Selective glycogen depletion in skeletal muscle fibres of man following sustained contractions

Brown

Proc. Natl. Acad. Sci. U.S.A.

et al. 1987

Quantitative probing of heterogeneous regions in muscle is feasible with phosphorus-31 magnetic resonance spectroscopy because of the differentiation of metabolic patterns of glycolytic and oxidative fibers. A differential recruitment of oxidative and glycolytic fibers during exercise was demonstrated in 4 of 10 untrained young men by following changes in phosphate metabolites. Concentrations of inorganic phosphate (Pi), phosphocreatine, and ATP were estimated in the wrist flexor muscles of the forearm at rest, during two cycles of three grades ofexercise, and in recovery. At high work levels (40% of maximum strength), two distinct Pi peaks were observed and identified with Pi pools at pH 6.9 and pH 5.9-6.4, respectively. These could be accounted for as follows. At the lowest level of work (using 20% of maximum strength), early recruitment primarily of oxidative (type I) and possibly some intermediate (type HA) muscle fibers occurs with relatively little net lactate production and consequently little decrease in pH. At higher work loads, however, primarily glycolytic (type UB) muscle fibers are recruited, which have relatively high net lactate production and therefore generate a second pool of Pi at low pH. ATP depletion (35-54%) and Pi losses accompanied the reduction in ability to perform during the first exercise cycle. When the cycle of graded exercise was repeated immediately, the total Pi remained high but gave rise to only one peak at pH 6.8-7.0. These observations indicated exhaustion of glycolytic type IIB fibers, removal of lactate by high local blood flow, and sustained contractions largely by oxidative type I and HA fibers. A functional differentiation of fiber types could also be demonstrated during recovery if exercise was stopped while two pools of P1 were still apparent. In the first 3 min of recovery, the Pi peak at pH 6.86.9 disappeared almost entirely, whereas the Pi peak at pH 6.0 remained unaltered, reflecting the faster recovery of oxidative type I fibers. The potential of magnetic resonance spectroscopy to characterize oxidative and glycolytic fibers, predict capacity for aerobic performance, and signal the presence of muscle pathology is discussed. By determining the enzyme and metabolite levels in muscle homogenates and single muscle fibers from serial biopsy samples, the recruitment of oxidative and glycolytic fibers in endurance athletes and untrained subjects has been studied (9)(10)(11)(12)(13)(14). It was concluded that the oxidative muscle fibers, types I and IIA, are recruited first at low work loads (15-18) and the fast-twitch glycolytic fibers, type IIB, are predominantly recruited when the work load is raised (18,19). The oxidative fibers break down relatively little glycogen, oxidize pyruvate readily, thereby minimizing net lactate production, and maintain their content of ATP and phosphocreatine (PCr) during work relatively well. The glycolytic fibers, on the other hand, break down much more glycogen and produce more lactic acid, which they can oxidize only slowly. ...

supporting

confidence: 83%

mentioning

confidence: 99%

mentioning

confidence: 99%

See 1 more Smart Citation

Functional pools of oxidative and glycolytic fibers in human muscle observed by 31P magnetic resonance spectroscopy during exercise.

Brown

Proc. Natl. Acad. Sci. U.S.A.

et al. 1987

Proc. Natl. Acad. Sci. U.S.A.

“…However, Gollnick and colleagues reported that at 3 h of exercise at 15% of maximal voluntary contraction, all slow-twitch fibers and 40% of all fast-twitch fibers were at least partially glycogen depleted (24). Furthermore, 90% of the slow-twitch fibers were significantly depleted.…”

Section: Muscle Fibers and The Lactate Shuttlementioning

confidence: 99%

The “glycogen shunt” in exercising muscle: A role for glycogen in muscle energetics and fatigue

Shulman

Rothman

2001

143

103

Stimulated by recent 13 C and 31 P NMR studies of exercising muscle, we propose a model of the energetics of contraction. Previous studies of energetics have followed energy consumption. However, the rapidity of contraction, in 10 -40 msec, requires that energy be delivered rapidly, so that the muscle has power requirements of rapid energy expenditure that are ultimately met by the slower averaged consumption of carbon and oxygen from blood. We propose that energy is supplied in milliseconds by glycogenolysis and that between contractions, glycogenesis refills the pools. The energy for glycogenesis is supplied by oxidative phosphorylation. This mechanism utilizes the rapid conversion of glycogen phosphorylase, the ''fight-or-flight'' enzyme, to its active form. Lactate is necessarily generated by this pathway to serve as a time buffer between fast and slow energy needs, which resolves the paradoxical generation of lactate in well oxygenated tissue. Consequences of the glycogen shunt are compatible with numerous biochemical and physiological experiments. The model provides a possible mechanism for muscle fatigue, suggesting that at low but nonzero glycogen concentrations, there is not enough glycogen to supply millisecond energy needs.

“…IIC). For this reason, in human muscles, glycogen decreases first in type 1 and type 2A fibers and then in type 2X fibers (272,833). Glycogen depletion is followed by glycogen resynthesis.…”

mentioning

confidence: 99%

Fiber Types in Mammalian Skeletal Muscles

Schiaffino

Reggiani

2011

Physiological Reviews

2,324

2,465

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.