Transgenic mice overexpressing GLUT-1 protein in muscle exhibit increased muscle glycogenesis after exercise

The purpose of the present study was to determine the rates of muscle glycogenolysis and glycogenesis during and after exercise in GLUT-1 transgenic mice and their age-matched littermates. Male transgenic mice (TG) expressing a high level of human GLUT-1 and their nontransgenic (NT) littermates underwent 3 h of swimming. Glycogen concentration was determined in gastrocnemius and extensor digitorum longus (EDL) muscles before exercise and at 0, 5, and 24 h postexercise, during which food (chow) and 10% glucose solution (as drinking water) were provided. Exercise resulted in approximately 90% reduction in muscle glycogen in both NT (from 11.2 +/- 1.4 to 2. 1 +/- 1.3 micromol/g) and TG (from 99.3 +/- 4.7 to 11.8 +/- 4.3 micromol/g) in gastrocnemius muscle. During recovery from exercise, the glycogen concentration increased to 38.2 +/- 7.3 (5 h postexercise) and 40.5 +/- 2.8 micromol/g (24 h postexercise) in NT mice. In TG mice, however, the increase in muscle glycogen concentration during recovery was greater (to 57.5 +/- 7.4 and 152.1 +/- 15.7 micromol/g at 5 and 24 h postexercise, respectively). Similar results were obtained from EDL muscle. The rate of 2-deoxyglucose uptake measured in isolated EDL muscles was 7- to 10-fold higher in TG mice at rest and at 0 and 5 h postexercise. There was no difference in muscle glycogen synthase activation measured in gastrocnemius muscles between NT and TG mice immediately after exercise. These results demonstrate that the rate of muscle glycogen accumulation postexercise exhibits two phases in TG: 1) an early phase (0-5 h), with rapid glycogen accumulation similar to that of NT mice, and 2) a progressive increase in muscle glycogen concentration, which differs from that of NT mice, during the second phase (5-24 h). Our data suggest that the high level of steady-state muscle glycogen in TG mice is due to the increase in muscle glucose transport activity.

Effect of AMPK activation on muscle glucose metabolism in conscious rats

The effect of AMP-activated protein kinase (AMPK) activation on skeletal muscle glucose metabolism was examined in awake rats by infusing them with 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside (AICAR; 40 mg/kg bolus and 7.5 mg. kg-1. min-1 constant infusion) along with a variable infusion of glucose (49.1 +/- 2.4 micromol. kg-1. min-1) to maintain euglycemia. Activation of AMPK by AICAR caused 2-deoxy-D-[1,2-3H]glucose (2-DG) uptake to increase more than twofold in the soleus and the lateral and medial gastrocnemius compared with saline infusion and occurred without phosphatidylinositol 3-kinase activation. Glucose uptake was also assessed in vitro by use of the epitrochlearis muscle incubated either with AICAR (0.5 mM) or insulin (20 mU/ml) or both in the presence or absence of wortmannin (1.0 microM). AICAR and insulin increased muscle 2-DG uptake rates by approximately 2- and 2.7-fold, respectively, compared with basal rates. Combining AICAR and insulin led to a fully additive effect on muscle glucose transport activity. Wortmannin inhibited insulin-stimulated glucose uptake. However, neither wortmannin nor 8-(p-sulfophenyl)-theophylline (10 microM), an adenosine receptor antagonist, inhibited the AICAR-induced activation of glucose uptake. Electrical stimulation led to an about threefold increase in glucose uptake over basal rates, whereas no additive effect was found when AICAR and contractions were combined. In conclusion, the activation of AMPK by AICAR increases skeletal muscle glucose transport activity both in vivo and in vitro. This cellular pathway may play an important role in exercise-induced increase in glucose transport activity.

Estradiol protects against ischemic injury

Clinical studies demonstrate that estrogen replacement therapy in postmenopausal women may enhance cognitive function and reduce neurodegeneration associated with Alzheimer's disease and stroke. This study assesses whether physiologic levels of estradiol prevent brain injury in an in vivo model of permanent focal ischemia. Sprague-Dawley rats were ovariectomized; they then were implanted, immediately or at the onset of ischemia, with capsules that produced physiologically low or physiologically high 17beta-estradiol levels in serum (10 or 60 pg/mL, respectively). One week after ovariectomy, ischemia was induced. Estradiol pretreatment significantly reduced overall infarct volume compared with oil-pretreated controls (mean+/-SD: oil = 241+/-88; low = 139+/-91; high = 132+/-88 mm3); this protective effect was regionally specific to the cortex, since no protection was observed in the striatum. Baseline and ischemic regional CBF did not differ between oil and estradiol pretreated rats, as measured by laser Doppler flowmetry. Acute estradiol treatment did not protect against ischemic injury. Our finding that estradiol pretreatment reduces injury demonstrates that physiologic levels of estradiol can protect against neurodegeneration.

Transgenic mice deficient in the LAR protein-tyrosine phosphatase exhibit profound defects in glucose homeostasis

Protein-tyrosine phosphatases (PTPases) play an integral role in the regulation of cellular insulin action. LAR, a transmembrane PTPase expressed in insulin-sensitive tissues, acts as a negative regulator of insulin signaling in intact cell models. The physiological role of LAR was studied in mice in which LAR expression was eradicated by insertional mutagenesis. In the fasting state, adult male homozygous LAR (-/-) mice had significantly lower plasma levels of insulin and glucose, as well as a reduced rate of hepatic glucose production compared with wild-type controls, suggesting a heightened level of insulin sensitivity. In euglycemic clamp studies, the LAR (-/-) mice exhibited a significant resistance to insulin-stimulated glucose disposal and suppression of hepatic glucose output. Examination of hepatic insulin action demonstrated that the major alteration involved a 47% reduction in insulin-stimulated phosphatidylinositol 3'-kinase (PI 3-kinase) activity in the knockout mice, indicating a post-receptor signaling defect. Taken together with previous work on the cellular effects of LAR, the present results are consistent with a physiological role for LAR in the negative regulation of insulin action, with secondary abnormalities that contribute to the resistance to insulin-stimulated signaling in the knockout mice. Overall, these data provide further evidence for an important role for LAR in the regulation of insulin action and glucose homeostasis in intact animals.

Disruption of IRS-2 causes type 2 diabetes in mice

Human type 2 diabetes is characterized by defects in both insulin action and insulin secretion. It has been difficult to identify a single molecular abnormality underlying these features. Insulin-receptor substrates (IRS proteins) may be involved in type 2 diabetes: they mediate pleiotropic signals initiated by receptors for insulin and other cytokines. Disruption of IRS-1 in mice retards growth, but diabetes does not develop because insulin secretion increases to compensate for the mild resistance to insulin. Here we show that disruption of IRS-2 impairs both peripheral insulin signalling and pancreatic beta-cell function. IRS-2-deficient mice show progressive deterioration of glucose homeostasis because of insulin resistance in the liver and skeletal muscle and a lack of beta-cell compensation for this insulin resistance. Our results indicate that dysfunction of IRS-2 may contribute to the pathophysiology of human type 2 diabetes.

Time window of infarct reduction by intravenous basic fibroblast growth factor in focal cerebral ischemia

Basic fibroblast growth factor (bFGF) is a heparin-binding polypeptide with potent trophic and protective effects on brain neurons, glia and endothelia. In previous studies, we showed that intravenously administered bFGF reduced the volume of cerebral infarcts following permanent occlusion of the middle cerebral artery in rats. In the current study, we examined the time dependence of bFGF infusion on infarct reduction, and the effect of co-infusion of bFGF with heparin. We found a significant reduction in infarct volume when the bFGF infusion (50 microg/kg per h for 3 h) was begun up to 3 h, but not 4 h after the onset of ischemia. The infarct reducing effects of bFGF were not altered by co-infusion of heparin. These results are potentially important in light of the ongoing clinical trials of intravenous bFGF in acute stroke.

Insulin signaling in mice expressing reduced levels of Syp

Syp is a protein tyrosine phosphatase implicated in insulin and growth factor signaling. To evaluate the role of syp in insulin's regulation of plasma glucose, we generated knockout mice. Homozygous knockout mice die prior to day 10.5 of embryonic development. Hemizygous mice express half the levels of syp protein compared with their wild type littermates but do not display any gross morphological changes. Total body weight (age 2-10 weeks) and plasma insulin and glucose levels both in fasting and glucose-challenged states were comparable in the wild type and the hemizygous mice. No differences were observed in insulin-induced glucose uptake in soleus muscle and epididymal fat; insulin inhibition of lipolysis was also similar. We injected insulin into the portal vein of the mice to examine upstream events of the insulin signaling cascade. Tyrosine phosphorylation of insulin receptor and insulin receptor substrate-1 (IRS-1) from hemizygous tissue was similar to that of wild type tissue. Association of the p85 subunit of phosphatidylinositol 3-kinase to IRS-1 increased an average of 2-fold in both groups. We did not observe an increase of IRS-1/syp association after insulin administration, but we did note a significant basal association in both wild type and hemizygous tissue. Our results do not support a major role for syp in the acute in vivo metabolic actions of insulin.

Effects of CGS 9343B (a putative calmodulin antagonist) on isolated skeletal muscle. Dissociation of signaling pathways for insulin-mediated activation of glycogen synthase and hexose transport

The role of calmoudulin in control of carbohydrate metabolism in the absence and presence of insulin in isolated mouse soleus muscle was investigated. The calmodulin antagonist CGS 9343B had no effect on basal glycogen synthase activity, the contents of high energy phosphates, glucose-6-P, or glycogen synthesis. However, CGS 9343B inhibited the basal rates of 2-deoxyglucose uptake and 3-O-methylglucose transport by 30% (p < 0.05) and 40% (p < 0.001), respectively. Insulin activated glycogen synthase by almost 40% (p < 0.01) and this increase was not altered in the presence of CGS 9343B. Insulin increased the muscle content of glucose-6-P (approximately equal to 2-fold), as well as glycogen synthesis (approximately equal to 8-fold), 2-deoxyglucose uptake (approximately equal to 3-fold), and 3-O-methylglucose transport (approximately equal to 2-fold), and these increases were inhibited by CGS 9343B. In additional experiments on isolated rat epitrochlearis muscle, it was found that the hypoxia-mediated activation of 3-O-methylglucose transport was also inhibited by CGS 9343B. These data demonstrate that: 1) hexose transport, both in the absence and presence of external stimuli (insulin and hypoxia), requires functional calmodulin; and 2) insulin-mediated activation of glycogen synthase does not require functional calmodulin, nor can it be accounted for by increases in glucose transport or glucose-6-P.

Effects of beta-guanidinopropionic acid-feeding on the patterns of myosin isoforms in rat fast-twitch muscle

Administration of beta-guanidinopropionic acid (beta-GPA) to rats as 1% of their diet for 6 weeks led to an accumulation of beta-GPA and beta-GPA-phosphate and to a depletion of creatine and phosphocreatine in the fast-twitch plantaris muscle. Adenosine triphosphate concentration was also decreased. Electrophoretic analyses were performed to investigate the effects of beta-GPA on the patterns of fast (FM) and slow (SM) isomyosins, myosin heavy chain (HC) isoforms and myosin light chain (LC) isoforms. The relative concentrations of fast isomyosins FM1 and FM2 decreased, whereas slow isomyosin SM increased. The increase in slow isomyosin corresponded to an increase in the relative concentration of the slow myosin HCI. The changes of the myosin light chain pattern consisted of increases in the relative concentrations of the two slow isoforms, LC1sb and LC2s, and decreases in the fast isoforms LC2f and LC3f. These results demonstrate that beta-GPA administration, leading to a depletion in energy-rich phosphates and a reduced phosphorylation potential, has an impact on myosin isoform expression in rat fast-twitch skeletal muscle.

Overexpression of Glut4 protein in muscle increases basal and insulin-stimulated whole body glucose disposal in conscious mice

The effect of increased Glut4 protein expression in muscle and fat on the whole body glucose metabolism has been evaluated by the euglycemic hyperinsulinemic clamp technique in conscious mice. Fed and fasting plasma glucose concentrations were 172 +/- 7 and 78 +/- 7 mg/dl, respectively, in transgenic mice, and were significantly lower than that of nontransgenic littermates (208 +/- 5 mg/dl in fed; 102 +/- 5 mg/dl in fasting state). Plasma lactate concentrations were higher in transgenic mice, (6.5 +/- 0.7 mM in the fed and 5.8 +/- 1.0 mM in fasting state) compared with that of non-transgenic littermates (4.7 +/- 0.3 mM in the fed and 4.2 +/- 0.5 mM in fasting state). In the fed state, the rate of whole body glucose disposal was 70% higher in transgenic mice in the basal state, 81 and 54% higher during submaximal and maximal insulin stimulation. In the fasting state, insulin-stimulated whole body glucose disposal was also higher in the transgenic mice. Hepatic glucose production after an overnight fast was 24.8 +/- 0.7 mg/kg per min in transgenic mice, and 25.4 +/- 2.7 mg/kg per min in nontransgenic mice. Our data demonstrate that overexpression of Glut4 protein in muscle increases basal as well as insulin-stimulated whole body glucose disposal. These results suggest that skeletal muscle glucose transport is rate-limiting for whole body glucose disposal and that the Glut4 protein is a potential target for pharmacological or genetic manipulation for treatment of patients with non-insulin-dependent diabetes mellitus.

An investigation of pedigrees of 110 patients with Graves' disease and the clinical significance of determinations of antithyroid antibodies of their first-degree relatives

Eight hundred and ten pedigree members of 110 patients with Graves' disease were studied. In 700 first-degree relatives, inquiry of medical history, physical examination (including eyes, thyroid, heart rate, etc), thyroid function tests (serum T3, T4 and TSH levels), determinations of thyroglobulin antibodies (TgAb) and thyroid microsomal antibodies (TmAb) were performed. For male (female) probands, the incidence of Graves' disease in male (female) first-degree relatives were investigated and their serum TgAb and TmAb were analysed. The incidence of these two kinds of autoantibodies in the male (female) first-degree relatives of familial and nonfamilial Graves' disease were analysed. Eighteen persons with positive TgAb and TmAb from 5 pedigrees had been followed up one year after initial determinations. Our results suggest that the positive rates of TgAb and TmAb in the first-degree relatives of Graves' disease were coincident with the incidence of Graves' disease, and the positive results of TgAb and TmAb in the first-degree relatives of Graves' disease may be an indicator of pre-Graves' disease or pre-autoimmune thyroid diseases.

Glucose transport activity in skeletal muscles from transgenic mice overexpressing GLUT1. Increased basal transport is associated with a defective response to diverse stimuli that activate GLUT4

Glucose transport activity was examined in transgenic mice overexpressing the human GLUT1 glucose transporter in skeletal muscles. Basal transport activity measured in vitro with the glucose analog 2-deoxy-D-glucose (1 mM) was increased 2-8-fold in four different muscle preparations. Incubation of muscles from control nontransgenic littermates with a maximally effective concentration of insulin or with insulin-like growth factor-1 resulted in glucose transport rates that were 2-3-fold higher than basal. In contrast, insulin did not stimulate glucose transport activity in three different muscle preparations from transgenic animals; insulin-like growth factor-1 was similarly ineffective. Activation of System A amino acid transport activity (measured with the nonmetabolizable analog alpha-methylaminoisobutyrate) by insulin was not impaired in muscles from transgenic mice, indicating that the defect does not involve the insulin receptor. In skeletal muscle, glucose transport can be activated by muscle contractions or hypoxia via a pathway separate from that activated by insulin. Incubation of muscles under hypoxic conditions or stimulation of muscles to contract in situ did not increase glucose transport activity in muscles from GLUT1-overexpressing mice, in contrast to the stimulatory effects measured in muscles from control animals. These data suggest that increased glucose flux per se into skeletal muscle results in resistance of GLUT4 to activation by insulin and various other stimuli that activate glucose transport by mechanisms distinct from that of insulin. GLUT1-overexpressing mice thus provide a new model system for studying the effects of glucose-induced resistance to activation of glucose transport.

Exercise induces rapid increases in GLUT4 expression, glucose transport capacity, and insulin-stimulated glycogen storage in muscle

GLUT4 glucose transporter content and glucose transport capacity are closely correlated in skeletal muscle. In this study, we tested the hypothesis that a rapid increase in GLUT4 expression occurs as part of the early adaptive response of muscle to exercise and serves to enhance glycogen storage. Rats exercised by swimming had a approximately 2-fold increase in GLUT4 mRNA and a 50% increase in GLUT4 protein expression in epitrochlearis muscle 16 h after one prolonged exercise session. After a 2nd day of exercise, muscle GLUT4 protein was increased further to approximately 2-fold while there was no additional increase in GLUT4 mRNA. Muscle hexokinase activity also doubled in response to 2 days of exercise. Glucose transport activity maximally stimulated with insulin, contractions, or hypoxia was increased roughly in proportion to the adaptive increase in GLUT4 protein in epitrochlearis muscles. Treatment with insulin prior to subcellular fractionation of muscle resulted in a approximately 2-fold greater increase in GLUT4 content of a plasma membrane fraction in the 2-day swimmers than in controls. When epitrochlearis muscles were incubated with glucose and insulin, glycogen accumulation over 3 h was twice as great in muscles from 2-day swimmers as in control muscles. Our results show that a rapid increase in GLUT4 expression is an early adaptive response of muscle to exercise. This adaptation appears to be mediated by pretranslational mechanisms. We hypothesize that the physiological role of this adaptation is to enhance replenishment of muscle glycogen stores.

[Pedigrees of 110 patients with Graves' disease and the determinations of antithyroid antibodies in their first-degree relatives]

We studied 810 pedigree members of 110 patients with Graves' disease. In 700 first-degree relatives, inquiry of medical history, physical examination (eyes, thyroid, heart rate, etc), thyroid function tests (serum T3, T4 and TSH), determinations of thyroglobulin antibodies (TgAb) and thyroid microsomal antibodies (TmAb) were carried out. For male (female) probands, the incidence of Graves' disease in first-degree relatives was investigated and their serum TgAb and TmAb were analyzed. In addition, we also analyzed the positive rates of these two kinds of autoantibodies of male (female) first-degree relatives of familial and nonfamilial Graves' disease. Eighteen persons with positive TgAb and TmAb from 5 pedigrees were followed up, one year after initial determinations. The morbidity of male (female) patients of male (female) probands of Graves' disease, the results of the determinations of TgAb and TmAb of male (female) first-degree relatives, the changes of TgAb and TmAb of the male (female) first-degree relatives of familial and nonfamilial Graves' disease are discussed.

Germline manipulation of glucose homeostasis via alteration of glucose transporter levels in skeletal muscle

Transgenic mice were constructed that overexpress the human Glut1 glucose transporter in skeletal muscle. Transcription of the human Glut1 cDNA was driven by the rat myosin light chain 2 promoter. Soleus and quadriceps muscles from transgenic mice expressed increased levels of Glut1 protein relative to muscles obtained from nontransgenic littermates, but there was no difference in the level of Glut4 protein between the two groups. Skeletal muscles isolated from the transgenic animals exhibited 3-4-fold increases in basal glucose uptake relative to muscles obtained from nontransgenic littermates. Muscles isolated from nontransgenic littermates exhibited 2-3-fold increases in glucose transport after incubation in the presence of insulin, but no insulin-stimulated increase in transport was observed in the muscles of transgenic mice. Plasma glucose levels were reduced by 18 and 30%, respectively, in fed and fasted transgenic mice relative to their nontransgenic siblings, but insulin and glucagon levels were not significantly different between the two groups. Glucose disposal following an oral glucose load was markedly enhanced in the transgenic animals, and plasma lactate and beta-OH-butyrate levels were elevated in both fed and fasted transgenic mice. These data strongly support the hypothesis that glucose transport plays a key role in whole body glucose homeostasis. They also demonstrate that the level of a glucose transporter in skeletal muscle can significantly influence the blood glucose set point and alter the levels of other fuel metabolites in the blood.

Evidence from transgenic mice that glucose transport is rate-limiting for glycogen deposition and glycolysis in skeletal muscle

A line of transgenic mice was constructed in which the human Glut1 glucose transporter is overexpressed in skeletal muscle. Overexpression of Glut1 protein was evident in epitrochlearis, extensor digitorum longus (EDL), and quadriceps muscles, and resulted in 6.6-7.4-fold elevations in basal glucose transport activity as measured in isolated muscles in vitro. The elevated glucose transporter activity in the skeletal muscles of transgenic mice was associated with a 10-fold increase in glycogen concentration in EDL and quadriceps muscles that was not due to an increase in muscle glycogen synthase activity or a decrease in glycogen phosphorylase activity. The increased glucose transport activity also resulted in a 2-fold increase in muscle lactate concentration, with no increase in muscle glucose 6-phosphate. Despite a slight (10%) increase in muscle hexokinase activity, there was a 4-fold increase in total muscle free glucose in transgenic mice, indicating that hexokinase becomes rate-limiting for glucose uptake when the rate of glucose transport is very high. These results demonstrate that the muscle glycogen content can be dramatically elevated by increasing the muscle Glut1 protein level and that glucose transport is a rate-limiting step for muscle glucose disposal in normal, resting mice.

Effects of alkaline pH on the stimulation of glucose transport in rat skeletal muscle

Alkaline pH has been reported to cause release of Ca2+ from skeletal muscle sarcoplasmic reticulum (SR). Elevation of sarcoplasmic Ca2+ concentration is thought to stimulate glucose transport in skeletal muscle. In this context, we examined the effect of alkaline pH (extracellular pH of 8.6) on 3-O-methylglucose transport in skeletal muscle. Incubation of rat epitrochlearis muscles at pH 8.6 for 45 min resulted in an approx. 3-fold increase in glucose transport activity, which was not affected by reducing Ca2+ concentration in the incubation medium and essentially completely blocked by 25 microM dantrolene, an inhibitor of SR Ca2+ release. In addition to stimulating glucose transport by itself, alkaline pH may partially inhibit the stimulation of sugar transport by insulin hypoxia and contractions, as the combined effect of alkaline pH and the maximal effect of insulin, contractions, or hypoxia on glucose transport are not different from the maximal effects of insulin, hypoxia, or contractions alone. The maximal effects of insulin and contractions, and of insulin and hypoxia, on glucose transport are normally additive in muscle. Alkaline pH completely prevented this additivity. In summary, our results show that alkaline pH stimulates glucose transport activity in skeletal muscle and provide evidence suggesting that this effect is mediated by Ca2+. They further show that alkaline pH blocks the additivity of the maximal effects of insulin and contractions or hypoxia suggesting that alkaline pH may partially inhibit the stimulation of glucose transport by insulin, contraction and hypoxia.

Adaptation of muscle to creatine depletion: effect on GLUT-4 glucose transporter expression

Feeding rats beta-guanidinopropionic acid (beta-GPA), a creatine analogue, results in depletion of creatine and phosphocreatine and induces increases in mitochondrial oxidative enzymes and hexokinase in skeletal muscle. Comparisons of different muscle types and studies of the adaptation to exercise suggest that 1) the levels of the insulin-responsive glucose transporter (GLUT-4), mitochondrial oxidative enzymes, and hexokinase may be coregulated and 2) GLUT-4 content can determine maximal glucose transport activity in muscle. To further evaluate these possibilities, we examined the effects of feeding rats 1% beta-GPA in their diet for 6 wk on muscle GLUT-4 expression and glucose transport activity. beta-GPA feeding induced 40-50% increases in cytochrome c concentration, citrate synthase activity, and hexokinase activity in plantaris muscle. GLUT-4 protein concentration was increased approximately 50% in plantaris and epitrochlearis muscles, while GLUT-4 mRNA was increased approximately 40% in plantaris muscles of beta-GPA-fed rats. Glucose transport activity maximally stimulated by insulin was increased in parallel with GLUT-4 protein concentration in the epitrochlearis. These results provide evidence that chronic creatine depletion increases GLUT-4 expression by pretranslational mechanisms. They support the hypothesis that the levels of mitochondrial enzymes, hexokinase, and GLUT-4 protein are coregulated in striated muscles. They also support the concept that the GLUT-4 content of a muscle determines its maximal glucose transport activity when the signaling pathways for glucose transport activation are intact.

Energy metabolism in single human muscle fibres during intermittent contraction with occluded circulation

1. Glycogenolysis in type I and II muscle fibres was investigated in five healthy volunteers during electrical stimulation of the quadriceps muscle group with blood flow occluded. 2. The quadriceps femoris muscles were stimulated intermittently (1.6 s stimulation, 1.6 s rest) at a frequency of 50 Hz for 64 s and isometric contraction force was recorded. Muscle biopsies were obtained at rest prior to and immediately after stimulation. Single muscle fibres were dissected free and were identified as type I and II fibres. ATP, phosphocreatine (PCr) and glycogen contents were measured luminometrically and enzymatically in single fibres and mixed fibre muscle. 3. Electrical stimulation resulted in a marked decline in contraction force and near total depletion of PCr in both fibre types. The ATP turnover rate (P < 0.05) and the magnitude of the decline in ATP (P < 0.05) were greater in type II fibres. Prior to stimulation the muscle glycogen content was 32% higher in type II fibres compared with type I fibres (P < 0.01). During stimulation the rate of glycogenolysis in type II fibres (4.32 +/- 0.54 mmol (kg dry matter (DM)-1 s-1 was twofold greater than the rate in type I fibres (2.05 +/- 0.70 mmol (kg DM)-1 s-1, P < 0.05). 4. The data suggest that the relatively higher rate of glycogenolysis observed in type I fibres during intermittent electrical stimulation with occluded circulation (2.05 +/- 0.70 mmol (kg DM)-1 s-1), when compared with the corresponding rate recorded during intense contraction with circulation intact (0.18 +/- 0.14 mmol (kg DM)-1 s-1, P < 0.05), may result from an accelerated ATP turnover rate in this fibre type increasing the cellular concentrations of free AMP and inosine 5'-monophosphate (IMP), which are known activators of glycogen phosphorylase. 5. The similarity in the rate of type II fibre glycogenolysis during contraction with circulatory occlusion (4.32 +/- 0.54 mmol (kg DM)-1 s-1), when compared with the corresponding rate recorded during non-occluded circulation (3.54 +/- 0.53 mmol (kg DM)-1 s-1, P > 0.05), is in agreement with the suggestion that glycogenolysis in this fibre type is already occurring at a near-maximal rate with circulation intact.

Hypoxia causes glycogenolysis without an increase in percent phosphorylase a in rat skeletal muscle

Stimulation of skeletal muscle to contract activates phosphorylase b-to-a conversion and glycogenolysis. Despite reversal of the increase in percentage of phosphorylase a after a few minutes, continued glycogen breakdown can occur during strenuous exercise. Hypoxia causes sustained glycogenolysis in skeletal muscle without an increase in percentage of phosphorylase a. We used this model to obtain insights regarding how glycogenolysis is mediated in the absence of an increase in percentage of phosphorylase a. Hypoxia caused a 70% decrease in glycogen in epitrochlearis muscles during an 80-min incubation despite no increase in percentage of phosphorylase a above the basal level of approximately 10%. Muscle Pi concentration increased from 3.8 to 8.6 mumol/g muscle after 5 min and 15.7 mumol/g after 20 min. AMP concentration doubled, attaining a steady state of 0.23 mumol/g in 5 min. Incubation of oxygenated muscles with 0.1 microM epinephrine induced an approximately sixfold increase in percentage of phosphorylase a but resulted in minimal glycogenolysis. Muscle Pi concentration was not altered by epinephrine. Despite no increase in percentage of phosphorylase a, hypoxia resulted in a fivefold greater depletion of glycogen over 20 min than did epinephrine. To evaluate the role of phosphorylase b, muscles were loaded with 2-deoxyglucose 6-phosphate, which inhibits phosphorylase b. The rate of glycogenolysis during 60 min of hypoxia was reduced by only approximately 14% in 2-deoxyglucose 6-phosphate-loaded muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation of rat skeletal muscle to creatine depletion: AMP deaminase and AMP deamination

AMP deaminase catalyzes deamination of the AMP formed in contracting muscles to inosine 5'-monophosphate (IMP). Slow-twitch muscle has only approximately 30% as high a level of AMP deaminase activity as fast-twitch muscle in the rat, and rates of IMP formation during intense contractile activity are much lower in slow-twitch muscle. We found that feeding the creatine analogue beta-guanidinopropionic acid (beta-GPA) to rats, which results in creatine depletion, causes a large decrease in muscle AMP deaminase. This adaptation was used to evaluate the role of AMP deaminase activity level in accounting for differences in IMP production in slow-twitch and fast-twitch muscles. beta-GPA feeding for 3 wk lowered AMP deaminase activity in fast-twitch epitrochlearis muscle to a level similar to that found in the normal slow-twitch soleus muscle but had no effect on the magnitude of the increase in IMP in response to intense contractile activity. Despite a similar decrease in ATP in the normal soleus and the epitrochlearis from beta-GPA-fed rats, the increase in IMP was only approximately 30% as great in the soleus in response to intense contractile activity. These results demonstrate that the accumulation of less IMP in slow- compared with fast-twitch skeletal muscle during contractile activity is not due to the lower level of AMP deaminase in slow-twitch muscle.

Energy metabolism in single human muscle fibers during contraction without and with epinephrine infusion

The concentrations of glycogen, ATP, and phosphocreatine were analyzed in types I and II muscle fibers separated from biopsy samples of the quadriceps femoris muscle in five healthy volunteers. Muscle samples were obtained before and after 64 s of intermittent electrical stimulation. The experiment was carried out without and with epinephrine (Epi) infusion. Before stimulation the glycogen concentration was 11% higher in type II than in type I fibers (P less than 0.05). During electrical stimulation, rapid glycogenolysis occurred in type II fibers with hardly any detectable glycogenolysis in type I fibers. The calculated rates of glycogenolysis were 0.18 +/- 0.14 and 3.54 +/- 0.53 mmol glucose.kg dry muscle-1.s-1 in types I and II fibers, respectively. Epi infusion increased the rate of glycogenolysis during electrical stimulation in type I fibers (10-fold) but did not enhance the rate in type II fibers (P greater than 0.05). It is considered that, during short-term maximal muscle contraction, rapid muscle glycogenolysis occurs predominantly in type II fibers even though types I and II fibers are recruited and that, when Epi stimulation of glycogenolysis occurs, this is predominantly limited to type I fibers.

Regulation of phosphorylase a activity in human skeletal muscle

The control mechanism of glycogenolysis by phosphorylase a in contracting muscle has been investigated. The quadriceps femoris muscles of six subjects were intermittently stimulated at 15 and 50 Hz. The stimulation lasted 9.6 s and was performed twice at 15 Hz and once at 50 Hz. Epinephrine was infused continuously during the experiment. The force generation and ATP turnover rate were nearly twofold higher at 50 Hz than at 15 Hz. Calculated mean Pi was 5.7 and 10.0 mM during the two 15-Hz stimulations and 8.1 mM during the 50-Hz stimulation. Phosphorylase a varied between 85.5 and 91.5% without significant differences between periods. However, the rate of glycogenolysis was twofold higher during the stimulation at 50 Hz than it was at 15 Hz (P less than 0.05) and was related to the ATP turnover rate (r = 0.992). These results demonstrate that rapid glycogen breakdown during muscle contraction cannot be solely explained by transformation of phosphorylase b to a and increased Pi concentration. The contraction intensity may determine the glycogenolytic rate through a transient increase in free AMP level related to the ATP turnover rate.