Effects of aging on the responsiveness and sensitivity of glucose metabolism to insulin in the incubated soleus muscle isolated from Sprague-Dawley and Wistar rats.

1. The effects of aging on the sensitivity and responsiveness of glucose transport, lactate formation and glycogen synthesis to insulin were studied in the incubated stripped soleus muscle isolated from aging Sprague-Dawley and Wistar rats. 2. As Sprague-Dawley rats aged from 5 to 13 weeks, there were marked increases in the concentrations of insulin that were required for half-maximal stimulation (i.e. EC50 value, which is a measure of sensitivity) of glucose transport, lactate formation and glycogen synthesis. 3. In marked contrast, there were no alterations in sensitivities of any of these processes to insulin in soleus muscle prepared from Wistar rats aged between 6 and 12 weeks. 4. However, in soleus muscles from 85-week-old Wistar rats the rates of glycogen synthesis in response to basal, sub-maximal and maximal concentrations of insulin were markedly decreased. The insulin EC50 value of glycogen synthesis was increased 4-fold, but was unchanged for lactate formation. 5. The insulin-stimulated rates of glucose transport in soleus muscles from 5- or 85-week-old Wistar rats were not significantly different.     

Effects of insulin and transgenic overexpression of UDP-glucose pyrophosphorylase on UDP-glucose and glycogen accumulation in skeletal muscle fibers

UDP-glucose (UDP-Glc) and glycogen levels in skeletal muscle fibers of defined fiber type were measured using microanalytical methods. Infusing rats with insulin increased glycogen in both Type I and Type II fibers. Insulin was without effect on UDP-Glc in Type I fibers but decreased UDP-Glc by 35-40% in Type IIA/D and Type IIB fibers. The reduction in UDP-Glc suggested that UDP-Glc pyrophosphorylase (PPL) activity might limit glycogen synthesis in response to insulin. To explore this possibility, we generated mice overexpressing a UDP-Glc PPL transgene in skeletal muscle. The transgene increased both UDP-Glc PPL activity and levels of UDP-Glc in skeletal muscles by approximately 3-fold. However, overexpression of UDP-Glc PPL was without effect on either the levels of skeletal muscle glycogen or glucose tolerance in vivo. The transgene was also without effect on either control or insulin-stimulated rates of (14)C-glucose incorporation into glycogen in muscles incubated in vitro. The results indicate that UDP-Glc PPL activity is not limiting for glycogen synthesis.     

Contraction activates glucose uptake and glycogen synthase normally in muscles from dexamethasone-treated rats

Glucocorticoids cause insulin resistance in skeletal muscle. The aims of the present study were to investigate the effects of contraction on glucose uptake, insulin signaling, and regulation of glycogen synthesis in skeletal muscles from rats treated with the glucocorticoid analog dexamethasone (1 mg x kg(-1) x day(-1) ip for 12 days). Insulin resistance in dexamethasone-treated rats was confirmed by reduced insulin-stimulated glucose uptake (approximately 35%), glycogen synthesis (approximately 70%), glycogen synthase activation (approximately 80%), and PKB Ser(473) phosphorylation (approximately 40%). Chronic dexamethasone treatment did not impair glucose uptake during contraction in soleus or epitrochlearis muscles. In epitrochlearis (but not in soleus), the presence of insulin during contraction enhanced glucose uptake to similar levels in control and dexamethasone-treated rats. Contraction also increased glycogen synthase fractional activity and dephosphorylated glycogen synthase at Ser(645), Ser(649), Ser(653), and Ser(657) normally in muscles from dexamethasone-treated rats. After contraction, insulin-stimulated glycogen synthesis was completely restored in epitrochlearis and improved in soleus from dexamethasone-treated rats. Contraction did not increase insulin-stimulated PKB Ser(473) or glycogen synthase kinase-3 (GSK-3) phosphorylation. Instead, contraction increased GSK-3beta Ser(9) phosphorylation in epitrochlearis (but not in soleus) in muscles from control and dexamethasone-treated rats. In conclusion, contraction stimulates glucose uptake normally in dexamethasone-induced insulin resistant muscles. After contraction, insulin's ability to stimulate glycogen synthesis was completely restored in epitrochlearis and improved in soleus from dexamethasone-treated rats.     

Anaerobic energy provision in aged skeletal muscle during tetanic stimulation

The effect of age on skeletal muscle anaerobic energy metabolism was investigated in adult (11 mo) and aged (25 mo) Fischer 344 rats. Hindlimb skeletal muscles innervated by the sciatic nerve were stimulated to contract with trains of supramaximal impulses (100 ms, 80 Hz) at a train rate of 1 Hz for 60 s, with an occluded circulation. Soleus, plantaris, and red and white gastrocnemius (WG) were sampled from control and stimulated limbs. All muscle masses were reduced with age (9-13%). Peak isometric tensions, normalized per gram of wet muscle, were lower throughout the stimulation in the aged animals (28%). The potential for anaerobic ATP provision was unaltered with age in all muscles, because resting high-energy phosphates and glycogen contents were similar to adult values. Anaerobic ATP provision during stimulation was unaltered by aging in soleus, plantaris, and red gastrocnemius muscles. In the WG, containing mainly fast glycolytic (FG) fibers, ATP and phosphocreatine contents were depleted less in aged muscle. In situ glycogenolysis and glycolysis were 90.0 +/- 4.8 and 69.3 +/- 2.6 mumol/g dry muscle (dm) in adult WG and reduced to 62.3 +/- 6.9 and 51.5 +/- 5.5 mumol/g dm, respectively, in aged WG. Consequently, total anaerobic ATP provision was lower in aged WG (224.5 +/- 20.9 mumol/g dm) vs. adult (292.6 +/- 7.6 mumol/g dm) WG muscle. In summary, the decreased tetanic tension production in aged animals was associated with a decreased anaerobic energy production in FG fibers. Reduced high-energy phosphate use and a greater energy charge potential after stimulation suggested that the energy demand was reduced in aged FG fibers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of continuous low-carbohydrate diet after long-term exercise on GLUT-4 protein content in rat skeletal muscle.

Stimulation of AMPK and decreased glycogen levels in skeletal muscle have a deep involvement in enhanced insulin action and GLUT-4 protein content after exercise training. The present study examined the chronic effects of a continuous low-carbohydrate diet after long-term exercise on GLUT-4 protein content, glycogen content, AMPK, and insulin signaling in skeletal muscle. Rats were divided randomly into four groups: normal chow diet sedentary (N-Sed), low carbohydrate diet sedentary (L-Sed), normal chow diet exercise (N-Ex), and low carbohydrate diet exercise (L-Ex) groups. Rats in the exercise groups (N-Ex and L-Ex) were exercised by swimming for 6 hours/day in two 3-hour bouts separated by 45 minutes of rest. The 10-day exercise training resulted in a significant increase in the GLUT-4 protein content (p<0.01). Additionally, the GLUT-4 protein content in L-Ex rats was increased by 29% above that in N-Ex rats (p<0.01). Finally, the glycogen content in skeletal muscle of L-Ex rats was decreased compared with that of N-Ex rats. Taken together, we suggest that the maintenance of glycogen depletion after exercise by continuous low carbohydrate diet results in the increment of the GLUT-4 protein content in skeletal muscle.     

Decreased insulin action in skeletal muscle from patients with McArdle's disease

Insulin action is decreased by high muscle glycogen concentrations in skeletal muscle. Patients with McArdle's disease have chronic high muscle glycogen levels and might therefore be at risk of developing insulin resistance. In this study, six patients with McArdle's disease and six matched control subjects were subjected to an oral glucose tolerance test and a euglycemic-hyperinsulinemic clamp. The muscle glycogen concentration was 103 +/- 45% higher in McArdle patients than in controls. Four of six McArdle patients, but none of the controls, had impaired glucose tolerance. The insulin-stimulated glucose utilization and the insulin-stimulated increase in glycogen synthase activity during the clamp were significantly lower in the patients than in controls (51.3 +/- 6.0 vs. 72.6 +/- 13.1 micromol x min(-1) x kg lean body mass(-1), P < 0.05, and 53 +/- 15 vs. 79 +/- 9%, P < 0.05, n = 6, respectively). The difference in insulin-stimulated glycogen synthase activity between the pairs was significantly correlated (r = 0.96, P < 0.002) with the difference in muscle glycogen level. The insulin-stimulated increase in Akt phosphorylation was smaller in the McArdle patients than in controls (45 +/- 13 vs. 76 +/- 13%, P < 0.05, respectively), whereas basal and insulin-stimulated glycogen synthase kinase 3alpha and protein phosphatase-1 activities were similar in the two groups. Furthermore, the ability of insulin to decrease and increase fat and carbohydrate oxidation, respectively, was blunted in the patients. In conclusion, these data show that patients with McArdle's glycogen storage disease are insulin resistant in terms of glucose uptake, glycogen synthase activation, and alterations in fuel oxidation. The data further suggest that skeletal muscle glycogen levels play an important role in the regulation of insulin-stimulated glycogen synthase activity.     

The lumbrical muscle: a novel in situ system to evaluate adult skeletal muscle proteolysis and anticatabolic drugs for therapeutic purposes

The molecular regulation of skeletal muscle proteolysis and the pharmacological screening of anticatabolic drugs have been addressed by measuring tyrosine release from prepubertal rat skeletal muscles, which are thin enough to allow adequate in vitro diffusion of oxygen and substrates. However, the use of muscle at accelerated prepubertal growth has limited the analysis of adult muscle proteolysis or that associated with aging and neurodegenerative diseases. Here we established the adult rat lumbrical muscle (4/hindpaw; 8/rat) as a new in situ experimental model for dynamic measurement of skeletal muscle proteolysis. By incubating lumbrical muscles attached to their individual metatarsal bones in Tyrode solution, we showed that the muscle proteolysis rate of adult and aged rats (3-4 to 24 mo old) is 45-25% of that in prepubertal animals (1 mo old), which makes questionable the usual extrapolation of proteolysis from prepubertal to adult/senile muscles. While acute mechanical injury or 1- to 7-day denervation increased tyrosine release from adult lumbrical muscle by up to 60%, it was reduced by 20-28% after 2-h incubation with β-adrenoceptor agonists, forskolin or phosphodiesterase inhibitor IBMX. Using inhibitors of 26S-proteasome (MG132), lysosome (methylamine), or calpain (E64/leupeptin) systems, we showed that ubiquitin-proteasome is accountable for 40-50% of total lumbrical proteolysis of adult, middle-aged, and aged rats. In conclusion, the lumbrical model allows the analysis of muscle proteolysis rate from prepubertal to senile rats. By permitting eight simultaneous matched measurements per rat, the new model improves similar protocols performed in paired extensor digitorum longus (EDL) muscles from prepubertal rats, optimizing the pharmacological screening of drugs for anticatabolic purposes.     

Transgenic overexpression of hexokinase II in skeletal muscle does not increase glucose disposal in wild-type or Glut1-overexpressing mice

Glut1 transgenic mice were bred with transgenic mice that overexpress hexokinase II in skeletal muscle in order to determine whether whole-body glucose disposal could be further augmented in mice overexpressing glucose transporters. Overexpression of hexokinase alone in skeletal muscle had no effect on glucose transport or metabolism in isolated muscles, nor did it alter blood glucose levels or the rate of whole-body glucose disposal. Expression of the hexokinase transgene in the context of the Glut1 transgenic background did not alter glucose transport in isolated muscles but did cause additional increases in steady-state glucose 6-phosphate (3.2-fold) and glycogen (7.5-fold) levels compared with muscles that overexpress the Glut1 transporter alone. Surprisingly, however, these increases were not accompanied by a change in basal or insulin-stimulated whole-body glucose disposal in the doubly transgenic mice compared with Glut1 transgenic mice, probably due to an inhibition of de novo glycogen synthesis as a result of the high levels of steady-state glycogen in the muscles of doubly transgenic mice (430 micromol/g versus 10 micromol/g in wild-type mice). We conclude that the hexokinase gene may not be a good target for therapies designed to counteract insulin resistance or hyperglycemia.     

Noninvasive Measurements of [1-13C] Glycogen Concentrations and Metabolism in Rat Brain In Vivo

Using a specific 13C NMR localization method, 13C label incorporation into the glycogen C1 resonance was measured while infusing [1-(13)C]glucose in intact rats. The maximal concentration of [1-(13)C]glycogen was 5.1 +/- 0.6 micromol g(-1) (mean +/- SE, n = 8). During the first 60 min of acute hyperglycemia, the rate of 13C label incorporation (synthase flux) was 2.3 +/- 0.7 micromol g(-1) h(-1) (mean +/- SE, n = 9 rats), which was higher (p < 0.01) than the rate of 0.49 +/- 0.14 micromol g(-1) h(-1) measured > or = 2 h later. To assess whether the incorporation of 13C label was due to turnover or net synthesis, the infusion was continued in seven rats with unlabeled glucose. The rate of 13C label decline (phosphorylase flux) was lower (0.33 +/- 0.10 micromol g(-1) h(-1)) than the initial rate of label incorporation (p < 0.01) and appeared to be independent of the duration of the preceding infusion of [1-(13)C]glucose (p > 0.05 for correlation). The results implied that net glycogen synthesis of approximately 3 micromol g(-1) had occurred, similar to previous reports. When infusing unlabeled glucose before [1-(13)C]glucose in three studies, the rate of glycogen C1 accumulation was 0.46 +/- 0.08 micromol g(-1) h(-1). The results suggest that steady-state glycogen turnover rates during hyperglycemia are approximately 1% of glucose consumption.     

Sensitivity of the soleus muscle to insulin in resting and exercising rats with experimental hypo- and hyper-thyroidism.

1. The effects of hypothyroidism (caused by surgical thyroidectomy followed by treatment for 1 month with propylthiouracil) and of hyperthyroidism [induced by subcutaneous administration of L-tri-iodothyronine (T3)] on glucose tolerance and skeletal-muscle sensitivity to insulin were examined in rats. Glucose tolerance was estimated during 2 h after subcutaneous glucose injection (1 g/kg body wt.). The sensitivity of the soleus muscle to insulin was studied in vitro in sedentary and acutely exercised animals. 2. Glucose tolerance was impaired in both hypothyroid and hyperthyroid rats in comparison with euthyroid controls. 3. In the soleus muscle, responsiveness of the rate of lactate formation to insulin was abolished in hypothyroid rats, whereas the sensitivity of the rate of glycogen synthesis to insulin was unchanged. In hyperthyroid animals, opposite changes were found, i.e. responsiveness of the rate of glycogen synthesis was inhibited and the sensitivity of the rate of lactate production did not differ from that in control sedentary rats. 4. A single bout of exercise for 30 min potentiated the stimulatory effect of insulin on lactate formation in hyperthyroid rats and on glycogen synthesis in hypothyroid animals. 5. The data suggest that thyroid hormones exert an interactive effect with insulin in skeletal muscle. This is likely to be at the post-receptor level, inhibiting the effect of insulin on glycogen synthesis and stimulating oxidative glucose utilization.     

Effects of an adenosine-receptor antagonist on insulin-resistance in soleus muscle from obese Zucker rats.

The decreased sensitivity of glycolysis to insulin seen in isolated soleus muscles from genetically obese Zucker rats was abolished by addition of the adenosine-receptor antagonist 8-phenyltheophylline to the incubation medium; 8-phenyltheophylline had no effect on the sensitivity of glycogen synthesis to insulin. These findings suggest that changes in the sensitivity of glucose utilization by muscles of genetically obese rats may be explained, in part, by a modification in either the concentration of adenosine or the affinity of adenosine receptors in skeletal muscle.     

Insulin resistance after a 72-h fast is associated with impaired AS160 phosphorylation and accumulation of lipid and glycogen in human skeletal muscle

During fasting, human skeletal muscle depends on lipid oxidation for its energy substrate metabolism. This is associated with the development of insulin resistance and a subsequent reduction of insulin-stimulated glucose uptake. The underlying mechanisms controlling insulin action on skeletal muscle under these conditions are unresolved. In a randomized design, we investigated eight healthy subjects after a 72-h fast compared with a 10-h overnight fast. Insulin action on skeletal muscle was assessed by a hyperinsulinemic euglycemic clamp and by determining insulin signaling to glucose transport. In addition, substrate oxidation, skeletal muscle lipid content, regulation of glycogen synthesis, and AMPK signaling were assessed. Skeletal muscle insulin sensitivity was reduced profoundly in response to a 72-h fast and substrate oxidation shifted to predominantly lipid oxidation. This was associated with accumulation of both lipid and glycogen in skeletal muscle. Intracellular insulin signaling to glucose transport was impaired by regulation of phosphorylation at specific sites on AS160 but not TBC1D1, both key regulators of glucose uptake. In contrast, fasting did not impact phosphorylation of AMPK or insulin regulation of Akt, both of which are established upstream kinases of AS160. These findings show that insulin resistance in muscles from healthy individuals is associated with suppression of site-specific phosphorylation of AS160, without Akt or AMPK being affected. This impairment of AS160 phosphorylation, in combination with glycogen accumulation and increased intramuscular lipid content, may provide the underlying mechanisms for resistance to insulin in skeletal muscle after a prolonged fast.     

Acclimation temperature affects the metabolic response of amphibian skeletal muscle to insulin

Frog skeletal muscle mainly utilizes the substrates glucose and lactate for energy metabolism. The goal of this study was to determine the effect of insulin on the uptake and metabolic fate of lactate and glucose at rest in skeletal muscle of the American bullfrog, Lithobates catesbeiana, under varying temperature regimens. We hypothesize that lactate and glucose metabolic pathways will respond differently to the presence of insulin in cold versus warm acclimated frog tissues, suggesting an interaction between temperature and metabolism under varying environmental conditions. We employed radiolabeled tracer techniques to measure in vitro uptake, oxidation, and incorporation of glucose and lactate into glycogen by isolated muscles from bullfrogs acclimated to 5 °C (cold) or 25 °C (warm). Isolated bundles from Sartorius muscles were incubated at 5 °C, 15 °C, or 25 °C, and in the presence and absence of 0.05 IU/mL bovine insulin. Insulin treatment in the warm acclimated and incubated frogs resulted in an increase in glucose incorporation into glycogen, and an increase in intracellular [glucose] of 0.5 μmol/g (P<0.05). Under the same conditions lactate incorporation into glycogen was reduced (P<0.05) in insulin-treated muscle. When compared to the warm treatment group, cold acclimation and incubation resulted in increased rates of glucose oxidation and glycogen synthesis, and a reduction in free intracellular glucose levels (P<0.05). When muscles from either acclimation group were incubated at an intermediate temperature of 15 °C, insulin's effect on substrate metabolism was attenuated or even reversed. Therefore, a significant interaction between insulin and acclimation condition in controlling skeletal muscle metabolism appears to exist. Our findings further suggest that one of insulin's actions in frog muscle is to increase glucose incorporation into glycogen, and to reduce reliance on lactate as the primary metabolic fuel.     

Muscle glycogen inharmoniously regulates glycogen synthase activity, glucose uptake, and proximal insulin signaling

Insulin-stimulated glucose uptake and incorporation of glucose into skeletal muscle glycogen contribute to physiological regulation of blood glucose concentration. In the present study, glucose handling and insulin signaling in isolated rat muscles with low glycogen (LG, 24-h fasting) and high glycogen (HG, refed for 24 h) content were compared with muscles with normal glycogen (NG, rats kept on their normal diet). In LG, basal and insulin-stimulated glycogen synthesis and glycogen synthase activation were higher and glycogen synthase phosphorylation (Ser(645), Ser(649), Ser(653), Ser(657)) lower than in NG. GLUT4 expression, insulin-stimulated glucose uptake, and PKB phosphorylation were higher in LG than in NG, whereas insulin receptor tyrosyl phosphorylation, insulin receptor substrate-1-associated phosphatidylinositol 3-kinase activity, and GSK-3 phosphorylation were unchanged. Muscles with HG showed lower insulin-stimulated glycogen synthesis and glycogen synthase activation than NG despite similar dephosphorylation. Insulin signaling, glucose uptake, and GLUT4 expression were similar in HG and NG. This discordant regulation of glucose uptake and glycogen synthesis in HG resulted in higher insulin-stimulated glucose 6-phosphate concentration, higher glycolytic flux, and intracellular accumulation of nonphosphorylated 2-deoxyglucose. In conclusion, elevated glycogen synthase activation, glucose uptake, and GLUT4 expression enhance glycogen resynthesis in muscles with low glycogen. High glycogen concentration per se does not impair proximal insulin signaling or glucose uptake. "Insulin resistance" is observed at the level of glycogen synthase, and the reduced glycogen synthesis leads to increased levels of glucose 6-phosphate, glycolytic flux, and accumulation of nonphosphorylated 2-deoxyglucose.     

The effect of glucagon administration on protein synthesis in skeletal muscles, heart and liver in vivo.

Infusion of glucagon (0.5 mg/h per 100 g body wt.) into fed rats for 6 h inhibited protein synthesis in skeletal muscle, but not in heart. The order of sensitivity of three muscles was plantaris greater than gastrocnemius greater than soleus. Treatment with glucagon for periods of 1 h or less had no effect. Liver protein synthesis was inhibited by glucagon treatment for 10 min, but stimulated after 6 h. The effect of glucagon on muscle was not secondary to impaired food absorption or to depletion of amino acids by increased gluconeogenesis, since the inhibition of protein synthesis was observed in postabsorptive and amino acid-infused rats. The failure of glucagon to inhibit muscle protein synthesis after 1 h may have been caused by the increase in plasma insulin that occurred at this time, since an inhibition was detected in insulin-treated diabetic rats. The lowest infusion rate that gave a significant decrease in muscle protein synthesis was 6 micrograms/h per 100 g body wt., despite a small increase in plasma insulin. This gave plasma glucagon concentrations in the high pathophysiological range, suggesting that glucagon may be significant in the pathogenesis of muscle wasting in metabolic stresses such as diabetes and starvation.     

Death receptor-associated pro-apoptotic signaling in aged skeletal muscle

Tumor necrosis factor-alpha (TNF-α) is elevated in the serum as a result of aging and it promotes pro-apoptotic signaling upon binding to the type I TNF receptor. It is not known if activation of this apoptotic pathway contributes to the well-documented age-associated decline in muscle mass (i.e. sarcopenia). We tested the hypothesis that skeletal muscles from aged rodents would exhibit elevations in markers involved in the extrinsic apoptotic pathway when compared to muscles from young adult rodents, thereby contributing to an increased incidence of nuclear apoptosis in these muscles. The plantaris (fast) and soleus (slow) muscles were studied in young adult (5–7 mo, n=8) and aged (33 mo, n=8) Fischer₃₄₄ × Brown Norway rats. Muscles from aged rats were significantly smaller while exhibiting a greater incidence of apoptosis. Furthermore, muscles from aged rats had higher type I TNF receptor and Fas associated death domain protein (FADD) mRNA, protein contents for FADD, BCL-2 Interacting Domain (Bid), FLICE-inhibitory protein (FLIP), and enzymatic activities of caspase-8 and caspase-3 than muscles from young adult rats. Significant correlations were observed in the plantaris muscle between caspase activity and muscle weight and the apoptotic index, while similar relationships were not found in the soleus. These data demonstrate that pro-apoptotic signaling downstream of the TNF receptor is active in aged muscles. Furthermore, our data extend the previous demonstration that type II fibers are preferentially affected by aging and support the hypothesis that type II fiber containing skeletal muscles may be more susceptible to muscle mass loses via the extrinsic apoptotic pathway.     

Mitochondrial energetics is impaired in vivo in aged skeletal muscle

With aging, most skeletal muscles undergo a progressive loss of mass and strength, a process termed sarcopenia. Aging‐related defects in mitochondrial energetics have been proposed to be causally involved in sarcopenia. However, changes in muscle mitochondrial oxidative phosphorylation with aging remain a highly controversial issue, creating a pressing need for integrative approaches to determine whether mitochondrial bioenergetics are impaired in aged skeletal muscle. To address this issue, mitochondrial bioenergetics was first investigated in vivo in the gastrocnemius muscle of adult (6 months) and aged (21 months) male Wistar rats by combining a modular control analysis approach with ³¹P magnetic resonance spectroscopy measurements of energetic metabolites. Using this innovative approach, we revealed that the in vivo responsiveness (‘elasticity’) of mitochondrial oxidative phosphorylation to contraction‐induced increase in ATP demand is significantly reduced in aged skeletal muscle, a reduction especially pronounced under low contractile activities. In line with this in vivo aging‐related defect in mitochondrial energetics, we found that the mitochondrial affinity for ADP is significantly decreased in mitochondria isolated from aged skeletal muscle. Collectively, the results of this study demonstrate that mitochondrial bioenergetics are effectively altered in vivo in aged skeletal muscle and provide a novel cellular basis for this phenomenon.     

No limiting role for glycogenin in determining maximal attainable glycogen levels in rat skeletal muscle

We examined whether the protein level and/or activity of glycogenin, the protein core upon which glycogen is synthesized, is limiting for maximal attainable glycogen levels in rat skeletal muscle. Glycogenin activity was 27.5 +/- 1.4, 34.7 +/- 1.7, and 39.7 +/- 1.3 mU/mg protein in white gastrocnemius, red gastrocnemius, and soleus muscles, respectively. A similar fiber type dependency of glycogenin protein levels was seen. Neither glycogenin protein level nor the activity of glycogenin correlated with previously determined maximal attainable glycogen levels, which were 69.3 +/- 5.8, 137.4 +/- 10.1, and 80.0 +/- 5.4 micromol/g wet wt in white gastrocnemius, red gastrocnemius, and soleus muscles, respectively. In additional experiments, rats were exercise trained by swimming, which resulted in a significant increase in the maximal attainable glycogen levels in soleus muscles ( approximately 25%). This increase in maximal glycogen levels was not accompanied by an increase in glycogenin protein level or activity. Furthermore, even in the presence of very high glycogen levels ( approximately 170 micromol/g wet wt), approximately 30% of the total glycogen pool continued to be present as unsaturated glycogen molecules (proglycogen). Therefore, it is concluded that glycogenin plays no limiting role for maximal attainable glycogen levels in rat skeletal muscle.