Mechanism of glycogen supercompensation in rat skeletal muscle cultures

The role of glycogen phosphorylase in glycogen biogenesis in skeletal muscle after exercise

Initially it was believed that phosphorylase was responsible for both glycogen breakdown and synthesis in the living cell. The discovery of glycogen synthase and McArdle's disease (lack of phosphorylase activity), together with the high P_i/glucose 1-P ratio in skeletal muscle, demonstrated that glycogen synthesis could not be attributed to reversal of the phosphorylase reaction. Rather, glycogen synthesis was attributable solely to the activity of glycogen synthase, subsequent to the transport of glucose into the cell. However, the well-established observation that phosphorylase was inactivated (i.e., dephosphorylated) during the initial recovery period after prior exercise, when the rate of glycogen accumulation is highest and independent of insulin, suggested that phosphorylase could play an active role in glycogen accumulation. But the quantitative contribution of phosphorylase inactivation was not established until recently, when studying isolated murine muscle preparations during recovery from repeated contractions at temperatures ranging from 25 to 35 °C. Thus, in both slow-twitch, oxidative and fast-twitch, glycolytic muscles, inactivation of phosphorylase accounted for 45%-75% of glycogen accumulation during the initial hours of recovery following repeated contractions. Such data indicate that phosphorylase inactivation may be the most important mechanism for glycogen accumulation under defined conditions. These results support the initial belief that phosphorylase plays a quantitative role in glycogen formation in the living cell. However, the mechanism is not via activation of phosphorylase, but rather via inactivation of the enzyme.

A century of exercise physiology: key concepts in regulation of glycogen metabolism in skeletal muscle

Glycogen is a branched, glucose polymer and the storage form of glucose in cells. Glycogen has traditionally been viewed as a key substrate for muscle ATP production during conditions of high energy demand and considered to be limiting for work capacity and force generation under defined conditions. Glycogenolysis is catalyzed by phosphorylase, while glycogenesis is catalyzed by glycogen synthase. For many years, it was believed that a primer was required for de novo glycogen synthesis and the protein considered responsible for this process was ultimately discovered and named glycogenin. However, the subsequent observation of glycogen storage in the absence of functional glycogenin raises questions about the true role of the protein. In resting muscle, phosphorylase is generally considered to be present in two forms: non-phosphorylated and inactive (phosphorylase b) and phosphorylated and constitutively active (phosphorylase a). Initially, it was believed that activation of phosphorylase during intense muscle contraction was primarily accounted for by phosphorylation of phosphorylase b (activated by increases in AMP) to a, and that glycogen synthesis during recovery from exercise occurred solely through mechanisms controlled by glucose transport and glycogen synthase. However, it now appears that these views require modifications. Moreover, the traditional roles of glycogen in muscle function have been extended in recent years and in some instances, the original concepts have undergone revision. Thus, despite the extensive amount of knowledge accrued during the past 100 years, several critical questions remain regarding the regulation of glycogen metabolism and its role in living muscle.

Hypoxia-induced GBE1 expression promotes tumor progression through metabolic reprogramming in lung adenocarcinoma

Hypoxia mediates a metabolic switch from oxidative phosphorylation to glycolysis and increases glycogen synthesis. We previously found that glycogen branching enzyme (GBE1) is downstream of the hypoxia-inducible factor-1 (HIF1) signaling pathway in lung adenocarcinoma (LUAD) cells; however, the molecular mechanism underlying HIF1 regulation of GBE1 expression remains unknown. Herein, the effect of GBE1 on tumor progression via changes in metabolic signaling under hypoxia in vitro and in vivo was evaluated, and GBE1-related genes from human specimens and data sets were analyzed. Hypoxia induced GBE1 upregulation in LUAD cells. GBE1-knockdown A549 cells showed impaired cell proliferation, clone formation, cell migration and invasion, angiogenesis, tumor growth, and metastasis. GBE1 mediated the metabolic reprogramming of LUAD cells. The expression of gluconeogenesis pathway molecules, especially fructose-1,6-bisphosphatase (FBP1), was markedly higher in shGBE1 A549 cells than it was in the control cells. FBP1 inhibited the tumor progression of LUAD. GBE1-mediated FBP1 suppression via promoter methylation enhanced HIF1α levels through NF-κB signaling. GBE1 may be a negative prognostic biomarker for LUAD patients. Altogether, hypoxia-induced HIF1α mediated GBE1 upregulation, suppressing FBP1 expression by promoter methylation via NF-κB signaling in LUAD cells. FBP1 blockade upregulated HIF1α, triggered the switch to anaerobic glycolysis, and enhanced glucose uptake. Therefore, targeting HIF1α/GBE1/NF-κB/FBP1 signaling may be a potential therapeutic strategy for LUAD.

Gintonin, a Ginseng-Derived Exogenous Lysophosphatidic Acid Receptor Ligand, Protects Astrocytes from Hypoxic and Re-oxygenation Stresses Through Stimulation of Astrocytic Glycogenolysis

Astrocytes are a unique brain cell-storing glycogen and express lysophosphatidic acid (LPA) receptors. Gintonin is a ginseng-derived exogenous G protein-coupled LPA receptor ligand. Accumulating evidence shows that astrocytes serve as an energy supplier to neurons through astrocytic glycogenolysis under physiological and pathophysiological conditions. However, little is known about the relationships between LPA receptors and astrocytic glycogenolysis or about the roles of LPA receptors in hypoxia and re-oxygenation stresses. In the present study, we examined the functions of gintonin-mediated astrocytic glycogenolysis in adenosine triphosphate (ATP) production, glutamate uptake, and cell viability under normoxic, hypoxic, and re-oxygenation conditions. The application of gintonin or LPA to astrocytes induced glycogenolysis in concentration- and time-dependent manners. The stimulation of gintonin-mediated astrocytic glycogenolysis was achieved through the LPA receptor-Gα_q/11 protein-phospholipase C-inositol 1,4,5-trisphosphate receptor-intracellular calcium ([Ca²⁺]_i) transient pathway. Gintonin treatment to astrocytes increased the phosphorylation of brain phosphorylase kinase, with sensitive manner to K252a, an inhibitor of phosphorylase kinase. Gintonin-mediated astrocytic glycogenolysis was blocked by isofagomine, a glycogen phosphorylase inhibitor. Gintonin additionally increased astrocytic glycogenolysis under hypoxic and re-oxygenation conditions. Moreover, gintonin increased ATP production, glutamate uptake, and cell viability under the hypoxic and re-oxygenation conditions. Collectively, we found that the gintonin-mediated [Ca²⁺]_i transients regulated by LPA receptors were coupled to astrocytic glycogenolysis and that stimulation of gintonin-mediated astrocytic glycogenolysis was coupled to ATP production and glutamate uptake under hypoxic and re-oxygenation conditions, ultimately protecting astrocytes. Hence, the gintonin-mediated astrocytic energy that is modulated via LPA receptors helps to protect astrocytes under hypoxia and re-oxygenation stresses.

Leucine-Enriched Essential Amino Acids Augment Muscle Glycogen Content in Rats Seven Days after Eccentric Contraction

Eccentric contractions induce muscle damage, which impairs recovery of glycogen and adenosine tri-phosphate (ATP) content over several days. Leucine-enriched essential amino acids (LEAAs) enhance the recovery in muscles that are damaged after eccentric contractions. However, the role of LEAAs in this process remains unclear. We evaluated the content in glycogen and high energy phosphates molecules (phosphocreatine (PCr), adenosine di-phosphate (ADP) and ATP) in rats that were following electrically stimulated eccentric contractions. Muscle glycogen content decreased immediately after the contraction and remained low for the first three days after the stimulation, but increased seven days after the eccentric contraction. LEAAs administration did not change muscle glycogen content during the first three days after the contraction. Interestingly, however, it induced a further increase in muscle glycogen seven days after the stimulation. Contrarily, ATP content decreased immediately after the eccentric contraction, and remained lower for up to seven days after. Additionally, LEAAs administration did not affect the ATP content over the experimental period. Finally, ADP and PCr levels did not significantly change after the contractions or LEAA administration. LEAAs modulate the recovery of glycogen content in muscle after damage-inducing exercise.

Electrical pulse stimulation of cultured skeletal muscle cells as a model for in vitro exercise - possibilities and limitations

The beneficial health-related effects of exercise are well recognized, and numerous studies have investigated underlying mechanism using various in vivo and in vitro models. Although electrical pulse stimulation (EPS) for the induction of muscle contraction has been used for quite some time, its application on cultured skeletal muscle cells of animal or human origin as a model of in vitro exercise is a more recent development. In this review, we compare in vivo exercise and in vitro EPS with regard to effects on signalling, expression level and metabolism. We provide a comprehensive overview of different EPS protocols and their applications, discuss technical aspects of this model including critical controls and the importance of a proper maintenance procedure and finally discuss the limitations of the EPS model.

Weak electromagnetic fields alter Ca(2+) handling and protect against hypoxia-mediated damage in primary newborn rat myotube cultures

Weak electromagnetic fields (WEF) enhance Ca(2+) entry into cells via voltage-gated Ca(2+) channels and affect various aspects of metabolism, structure, and function. However, little information is available on the effect of WEF on skeletal muscle, which depends primarily on intracellular Ca(2+) stores for function and metabolism. Here, we examine the effects of 30 min exposure of rat primary myotube cultures to WEF (1.75 μT, 16 Hz) on Ca(2+) handling and creatine kinase (CK) release. Free myoplasmic Ca(2+) concentration ([Ca(2+) i]) was measured with the ratiometric dye indo-1. WEF did not affect basal [Ca(2+)]i but decreased the twitch [Ca(2+)]i transient in a time-dependent manner, and the twitch amplitude was decreased to ∼30 % after 30 min. WEF completely abolished the increase in [Ca(2+)]i induced by potassium chloride (∼60 mM) but had no effect on the increase induced by caffeine (∼6 mM). Hypoxia (2 h exposure to 100 % argon) resulted in a marked loss of CK into the medium (400 % of normoxic value), as well as a rapid (within 20 min) and sustained increase in basal [Ca(2+)]i (∼20 % above baseline). However, during exposure to WEF, basal [Ca(2+)]i remained constant during the initial 60 min of hypoxia and, thereafter, increased to levels similar to those observed in the absence of WEF. Finally, WEF blocked about 80 % of hypoxia-mediated CK release (P < 0.05). These data demonstrate that WEF inhibits increases in [Ca(2+)]i by interfering with muscle excitation and protects against muscle damage induced by hypoxia. Thus, WEF may have therapeutic/protective effects on skeletal muscle.

Glycogen metabolism in cancer

Since its identification more than 150 years ago, there has been an extensive characterisation of glycogen metabolism and its regulatory pathways in the two main glycogen storage organs of the body, i.e. liver and muscle. In recent years, glycogen metabolism has also been demonstrated to be upregulated in many tumour types, suggesting it is an important aspect of cancer cell pathophysiology. Here, we provide an overview of glycogen metabolism and its regulation, with a focus on its role in metabolic reprogramming of cancer cells. The various methods to detect glycogen in tumours in vivo are also reviewed. Finally, we discuss the targeting of glycogen metabolism as a strategy for cancer treatment.

Molecular basis of impaired glycogen metabolism during ischemic stroke and hypoxia

Background

Ischemic stroke is the combinatorial effect of many pathological processes including the loss of energy supplies, excessive intracellular calcium accumulation, oxidative stress, and inflammatory responses. The brain's ability to maintain energy demand through this process involves metabolism of glycogen, which is critical for release of stored glucose. However, regulation of glycogen metabolism in ischemic stroke remains unknown. In the present study, we investigate the role and regulation of glycogen metabolizing enzymes and their effects on the fate of glycogen during ischemic stroke.

Results

Ischemic stroke was induced in rats by peri-vascular application of the vasoconstrictor endothelin-1 and forebrains were collected at 1, 3, 6 and 24 hours post-stroke. Glycogen levels and the expression and activity of enzymes involved in glycogen metabolism were analyzed. We found elevated glycogen levels in the ipsilateral hemispheres compared with contralateral hemispheres at 6 and 24 hours (25% and 39% increase respectively; P<0.05). Glycogen synthase activity and glycogen branching enzyme expression were found to be similar between the ipsilateral, contralateral, and sham control hemispheres. In contrast, the rate-limiting enzyme for glycogen breakdown, glycogen phosphorylase, had 58% lower activity (P<0.01) in the ipsilateral hemisphere (24 hours post-stroke), which corresponded with a 48% reduction in cAMP-dependent protein kinase A (PKA) activity (P<0.01). In addition, glycogen debranching enzyme expression 24 hours post-stroke was 77% (P<0.01) and 72% lower (P<0.01) at the protein and mRNA level, respectively. In cultured rat primary cerebellar astrocytes, hypoxia and inhibition of PKA activity significantly reduced glycogen phosphorylase activity and increased glycogen accumulation but did not alter glycogen synthase activity. Furthermore, elevated glycogen levels provided metabolic support to astrocytes during hypoxia.

Conclusion

Our study has identified that glycogen breakdown is impaired during ischemic stroke, the molecular basis of which includes reduced glycogen debranching enzyme expression level together with reduced glycogen phosphorylase and PKA activity.

Inducible metabolic adaptation promotes mesenchymal stem cell therapy for ischemia: a hypoxia-induced and glycogen-based energy prestorage strategy

OBJECTIVE

Ischemic tissue is an environment with limited oxygen and nutrition availability. The poor retention of mesenchymal stem cells (MSC) in ischemic tissues greatly limits their therapeutic potential. The aim of this study was to determine whether and how inducible metabolic adaptation enhances MSC survival and therapy under ischemia.

APPROACH AND RESULTS

MSC were subjected to glycogen synthase 1-specific small interfering RNA or vehicle treatment, and then sublethal hypoxic preconditioning (HP) was applied to induce glycogenesis. The treated cells were subjected to ischemic challenge. The results exhibited that HP of MSC induced glycogen storage and stimulated glycogen catabolism and cellular ATP production, thereby preserving cell viability in long-term ischemia. In vivo study using the mouse limb ischemia model transplanted with HP or control MSC into the ischemic thigh muscles revealed a significant increased retention of MSC with glycogen storage associated with improved limb salvage, perfusion recovery and angiogenesis in the ischemic muscles. In contrast, glycogen synthesis inhibition significantly abolished these improvements. Further molecular analysis indicated that phosphoinositide 3-kinase/AKT, hypoxia-inducible factor-1, and glycogen synthase kinase-3β regulated expression of glycogenesis genes, including glucose transporter 1, hexokinase, phosphoglucomutase 1, glycogen synthase 1, and glycogen phosphorylase, thereby regulating glycogen metabolism of stem cell during HP.

CONCLUSIONS

HP-induced glycogen storage improves MSC survival and therapy in ischemic tissues. Thus, inducible metabolic adaptation in stem cells may be considered as a novel strategy for potentiating stem cell therapy for ischemia.

Glucose utilization via glycogen phosphorylase sustains proliferation and prevents premature senescence in cancer cells

Metabolic reprogramming of cancer cells provides energy and multiple intermediates critical for cell growth. Hypoxia in tumors represents a hostile environment that can encourage these transformations. We report that glycogen metabolism is upregulated in tumors in vivo and in cancer cells in vitro in response to hypoxia. In vitro, hypoxia induced an early accumulation of glycogen, followed by a gradual decline. Concordantly, glycogen synthase (GYS1) showed a rapid induction, followed by a later increase of glycogen phosphorylase (PYGL). PYGL depletion and the consequent glycogen accumulation led to increased reactive oxygen species (ROS) levels that contributed to a p53-dependent induction of senescence and markedly impaired tumorigenesis in vivo. Metabolic analyses indicated that glycogen degradation by PYGL is important for the optimal function of the pentose phosphate pathway. Thus, glycogen metabolism is a key pathway induced by hypoxia, necessary for optimal glucose utilization, which represents a targetable mechanism of metabolic adaptation.

Hexokinase 2, glycogen synthase and phosphorylase play a key role in muscle glycogen supercompensation

Background

Glycogen-depleting exercise can lead to supercompensation of muscle glycogen stores, but the biochemical mechanisms of this phenomenon are still not completely understood.

Methods

Using chronic low-frequency stimulation (CLFS) as an exercise model, the tibialis anterior muscle of rabbits was stimulated for either 1 or 24 hours, inducing a reduction in glycogen of 90% and 50% respectively. Glycogen recovery was subsequently monitored during 24 hours of rest.

Results

In muscles stimulated for 1 hour, glycogen recovered basal levels during the rest period. However, in those stimulated for 24 hours, glycogen was supercompensated and its levels remained 50% higher than basal levels after 6 hours of rest, although the newly synthesized glycogen had fewer branches. This increase in glycogen correlated with an increase in hexokinase-2 expression and activity, a reduction in the glycogen phosphorylase activity ratio and an increase in the glycogen synthase activity ratio, due to dephosphorylation of site 3a, even in the presence of elevated glycogen stores. During supercompensation there was also an increase in 5′-AMP-activated protein kinase phosphorylation, correlating with a stable reduction in ATP and total purine nucleotide levels.

Conclusions

Glycogen supercompensation requires a coordinated chain of events at two levels in the context of decreased cell energy balance: First, an increase in the glucose phosphorylation capacity of the muscle and secondly, control of the enzymes directly involved in the synthesis and degradation of the glycogen molecule. However, supercompensated glycogen has fewer branches.

Hypoxia-inducible factor 1-mediated regulation of PPP1R3C promotes glycogen accumulation in human MCF-7 cells under hypoxia

Hundreds of genes can be regulated by hypoxia-inducible factor 1 (HIF1) under hypoxia. Here we demonstrated a HIF1-mediated induction of protein phosphatase 1, regulatory subunit 3C gene (PPP1R3C) in human MCF7 cells under hypoxia. By mutation analysis we confirmed the presence of a functional hypoxia response element that is located 229bp upstream from the PPP1R3C gene. PPP1R3C induction correlates with a significant glycogen accumulation in MCF7 cells under hypoxia. Knockdown of either HIF1α or PPP1R3C attenuated hypoxia-induced glycogen accumulation significantly. Knockdown of HIF2α reduced hypoxia-induced glycogen accumulation slightly (but not significantly). Our results demonstrated that HIF1 promotes glycogen accumulation through regulating PPP1R3C expression under hypoxia, which revealed a novel metabolic adaptation of cells to hypoxia.

Effects of acute ethionine injection on plasma ghrelin and obestatin levels in trained male rats

Ghrelin and obestatin are orexigenic and anorexigenic peptides, respectively, that are secreted from the stomach mucosa into the circulation. These peptides have opposing actions on food intake, weight gain, and adiposity. It is thought that ghrelin is sensitive to a negative energy environment and also plays a considerable role in short- and long-term energy balance and glucose homeostasis. It has been suggested that the levels of ghrelin and obestatin are upregulated by fasting, hypoglycemic status, and a physical-exercise-induced energy deficit. Ethionine (ETH), the ethyl analogue of methionine, has been shown to increase food intake, decrease adenosine triphosphate (ATP) and glycogen levels, and inhibit protein synthesis in the liver. The purpose of this study was to examine the effect of a single dose of ETH (0.7 mg/g of body weight) injection on resting plasma total ghrelin and obestatin concentrations in male trained rats. Thirty-two adult Wistar male rats weighing 180 to 200 g were randomly assigned to control (n = 16) and training (n =16) groups. The training group was exercised for 10 weeks (25 m/min, 0% grade, 60 minutes, and 5 d/wk). Seventy-two hours after the last exercise session, rats were injected with either saline (NaCl) or ETH and then killed. Ethionine compared with a NaCl injection resulted in significant (P < .013) reductions in resting hepatic ATP and glycogen levels, and in a significant (P < .001) increase in concentrations of plasma total ghrelin but not obestatin. The results indicate that ETH-induced liver ATP and glycogen deficiency could exert a powerful regulatory influence on plasma total ghrelin, but this is not the case for obestatin. Findings demonstrate the short-term energy-regulating capacity of ghrelin.

Hypoxia promotes glycogen accumulation through hypoxia inducible factor (HIF)-mediated induction of glycogen synthase 1

When oxygen becomes limiting, cells reduce mitochondrial respiration and increase ATP production through anaerobic fermentation of glucose. The Hypoxia Inducible Factors (HIFs) play a key role in this metabolic shift by regulating the transcription of key enzymes of glucose metabolism. Here we show that oxygen regulates the expression of the muscle glycogen synthase (GYS1). Hypoxic GYS1 induction requires HIF activity and a Hypoxia Response Element within its promoter. GYS1 gene induction correlated with a significant increase in glycogen synthase activity and glycogen accumulation in cells exposed to hypoxia. Significantly, knockdown of either HIF1α or GYS1 attenuated hypoxia-induced glycogen accumulation, while GYS1 overexpression was sufficient to mimic this effect. Altogether, these results indicate that GYS1 regulation by HIF plays a central role in the hypoxic accumulation of glycogen. Importantly, we found that hypoxia also upregulates the expression of UTP:glucose-1-phosphate urydylyltransferase (UGP2) and 1,4-α glucan branching enzyme (GBE1), two enzymes involved in the biosynthesis of glycogen. Therefore, hypoxia regulates almost all the enzymes involved in glycogen metabolism in a coordinated fashion, leading to its accumulation. Finally, we demonstrated that abrogation of glycogen synthesis, by knock-down of GYS1 expression, impairs hypoxic preconditioning, suggesting a physiological role for the glycogen accumulated during chronic hypoxia. In summary, our results uncover a novel effect of hypoxia on glucose metabolism, further supporting the central importance of metabolic reprogramming in the cellular adaptation to hypoxia.

Glycogen: an overview of possible regulatory roles of the proteins associated with the granule

While scientists have routinely measured muscle glycogen in many metabolic situations for over 4 decades, there is surprisingly little known regarding its regulation. In the past decade, considerable evidence has illustrated that the carbohydrate stores in muscle are not homogeneous, and it is very likely that metabolic pools exist or that each granule has independent regulation. The fundamental aspects appear to be associated with a complex set of proteins that associate with both the granule and each other in a dynamic fashion. Some of the proteins are enzymes and others play scaffolding roles. A number of the proteins can translocate, depending on the metabolic stimulus. These various processes appear to be the mechanisms that give the glycogen granule precise yet dynamic regulation. This may also allow the stores to serve as an important metabolic regulator of other metabolic events.

A role for AGL ubiquitination in the glycogen storage disorders of Lafora and Cori's disease

Cori's disease is a glycogen storage disorder characterized by a deficiency in the glycogen debranching enzyme, amylo-1,6-glucosidase,4-alpha-glucanotransferase (AGL). Here, we demonstrate that the G1448R genetic variant of AGL is unable to bind to glycogen and displays decreased stability that is rescued by proteasomal inhibition. AGL G1448R is more highly ubiquitinated than its wild-type counterpart and forms aggresomes upon proteasome impairment. Furthermore, the E3 ubiquitin ligase Malin interacts with and promotes the ubiquitination of AGL. Malin is known to be mutated in Lafora disease, an autosomal recessive disorder clinically characterized by the accumulation of polyglucosan bodies resembling poorly branched glycogen. Transfection studies in HepG2 cells demonstrate that AGL is cytoplasmic whereas Malin is predominately nuclear. However, after depletion of glycogen stores for 4 h, approximately 90% of transfected cells exhibit partial nuclear staining for AGL. Furthermore, stimulation of cells with agents that elevate cAMP increases Malin levels and Malin/AGL complex formation. Refeeding mice for 2 h after an overnight fast causes a reduction in hepatic AGL levels by 48%. Taken together, these results indicate that binding to glycogen crucially regulates the stability of AGL and, further, that its ubiquitination may play an important role in the pathophysiology of both Lafora and Cori's disease.

Regulation of the mouse protein targeting to glycogen (PTG) promoter by the FoxA2 forkhead protein and by 3',5'-cyclic adenosine 5'-monophosphate in H4IIE hepatoma cells

The scaffolding protein, protein targeting to glycogen (PTG), orchestrates the signaling of several metabolic enzymes involved in glycogen synthesis. However, little is known concerning the regulation of PTG itself. In this study, we have cloned and characterized the mouse promoter of PTG. We identified multiple FoxA2 binding sites within this region. FoxA2 is a member of the forkhead family of transcription factors that has recently been implicated in the cAMP-dependent regulation of several genes involved in liver metabolism. Using luciferase reporter constructs, we demonstrate that FoxA2 transactivates the PTG promoter in H4IIE hepatoma cells. Nuclear extracts prepared from mouse liver and H4IIE cells were able to bind a FoxA2-specific probe derived within the PTG promoter region. Chromatin immunoprecipitation experiments further demonstrate that FoxA2 binds to the PTG promoter in vivo. Finally, we show that treatment with cAMP analogs activates the PTG promoter and significantly increases PTG levels in H4IIE cells. Our results provide a framework to investigate how additional transcription factors may regulate PTG expression in other cell types.

Glycogen metabolism in rat heart muscle cultures after hypoxia

Elevated glycogen levels in heart have been shown to have cardioprotective effects against ischemic injury. We have therefore established a model for elevating glycogen content in primary rat cardiac cells grown in culture and examined potential mechanisms for the elevation (glycogen supercompensation). Glycogen was depleted by exposing the cells to hypoxia for 2 h in the absence of glucose in the medium. This was followed by incubating the cells with 28 mM glucose in normoxia for up to 120 h. Hypoxia decreased glycogen content to about 15% of control, oxygenated cells. This was followed by a continuous increase in glycogen in the hypoxia treated cells during the 120 h recovery period in normoxia. By 48 h after termination of hypoxia, the glycogen content had returned to baseline levels and by 120 h glycogen was about 150% of control. The increase in glycogen at 120 h was associated with comparable relative increases in glucose uptake (approximately 180% of control) and the protein level of the glut-1 transporter (approximately 170% of control), whereas the protein level of the glut-4 transporter was decreased to < 10% of control. By 120 h, the hypoxia-treated cells also exhibited marked increases in the total (approximately 170% of control) and fractional activity of glycogen synthase (control, approximately 15%; hypoxia-treated, approximately 30%). Concomitantly, the hypoxia-treated cells also exhibited marked decreases in the total (approximately 50% of control) and fractional activity of glycogen phosphorylase (control, approximately 50%; hypoxia-treated, approximately 25%). Thus, we have established a model of glycogen supercompensation in cultures of cardiac cells that is explained by concerted increases in glucose uptake and glycogen synthase activity and decreases in phosphorylase activity. This model should prove useful in studying the cardioprotective effects of glycogen.

Insulin-independent glycogen supercompensation in isolated mouse skeletal muscle: role of phosphorylase inactivation

Glycogen supercompensation (increase in muscle glycogen content above basal) is an established phenomenon induced by unknown mechanisms. It consists of both insulin-dependent and -independent components. Here, we investigate insulin-independent glycogen supercompensation in isolated, intact extensor digitorum longus muscles from mice. Muscles were stimulated electrically, incubated in vitro with 5.5 mM glucose for up to 16 h and then analysed for glycogen, glucose uptake and enzyme activities. Basal glycogen was 84+/-6 micro mol glucosyl units/g dry muscle and was depleted by 80% after 10 min contraction. Glycogen increased after contraction, reaching a peak value of 113+/-9 micro mol glucosyl units/g dry muscle ( P<0.05 vs. basal) by 6 h, and returned to basal values by 16 h (84+/-8). Maximal activities of glycogen synthase, phosphorylase and alpha-glucosidase were not significantly altered by contraction or during the 6-h recovery period. Glycogen synthase fractional activity (0.17/7.2 mM glucose-6-P; inversely related to phosphorylation state of the enzyme) was increased about twofold early after contraction but then decreased and was slightly lower than baseline during the period of supercompensation (4-6 h). Phosphorylase fractional activity (+/-adenosine monophosphate; directly related to phosphorylation state of the enzyme) decreased to 60% of basal after contraction and decreased further during the initial 4 h of recovery to 40% of basal ( P<0.01 vs. basal). After 4 h recovery, glucose uptake was slightly (50%) higher in the stimulated than in the non-stimulated muscle ( P<0.01). Thus, insulin-independent glycogen supercompensation involves inactivation of phosphorylase and hence an inhibition of glycogen breakdown.