Reduced glycogen phosphorylase activity in denervated hindlimb muscles of rat is related to muscle atrophy and fibre type

Effects of oleanolic acid on the insulin signaling pathway in skeletal muscle of streptozotocin-induced diabetic male Sprague-Dawley rats

BACKGROUND

The pant-derived triterpene oleanolic acid (OA) has been shown to have antidiabetic effects, but its action on the insulin signaling cascade has not been fully elucidated. The aim of the present study was to investigate the effects of OA on aspects of the phosphatidylinositol 3-kinase/Akt insulin signaling cascade in skeletal muscle of streptozotocin-induced type 1 diabetic male Sprague-Dawley rats.

METHODS

Diabetic and non-diabetic rats were treated with insulin (4 IU/kg), OA (80 mg/kg), and the combination of OA + insulin in acute 60-min and sub-chronic 14-day studies. Single and daily doses were administered in the acute and sub-chronic studies, respectively. In acute studies, phosphorylated (p-) Akt and p-glycogen synthase (GS) expression was evaluated. In sub-chronic studies, GS and glycogen phosphorylase (GP) expression and activity were evaluated, as were glycogen levels.

RESULTS

The findings show that OA enhances insulin-stimulated hypoglycemic effects in diabetic rats. In the acute study, OA increased levels of p-Akt and decreased levels of p-GS. In the sub-chronic study, OA increased both GS and GP activity, whereas OA + insulin increased GS and decreased GP activity. Treatment of rats with OA and OA + insulin increased GS expression in the skeletal muscle of diabetic rats and decreased GP expression. Glycogen levels were increased by OA but decreased OA + insulin treatment.

CONCLUSION

Oleanolic acid in synergy with insulin can enhance activation of the insulin signaling pathway. Furthermore, the present study provides evidence of OA activation of insulin signaling enzymes independent of insulin.

Effect of Dipsaci Radix on Hind Limb Muscle Atrophy of Sciatic Nerve Transected Rats

It was reported that Dipsaci radix (DR) has a reinforcement effect on the bone-muscle dysfunction in the oriental medical classics and the experimental animal studies. The muscle atrophy was induced by unilateral transection of the sciatic nerve of the rats. Water-extract of DR was used as treatment once a day for 12 days. The muscle weights of the hind limb, atrophic changes, glycogen contents, compositions and cross-section areas of muscle fiber types in soleus and medial gastrocnemius were investigated. Muscle fiber type was classified to type-I and type-II with MHCf immunohistochemistry. Furthermore, Bax and Bcl-2 expressions were observed with immunohistochemiatry. DR treatment significantly increased muscle weights of soleus, medial gastrocnemius, lateral gastrocnemius, and posterior tibialis of the damaged hind limb. DR treatment reduced apoptotic muscle nuclei and hyaline-degenerated muscle fibers in soleus and medial gastrocnemius of the damaged hind limb. DR treatment also significantly increased glycogen contents in medial gastrocnemius of the damaged hind limb. DR treatment significantly attenuated the slow-to-fast shift in soleus of the damaged hind limb but not in medial gastrocnemius. DR treatment significantly increased cross-section areas of type-I and type-II fibers in soleus and medial gastrocnemius of the damaged hind limb. In soleus and medial gastrocnemius, DR treatment significantly reduced Bax positive muscle nuclei in the damaged hind limb. These results suggest that DR treatment has an anti-atrophic effect and an anti-apoptotic effect against myonuclear apoptosis induced by the peripheral nerve damage.

Effect of astragaloside IV on hepatic glucose-regulating enzymes in diabetic mice induced by a high-fat diet and streptozotocin

AIM

Hepatic glycogen phosphorylase (GP) and glucose-6-phosphatase (G6Pase) are important in control of blood glucose homeostasis, and are considered to be potential targets for antidiabetic drugs. Astragaloside IV has been reported to have a hypoglycemic effect. However, the biochemical mechanisms by which astragaloside IV regulates hepatic glucose-metabolizing enzymes remain unknown. The present study examines whether GP and G6Pase mediate the hypoglycemic effect of astragaloside IV.

METHODS

Type 2 diabetic mice were treated with astragaloside IV for 2 weeks. Blood glucose and insulin levels were measured by a glucometer and the ELISA method, respectively. Total cholesterol (TC) and triglyceride (TG) levels were determined using Labassay kits. Activities of hepatic GP and G6Pase were measured by the glucose-6-phosphate dehydrogenase-coupled reaction. The mRNA and protein levels of both enzymes were determined by real-time RT-PCR and Western blotting.

RESULTS

Astragaloside IV at 25 and 50 mg/kg significally decreased the blood glucose, TG and insulin levels, and inhibited the mRNA and protein expression as well as enzyme activity of GP and G6Pase in diabetic mice.

CONCLUSIONS

Astragaloside IV exhibited a hypoglycemic effect in diabetic mice. The hypoglycemic effect of this compound may be explained, in part, by its inhibition of hepatic GP and G6Pase activities.

Targeting the CNS to treat type 2 diabetes

Research on the role of peripheral organs in the regulation of glucose homeostasis has led to the development of various monotherapies that aim to improve glucose uptake and insulin action in these organs for the treatment of type 2 diabetes. It is now clear that the central nervous system (CNS) also plays an important part in orchestrating appropriate glucose metabolism, with accumulating evidence linking dysregulated CNS circuits to the failure of normal glucoregulatory mechanisms. There is evidence that there is substantial overlap between the CNS circuits that regulate energy balance and those that regulate glucose levels, suggesting that their dysregulation could link obesity and diabetes. These findings present new targets for therapies that may be capable of both inducing weight loss and improving glucose regulation.

Effect of geniposide, a hypoglycemic glucoside, on hepatic regulating enzymes in diabetic mice induced by a high-fat diet and streptozotocin

AIM

Hepatic glycogen phosphorylase (GP) and glucose-6-phosphatase (G6Pase) play an important role in the control of blood glucose homeostasis and are proposed to be potential targets for anti-diabetic drugs. Geniposide is an iridoid glucoside extracted from Gardenia jasminoides Ellis fruits and has been reported to have a hypoglycemic effect. However, little is known about the biochemical mechanisms by which geniposide regulates hepatic glucose-metabolizing enzymes. The present study investigates whether the hypoglycemic effect of geniposide is mediated by GP or G6Pase.

METHODS

Type 2 diabetic mice, induced by a high-fat diet and streptozotocin injection, were treated with or without geniposide for 2 weeks. Blood glucose levels were monitored by a glucometer. Insulin concentrations were analyzed by the ELISA method. Total cholesterol (TC) and triglyceride (TG) levels were measured using Labassay kits. Activities of hepatic GP and G6Pase were measured by glucose-6-phosphate dehydrogenase-coupled reaction. Real-time RT-PCR and Western blotting were used to determine the mRNA and protein levels of both enzymes.

RESULTS

Geniposide (200 and 400 mg/kg) significantly decreased the blood glucose, insulin and TG levels in diabetic mice in a dose-dependent manner. This compound also decreased the expression of GP and G6Pase at mRNA and immunoreactive protein levels, as well as enzyme activity.

CONCLUSION

Geniposide is an effective hypoglycemic agent in diabetic mice. The hypoglycemic effect of this compound may be mediated, at least in part, by inhibiting the GP and G6Pase activities.

The integrative role of CNS fuel-sensing mechanisms in energy balance and glucose regulation

The incidences of both obesity and type 2 diabetes mellitus are rising at epidemic proportions. Despite this, the balance between caloric intake and expenditure is tremendously accurate under most circumstances. Growing evidence suggests that nutrient and hormonal signals converge and directly act on brain centers, leading to changes in fuel metabolism and, thus, stable body weight over time. Growing evidence also suggests that these same signals act on the central nervous system (CNS) to regulate glucose metabolism independently. We propose that this is not coincidental and that the CNS responds to peripheral signals to orchestrate changes in both energy and glucose homeostasis. In this way the CNS ensures that the nutrient demands of peripheral tissues (and likely of the brain itself) are being met. Consequently, dysfunction of the ability of the CNS to integrate fuel-sensing signals may underlie the etiology of metabolic diseases such as obesity and diabetes.

Transcriptional pathways associated with skeletal muscle disuse atrophy in humans

Disuse atrophy is a common clinical phenomenon that significantly impacts muscle function and activities of daily living. The purpose of this study was to implement genome-wide expression profiling to identify transcriptional pathways associated with muscle remodeling in a clinical model of disuse. Skeletal muscle biopsies were acquired from the medial gastrocnemius in patients with an ankle fracture and from healthy volunteers subjected to 4-11 days of cast immobilization. We identified 277 misregulated transcripts in immobilized muscles of patients, of which the majority were downregulated. The most broadly affected pathways were involved in energy metabolism, mitochondrial function, and cell cycle regulation. We also found decreased expression in genes encoding proteolytic proteins, calpain-3 and calpastatin, and members of the myostatin and IGF-I pathway. Only 26 genes showed increased expression in immobilized muscles, including apolipoprotein (APOD) and leptin receptor (LEPR). Upregulation of APOD (5.0-fold, P < 0.001) and LEPR (5.7-fold, P < 0.05) was confirmed by quantitative RT-PCR and immunohistochemistry. In addition, atrogin-1/MAFbx was found to be 2.4-fold upregulated (P < 0.005) by quantitative RT-PCR. Interestingly, 96% of the transcripts differentially regulated in immobilized limbs also showed the same trend of change in the contralateral legs of patients but not the contralateral legs of healthy volunteers. Information obtained in this study complements findings in animal models of disuse and provides important feedback for future clinical studies targeting the restoration of muscle function following limb disuse in humans.

Atrophy, but not necrosis, in rabbit skeletal muscle denervated for periods up to one year

Our understanding of the effects of long-term denervation on skeletal muscle is heavily influenced by an extensive literature based on the rat. We have studied physiological and morphological changes in an alternative model, the rabbit. In adult rabbits, tibialis anterior muscles were denervated unilaterally by selective section of motor branches of the common peroneal nerve and examined after 10, 36, or 51 wk. Denervation reduced muscle mass and cross-sectional area by 50-60% and tetanic force by 75%, with no apparent reduction in specific force (force per cross-sectional area of muscle fibers). The loss of mass was associated with atrophy of fast fibers and an increase in fibrous and adipose connective tissue; the diameter of slow fibers was preserved. Within fibers, electron microscopy revealed signs of ultrastructural disorganization of sarcomeres and tubular systems. This, rather than the observed transformation of fiber type from IIx to IIa, was probably responsible for the slow contractile speed of the muscles. The muscle groups denervated for 10, 36, or 51 wk showed no significant differences. At no stage was there any evidence of necrosis or regeneration, and the total number of fibers remained constant. These changes are in marked contrast to the necrotic degeneration and progressive decline in mass and force that have previously been found in long-term denervated rat muscles. The rabbit may be a better choice for a model of the effects of denervation in humans, at least up to 1 yr after lesion.

Evaluation of diffusional anisotropy and microscopic structure in skeletal muscles using magnetic resonance

The pulsed-gradient spin-echo (PGSE) nuclear magnetic resonance (NMR) method is used for detecting the diffusion of water molecules in biological tissues. Because tissues generally have diffusional anisotropy, their diffusion properties are denoted by a tensor. In this study, we evaluated the diffusional anisotropy and microscopic structure in atrophied skeletal muscles using the PGSE NMR method. The left sciatic nerve was severed in twelve 9-week-old rats. Neurotomy caused neurogenic muscular atrophy at the left gastrocnemius. At 2, 4 and 8 weeks after neurotomy, magnetic resonance signals were selectively acquired from a 2 x 2 x 2 mm(3) voxel, which was located on the left gastrocnemius. The diffusion tensor, the mean diffusivity (MD) and the fractional anisotropy (FA) were calculated from the signals. A theoretical model of the diffusion in muscles was derived from Tanner's equation. The muscle fiber diameter was estimated by fitting the model to the measured signals. The measurements were also performed for normal rats as controls. No significant difference was found in the MD and the estimated intracellular diffusion coefficient between the control group and the denervated group. The denervated group had significantly higher FA compared with the control group (P<.05). The estimated muscle fiber diameter of the denervated group was significantly smaller than the estimated value of the control group (P<.05). These differences were found at 8 weeks after neurotomy. The proposed method is effective for evaluating changes in the microscopic structure of skeletal muscles.

New insights into the skeletal muscle phenotype of equine motor neuron disease: a quantitative approach

Equine motor neuron disease (EMND) is a neurodegenerative disorder similar to the sporadic form of human amyotrophic lateral sclerosis. This study was conducted to quantify myofiber plasticity in response to EMND. Deep M. gluteus medius biopsy samples from eight horses with an ante mortem diagnosis of EMND, which in five cases was later confirmed by post mortem examination of spinal cord and peripheral nerves, were examined by combined methodologies of electrophoresis of myosin heavy chains (MyHC), muscle enzymes and substrate biochemistry, immunohistochemistry of MyHCs and sarcoendoplasmic Ca2+-ATPase (SERCA) isoforms, quantitative histochemistry of succinic dehydrogenase, glycerol-3-phosphate dehydrogenase, periodic acid-Schiff and capillaries, and photometric image analysis. The data were compared with muscle biopsies from healthy controls. Histopathological findings of EMND were observed in muscle biopsy specimens from all cases, but the severity and intra-biopsy extent varied from case to case. Compared with controls, muscle biopsy samples from EMND horses had a lower percentage of MyHC type I fibers, higher percentages of hybrid IIAX and pure IIX fibers, significant atrophy of all muscle fiber types, reduced oxidative capacity, increased glycolytic capacity, diminished intramuscular glycogen, lower capillary-to-fiber ratio, a higher ratio of myofibers expressing SERCA1a to SERCA2a isoforms, and a lower percentage of fibers expressing phospholamban. Objective discrimination of muscle biopsy specimens according to their healthy status (EMND vs controls) was possible on the basis of their muscular characteristics. A coordinated shift from slow to fast muscle characteristics in contractile and metabolic features of muscle fiber types, together with generalized myofiber atrophy, occurs in EMND and the extent of this change seems to be related to the duration of the disease.

Regulation of dihydropyridine receptor gene expression in mouse skeletal muscles by stretch and disuse

This study examined dihydropyridine receptor (DHPR) gene expression in mouse skeletal muscles during physiological adaptations to disuse. Disuse was produced by three in vivo models-denervation, tenotomy, and immobilization-and DHPR alpha1s mRNA was measured by quantitative Northern blot. After 14-day simultaneous denervation of the soleus (Sol), tibialis anterior (TA), extensor digitorum longus (EDL), and gastrocnemius (Gastr) muscles by sciatic nerve section, DHPR mRNA increased preferentially in the Sol and TA (+1.6-fold), whereas it increased in the EDL (+1.6-fold) and TA (+1.8-fold) after selective denervation of these muscles by peroneal nerve section. It declined in all muscles (-1.3- to -2.6-fold) after 14-day tenotomy, which preserves nerve input but removes mechanical tension. Atrophy was comparable in denervated and tenotomized muscles. These results suggest that factor(s) in addition to inactivity per se, muscle phenotype, or associated atrophy can regulate DHPR gene expression. To test the contribution of passive tension to this regulation, we subjected the same muscles to disuse by limb immobilization in a maximally dorsiflexed position. DHPR alpha1s mRNA increased in the stretched muscles (Sol, +2.3-fold; Gastr, +1.5-fold) and decreased in the shortened muscles (TA, -1.4-fold; EDL, -1.3-fold). The effect of stretch was confirmed in vitro. DHPR protein did not change significantly after 4-day immobilization, suggesting that additional levels of regulation may exist. These results demonstrate that DHPR alpha1s gene expression is regulated as an integral part of the adaptive response of skeletal muscles to disuse in both slow- and fast-twitch muscles and identify passive tension as an important signal for its regulation in vivo.

Muscle fiber type IIX atrophy is involved in the loss of fat-free mass in chronic obstructive pulmonary disease

BACKGROUND

Although the loss of peripheral muscle mass has been shown convincingly in chronic obstructive pulmonary disease (COPD), the underlying pathogenesis remains unclear.

OBJECTIVE

The aim of the present study was to determine the relations between skeletal muscle fiber types, fiber cross-sectional area (CSA), enzyme activities, and fat-free mass (FFM) in patients with COPD and in control subjects.

DESIGN

In 15 patients with COPD and 15 healthy, age-matched control subjects, FFM was determined by dual-energy X-ray absorptiometry and bioelectrical impedance analysis. In biopsy specimens from the vastus lateralis fiber types, fiber CSA and activities of cytochrome oxidase (EC 1.9.3.1), succinate dehydrogenase (EC 1.3.99.1), and glycogen phosphorylase (EC 2.4.1.1) were examined immunohistochemically and histochemically.

RESULTS

Compared with control subjects, patients with COPD had less FFM (49 compared with 59 kg, P = 0.030) and lower mean fiber CSA (3839 compared with 4647 microm(2), P = 0.037). A strong correlation (r = 0.87, P < 0.001) was observed between the FFM measured by bioelectrical impedance analysis and mean fiber CSA in patients with COPD. Within fiber-type categories the mean CSA of only the IIA/IIX and IIX fiber types was lower in patients than in control subjects [3358 compared with 4428 microm(2) (P = 0.022) and 2566 compared with 4248 microm(2) (P = 0.003), respectively]. In COPD, 20% of the type IIX fibers lacked stainable activities of cytochrome oxidase, succinate dehydrogenase, and glycogen phosphorylase, and this proportion correlated negatively with type IIX fiber CSA (r = -0.65, P = 0.012).

CONCLUSIONS

Muscle fiber atrophy occurs in the vastus lateralis in patients with COPD and contributes to the loss of muscle mass in COPD. Atrophy is specific to fiber types IIA/IIX and IIX and is associated with a disturbed metabolic capacity.