Physiological role of skeletal muscle glycogen in starved mice

Reduced metabolic capacity in fast and slow skeletal muscle via oxidative stress and the energy-sensing of AMPK/SIRT1 in malnutrition

The effects of malnutrition on skeletal muscle result in not only the loss of muscle mass but also fatigue intolerance. It remains unknown whether the metabolic capacity is related to the fiber type composition of skeletal muscle under malnourished condition although malnutrition resulted in preferential atrophy in fast muscle. The purpose of the present study was to investigate the effects of metabolic capacity in fast and slow muscles via the energy-sensing of AMPK and SIRT1 in malnutrition. Wistar rats were randomly divided into control and malnutrition groups. The rats in the malnutrition group were provided with a low-protein diet, and daily food intake was limited to 50% for 12 weeks. Malnutrition with hypoalbuminemia decreased the body weight and induced the loss of plantaris muscle mass, but there was little change in the soleus muscle. An increase in the superoxide level in the plasma and a decrease in SOD-2 protein expression in both muscles were observed in the malnutrition group. In addition, the expression level of AMPK in the malnutrition group increased in both muscles. Conversely, the expression level of SIRT1 decreased in both muscles of the malnutrition group. In addition, malnutrition resulted in a decrease in the expression levels of PGC-1α and PINK protein, and induced a decrease in the levels of two key mitochondrial enzymes (succinate dehydrogenase and citrate synthase) and COX IV protein expression in both muscles. These results indicate that malnutrition impaired the metabolic capacity in both fast and slow muscles via AMPK-independent SIRT1 inhibition induced by increased oxidative stress.

Characterising the dynamics of placental glycogen stores in the mouse

INTRODUCTION

The placenta performs a range of functions to support fetal growth. In addition to facilitating nutrient transport, the placenta also stores glucose as glycogen, which is thought to maintain fetal glucose supply during late gestation. However, evidence to support such a role is currently lacking. Similarly, our understanding of the dynamics of placental glycogen metabolism in normal mouse pregnancy is limited.

METHODS

We quantified the placental glycogen content of wild type C57BL/6JOlaHsd mouse placentas from mid (E12.5) to late (E18.5) gestation, alongside characterising the temporal expression pattern of genes encoding glycogenesis and glycogenolysis pathway enzymes. To assess the potential of the placenta to produce glucose, we investigated the spatiotemporal expression of glucose 6-phosphatase by qPCR and in situ hybridisation. Separate analyses were undertaken for placentas of male and female conceptuses to account for potential sexual dimorphism.

RESULTS

Placental glycogen stores peak at E15.5, having increased over 5-fold from E12.5, before declining by a similar extent by E18.5. Glycogen stores were 17% higher in male placentas than in females at E15.5. Expression of glycogen branching enzyme (Gbe1) was reduced ~40% towards term. Expression of the glucose 6-phosphatase isoform G6pc3 was enriched in glycogen trophoblast cells and increased towards term.

DISCUSSION

Reduced expression of Gbe1 suggests a decline in glycogen branching towards term. Expression of G6pc3 by glycogen trophoblasts is consistent with an ability to produce and release glucose from glycogen stores. However, the ultimate destination of the glucose generated from placental glycogen remains to be elucidated.

Vitamin A Improves Hyperglycemia and Glucose-Intolerance through Regulation of Intracellular Signaling Pathways and Glycogen Synthesis in WNIN/GR-Ob Obese Rat Model

Vitamin A and its metabolites modulate insulin resistance and regulate stearoyl-CoA desaturase 1 (SCD1), which are also known to affect insulin resistance. Here, we tested, whether vitamin A-mediated changes in insulin resistance markers are associated with SCD1 regulation or not. For this purpose, 30-week old male lean and glucose-intolerant obese rats of WNIN/GR-Ob strain were given either a stock or vitamin A-enriched diet, i.e. 2.6 mg or 129 mg vitamin A/kg diet, for 14 weeks. Compared to the stock diet, vitamin A-enriched diet feeding improved hyperglycemia and glucose-clearance rate in obese rats and no such changes were seen in lean rats receiving identical diets. These changes were corroborated with concomitant increase in circulatory insulin and glycogen levels of liver and muscle (whose insulin signaling pathway genes were up-regulated) in obese rats. Further, the observed increase in muscle glycogen content in these obese rats could be explained by increased levels of the active form of glycogen synthase, the key regulator of glycogen synthesis pathway, possibly inactivated through increased phosphorylation of its upstream inhibitor, glycogen synthase kinase. However, the unaltered hepatic SCD1 protein expression (despite decreased mRNA level) and increased muscle-SCD1 expression (both at gene and protein levels) suggest that vitamin A-mediated changes on glucose metabolism are not associated with SCD1 regulation. Chronic consumption of vitamin A-enriched diet improved hyperglycemia and glucose-intolerance, possibly, through the regulation of intracellular signaling and glycogen synthesis pathways of muscle and liver, but not associated with SCD1.

Liver glycogen in type 2 diabetic mice is randomly branched as enlarged aggregates with blunted glucose release

Glycogen is a vital highly branched polymer of glucose that is essential for blood glucose homeostasis. In this article, the structure of liver glycogen from mice is investigated with respect to size distributions, degradation kinetics, and branching structure, complemented by a comparison of normal and diabetic liver glycogen. This is done to screen for differences that may result from disease. Glycogen α-particle (diameter ∼ 150 nm) and β-particle (diameter ∼ 25 nm) size distributions are reported, along with in vitro γ-amylase degradation experiments, and a small angle X-ray scattering analysis of mouse β-particles. Type 2 diabetic liver glycogen upon extraction was found to be present as large loosely bound, aggregates, not present in normal livers. Liver glycogen was found to aggregate in vitro over a period of 20 h, and particle size is shown to be related to rate of glucose release, allowing a structure-function relationship to be inferred for the tissue specific distribution of particle types. Application of branching theories to small angle X-ray scattering data for mouse β-particles revealed these particles to be randomly branched polymers, not fractal polymers. Together, this article shows that type 2 diabetic liver glycogen is present as large aggregates in mice, which may contribute to the inflexibility of interconversion between glucose and glycogen in type 2 diabetes, and further that glycogen particles are randomly branched with a size that is related to the rate of glucose release.

Hindlimb Muscle Atrophy Occurs From Short-Term Undernutrition in Rats

The purpose of this study was to examine the effect of short-term (7 days) undernutrition on Type I (soleus) and Type II (plantaris, gastrocnemius) muscles in rats. Male Sprague-Dawley rats ( N = 20) were randomly assigned to one of two groups: a control group ( n = 10) in which animals were allowed to have water and pellets ad libitum and an undernourished group ( n = 10) in which animals were allowed to have 37% of the total food intake of the control group and water ad libitum. Body weight and food intake were measured daily. After 7 days, rats were anesthetized and the soleus, plantaris, and gastrocnemius muscles and liver were dissected. Body weight, liver weight, muscle weight, Types I and II fiber cross-sectional area, and myofibrillar protein content were determined. After 7 days of undernutrition, the undernourished group showed significant decreases ( p < .05) compared to the control group in body weight, liver weight, muscle weight of soleus, plantaris, and gastrocnemius muscles, and cross-sectional areas of Types I and II fiber of the plantaris and gastrocnemius muscles.

The structure of cardiac glycogen in healthy mice

Transmission electron micrographs of glycogen extracted from healthy mouse hearts reveal aggregate structures around 133 nm in diameter. These structures are similar to, but on average somewhat smaller than, the α-particles of glycogen found in mammalian liver. Like the larger liver glycogens, these new particles in cardiac tissue appear to be aggregates of β-particles. Free β-particles are also present in liver, and are the only type of particle seen in skeletal muscle. They have diameters from 20 to 50 nm. We discuss the number distributions of glycogen particle diameters and the implications for the structure-function relationship of glycogens in these tissues. We point out the possible implications for the study of glycogen storage diseases, and of non-insulin dependent diabetes mellitus.

The effect of fasting on the glycogen metabolism in heat-acclimated rats

We investigated the influence of successive fasting for 24,48,72, and 96 h on some key enzymes and substrates of liver, kidney, and muscle in control and heat-acclimated (30days at 35 ? 1?C)rats. Short-term fasting (for 24 and 48 h)resulted in decrease of liver glycogen content, blood glucose level, and concentration of glucose-6-phosphate, as well as increase of glucose-6-phosphatase activity, regardless of the previous temperature of acclimation. During a period of prolonged fasting (for 72 and 96 h),there was a rebound of liver glycogen content only in animals kept at room temperature. Fasting induced increase of renal glycogen content in animals kept at room temperature and increase of renal glucose-6-phosphatase activity in both experimental groups. As for muscle metabolism, endogenous nutrition resulted in decrease of muscle glycogen content in heat-acclimated animals. Activity of muscle glycogen phosphorylase (a+b)was decreased in the control and increased in heat-acclimated animals. The obtained results indicate that the examined carbohydrate-related parameters show time-dependent changes during 4 days of fasting. Twenty-four- and 48-h fasting intensifies glycogenolytic processes, while 72- and 96-h fasting intensifies gluconeogenic processes, doing so to a lesser extent in heat-acclimated animals. The changes caused by the fasting were modified by acclimation to moderate heat, primarily in the liver and to a lesser extent in the kidney and muscle.

Age-related changes in fibre number, fibre size, fibre type composition and adenosine triphosphatase activity in rat soleus muscle

To study the aging of muscle fibres in red skeletal muscle, fibre number, fibre diameter and fibre type composition in the soleus muscle of male rats of 3, 12 and 24 months old were examined. The total number of muscle fibres remained unchanged, while average diameter increased slightly with increasing age. The staining intensity of myosin adenosine triphosphatase (ATPase) activity in the fibres decreased with advancing age. Therefore, observation on the basis of myosin ATPase histochemistry alone is not adequate to study the aging of muscle fibres. In the muscles of 24 month-old animals, four fibre types were recognized; 1) many (52%) type I-O fibres showing weak ATPase and succinate dehydrogenase (SDH) reactions with slight subsarcolemmal aggregates of diformazan (SAD); 2) some (33%) type M fibres showing weak ATPase and intense SDH reactions with marked SAD; 3) a few (12%) type O fibres showing weak ATPase and intense SDH reactions without SAD; and 4) very few (4%) type IIA fibres. Histochemical and morphometric results suggest that type I-O, type M and type O fibres are derived from type I, type I and type IIA fibres, respectively. Furthermore, no transitional fibres from type IIA to type I were observed. Therefore, age-related changes in fibre type composition in the muscle cannot be explained by the simple idea that most type IIA fibres are transformed into type I fibres.

High glucose-6-phosphatase activity in osteoblasts in the metaphysis of femur of growing rats

Glucose-6-phosphatase (G6Pase) activity was examined cytochemically in the metaphysis of femurs of 3- and 7-day-old rats. G6Pase and hexokinase activities were also examined biochemically in the femur and tibia of 3-day-old animals. The reaction product for G6Pase activity was seen in the endoplasmic reticulum and nuclear envelope of all cell types composing the metaphysis. The amount of the reaction product was abundant in osteoblasts, moderate in osteocytes, and moderate to scarce in osteoclasts and capillary endothelial cells. Biochemical G6Pase activity in the bones was higher than that in the brain, submandibular gland, or pancreas of the animals. Hexokinase activity in the bones was not different from that in the submandibular gland, pancreas, or kidney. The activity ratio of G6Pase and hexokinase in the bones (0.603) was greater than that in the submandibular gland, pancreas, or brain and smaller than that in the kidney. Possible physiological significances of the higher G6Pase activity in osteoblasts are discussed.

Glucose 6-phosphatase and glycogen phosphorylase activities in chondrocytes in epiphyseal cartilage of growing rats

Glycogen, glycogen phosphorylase, and glucose 6-phosphatase (G6Pase) activities were examined cytochemically in chondrocytes of femoral epiphyseal cartilages and cartilaginous ribs of 3- and 7-day-old rats. G6Pase activity was also examined biochemically. Glycogen was abundant in chondrocytes of the reserve zone, while it became scarce in the cells of the proliferative zone. From the upper part (adjoining the proliferative zone) to the lower part of the hypertrophic zone, glycogen accumulated in chondrocytes and decreased in the cells of the degenerative zone. Inversely, glycogen phosphorylase a and G6Pase activities were relatively high in chondrocytes of the proliferative zone and upper hypertrophic zone and were low in the cells of the reserve zone, lower hypertrophic zone, and degenerative zone. The reaction product for G6Pase was present in the endoplasmic reticulum and nuclear envelope of all types of chondrocytes composing the cartilages, although the amounts of reaction product varied with the cell types in parallel with the histochemical results. Biochemical G6Pase activity was higher in epiphyseal cartilages than in cartilaginous ribs. The possible mechanism and significance of the accumulation and decrease of glycogen in chondrocytes of the epiphyseal cartilage were discussed.