOUR
T1 - Glycogen supercompensation is due to increased number, not size, of glycogen particles in human skeletal muscle
AU - Jensen, Rasmus
AU - Ørtenblad, Niels
AU - Stausholm, Marie Louise H.
AU - Skjærbæk, Mette C.
AU - Larsen, Daniel N.
AU - Hansen, Mette
AU - Holmberg, Hans Christer
AU - Plomgaard, Peter
AU - Nielsen, Joachim
PY - 2021
Y1 - 2021
N2 - New Findings: What is the central question of this study? Glycogen supercompensation after glycogen-depleting exercise can be achieved by consuming a carbohydrate-enriched diet, but the associated effects on the size, number and localization of intramuscular glycogen particles are unknown. What is the main finding and its importance? Using transmission electron microscopy to inspect individual glycogen particles visually, we show that glycogen supercompensation is achieved by increasing the number of particles while keeping them at submaximal sizes. This might be a strategy to ensure that glycogen particles can be used fast, because particles that are too large might impair utilization rate. Abstract: Glycogen supercompensation after glycogen-depleting exercise can be achieved by consuming a carbohydrate-enriched diet, but the associated effects on the size, number and localization of intramuscular glycogen particles are unknown. We investigated how a glycogen-loading protocol affects fibre type-specific glycogen volume density, particle diameter and numerical density in three subcellular pools: between (intermyofibrillar) or within (intramyofibrillar) the myofibrils or beneath the sarcolemma (subsarcolemmal). Resting muscle biopsies from 11 physically active men were analysed using transmission electron microscopy after mixed (MIX), LOW or HIGH carbohydrate consumption separated by glycogen-lowering cycling at 75% of maximal oxygen consumption until exhaustion. After HIGH, the total volumetric glycogen content was 40% [95% confidence interval 16, 68] higher than after MIX in type I fibres (P < 0.001), with little to no difference in type II fibres (9% [95% confidence interval −9, 27]). Median particle diameter was 22.5 (interquartile range 20.8–24.7) nm across glycogen pools and fibre types, and the numerical density was 61% [25, 107] and 40% [9, 80] higher in the subsarcolemmal (P < 0.001) and intermyofibrillar (P < 0.01) pools of type I fibres, respectively, with little to no difference in the intramyofibrillar pool (3% [−20, 32]). In LOW, total glycogen was in the range of 21–23% lower, relative to MIX, in both fibre types, reflected in a 21–46% lower numerical density across pools. In comparison to MIX, particle diameter was unaffected by other diets ([−1.4, 1.3] nm). In conclusion, glycogen supercompensation after prolonged cycling is exclusive to type I fibres, predominantly in the subsarcolemmal pool, and involves an increase in the numerical density rather than the size of existing glycogen particles.
AB - New Findings: What is the central question of this study? Glycogen supercompensation after glycogen-depleting exercise can be achieved by consuming a carbohydrate-enriched diet, but the associated effects on the size, number and localization of intramuscular glycogen particles are unknown. What is the main finding and its importance? Using transmission electron microscopy to inspect individual glycogen particles visually, we show that glycogen supercompensation is achieved by increasing the number of particles while keeping them at submaximal sizes. This might be a strategy to ensure that glycogen particles can be used fast, because particles that are too large might impair utilization rate. Abstract: Glycogen supercompensation after glycogen-depleting exercise can be achieved by consuming a carbohydrate-enriched diet, but the associated effects on the size, number and localization of intramuscular glycogen particles are unknown. We investigated how a glycogen-loading protocol affects fibre type-specific glycogen volume density, particle diameter and numerical density in three subcellular pools: between (intermyofibrillar) or within (intramyofibrillar) the myofibrils or beneath the sarcolemma (subsarcolemmal). Resting muscle biopsies from 11 physically active men were analysed using transmission electron microscopy after mixed (MIX), LOW or HIGH carbohydrate consumption separated by glycogen-lowering cycling at 75% of maximal oxygen consumption until exhaustion. After HIGH, the total volumetric glycogen content was 40% [95% confidence interval 16, 68] higher than after MIX in type I fibres (P < 0.001), with little to no difference in type II fibres (9% [95% confidence interval −9, 27]). Median particle diameter was 22.5 (interquartile range 20.8–24.7) nm across glycogen pools and fibre types, and the numerical density was 61% [25, 107] and 40% [9, 80] higher in the subsarcolemmal (P < 0.001) and intermyofibrillar (P < 0.01) pools of type I fibres, respectively, with little to no difference in the intramyofibrillar pool (3% [−20, 32]). In LOW, total glycogen was in the range of 21–23% lower, relative to MIX, in both fibre types, reflected in a 21–46% lower numerical density across pools. In comparison to MIX, particle diameter was unaffected by other diets ([−1.4, 1.3] nm). In conclusion, glycogen supercompensation after prolonged cycling is exclusive to type I fibres, predominantly in the subsarcolemmal pool, and involves an increase in the numerical density rather than the size of existing glycogen particles.
KW - carbohydrate
KW - compartmentalization
KW - diet
KW - glycogen loading
KW - transmission electron microscopy
U2 - 10.1113/EP089317
DO - 10.1113/EP089317
M3 - Journal article
C2 - 33675088
AN - SCOPUS:85103212854
VL - 106
SP - 1272
EP - 1284
JO - Experimental Physiology
JF - Experimental Physiology
SN - 0958-0670
IS - 5
ER -