The Role of Skeletal Muscle Glycogen Breakdown for Regulation of Insulin Sensitivity by Exercise
Glycogen is the storage form of carbohydrates in mammals. In humans the majority of glycogen is stored in skeletal muscles (~500
g) and the liver (~100
g). Food is supplied in larger meals, but the blood glucose concentration has to be kept within narrow limits to survive and stay healthy. Therefore, the body has to cope with periods of excess carbohydrates and periods without supplementation. Healthy persons remove blood glucose rapidly when glucose is in excess, but insulin-stimulated glucose disposal is reduced in insulin resistant and type 2 diabetic subjects. During a hyperinsulinemic euglycemic clamp, 70–90% of glucose disposal will be stored as muscle glycogen in healthy subjects. The glycogen stores in skeletal muscles are limited because an efficient feedback-mediated inhibition of glycogen synthase prevents accumulation. De novo lipid synthesis can contribute to glucose disposal when glycogen stores are filled. Exercise physiologists normally consider glycogen’s main function as energy substrate. Glycogen is the main energy substrate during exercise intensity above 70% of maximal oxygen uptake (<nobr style="-webkit-transition: none; border: 0px; padding: 0px; margin: 0px; max-width: 5000em; max-height: 5000em; vertical-align: 0px; ">Vo2max</nobr>) and fatigue develops when the glycogen stores are depleted in the active muscles. After exercise, the rate of glycogen synthesis is increased to replete glycogen stores, and blood glucose is the substrate. Indeed insulin-stimulated glucose uptake and glycogen synthesis is elevated after exercise, which, from an evolutional point of view, will favor glycogen repletion and preparation for new “fight or flight” events. In the modern society, the reduced glycogen stores in skeletal muscles after exercise allows carbohydrates to be stored as muscle glycogen and prevents that glucose is channeled to de novo lipid synthesis, which over time will causes ectopic fat accumulation and insulin resistance. The reduction of skeletal muscle glycogen after exercise allows a healthy storage of carbohydrates after meals and prevents development of type 2 diabetes.
Keywords: glycogen phosphorylase, glycogen synthase, exercise, type 2 diabetes, insulin resistance, exercise, de novo lipogenesis
Introduction
Exercise is considered a cornerstone in prevention and treatment of type 2 diabetes and several mechanisms may contribute to the benefits of exercise. Acutely, exercise improves insulin sensitivity in both healthy subjects and insulin resistant people (Heath et al., 1983; Mikines et al., 1988). The improved insulin sensitivity after a single bout of exercise is short-lived but repeated bouts of endurance training improve insulin sensitivity beyond the acute effect of the last training session, and insulin sensitivity correlates with oxidative capacity in skeletal muscles (Koivisto et al., 1986; Bruce et al., 2003). Importantly, the risk for development of type 2 diabetes is reduced by yearlong training (Knowler et al., 2002).
Skeletal muscles are the tissue that transforms chemical energy to mechanical work and therefore uses the majority of energy during exercise; glycogen is the main substrate during high intensity exercise (Hermansen et al., 1967; Romijn et al., 1993). Skeletal muscles are, however, also the major tissue where insulin stimulates glucose uptake to remove glucose from the blood, and the glucose taken up is incorporated into glycogen (DeFronzo et al., 1981b; Shulman et al., 1990). The logic link between glycogen content and insulin sensitivity is also supported experimentally (Jensen et al., 1997).
The flux by which glucose is removed from the blood into skeletal muscle glycogen is the major determinant of insulin sensitivity (Højlund and Beck-Nielsen, 2006). Insulin stimulates glucose uptake via translocation of GLUT4 (Etgen et al., 1996; Larance et al., 2008). Endurance training increases expression of GLUT4 and other proteins involved in insulin signaling and glucose metabolism (Houmard et al., 1993), but the mechanism determining insulin sensitivity remains poorly understood. Nevertheless, the major defect in insulin resistant people is that the non-oxidative glucose disposal (glycogen synthesis) is reduced (Højlund and Beck-Nielsen, 2006). Several reviews have discussed the effect of endurance training on insulin sensitivity from a molecular point of view (Wojtaszewski et al., 2002; Maarbjerg et al., 2011).
Exercise physiologists have performed numerous studies on glycogen utilization during exercise and studied the effects of nutritional supply for optimal glycogen repletion after exercise (Ivy, 2001; Betts and Williams,2010). Rapid glycogen repletion requires that high rates of blood glucose must be taken up by skeletal muscles, and insulin sensitivity is high after exercise. Diabetes is defined by elevated blood glucose and a major defect is that insulin-stimulated glucose uptake and glycogen synthesis is impaired in skeletal muscle (Shulman et al., 1990). A common point at issue for both diabetologists and exercise physiologists is: How can blood glucose rapidly be converted into skeletal muscle glycogen? In the present review we have taken the view of exercise physiologists to discuss the role of skeletal muscle glycogen in regulation of insulin sensitivity.
Glycogen is the molecular form of carbohydrates stored in humans and other mammals. A glycogen particles in skeletal muscles can contain as much as 50,000 glucose moieties linked with α(1
→
4) bonds and branched by α(1
→
6) bonds (Meléndez et al., 1999). In humans, ~80% of the glycogen is stored in skeletal muscles, simply because skeletal muscles account for ~40–50% of body weight in healthy young men and the glycogen concentration is 80–150
mmol
kg
ww[SUP]−1[/SUP] (Ivy et al., 1988; Hawley et al., 1997; Jensen et al., 2011). The liver has a higher glycogen concentration, but as the liver is much smaller (~1.5
kg) and the total amount of liver glycogen is ~100
g (Taylor et al., 1996). Other tissue, like the heart and brain contains minor glycogen stores with important physiological function.
A main function of glycogen is to maintain a physiological blood glucose concentration, but only liver glycogen directly contributes to release of glucose into the blood. Skeletal muscles are unable to release glucose (because muscles lack glucose 6-phosphatase) and muscles glycogen is mainly a local energy substrate for exercise, rather than an energy source to maintain blood glucose concentration during fasting. Indeed, muscle glycogen can be broken down to lactate, which can be transported to the liver and via gluconeogenesis in the liver contribute to maintaining euglycemia (Cori cycle). However, humans do not show major decrease in muscle glycogen content during fasting (Nieman et al., 1987; Vendelbo et al., 2011). In contrast, the liver glycogen content decreases rapidly during fasting and the liver glycogen content has decreased by ~65% after 24
h fasting (Magnusson et al., 1992). So, why is the majority of glycogen stored in muscles?
We believe that the main function of skeletal muscle glycogen, from an evolutional point of view, is to serve as an energy store in “fight or flight” situations. In the heart and the brain, glycogen is also the energy substrate that can generate anaerobic energy during short-term oxygen deficiency contributing to survival (Prebil et al., 2011). Indeed, reduced glycogen content in skeletal muscles increases insulin sensitivity (Jensen et al., 1997), but the increased insulin sensitivity can again be related to the importance to restore glycogen content rapidly for new challenges. Glycogen stored intracellularly is immediately available for energy production, and the rate of energy production far exceeds the flux of glucose into skeletal muscles. Therefore, muscle glycogen may have been important for survival during acute emergencies as substrate for “fight or flight” reactions, whereas accumulated fat has its importance for survival during starvation.
The glycogen content increases slightly by acute intake of large amount of carbohydrates (Hawley et al.,1997). However, an acute bout of glycogen depleting exercise can double glycogen content in skeletal muscles if high amount of carbohydrates are ingested for 3
days (Bergström and Hultman, 1966); this phenomenon is called super compensation. The glycogen content is higher in endurance trained subjects compared to untrained subjects (Hickner et al., 1997), and glycogen content increases in muscles after endurance training (Burgomaster et al., 2005). In contrast, prolonged intake of high amount of carbohydrates does not increase glycogen content in skeletal muscles, and the excess carbohydrate ingested is converted to lipid (Acheson et al., 1988; Jensen, 2009). Therefore, the glycogen content in skeletal muscles from obese and type 2 diabetes subjects is comparable to lean subjects or may even be reduced (Shulman et al., 1990; He and Kelley, 2004). Since exercise increases the glycogen storage capacity in skeletal muscles, it is likely that inactivity will reduce storage capacity. Interestingly, the ratio between glycogen content and oxidative capacity was increased in muscles from obese subjects (He and Kelley, 2004). Is this indicating increased glycogen content relative to the storage capacity in muscles from obese subjects? A reduced glycogen storage capacity in muscles from insulin resistant subjects will cause a stronger feedback inhibition of glycogen synthase at similar glycogen content and deteriorate glucose regulation, and the glycogen content relative to glycogen storage capacity may regulate insulin sensitivity. Indeed, it has been reported that hyperglycemia compensate for impaired insulin-mediated activation of glycogen synthase and glycogen storage in type 2 diabetic subjects (Kelley and Mandarino, 1990; Vaag et al., 1992; Mevorach et al., 1998), but these data also show a defect in regulation of glycogen storage as a higher glucose concentration is required to uphold glycogen synthesis. Such forced glycogen synthesis may increase metabolic stress.
In rats, glycogen content is increased the day after exercise when fed normal chow (Hespel and Richter,1990; Kawanaka et al., 2000) and increased even more when rats have free access to chow and given drink containing glucose (Hespel and Richter, 1990; Derave et al., 2000). Glycogen content is also increased in epitrochlearis muscles when 24
h fasted rats are fed chow for another 24
h; the glycogen content is twice as high in epitrochlearis muscles from fasted–refed rats compared to rats with free access to chow continuously (Jensen et al., 1997, 2006; Lai et al., 2007). Both exercise and fasting decrease glycogen in the muscle where supercompensation occurs (Hespel and Richter, 1990; Jensen et al., 1997, 2006), but is not understood why glycogen content is increased after glycogen depletion.
Insulin regulates many biological functions in skeletal muscle and stimulation of skeletal muscle glucose uptake is one of the most important processes regulated by insulin (Taniguchi et al., 2006). Skeletal muscle has been reported to account for 70–75% of insulin-stimulated glucose disposal during hyperinsulinemic clamps and, therefore, represents a principle tissue mediating whole body glucose homeostasis (DeFronzo et al., 1981a; Shulman et al., 1990). After an oral glucose tolerance test, skeletal muscles also dispose a substantial part of the glucose. It has been reported that 30–40% of the glucose is immediately oxidized after an oral glucose tolerance test, and ~15% of the ingested glucose is stored as muscle glycogen (Kelley et al.,1988). However, after glycogen depleting exercise, more 40% of the ingested glucose can be stored as skeletal muscles glycogen of trained subjects (Hickner et al., 1997; Greiwe et al., 1999). Untrained subjects have lower capacity to store ingested carbohydrates after exercise than endurance trained subjects (Hickner et al., 1997; Greiwe et al., 1999), but exercise will still channel more of the ingested glucose into skeletal muscles glycogen and reduces metabolic stress in untrained subjects.
Insulin stimulates skeletal muscle glucose uptake through an increase of GLUT4 translocation from intracellular storage vesicles to the plasma membrane and transverse tubules (Etgen et al., 1996; Lauritzen et al., 2008). Insulin initiates its effect in skeletal muscle by binding to the insulin receptor, followed by receptor auto-phosphorylation. This induces a series of phosphorylation and protein–protein interactions mediating insulin signaling (Shepherd, 2005). In brief, insulin activates insulin receptor tyrosine kinase activity that increases the tyrosine phosphorylation of insulin receptor substrate (IRS) proteins, which recruit and activates class 1A phosphatidylinositol 3-kinase (PI3K; Figure Figure1).1). Activation of PI3K catalyzes the formation of phosphatidylinositol 3,4,5-trisphosphate (PIP3), which recruits both PDK1 and PKB to the phospholipid, and subsequently allows PKB to be activated through phosphorylation by PDK1 at threonine 308 (Alessi and Cohen, 1998). The mammalian target of rapamycin complexed with Rictor (mTORC2) phosphorylates PKB at serine 473, and phosphorylation of both sites is required for full PKB activity (Alessi and Cohen, 1998; Sarbassov et al., 2005). Several lines of evidence have indicated the critical role of PKB phosphorylation and activation in the regulation of insulin-stimulated glucose uptake (Larance et al., 2008). It is the PKBβ isoform that controls whole body glucose homeostasis (Cleasby et al., 2007; Schultze et al., 2011).
Figure 1
Insulin signaling pathways regulating glucose transport and glycogen synthase in skeletal muscle. Insulin activates protein kinase B (PKB) through phosphatidylinositol 3-kinase (PI3K) and two upstream kinases; namely phosphoinositide-dependent protein ...
PKB-mediated phosphorylation of AS160 and TBC1D1 has recently emerged to regulate insulin-stimulated GLUT4 translocation beyond PKB (Arias et al., 2007; Sakamoto and Holman, 2008). Insulin-stimulated phosphorylation of AS160 and TBC1D1 seems, however, not to be regulated by glycogen content as we did not find correlation between insulin-stimulated glucose uptake and AS160 phosphorylation using the phospho-Akt substrate (PAS) antibody (Lai et al., 2010b).
Insulin also activates glycogen synthase (Cohen, 1993; Jensen and Lai, 2009). Glycogen synthase (GS) is phosphorylated at nine sites and insulin stimulates dephosphorylation of glycogen synthase (Cohen, 1993; Jensen and Lai, 2009). Insulin stimulates dephosphorylation of glycogen synthase via PKB-mediated phosphorylation of GSK3 (McManus et al., 2005; Bouskila et al., 2008; Jensen and Lai, 2009). Phosphorylation of GSK3 decreases kinase activity which will decrease phosphorylation of GS and increase glycogens synthase fractional activity (Lai et al., 2007, 2010b; Jensen and Lai, 2009).
Glycogen synthase is also activated by glucose 6-phosphate and allosteric activation is necessary for normal rate of glycogen synthesis (Jensen and Lai, 2009; Bouskila et al., 2010). Glycogen synthase activity with high concentrations of glucose 6-phosphate (>8
mM) is independent of phosphorylation; activity with high glucose 6-phosphate concentration is called total activity. However, dephosphorylation of glycogen synthase increases affinity for glucose 6-phosphate and glycogen synthase activity with a physiological concentration of glucose 6-phosphate (e.g., 0.17
mM) describes activation of glycogen synthase (Jensen and Lai, 2009).
Recently, a mutated glycogen synthase was developed where phosphorylation-mediated regulation was normal, but allosteric activation by glucose 6-phosphate was abolished (Bouskila et al., 2010). Data achieved with the knockin mice expressing a GS without glucose 6-phophate activation provided seminal information about regulation of glycogen synthase (Brady, 2010). Bouskila et al. (2010) showed that allosteric activation of GS is necessary for regulation of glycogen synthesis in skeletal muscles. Therefore, dephosphorylation of glycogen synthase increases glycogen synthesis mainly by increasing GS affinity for glucose 6-phosphate and allosteric activation. The GS knockin mice without allosteric activation by glucose 6-phosphate also answered the challenging question why AICAR (AMPK activator), which reduces GS fractional activity, increases glycogen content: AICAR stimulates glucose uptake and glucose 6-phosphate mediated GS activation stimulates glycogen synthesis (Hunter et al., 2011).
Impaired insulin-stimulated disposal is a common feature in people with type 2 diabetes, and causes inability to maintain blood glucose in a normal range. Insulin-stimulated glycogen synthesis is reduced in skeletal muscle in insulin resistant people and prevent proper regulation of blood glucose (Shulman et al., 1990) and particularly non-oxidative glucose metabolism is reduced in insulin resistant subjects (Højlund and Beck-Nielsen, 2006). It is also a consistent finding that insulin signaling is reduced at several sites, like PI3K, PKB, GSK3, and GS, in muscle from insulin resistance (Kim et al., 2000; Morino et al., 2005; Højlund and Beck-Nielsen, 2006). Obesity is a strong risk factor for insulin resistance but accumulation of fat per se does not cause insulin resistance, as mice depleted for adipose triglyceride lipase (ATGL) accumulates fat in muscles and heart, but do not develop insulin resistance (Haemmerle et al., 2006). This finding suggest that lipid intermediates like long chain acyl-CoA, diacylglycerol, or ceramides causes insulin resistance (Franch et al.,2002; Samuel et al., 2010).
When insulin is administrated immediately after contraction or exercise, there is an additive increase in glucose uptake. This increased glucose uptake immediately after exercise occurs because the effect of muscle contraction on glucose uptake is still present; e.g., AMPK and glycogen synthase remains activated (Franch et al., 1999; Musi et al., 2001). Insulin-mediated activation of the proximal insulin signaling at the level of IRS1 and PI3K is unchanged after exercise (Wojtaszewski et al., 1999; Jessen et al., 2003). Most studies also report that insulin-stimulated PKB activity is unchanged after exercise (Wojtaszewski et al., 1999; Jessen et al., 2003), but some recent studies revealed that prior contractile activity induces higher insulin-stimulated PKB threonine 308 phosphorylation compared to rested muscles, whereas insulin-stimulated PKB phosphorylation at serine 473 was unchanged by exercise (Arias et al., 2007; Lai et al., 2009). Whether this increased site specific PKB phosphorylation contributes to training-enhanced insulin sensitivity is currently unknown. However, insulin-stimulated phosphorylation of GSK3, the critical regulator of GS activity, was not increased after muscle contraction (Lai et al., 2009, 2010b).
Exercise training enhances insulin sensitivity. It is well established that the enhanced insulin sensitivity after training is associated with adaptations in skeletal muscles such as increased expression of key proteins like GLUT4, hexokinase II, and GS, involved in insulin-stimulated glucose metabolism (Dela et al., 1993; Frosig et al., 2007). However, the signaling event that leads to enhanced insulin sensitivity after exercise training is not conclusive. It has been reported that short-term exercise training increased insulin-stimulated PI3K activity (Houmard et al., 1999), but other studies have reported that insulin-stimulated IRS1-associated PI3K activity is unchanged or reduced after training (Christ-Roberts et al., 2004; Frosig et al., 2007). While the training effect on PI3K activity is inconsistent, several studies have reported that enhanced insulin sensitivity was associated with increased PKB phosphorylation and expression (Christ-Roberts et al., 2004; Frosig et al.,2007; Wadley et al., 2007). Consistent with the increased PKB activation after training, it has also been demonstrated that insulin-mediated AS160 phosphorylation is enhanced after training (Frosig et al., 2007; Vind et al., 2011). However, exercise normalized insulin-mediated AS160 phosphorylation in skeletal muscle from type 2 diabetic subjects but without normalizing insulin-stimulated glucose disposal (Vind et al., 2011).
Exercise training also increases insulin-stimulated glucose uptake and GLUT4 translocation in muscles from obese Zucker rats (Etgen et al., 1997). Skeletal muscles from the obese Zucker rats develop severe insulin resistance and impaired insulin signaling (Christ et al., 2002). However, although training increases insulin-stimulated glucose uptake in skeletal from obese Zucker rats, insulin-mediated activation of PI3K and PKB remained low after training (Christ et al., 2002). The signaling mechanisms which increase insulin-stimulated glucose uptake after training remain to be determined.
Energy consumption at rest is low; oxygen uptake at rest is typically ~0.25
L O[SUB]2[/SUB] and carbohydrate oxidation is ~0.1
g
min[SUP]−1[/SUP] (Hermansen et al., 1967; van Loon et al., 2001), and the rate of carbohydrate oxidation gradually decreases during fasting. At rest, the rate of carbohydrate oxidation depends mainly on the diet and exercise prior to measurements, and the glycogen utilization in skeletal muscles at rest is low or absent (van Loon et al., 2001).
The utilization of carbohydrate during exercise can easily be calculated from oxygen uptake(<nobr style="-webkit-transition: none; border: 0px; padding: 0px; margin: 0px; max-width: 5000em; max-height: 5000em; vertical-align: 0px; ">VO2</nobr>) and respiratory exchange ratio (RER). Normally carbohydrate oxidation is calculated without taking protein oxidation in consideration; tables and formulae have been published for such calculations (Frayn, 1983; Peronnet and Massicotte, 1991). The relative (as well as absolute) rate of carbohydrate oxidation depends on exercise intensity and well-trained persons have a much higher capacity to metabolize glucose and fat compared to untrained persons. During exercise above 70% the major carbohydrate source is muscle glycogen (Romijn et al., 1993; van Loon et al., 2001).
The physical form of humans are determined by their capacity to oxidize energy substrates (carbohydrates and fat), which is reflected in ability to utilize oxygen. Maximal oxygen uptake is used to describe oxidative capacity, and values of 40–50
ml
kg[SUP]−1[/SUP]
min[SUP]−1[/SUP] are common in healthy young men. However,<nobr style="-webkit-transition: none; border: 0px; padding: 0px; margin: 0px; max-width: 5000em; max-height: 5000em; vertical-align: 0px; ">VO2max</nobr> can vary from below 15
ml
kg[SUP]−1[/SUP]
min[SUP]−1[/SUP] in elderly people to more than 90
ml
kg[SUP]−1[/SUP]
min[SUP]−1[/SUP] in some endurance athletes. Capacity for carbohydrate oxidation varies correspondingly. Although, well-trained people utilize more fat during exercise, there is huge variation in carbohydrate oxidation. Well-trained subjects can more than oxidize 3
g
min[SUP]−1[/SUP] (Hermansen et al., 1967) which results in oxidation of 180
g carbohydrate during 1
h of intense exercise.
During cycling, ~20
kg of muscle is active (Boushel et al., 2011) and cycling is the preferred type of activity in exercise physiology. Several studies have investigated glycogen breakdown during cycling and exercise intensity cannot be maintained when the active muscles are depleted for glycogen (Hermansen et al., 1967). Hermansen et al. (1967) reported a glycogen content of only 7
mmol
kg
ww[SUP]−1[/SUP]at exhaustion after cycling at 75% of <nobr style="-webkit-transition: none; border: 0px; padding: 0px; margin: 0px; max-width: 5000em; max-height: 5000em; vertical-align: 0px; ">VO2max</nobr> Most studies find low glycogen content at exhaustion, but the degree of depletion depends of the exercise intensity, and the glycogen depletion is most pronounced when cycling to exhaustion at ~75% of<nobr style="-webkit-transition: none; border: 0px; padding: 0px; margin: 0px; max-width: 5000em; max-height: 5000em; vertical-align: 0px; ">VO2max</nobr> (Saltin and Karlsson, 1971). Most studies report glycogen concentration of 7–20
mmol
kg
ww[SUP]−1[/SUP] in m. vastus lateralis after cycling to exhaustion (Hermansen et al., 1967; Nieman et al., 1987; Hickner et al.,1997). Glycogen concentration in m. vastus lateralis is typically 80–150
mmol
kg
ww[SUP]−1[/SUP] in rested muscles (Coyle et al., 1986; Nieman et al., 1987; Hawley et al., 1997; van Loon et al., 2001).
During running, the energy consumption is ~1
kcal
kg[SUP]−1[/SUP]
km[SUP]−1[/SUP] (Åstrand and Rodahl, 1992). This means that an 85-kg person will use about 850
kcal during a 10-km run; 850
kcal corresponds to ~200
g carbohydrate or ~90
g fat. During exercise, carbohydrates and fat are used simultaneously. During running, a larger muscle mass is used and less glycogen is broken down in the leg muscles and m. gastrocnemius is not depleted for glycogen at exhaustion (Madsen et al., 1990). Cross-country skiing mainly depletes glycogen stores in arms (Ortenblad et al., 2011).
The intensity of exercise, together with duration, determines the amount of energy used in the training session. High intensity intermittent training (HIT) is often performed as 30
s “all-out” cycling in experiments. The power that can be produced during 30
s “all-out” corresponds to ~250% of <nobr style="-webkit-transition: none; border: 0px; padding: 0px; margin: 0px; max-width: 5000em; max-height: 5000em; vertical-align: 0px; ">VO2max</nobr> (Gibala et al., 2006) and 3–5
min rest is typically allowed between bouts. The metabolism in skeletal muscles during the moderate intensity training and HIT differs dramatically. During HIT anaerobic provides the major part of energy, which is repaid with aerobic processes in the rest periods. During prolonged continuous exercise energy consumption will be rather stable, and skeletal muscle glycogen content will be reduced by 50–70% after 60
min cycling at 75% of <nobr style="-webkit-transition: none; border: 0px; padding: 0px; margin: 0px; max-width: 5000em; max-height: 5000em; vertical-align: 0px; ">VO2max</nobr> (Hermansen et al., 1967; Saltin and Karlsson, 1971).
During high intensity training the power output is high with substantial anaerobic energy turn over and high adrenaline concentration. Jacobs et al. (1982) reported that a single 30
s all-out cycling decreased glycogen content by 22% corresponding to ~20
mmol
kg
ww[SUP]−1[/SUP]. Esbjornsson-Liljedahl et al. (1999) also found that a single 30
s all-out cycling in males and females decreased glycogen content by ~25% in both type I and type II fibers. Furthermore, three bouts of 30
s all-out cycling with 20
min rest between sprints decreased glycogen content by more than 50% in type II fibers and nearly 50% in type I fibers in both females and males (Esbjornsson-Liljedahl et al., 2002). These data show that high intensity training effectively decreases glycogen content in skeletal muscles.
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AbstractGlycogen is the storage form of carbohydrates in mammals. In humans the majority of glycogen is stored in skeletal muscles (~500
Keywords: glycogen phosphorylase, glycogen synthase, exercise, type 2 diabetes, insulin resistance, exercise, de novo lipogenesis
Introduction
Exercise is considered a cornerstone in prevention and treatment of type 2 diabetes and several mechanisms may contribute to the benefits of exercise. Acutely, exercise improves insulin sensitivity in both healthy subjects and insulin resistant people (Heath et al., 1983; Mikines et al., 1988). The improved insulin sensitivity after a single bout of exercise is short-lived but repeated bouts of endurance training improve insulin sensitivity beyond the acute effect of the last training session, and insulin sensitivity correlates with oxidative capacity in skeletal muscles (Koivisto et al., 1986; Bruce et al., 2003). Importantly, the risk for development of type 2 diabetes is reduced by yearlong training (Knowler et al., 2002).
Skeletal muscles are the tissue that transforms chemical energy to mechanical work and therefore uses the majority of energy during exercise; glycogen is the main substrate during high intensity exercise (Hermansen et al., 1967; Romijn et al., 1993). Skeletal muscles are, however, also the major tissue where insulin stimulates glucose uptake to remove glucose from the blood, and the glucose taken up is incorporated into glycogen (DeFronzo et al., 1981b; Shulman et al., 1990). The logic link between glycogen content and insulin sensitivity is also supported experimentally (Jensen et al., 1997).
The flux by which glucose is removed from the blood into skeletal muscle glycogen is the major determinant of insulin sensitivity (Højlund and Beck-Nielsen, 2006). Insulin stimulates glucose uptake via translocation of GLUT4 (Etgen et al., 1996; Larance et al., 2008). Endurance training increases expression of GLUT4 and other proteins involved in insulin signaling and glucose metabolism (Houmard et al., 1993), but the mechanism determining insulin sensitivity remains poorly understood. Nevertheless, the major defect in insulin resistant people is that the non-oxidative glucose disposal (glycogen synthesis) is reduced (Højlund and Beck-Nielsen, 2006). Several reviews have discussed the effect of endurance training on insulin sensitivity from a molecular point of view (Wojtaszewski et al., 2002; Maarbjerg et al., 2011).
Exercise physiologists have performed numerous studies on glycogen utilization during exercise and studied the effects of nutritional supply for optimal glycogen repletion after exercise (Ivy, 2001; Betts and Williams,2010). Rapid glycogen repletion requires that high rates of blood glucose must be taken up by skeletal muscles, and insulin sensitivity is high after exercise. Diabetes is defined by elevated blood glucose and a major defect is that insulin-stimulated glucose uptake and glycogen synthesis is impaired in skeletal muscle (Shulman et al., 1990). A common point at issue for both diabetologists and exercise physiologists is: How can blood glucose rapidly be converted into skeletal muscle glycogen? In the present review we have taken the view of exercise physiologists to discuss the role of skeletal muscle glycogen in regulation of insulin sensitivity.
Go to:
GlycogenGlycogen is the molecular form of carbohydrates stored in humans and other mammals. A glycogen particles in skeletal muscles can contain as much as 50,000 glucose moieties linked with α(1
A main function of glycogen is to maintain a physiological blood glucose concentration, but only liver glycogen directly contributes to release of glucose into the blood. Skeletal muscles are unable to release glucose (because muscles lack glucose 6-phosphatase) and muscles glycogen is mainly a local energy substrate for exercise, rather than an energy source to maintain blood glucose concentration during fasting. Indeed, muscle glycogen can be broken down to lactate, which can be transported to the liver and via gluconeogenesis in the liver contribute to maintaining euglycemia (Cori cycle). However, humans do not show major decrease in muscle glycogen content during fasting (Nieman et al., 1987; Vendelbo et al., 2011). In contrast, the liver glycogen content decreases rapidly during fasting and the liver glycogen content has decreased by ~65% after 24
We believe that the main function of skeletal muscle glycogen, from an evolutional point of view, is to serve as an energy store in “fight or flight” situations. In the heart and the brain, glycogen is also the energy substrate that can generate anaerobic energy during short-term oxygen deficiency contributing to survival (Prebil et al., 2011). Indeed, reduced glycogen content in skeletal muscles increases insulin sensitivity (Jensen et al., 1997), but the increased insulin sensitivity can again be related to the importance to restore glycogen content rapidly for new challenges. Glycogen stored intracellularly is immediately available for energy production, and the rate of energy production far exceeds the flux of glucose into skeletal muscles. Therefore, muscle glycogen may have been important for survival during acute emergencies as substrate for “fight or flight” reactions, whereas accumulated fat has its importance for survival during starvation.
The glycogen content increases slightly by acute intake of large amount of carbohydrates (Hawley et al.,1997). However, an acute bout of glycogen depleting exercise can double glycogen content in skeletal muscles if high amount of carbohydrates are ingested for 3
In rats, glycogen content is increased the day after exercise when fed normal chow (Hespel and Richter,1990; Kawanaka et al., 2000) and increased even more when rats have free access to chow and given drink containing glucose (Hespel and Richter, 1990; Derave et al., 2000). Glycogen content is also increased in epitrochlearis muscles when 24
Go to:
Insulin-Stimulated Glucose UptakeInsulin regulates many biological functions in skeletal muscle and stimulation of skeletal muscle glucose uptake is one of the most important processes regulated by insulin (Taniguchi et al., 2006). Skeletal muscle has been reported to account for 70–75% of insulin-stimulated glucose disposal during hyperinsulinemic clamps and, therefore, represents a principle tissue mediating whole body glucose homeostasis (DeFronzo et al., 1981a; Shulman et al., 1990). After an oral glucose tolerance test, skeletal muscles also dispose a substantial part of the glucose. It has been reported that 30–40% of the glucose is immediately oxidized after an oral glucose tolerance test, and ~15% of the ingested glucose is stored as muscle glycogen (Kelley et al.,1988). However, after glycogen depleting exercise, more 40% of the ingested glucose can be stored as skeletal muscles glycogen of trained subjects (Hickner et al., 1997; Greiwe et al., 1999). Untrained subjects have lower capacity to store ingested carbohydrates after exercise than endurance trained subjects (Hickner et al., 1997; Greiwe et al., 1999), but exercise will still channel more of the ingested glucose into skeletal muscles glycogen and reduces metabolic stress in untrained subjects.
Insulin stimulates skeletal muscle glucose uptake through an increase of GLUT4 translocation from intracellular storage vesicles to the plasma membrane and transverse tubules (Etgen et al., 1996; Lauritzen et al., 2008). Insulin initiates its effect in skeletal muscle by binding to the insulin receptor, followed by receptor auto-phosphorylation. This induces a series of phosphorylation and protein–protein interactions mediating insulin signaling (Shepherd, 2005). In brief, insulin activates insulin receptor tyrosine kinase activity that increases the tyrosine phosphorylation of insulin receptor substrate (IRS) proteins, which recruit and activates class 1A phosphatidylinositol 3-kinase (PI3K; Figure Figure1).1). Activation of PI3K catalyzes the formation of phosphatidylinositol 3,4,5-trisphosphate (PIP3), which recruits both PDK1 and PKB to the phospholipid, and subsequently allows PKB to be activated through phosphorylation by PDK1 at threonine 308 (Alessi and Cohen, 1998). The mammalian target of rapamycin complexed with Rictor (mTORC2) phosphorylates PKB at serine 473, and phosphorylation of both sites is required for full PKB activity (Alessi and Cohen, 1998; Sarbassov et al., 2005). Several lines of evidence have indicated the critical role of PKB phosphorylation and activation in the regulation of insulin-stimulated glucose uptake (Larance et al., 2008). It is the PKBβ isoform that controls whole body glucose homeostasis (Cleasby et al., 2007; Schultze et al., 2011).
Insulin signaling pathways regulating glucose transport and glycogen synthase in skeletal muscle. Insulin activates protein kinase B (PKB) through phosphatidylinositol 3-kinase (PI3K) and two upstream kinases; namely phosphoinositide-dependent protein ...
PKB-mediated phosphorylation of AS160 and TBC1D1 has recently emerged to regulate insulin-stimulated GLUT4 translocation beyond PKB (Arias et al., 2007; Sakamoto and Holman, 2008). Insulin-stimulated phosphorylation of AS160 and TBC1D1 seems, however, not to be regulated by glycogen content as we did not find correlation between insulin-stimulated glucose uptake and AS160 phosphorylation using the phospho-Akt substrate (PAS) antibody (Lai et al., 2010b).
Insulin also activates glycogen synthase (Cohen, 1993; Jensen and Lai, 2009). Glycogen synthase (GS) is phosphorylated at nine sites and insulin stimulates dephosphorylation of glycogen synthase (Cohen, 1993; Jensen and Lai, 2009). Insulin stimulates dephosphorylation of glycogen synthase via PKB-mediated phosphorylation of GSK3 (McManus et al., 2005; Bouskila et al., 2008; Jensen and Lai, 2009). Phosphorylation of GSK3 decreases kinase activity which will decrease phosphorylation of GS and increase glycogens synthase fractional activity (Lai et al., 2007, 2010b; Jensen and Lai, 2009).
Glycogen synthase is also activated by glucose 6-phosphate and allosteric activation is necessary for normal rate of glycogen synthesis (Jensen and Lai, 2009; Bouskila et al., 2010). Glycogen synthase activity with high concentrations of glucose 6-phosphate (>8
Recently, a mutated glycogen synthase was developed where phosphorylation-mediated regulation was normal, but allosteric activation by glucose 6-phosphate was abolished (Bouskila et al., 2010). Data achieved with the knockin mice expressing a GS without glucose 6-phophate activation provided seminal information about regulation of glycogen synthase (Brady, 2010). Bouskila et al. (2010) showed that allosteric activation of GS is necessary for regulation of glycogen synthesis in skeletal muscles. Therefore, dephosphorylation of glycogen synthase increases glycogen synthesis mainly by increasing GS affinity for glucose 6-phosphate and allosteric activation. The GS knockin mice without allosteric activation by glucose 6-phosphate also answered the challenging question why AICAR (AMPK activator), which reduces GS fractional activity, increases glycogen content: AICAR stimulates glucose uptake and glucose 6-phosphate mediated GS activation stimulates glycogen synthesis (Hunter et al., 2011).
Impaired insulin-stimulated disposal is a common feature in people with type 2 diabetes, and causes inability to maintain blood glucose in a normal range. Insulin-stimulated glycogen synthesis is reduced in skeletal muscle in insulin resistant people and prevent proper regulation of blood glucose (Shulman et al., 1990) and particularly non-oxidative glucose metabolism is reduced in insulin resistant subjects (Højlund and Beck-Nielsen, 2006). It is also a consistent finding that insulin signaling is reduced at several sites, like PI3K, PKB, GSK3, and GS, in muscle from insulin resistance (Kim et al., 2000; Morino et al., 2005; Højlund and Beck-Nielsen, 2006). Obesity is a strong risk factor for insulin resistance but accumulation of fat per se does not cause insulin resistance, as mice depleted for adipose triglyceride lipase (ATGL) accumulates fat in muscles and heart, but do not develop insulin resistance (Haemmerle et al., 2006). This finding suggest that lipid intermediates like long chain acyl-CoA, diacylglycerol, or ceramides causes insulin resistance (Franch et al.,2002; Samuel et al., 2010).
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Effect of Exercise on Insulin Sensitivity and Insulin SignalingWhen insulin is administrated immediately after contraction or exercise, there is an additive increase in glucose uptake. This increased glucose uptake immediately after exercise occurs because the effect of muscle contraction on glucose uptake is still present; e.g., AMPK and glycogen synthase remains activated (Franch et al., 1999; Musi et al., 2001). Insulin-mediated activation of the proximal insulin signaling at the level of IRS1 and PI3K is unchanged after exercise (Wojtaszewski et al., 1999; Jessen et al., 2003). Most studies also report that insulin-stimulated PKB activity is unchanged after exercise (Wojtaszewski et al., 1999; Jessen et al., 2003), but some recent studies revealed that prior contractile activity induces higher insulin-stimulated PKB threonine 308 phosphorylation compared to rested muscles, whereas insulin-stimulated PKB phosphorylation at serine 473 was unchanged by exercise (Arias et al., 2007; Lai et al., 2009). Whether this increased site specific PKB phosphorylation contributes to training-enhanced insulin sensitivity is currently unknown. However, insulin-stimulated phosphorylation of GSK3, the critical regulator of GS activity, was not increased after muscle contraction (Lai et al., 2009, 2010b).
Exercise training enhances insulin sensitivity. It is well established that the enhanced insulin sensitivity after training is associated with adaptations in skeletal muscles such as increased expression of key proteins like GLUT4, hexokinase II, and GS, involved in insulin-stimulated glucose metabolism (Dela et al., 1993; Frosig et al., 2007). However, the signaling event that leads to enhanced insulin sensitivity after exercise training is not conclusive. It has been reported that short-term exercise training increased insulin-stimulated PI3K activity (Houmard et al., 1999), but other studies have reported that insulin-stimulated IRS1-associated PI3K activity is unchanged or reduced after training (Christ-Roberts et al., 2004; Frosig et al., 2007). While the training effect on PI3K activity is inconsistent, several studies have reported that enhanced insulin sensitivity was associated with increased PKB phosphorylation and expression (Christ-Roberts et al., 2004; Frosig et al.,2007; Wadley et al., 2007). Consistent with the increased PKB activation after training, it has also been demonstrated that insulin-mediated AS160 phosphorylation is enhanced after training (Frosig et al., 2007; Vind et al., 2011). However, exercise normalized insulin-mediated AS160 phosphorylation in skeletal muscle from type 2 diabetic subjects but without normalizing insulin-stimulated glucose disposal (Vind et al., 2011).
Exercise training also increases insulin-stimulated glucose uptake and GLUT4 translocation in muscles from obese Zucker rats (Etgen et al., 1997). Skeletal muscles from the obese Zucker rats develop severe insulin resistance and impaired insulin signaling (Christ et al., 2002). However, although training increases insulin-stimulated glucose uptake in skeletal from obese Zucker rats, insulin-mediated activation of PI3K and PKB remained low after training (Christ et al., 2002). The signaling mechanisms which increase insulin-stimulated glucose uptake after training remain to be determined.
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Glycogen Utilization During ExerciseEnergy consumption at rest is low; oxygen uptake at rest is typically ~0.25
The utilization of carbohydrate during exercise can easily be calculated from oxygen uptake(<nobr style="-webkit-transition: none; border: 0px; padding: 0px; margin: 0px; max-width: 5000em; max-height: 5000em; vertical-align: 0px; ">VO2</nobr>) and respiratory exchange ratio (RER). Normally carbohydrate oxidation is calculated without taking protein oxidation in consideration; tables and formulae have been published for such calculations (Frayn, 1983; Peronnet and Massicotte, 1991). The relative (as well as absolute) rate of carbohydrate oxidation depends on exercise intensity and well-trained persons have a much higher capacity to metabolize glucose and fat compared to untrained persons. During exercise above 70% the major carbohydrate source is muscle glycogen (Romijn et al., 1993; van Loon et al., 2001).
The physical form of humans are determined by their capacity to oxidize energy substrates (carbohydrates and fat), which is reflected in ability to utilize oxygen. Maximal oxygen uptake is used to describe oxidative capacity, and values of 40–50
During cycling, ~20
During running, the energy consumption is ~1
The intensity of exercise, together with duration, determines the amount of energy used in the training session. High intensity intermittent training (HIT) is often performed as 30
During high intensity training the power output is high with substantial anaerobic energy turn over and high adrenaline concentration. Jacobs et al. (1982) reported that a single 30
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