Relationship between muscle fibre composition, glucose transporter protein 4 and exercise training: possible consequences in non-insulin-dependent diabetes mellitus

Benefits beyond cardiometabolic health: the potential of frequent high intensity 'exercise snacks' to improve outcomes for those living with and beyond cancer

High intensity interval training (HIIT) has been shown to consistently elicit rapid and significant adaptations in a number of physiological systems, across many different healthy and clinical populations. In addition, there is increasing interest in how some acute, yet transient responses to high intensity exercise potentially reduce the risks of particular diseases. Recent work has shown that discrete, brief bouts of high intensity exercise (termed 'exercise snacks') can improve glucose control and vascular health and thus counter the negative cardiometabolic consequences of prolonged, uninterrupted periods of inactivity. In this brief review, we advance the case, using evidence available from pre-clinical studies in the exercise oncology literature, that brief, frequently completed bouts of high intensity exercise embedded within an individual's overall daily and weekly physical activity schedule, may transiently impact the tumour microenvironment and improve the health outcomes for those who have been diagnosed and treated for cancer.

A one-health approach to identifying and mitigating the impact of endocrine disorders on human and equine athletes

Endocrinopathies affect multiple species in ever-increasing percentages of their populations, creating an opportunity to apply one-health approaches to determining creative preventative measures and therapies in athletes. Obesity and alterations in insulin and glucose dynamics are medical concerns that play a role in whole-body health and homeostasis in both horses and humans. The role and impact of endocrine disorders on the musculoskeletal, cardiovascular, and reproductive systems are of particular interest to the athlete. Elucidation of both physiologic and pathophysiologic mechanisms involved in disease processes, starting in utero, is important for development of prevention and treatment strategies for the health and well-being of all species. This review focuses on the unrecognized effects of endocrine disorders associated with the origins of metabolic disease; inflammation at the intersection of endocrine disease and related diseases in the musculoskeletal, cardiovascular, and reproductive systems; novel interventions; and diagnostics that are informed via multiomic and one-health approaches. Readers interested in further details on specific equine performance conditions associated with endocrine disease are invited to read the companion Currents in One Health by Manfredi et al, JAVMA, February 2023.

The relationship between dynapenia and vitamin D level in geriatric women with type 2 diabetes mellitus

Objective

In this study, we examined the possible relationship between dynapenia and vitamin D (VD) levels in geriatric women with type 2 diabetes mellitus (T2DM).

Methods

One hundred and twenty-two geriatric female patients aged 65-80 years with a diagnosis of T2DM were included in this prospective study. Physical examinations of the patients were performed, and biochemical tests were analyzed. The muscle strength of the patients was measured with a hand dynamometer. Dynapenia was defined as low grip strength with normal skeletal muscle mass index. In muscle strength measurements, for female patients, over 20 kg was accepted as normal and below 20 kg as decreased muscle strength. Patients were separated into three groups as <10 ng/ml, 10-30 ng/ml, and >30 ng/ml according to VD levels; according to the status of dynapenia, they were divided into two groups as dynapenic and non-dynapenic. By comparing all these parameters between these groups, the relationship between VD level and dynapenia was evaluated. In statistical analysis, significance was accepted as p<0.05.

Results

While 54 of the patients (44.3%) met the dynapenia criterion, 68 patients (55.7%) were non-dynapenic. Patients were first compared according to their dynapenia status. VD level was significantly lower in the dynapenic group (p<0.05). In the correlation analysis, a moderate positive correlation was found between muscle strength and VD (p=0.033, r: 0.23). The patients were then compared according to the VD groups. In the VD insufficient group, muscle strength (p=0.015), body mass index (p=0.025), systolic blood pressure (p<0.01), and glucose (p<0.01) were statistically significantly higher.

Conclusion

In the present study, we found a considerable relationship between VD levels and dynapenia in geriatric women with T2DM.

Current understanding of glucose transporter 4 expression and functional mechanisms

Glucose is used aerobically and anaerobically to generate energy for cells. Glucose transporters (GLUTs) are transmembrane proteins that transport glucose across the cell membrane. Insulin promotes glucose utilization in part through promoting glucose entry into the skeletal and adipose tissues. This has been thought to be achieved through insulin-induced GLUT4 translocation from intracellular compartments to the cell membrane, which increases the overall rate of glucose flux into a cell. The insulin-induced GLUT4 translocation has been investigated extensively. Recently, significant progress has been made in our understanding of GLUT4 expression and translocation. Here, we summarized the methods and reagents used to determine the expression levels of Slc2a4 mRNA and GLUT4 protein, and GLUT4 translocation in the skeletal muscle, adipose tissues, heart and brain. Overall, a variety of methods such real-time polymerase chain reaction, immunohistochemistry, fluorescence microscopy, fusion proteins, stable cell line and transgenic animals have been used to answer particular questions related to GLUT4 system and insulin action. It seems that insulin-induced GLUT4 translocation can be observed in the heart and brain in addition to the skeletal muscle and adipocytes. Hormones other than insulin can induce GLUT4 translocation. Clearly, more studies of GLUT4 are warranted in the future to advance of our understanding of glucose homeostasis.

Cachexia, muscle wasting, and frailty in cardiovascular disease

The last several years have seen increasing interest in understanding cachexia, muscle wasting, and physical frailty across the broad spectrum of patients with cardiovascular illnesses. This interest originally started in the field of heart failure, but has recently been extended to other areas such as atrial fibrillation, coronary artery disease, peripheral artery disease as well as to patients after cardiac surgery or transcatheter aortic valve implantation. Tissue wasting and frailty are prevalent among many of the affected patients. The ageing process itself and concomitant cardiovascular illness decrease lean mass while fat mass is relatively preserved, making elderly patients particularly prone to develop wasting syndromes and frailty. The aim of this review is to provide an overview of the available knowledge of body wasting and physical frailty in patients with cardiovascular illness, particularly focussing on patients with heart failure in whom most of the available data have been gathered. In addition, mechanisms of wasting and possible therapeutic targets are discussed.

Reduced Skeletal Muscle Volume and Increased Skeletal Muscle Fat Deposition Characterize Diabetes in Individuals after Pancreatitis: A Magnetic Resonance Imaging Study

Background: Skeletal muscle has been implicated in the pathogenesis of type 2 diabetes but it has never been investigated in diabetes after pancreatitis. The aim was to investigate the relationship between psoas muscle volume (PMV) and diabetes in individuals after pancreatitis, as well as its associations with ectopic fat phenotypes and insulin traits. Methods: Individuals after an attack of pancreatitis and healthy individuals were studied in a cross-sectional fashion. All participants underwent magnetic resonance imaging, based on which PMV, skeletal muscle fat deposition (SMFD), as well as liver and intra-pancreatic fat depositions were derived. Fasting and postprandial blood samples were collected to calculate indices of insulin sensitivity and secretion. Linear regression analyses were conducted, adjusting for possible confounders (age, sex, body composition, comorbidities, use of insulin, and others). Results: A total of 153 participants were studied. PMV was significantly decreased in the diabetes group compared with healthy controls (β = −30.0, p = 0.034 in the most adjusted model). SMFD was significantly inversely associated with PMV (β = −3.1, p < 0.001 in the most adjusted model). The Matsuda index of insulin sensitivity was significantly directly associated with PMV (β = 1.6, p = 0.010 in the most adjusted model). Conclusions: Diabetes in individuals after pancreatitis is characterized by reduced PMV. Reduced PMV is associated with increased SMFD and decreased insulin sensitivity in individuals after pancreatitis.

Effects of a High-Intensity Exercise Program on Weight Regain and Cardio-metabolic Profile after 3 Years of Bariatric Surgery: A Randomized Trial

Weight regain is one of the most common problems in the long-term after bariatric surgery. It is unknown if high-intensity exercise programs applied in late phases of post-surgical follow-up could counteract this trend. After a 3-year follow-up, 21 patients underwent sleeve gastrectomy were randomized into an exercise group (EG, n = 11), that performed a 5-month supervised exercise program, and a control group (CG, n = 10), that followed the usual care. Body composition, cardiorespiratory fitness, glycaemia and blood cholesterol were evaluated before and after the intervention. Finally, the EG repeated the evaluations 2 months after the end of the exercise program. Both groups reached their maximum weight loss at the first year after surgery and showed significant weight regain by the end of the follow-up. After the exercise program, the EG showed reductions in fat mass (-2.5 ± 2.6  kg, P < 0.05), glycaemia (-13.4 ± 8.7  mg·dL-1, P < 0.01) and blood cholesterol (-24.6 ± 29.1  mg·dL-1, P < 0.05), whereas the CG during the same period showed increases in weight (1.5 ± 1.3  kg, P < 0.05) and fat mass (1.8 ± 0.9, P < 0.01). Two months after the end of the program, EG had increases in weight (1.1 ± 1.2  kg, P < 0.05), fat mass (2.6 ± 2.2  kg, P < 0.01), glycaemia (8.2 ± 11.6  mg·dL-1, P < 0.05) and blood cholesterol (20.0 ± 22.1  mg·dL-1, P < 0.05), when compared with the values after the exercise program. Therefore, in the medium-term after sleeve gastrectomy exercise may contribute to prevent weight regain and to reduce fat mass, glycaemia, and blood cholesterol.

Amino acids in post-stroke rehabilitation

Objectives: Patients with stroke are prone to disability due to muscle hypercatabolism. We aim to review this concept based on available data on benefits of amino acid supplementation in post-stroke rehabilitation.Method: The search was performed on Medline and Embase in January 2019. Randomized controlled studies, observational studies and case reports conducted in the last 15 years on the supplementation of amino acids in post-stroke rehabilitation patients were included.Result: Amino acids prevent muscle hypercatabolism in post stroke patients by suppressing myofibrillar protein and skeletal muscle degradation. Stroke patients supplemented with amino acids led to an improvement of functional and physical performance.Discussion: Muscle protein hypercatabolism and sequestration of amino acids from skeletal muscles occur cyclically in post-stroke patients to counter each other. There is a resultant deficit of amino acids which is unmet. Amino acids have antiproteolytic effect. Its supplementation prevents muscle wasting and improves rehabilitation by promoting physical performance, muscle strength, mass, and function.

EGF receptor (EGFR) inhibition promotes a slow-twitch oxidative, over a fast-twitch, muscle phenotype

A low quadriceps slow-twitch (ST), oxidative (relative to fast-twitch) fiber proportion is prevalent in chronic diseases such Chronic Obstructive Pulmonary Disease (COPD) and is associated with exercise limitation and poor outcomes. Benefits of an increased ST fiber proportion are demonstrated in genetically modified animals. Pathway analysis of published data of differentially expressed genes in mouse ST and FT fibers, mining of our microarray data and a qPCR analysis of quadriceps specimens from COPD patients and controls were performed. ST markers were quantified in C2C12 myotubes with EGF-neutralizing antibody, EGFR inhibitor or an EGFR-silencing RNA added. A zebrafish egfra mutant was generated by genome editing and ST fibers counted. EGF signaling was (negatively) associated with the ST muscle phenotype in mice and humans, and muscle EGF transcript levels were raised in COPD. In C2C12 myotubes, EGFR inhibition/silencing increased ST, including mitochondrial, markers. In zebrafish, egfra depletion increased ST fibers and mitochondrial content. EGF is negatively associated with ST muscle phenotype in mice, healthy humans and COPD patients. EGFR blockade promotes the ST phenotype in myotubes and zebrafish embryos. EGF signaling suppresses the ST phenotype, therefore EGFR inhibitors may be potential treatments for COPD-related muscle ST fiber loss.

Muscle wasting and cachexia in heart failure: mechanisms and therapies

Body wasting is a serious complication that affects a large proportion of patients with heart failure. Muscle wasting, also known as sarcopenia, is the loss of muscle mass and strength, whereas cachexia describes loss of weight. After reaching guideline-recommended doses of heart failure therapies, the most promising approach to treating body wasting seems to be combined therapy that includes exercise, nutritional counselling, and drug treatment. Nutritional considerations include avoiding excessive salt and fluid intake, and replenishment of deficiencies in trace elements. Administration of omega-3 polyunsaturated fatty acids is beneficial in selected patients. High-calorific nutritional supplements can also be useful. The prescription of aerobic exercise training that provokes mild or moderate breathlessness has good scientific support. Drugs with potential benefit in the treatment of body wasting that have been tested in clinical studies in patients with heart failure include testosterone, ghrelin, recombinant human growth hormone, essential amino acids, and β₂-adrenergic receptor agonists. In this Review, we summarize the pathophysiological mechanisms of muscle wasting and cachexia in heart failure, and highlight the potential treatment strategies. We aim to provide clinicians with the relevant information on body wasting to understand and treat these conditions in patients with heart failure.

Longer-term impact of hemiparetic stroke on skeletal muscle metabolism-A pilot study

BACKGROUND

Hemiparetic stroke leads to structural and metabolic alterations of skeletal muscle tissue, thereby contributing to functional impairment associated with stroke. In situ metabolic processes at tissue level in skeletal muscle have not been investigated. We hypothesize that muscular metabolic capacity is limited after hemiparetic stroke, and that changes affect rather the paretic than non-paretic limb.

METHODS

Nine male hemiparetic stroke survivors (age, 62±8years; BMI, 28±4kg/m²; median stroke latency, 23months ranging from 7 to 34months poststroke) underwent dynamic in situ measurements of carbohydrate and lipid metabolism at fasting condition and during oral glucose tolerance testing, using bilateral microdialysis. Results were compared to 8 healthy male subjects of similar age and BMI.

RESULTS

Tissue perfusion, fasting and postprandial profiles of interstitial metabolites glucose, pyruvate, lactate and glycerol did not differ between paretic and non-paretic muscle. Patients displayed higher fasting and postprandial dialysate glycerol levels compared to controls (P<0.001) with elevated plasma FFA (fasting FFA; 0.63±0.23 vs. 0.29±0.17mmol/L; P=0.004). Glycolytic activity was higher in patients vs. controls, with increased lactate production upon glucose load (P<0.001).

CONCLUSIONS

An elevated lipolytic and glycolytic activity on tissue level suggests an impaired substrate metabolism with blunted oxidative metabolism in bilateral skeletal muscle in patients after hemiparetic stroke. Muscular metabolic properties did not differ between paretic and non-paretic leg. Further work is needed to investigate the clinical impact of this impaired muscular metabolic capacity in post-stroke patients.

Strength Training for Skeletal Muscle Endurance after Stroke

BACKGROUND AND PURPOSE

Initial studies support the use of strength training (ST) as a safe and effective intervention after stroke. Our previous work shows that relatively aggressive, higher intensity ST translates into large effect sizes for paretic and non-paretic leg muscle volume, myostatin expression, and maximum strength post-stroke. An unanswered question pertains to how our unique ST model for stroke impacts skeletal muscle endurance (SME). Thus, we now report on ST-induced adaptation in the ability to sustain isotonic muscle contraction.

METHODS

Following screening and baseline testing, hemiparetic stroke participants were randomized to either ST or an attention-matched stretch control group (SC). Those in the ST group trained each leg individually to muscle failure (20 repetition sets, 3× per week for 3 months) on each of three pneumatic resistance machines (leg press, leg extension, and leg curl). Our primary outcome measure was SME, quantified as the number of submaximal weight leg press repetitions possible at a specified cadence. The secondary measures included one-repetition maximum strength, 6-minute walk distance (6MWD), 10-meter walk speeds, and peak aerobic capacity (VO₂ peak).

RESULTS

ST participants (N = 14) had significantly greater SME gains compared with SC participants (N = 16) in both the paretic (178% versus 12%, P < .01) and non-paretic legs (161% versus 12%, P < .01). These gains were accompanied by group differences for 6MWD (P < .05) and VO₂ peak (P < .05).

CONCLUSION

Our ST regimen had a large impact on the capacity to sustain submaximal muscle contraction, a metric that may carry more practical significance for stroke than the often reported measures of maximum strength.

Low-volume high-intensity swim training is superior to high-volume low-intensity training in relation to insulin sensitivity and glucose control in inactive middle-aged women

PURPOSE

We tested the hypothesis that low-volume high-intensity swimming has a larger impact on insulin sensitivity and glucose control than high-volume low-intensity swimming in inactive premenopausal women with mild hypertension.

METHODS

Sixty-two untrained premenopausal women were randomised to an inactive control (n = 20; CON), a high-intensity low-volume (n = 21; HIT) or a low-intensity high-volume (n = 21; LIT) training group. During the 15-week intervention period, HIT performed 3 weekly 6-10 × 30-s all-out swimming intervals (average heart rate (HR) = 86 ± 3 % HRmax) interspersed by 2-min recovery periods and LIT swam continuously for 1 h at low intensity (average HR = 73 ± 3 % HRmax). Fasting blood samples were taken and an oral glucose tolerance test (OGTT) was conducted pre- and post-intervention.

RESULTS

After HIT, resting plasma [insulin] was lowered (17 ± 34 %; P < 0.05) but remained similar after LIT and CON. Following HIT, 60-min OGTT plasma [insulin] and [glucose] was lowered (24 ± 30 % and 10 ± 16 %; P < 0.05) but remained similar after LIT and CON. Total area under the curve for plasma [glucose] was lower (P < 0.05) after HIT than LIT (660 ± 141 vs. 860 ± 325 mmol min L(-1)). Insulin sensitivity (HOMA-IR) had increased (P < 0.05) by 22 ± 34 % after HIT, with no significant change after LIT or CON, respectively. Plasma soluble intracellular cell adhesion molecule 1 was lowered (P < 0.05) by 4 ± 8 and 3 ± 9 % after HIT and CON, respectively, while plasma soluble vascular cell adhesion molecule 1 had decreased (P < 0.05) by 8 ± 23 % after HIT only.

CONCLUSIONS

These findings suggest that low-volume high-intensity intermittent swimming is an effective and time-efficient training strategy for improving insulin sensitivity, glucose control and biomarkers of vascular function in inactive, middle-aged mildly hypertensive women.

The Effects of 52 Weeks of Soccer or Resistance Training on Body Composition and Muscle Function in +65-Year-Old Healthy Males--A Randomized Controlled Trial

The effects of 52 weeks of soccer or resistance training were investigated in untrained elderly men. The subjects aged 68.1±2.1 yrs were randomised into a soccer (SG; n = 9), a resistance (RG; n = 9) and a control group (CG; n = 8). The subjects in SG and RG, respectively, trained 1.7±0.3 and 1.8±0.3 times weekly on average during the intervention period. Muscle function and body composition were determined before and after 16 and 52 weeks of the intervention period. In SG, BMI was reduced by 1.5% and 3.0% (p<0.05) after 16 and 52 weeks, respectively, unchanged in RG and 2% higher (p<0.05) in CG after 52 weeks of the intervention period. In SG, the response to a glucose tolerance test was 16% lower (p<0.05) after 16 wks, but not after 52 wks, compared to before the intervention period, and unchanged in RG and CG. In SG, superoxide dismutase-2 expression was 59% higher (p<0.05) after 52 wks compared to before the intervention period, and unchanged in RG and CG. In RG, upper body lean mass was 3 and 2% higher (p<0.05) after 16 and 52 wks, respectively, compared to before the intervention period, and unchanged in SG and CG. In RG, Akt-2 expression increased by 28% (p<0.01) and follistatin expression decreased by 38% (p<0.05) during the 52-wk intervention period, and was unchanged in SG and CG. Thus, long-term soccer training reduces BMI and improves anti-oxidative capacity, while long-term resistance training impacts muscle protein enzyme expression and increases lean body mass in elderly men. Trial Registration: ClinicalTrials.gov: NCT01530035.

The wasting continuum in heart failure: from sarcopenia to cachexia

Sarcopenia (muscle wasting) and cachexia share some pathophysiological aspects. Sarcopenia affects approximately 20 %, cachexia <10 % of ambulatory patients with heart failure (HF). Whilst sarcopenia means loss of skeletal muscle mass and strength that predominantly affects postural rather than non-postural muscles, cachexia means loss of muscle and fat tissue that leads to weight loss. The wasting continuum in HF implies that skeletal muscle is lost earlier than fat tissue and may lead from sarcopenia to cachexia. Both tissues require conservation, and therapies that stop the wasting process have tremendous therapeutic appeal. The present paper reviews the pathophysiology of muscle and fat wasting in HF and discusses potential treatments, including exercise training, appetite stimulants, essential amino acids, growth hormone, testosterone, electrical muscle stimulation, ghrelin and its analogues, ghrelin receptor agonists and myostatin antibodies.

Cardiovascular Health and Fitness After Stroke

Stroke patients have profound cardiovascular and muscular deconditioning, with metabolic fitness levels that are about half those found in age-matched sedentary controls. Physical deconditioning, along with elevated energy demands of hemiparetic gait, define a detrimental combination termed diminished physiological fitness reserve that can greatly limit that can greatly limit performance of activities of daily living. The physiological features that underlie worsening metabolic fitness in the chronic phase of stroke include gross muscular atrophy, altered muscle molecular phenotype, increased intramuscular area fat, elevated tissue inflammatory markers, and diminished peripheral blood flow dynamics. Epidemiological evidence further suggests that the reduced cardiovascular fitness and secondary biological changes in muscle may propagate components of the metabolic syndrome, conferring added morbidity and mortality risk. This article reviews some of the consequences of poor fitness in chronic stroke and the potential biological underpinnings that support a rationale for more aggressive approaches to exercise therapy in this population.

Genetic Dissection of the Physiological Role of Skeletal Muscle in Metabolic Syndrome

The primary deficiency underlying metabolic syndrome is insulin resistance, in which insulin-responsive peripheral tissues fail to maintain glucose homeostasis. Because skeletal muscle is the major site for insulin-induced glucose uptake, impairments in skeletal muscle’s insulin responsiveness play a major role in the development of insulin resistance and type 2 diabetes. For example, skeletal muscle of type 2 diabetes patients and their offspring exhibit reduced ratios of slow oxidative muscle. These observations suggest the possibility of applying muscle remodeling to recover insulin sensitivity in metabolic syndrome. Skeletal muscle is highly adaptive to external stimulations such as exercise; however, in practice it is often not practical or possible to enforce the necessary intensity to obtain measurable benefits to the metabolic syndrome patient population. Therefore, identifying molecular targets for inducing muscle remodeling would provide new approaches to treat metabolic syndrome. In this review, the physiological properties of skeletal muscle, genetic analysis of metabolic syndrome in human populations and model organisms, and genetically engineered mouse models will be discussed in regard to the prospect of applying skeletal muscle remodeling as possible therapy for metabolic syndrome.

The importance of the cellular stress response in the pathogenesis and treatment of type 2 diabetes

Organisms have evolved to survive rigorous environments and are not prepared to thrive in a world of caloric excess and sedentary behavior. A realization that physical exercise (or lack of it) plays a pivotal role in both the pathogenesis and therapy of type 2 diabetes mellitus (t2DM) has led to the provocative concept of therapeutic exercise mimetics. A decade ago, we attempted to simulate the beneficial effects of exercise by treating t2DM patients with 3 weeks of daily hyperthermia, induced by hot tub immersion. The short-term intervention had remarkable success, with a 1 % drop in HbA1, a trend toward weight loss, and improvement in diabetic neuropathic symptoms. An explanation for the beneficial effects of exercise and hyperthermia centers upon their ability to induce the cellular stress response (the heat shock response) and restore cellular homeostasis. Impaired stress response precedes major metabolic defects associated with t2DM and may be a near seminal event in the pathogenesis of the disease, tipping the balance from health into disease. Heat shock protein inducers share metabolic pathways associated with exercise with activation of AMPK, PGC1-a, and sirtuins. Diabetic therapies that induce the stress response, whether via heat, bioactive compounds, or genetic manipulation, improve or prevent all of the morbidities and comorbidities associated with the disease. The agents reduce insulin resistance, inflammatory cytokines, visceral adiposity, and body weight while increasing mitochondrial activity, normalizing membrane structure and lipid composition, and preserving organ function. Therapies restoring the stress response can re-tip the balance from disease into health and address the multifaceted defects associated with the disease.

A preliminary study: effects of football training on glucose control, body composition, and performance in men with type 2 diabetes

The effects of regular football training on glycemic control, body composition, and peak oxygen uptake (VO₂ peak) were investigated in men with type 2 diabetes mellitus (T2DM). Twenty-one middle-aged men (49.8 ± 1.7 years ± SEM) with T2DM were divided into a football training group (FG; n = 12) and an inactive control group (CG; n = 9) during a 24-week intervention period (IP). During a 1-h football training session, the distance covered was 4.7 ± 0.2 km, mean heart rate (HR) was 83 ± 2% of HRmax, and blood lactate levels increased (P < 0.001) from 2.1 ± 0.3 to 8.2 ± 1.3 mmol/L. In FG, VO₂ peak was 11% higher (P < 0.01), and total fat mass and android fat mass were 1.7 kg and 12.8% lower (P < 0.001), respectively, after IP. After IP, the reduction in plasma glucose was greater (P = 0.02) in FG than the increase in CG, and in FG, GLUT-4 tended to be higher (P = 0.072) after IP. For glycosylated hemoglobin (HbA1), an overall time effect (P < 0.01) was detected after 24 weeks. After IP, the number of capillaries around type I fibers was 7% higher (P < 0.05) in FG and 5% lower (P < 0.05) in CG. Thus, in men with T2DM, regular football training improves VO₂ peak, reduces fat mass, and may positively influence glycemic control.

Exercise, GLUT4, and skeletal muscle glucose uptake

Glucose is an important fuel for contracting muscle, and normal glucose metabolism is vital for health. Glucose enters the muscle cell via facilitated diffusion through the GLUT4 glucose transporter which translocates from intracellular storage depots to the plasma membrane and T-tubules upon muscle contraction. Here we discuss the current understanding of how exercise-induced muscle glucose uptake is regulated. We briefly discuss the role of glucose supply and metabolism and concentrate on GLUT4 translocation and the molecular signaling that sets this in motion during muscle contractions. Contraction-induced molecular signaling is complex and involves a variety of signaling molecules including AMPK, Ca(2+), and NOS in the proximal part of the signaling cascade as well as GTPases, Rab, and SNARE proteins and cytoskeletal components in the distal part. While acute regulation of muscle glucose uptake relies on GLUT4 translocation, glucose uptake also depends on muscle GLUT4 expression which is increased following exercise. AMPK and CaMKII are key signaling kinases that appear to regulate GLUT4 expression via the HDAC4/5-MEF2 axis and MEF2-GEF interactions resulting in nuclear export of HDAC4/5 in turn leading to histone hyperacetylation on the GLUT4 promoter and increased GLUT4 transcription. Exercise training is the most potent stimulus to increase skeletal muscle GLUT4 expression, an effect that may partly contribute to improved insulin action and glucose disposal and enhanced muscle glycogen storage following exercise training in health and disease.

Black tea high-molecular-weight polyphenol stimulates exercise training-induced improvement of endurance capacity in mouse via the link between AMPK and GLUT4

Aerobic exercise can promote “fast-to-slow transition” in skeletal muscles, i.e. an increase in oxidative fibers, mitochondria, and myoglobin and improvement in glucose and lipid metabolism. Here, we found that mice administered Mitochondria Activation Factor (MAF) combined with exercise training could run longer distances and for a longer time compared with the exercise only group; MAF is a high-molecular-weight polyphenol purified from black tea. Furthermore, MAF intake combined with exercise training increased phosphorylation of AMPK and mRNA level of glucose transporter 4 (GLUT4). Thus, our data demonstrate for the first time that MAF activates exercise training-induced intracellular signaling pathways that involve AMPK, and improves endurance capacity.

Expression of conventional and novel glucose transporters, GLUT1, -9, -10, and -12, in vascular smooth muscle cells

Intimal hyperplasia is characterized by exaggerated proliferation of vascular smooth muscle cells (VSMCs). Enhanced VSMC growth is dependent on increased glucose uptake and metabolism. Facilitative glucose transporters (GLUTs) are comprised of conventional GLUT isoforms (GLUT1-5) and novel GLUT isoforms (GLUT6-14). Previous studies demonstrate that GLUT1 overexpression or GLUT10 downregulation contribute to phenotypic changes in VSMCs. To date, the expression profile of all 14 GLUT isoforms has not been fully examined in VSMCs. Using the proliferative and differentiated phenotypes of human aortic VSMCs, the present study has determined the relative abundance of GLUT1-14 mRNAs by quantitative real-time PCR analysis. Twelve GLUT mRNAs excluding GLUT7 and GLUT14 were detectable in VSMCs. In the proliferative phenotype, the relative abundance of key GLUT mRNAs was GLUT1 (∼43%)>GLUT10 (∼26%)>GLUT9 (∼13%)>GLUT12 (∼4%), whereas in the differentiated phenotype the relative abundance was GLUT10 (∼28%)>GLUT1 (∼25%)>GLUT12 (∼20%)>GLUT9 (∼14%), together constituting 86-87% of total GLUT transcripts. To confirm the expression of key GLUT proteins, immunoblot and immunocytochemical analyses were performed using GLUT isoform-specific primary antibodies. The protein bands characteristic of GLUT1, -9, -10, and -12 were detected in VSMCs in parallel with respective positive controls. In particular, GLUT1 protein expression showed different molecular forms representative of altered glycosylation. While GLUT1 protein displayed a predominant distribution in the plasma membrane, GLUT9, -10, and -12 proteins were mostly distributed in the intracellular compartments. The present study provides the first direct evidence for GLUT9 and GLUT12 expression in VSMCs in conjunction with the previously identified GLUT1 and GLUT10.

Reduced cardiorespiratory fitness after stroke: biological consequences and exercise-induced adaptations

Evidence from several studies consistently shows decline in cardiorespiratory (CR) fitness and physical function after disabling stroke. The broader implications of such a decline to general health may be partially understood through negative poststroke physiologic adaptations such as unilateral muscle fiber type shifts, impaired hemodynamic function, and decrements in systemic metabolic status. These physiologic changes also interrelate with reductions in activities of daily living (ADLs), community ambulation, and exercise tolerance, causing a perpetual cycle of worsening disability and deteriorating health. Fortunately, initial evidence suggests that stroke participants retain the capacity to adapt physiologically to an exercise training stimulus. However, despite this evidence, exercise as a therapeutic intervention continues to be clinically underutilized in the general stroke population. Far more research is needed to fully comprehend the consequences of and remedies for CR fitness impairments after stroke. The purpose of this brief review is to describe some of what is currently known about the physiological consequences of CR fitness decline after stroke. Additionally, there is an overview of the evidence supporting exercise interventions for improving CR fitness, and associated aspects of general health in this population.

Glucose intolerance in the West African Diaspora: a skeletal muscle fibre type distribution hypothesis

In the United States, Black Americans are largely descendants of West African slaves; they have a higher relative proportion of obesity and experience a higher prevalence of diabetes than White Americans. However, obesity rates alone cannot explain the higher prevalence of type 2 diabetes. Type 2 diabetes is characterized by insulin resistance and beta-cell dysfunction. We hypothesize that the higher prevalence of type 2 diabetes in African Americans (as compared to White Americans) is facilitated by an inherited higher percentage of skeletal muscle fibre type II and a lower percentage of skeletal muscle fibre type I. Skeletal muscle fibre type II is less oxidative and more glycolytic than skeletal muscle fibre type I. Lower oxidative capacity is associated with lower fat oxidation and a higher disposal of lipids, which are stored as muscular adipose tissue in higher amounts in Black compared to White Americans. In physically active individuals, the influence of muscle fibre composition will not be as detrimental as in physically inactive individuals. This discrepancy is caused by the plasticity in the skeletal muscle fibre characteristics towards a higher activity of oxidative enzymes as a consequence of physical activity. We suggest that a higher percentage of skeletal muscle fibre type II combined with physical inactivity has an impact on insulin sensitivity and high prevalence of type 2 diabetes in Blacks of West African ancestry.

Clustering of GLUT4, TUG, and RUVBL2 protein levels correlate with myosin heavy chain isoform pattern in skeletal muscles, but AS160 and TBC1D1 levels do not

Skeletal muscle is a heterogeneous tissue. To further elucidate this heterogeneity, we probed relationships between myosin heavy chain (MHC) isoform composition and abundance of GLUT4 and four other proteins that are established or putative GLUT4 regulators [Akt substrate of 160 kDa (AS160), Tre-2/Bub2/Cdc 16-domain member 1 (TBC1D1), Tethering protein containing an UBX-domain for GLUT4 (TUG), and RuvB-like protein two (RUVBL2)] in 12 skeletal muscles or muscle regions from Wistar rats [adductor longus, extensor digitorum longus, epitrochlearis, gastrocnemius (mixed, red, and white), plantaris, soleus, tibialis anterior (red and white), tensor fasciae latae, and white vastus lateralis]. Key results were 1) significant differences found among the muscles (range of muscle expression values) for GLUT4 (2.5-fold), TUG (1.7-fold), RUVBL2 (2.0-fold), and TBC1D1 (2.7-fold), but not AS160; 2) significant positive correlations for pairs of proteins: GLUT4 vs. TUG (R = 0.699), GLUT4 vs. RUVBL2 (R = 0.613), TUG vs. RUVBL2 (R = 0.564), AS160 vs. TBC1D1 (R = 0.293), and AS160 vs. TUG (R = 0.246); 3) significant positive correlations for %MHC-I: GLUT4 (R = 0.460), TUG (R = 0.538), and RUVBL2 (R = 0.511); 4) significant positive correlations for %MHC-IIa: GLUT4 (R = 0.293) and RUVBL2 (R = 0.204); 5) significant negative correlations for %MHC-IIb vs. GLUT4 (R = -0.642), TUG (R = -0.626), and RUVBL2 (R = -0.692); and 6) neither AS160 nor TBC1D1 significantly correlated with MHC isoforms. In 12 rat muscles, GLUT4 abundance tracked with TUG and RUVBL2 and correlated with MHC isoform expression, but was unrelated to AS160 or TBC1D1. Our working hypothesis is that some of the mechanisms that regulate GLUT4 abundance in rat skeletal muscle also influence TUG and RUVBL2 abundance.

Improved fatigue resistance in Gsα-deficient and aging mouse skeletal muscles due to adaptive increases in slow fibers

Genetically modified mice with deficiency of the G protein α-subunit (G(s)α) in skeletal muscle showed metabolic abnormality with reduced glucose tolerance, low muscle mass, and low contractile force, along with a fast-to-slow-fiber-type switch (Chen M, Feng HZ, Gupta D, Kelleher J, Dickerson KE, Wang J, Hunt D, Jou W, Gavrilova O, Jin JP, Weinstein LS. Am J Physiol Cell Physiol 296: C930-C940, 2009). Here we investigated a hypothesis that the switching to more slow fibers is an adaptive response with specific benefit. The results showed that, corresponding to the switch of myosin isoforms, the thin-filament regulatory proteins troponin T and troponin I both switched to their slow isoforms in the atrophic soleus muscle of 3-mo-old G(s)α-deficient mice. This fiber-type switch involving coordinated changes of both thick- and thin-myofilament proteins progressed in the G(s)α-deficient soleus muscles of 18- to 24-mo-old mice, as reflected by the expression of solely slow isoforms of myosin and troponin. Compared with age-matched controls, G(s)α-deficient soleus muscles with higher proportion of slow fibers exhibited slower contractile and relaxation kinetics and lower developed force, but significantly increased resistance to fatigue, followed by a better recovery. G(s)α-deficient soleus muscles of neonatal and 3-wk-old mice did not show the increase in slow fibers. Therefore, the fast-to-slow-fiber-type switch in G(s)α deficiency at older ages was likely an adaptive response. The benefit of higher fatigue resistance in adaption to metabolic deficiency and aging provides a mechanism to sustain skeletal muscle function in diabetic patients and elderly individuals.

Cytokine abnormalities in the etiology of the cardiometabolic syndrome

The cardiometabolic syndrome comprises a cluster of risk factors, including abdominal obesity, dyslipidemia, hypertension, insulin resistance/glucose intolerance, and proteinuria. This syndrome is due, in part, to the accumulation of visceral fat, which promotes synthesis of proinflammatory adipokines resulting in a visceral adipose tissue-specific increase in reactive oxygen species derived from NADPH oxidase. Adipose tissue oxidative stress results in the development of systemic oxidative stress and inflammation, which further lead to development of metabolic dyslipidemia, impaired glucose metabolism, renal disease, and hypertension. Importantly, visceral-not subcutaneous-fat is the significant source of the circulating adipokines that promote these systemic abnormalities. Chronic low-grade inflammation develops within adipose tissue because of the additional infiltration and accumulation of inflammatory macrophages. There is evidence that lifestyle changes, bariatric surgery, and/or administration of insulin-sensitizing, anti-inflammatory, or antihypertensive drugs that address the risk factors promoting the cardiometabolic syndrome act, in part, by promoting an anti-inflammatory adipokine profile in visceral fat.

Impaired insulin-induced site-specific phosphorylation of TBC1 domain family, member 4 (TBC1D4) in skeletal muscle of type 2 diabetes patients is restored by endurance exercise-training

AIMS/HYPOTHESIS

Insulin-mediated glucose disposal rates (R(d)) are reduced in type 2 diabetic patients, a process in which intrinsic signalling defects are thought to be involved. Phosphorylation of TBC1 domain family, member 4 (TBC1D4) is at present the most distal insulin receptor signalling event linked to glucose transport. In this study, we examined insulin action on site-specific phosphorylation of TBC1D4 and the effect of exercise training on insulin action and signalling to TBC1D4 in skeletal muscle from type 2 diabetic patients.

METHODS

During a 3 h euglycaemic-hyperinsulinaemic (80 mU min⁻¹ m⁻²) clamp, we obtained M. vastus lateralis biopsies from 13 obese type 2 diabetic and 13 obese, non-diabetic control individuals before and after 10 weeks of endurance exercise-training.

RESULTS

Before training, reductions in insulin-stimulated R (d), together with impaired insulin-stimulated glycogen synthase fractional velocity, Akt Thr³⁰⁸ phosphorylation and phosphorylation of TBC1D4 at Ser³¹⁸, Ser⁵⁸⁸ and Ser⁷⁵¹ were observed in skeletal muscle from diabetic patients. Interestingly, exercise-training normalised insulin-induced TBC1D4 phosphorylation in diabetic patients. This happened independently of increased TBC1D4 protein content, but exercise-training did not normalise Akt phosphorylation in diabetic patients. In both groups, training-induced improvements in insulin-stimulated R(d) (~20%) were associated with increased muscle protein content of Akt, TBC1D4, α2-AMP-activated kinase (AMPK), glycogen synthase, hexokinase II and GLUT4 (20-75%).

CONCLUSIONS/INTERPRETATION

Impaired insulin-induced site-specific TBC1D4 phosphorylation may contribute to skeletal muscle insulin resistance in type 2 diabetes. The mechanisms by which exercise-training improves insulin sensitivity in type 2 diabetes may involve augmented signalling of TBC1D4 and increased skeletal muscle content of key insulin signalling and effector proteins, e.g., Akt, TBC1D4, AMPK, glycogen synthase, GLUT4 and hexokinase II.

Optimal control of blood glucose: the diabetic patient or the machine?

In this issue of Science Translational Medicine, El-Khatib et al. describe a "closed-loop" bihormonal artificial pancreas, designed to avert episodes of low blood sugar in patients with insulin-dependent diabetes. We discuss the benefits and challenges of therapy directed at tight control of blood glucose and ask whether this and similar technological breakthroughs can address as yet unanswered questions in the biology of diabetes.

Loss of Skeletal Muscle Mass after Stroke: a Systematic Review

Loss of muscle mass after stroke has implications for strength and functional ability and may also contribute to impaired glucose metabolism. Therefore, prevention of muscle loss is desirable. Before interventions to prevent loss of muscle can be designed and evaluated, the expected rate, magnitude and timing of muscle loss need to be understood. A systematic search was undertaken to identify all studies that investigated changes in skeletal muscle mass, volume or cross-sectional area in people after stroke. Studies that used either direct measures of muscle size (computer tomography, magnetic resonance imaging or ultrasound) or measures of lean tissue mass (dual X-ray absorptiometry) were included. Fourteen trials were found and the results were pooled for differences in lean tissue mass between the paretic and the nonparetic leg and arm as well as differences in the midthigh cross-sectional area. In individuals at least 6-month post-stroke, there was significantly less lean tissue mass in the paretic compared with the nonparetic lower limb (MD 342.3 g, 95% confidence interval 247.0–437.6 g) and upper limb (MD 239.9 g, 95% confidence interval 181.7–298.2 g), and significantly less midthigh muscle cross-sectional area (MD 15.4 cm2, 95% confidence interval 13.8–16.9 cm2). There were insufficient data to pool with regard to change in muscle mass over time. There is a significant difference in the regional muscle mass in the paretic vs. the nonparetic limb in individuals greater than 6-months poststroke but little is known about how early and how quickly changes in muscle mass occur.

Heart rate prescribed walking training improves cardiorespiratory fitness but not glycaemic control in people with type 2 diabetes

In this study, we examined the effects of a supervised, heart rate intensity prescribed walking training programme on cardiorespiratory fitness and glycaemic control in people with type 2 diabetes mellitus. After receiving local ethics approval, 27 individuals (21 males, 6 females) with type 2 diabetes were randomly assigned to an experimental ("walking") or control group. Participants completed a Balke-Ware test to determine peak heart rate, peak oxygen consumption (VO(2peak)), and peak gradient. The walking group then completed a 7-week (four sessions a week) supervised, heart rate prescribed walking training programme, whereas the control group continued daily life. After training, participants completed another Balke-Ware test. Fasting blood glucose and glycosylated haemoglobin were measured at rest. The results showed that walking training elicited 80% (s = 2) of peak heart rate and a rating of perceived exertion of 11 (s = 1). Peak heart rate and VO(2peak) were higher in the walking than in the control group after training (P < 0.05). Based on the peak gradient before training, the respiratory exchange ratio was significantly lower (P < 0.05) and there was a strong trend for VO(2) (P = 0.09) and heart rate (P = 0.09) to be lower after training at the same gradient in the walking compared with the control group. These improvements increased walking peak gradient by 5 min (s = 4 min) compared with the control (P < 0.05). There was no change in fasting blood glucose or glycosylated haemoglobin after training. Despite no change in glycaemic control, heart rate prescribed walking improved peak and sub-maximal cardiorespiratory responses. The beneficial adaptations support the use of heart rate monitoring during walking in people with type 2 diabetes mellitus.

Hemiparetic stroke alters vastus lateralis myosin heavy chain profiles between the paretic and nonparetic muscles

Skeletal muscle phenotype alterations following hemiparetic stroke contribute to disabilities associated with stroke. The phenotypic response following stroke is undefined. This investigation examined the myosin heavy chain (MHC) composition of the vastus lateralis (VL) of stroke survivors in paretic (P) and nonparetic (NP) muscle. Protein obtained from VL of 10 stroke survivors was isolated and purified, and MHC gel electrophoresis was performed. The MHC bands were quantified, and a paired sample two-tailed T test with significance set at p < or = 0.05 was performed. MHC I expression was significantly less in P versus NP VL (.93 vs. 1.00 arbitrary units [AU]). Significantly more IIx MHC was found in the P versus NP VL (1.33 vs. 1.0). No significant differences in type IIa MHC (1.07 P vs. 1.00 NP) were found. These changes in MHC composition suggest an alteration in muscle function due to stroke or the altered activity patterns of muscle following stroke.

G(s)alpha deficiency in skeletal muscle leads to reduced muscle mass, fiber-type switching, and glucose intolerance without insulin resistance or deficiency

The ubiquitously expressed G protein alpha-subunit G(s)alpha is required for receptor-stimulated intracellular cAMP responses and is an important regulator of energy and glucose metabolism. We have generated skeletal muscle-specific G(s)alpha-knockout (KO) mice (MGsKO) by mating G(s)alpha-floxed mice with muscle creatine kinase-cre transgenic mice. MGsKO mice had normal body weight and composition, and their serum glucose, insulin, free fatty acid, and triglyceride levels were similar to that of controls. However, MGsKO mice were glucose intolerant despite the fact that insulin sensitivity and glucose-stimulated insulin secretion were normal, suggesting an insulin-independent mechanism. Isolated muscles from MGsKO mice had increased basal glucose uptake and normal responses to a stimulator of AMP-activated protein kinase (AMPK), which indicates that AMPK and its downstream pathways are intact. Compared with control mice, MGsKO mice had reduced muscle mass with decreased cross-sectional area and force production. In addition, adult MGsKO mice showed an increased proportion of type I (slow-twitch, oxidative) fibers based on kinetic properties and myosin heavy chain isoforms, despite the fact that these muscles had reduced expression of peroxisome proliferator-activated receptor coactivator protein-1alpha (PGC-1alpha) and reduced mitochondrial content and oxidative capacity. Therefore G(s)alpha deficiency led to fast-to-slow fiber-type switching, which appeared to be dissociated from the expected change in oxidative capacity. MGsKO mice are a valuable model for future studies of the role of G(s)alpha signaling pathways in skeletal muscle adaptation and their effects on whole body metabolism.

Physical activity may facilitate diabetes prevention in adolescents

OBJECTIVE

The aim of this study was to examine the association of physical activity with glucose tolerance and resting energy expenditure (REE) among adolescents.

RESEARCH DESIGN AND METHODS

Subjects were 32 male and female adolescents aged 12-18 years. Intravenous glucose tolerance (K(g)) and REE were assessed under inpatient conditions after an overnight fast. K(g) was determined as the inverse slope of time versus (ln) glucose over minutes 8-19 of an intravenous glucose tolerance test. Physical activity was assessed over 8 days using accelerometry (counts per minute).

RESULTS

In multiple linear regression analysis, K(g) was positively associated with total physical activity (TPA), moderate physical activity (MPA), and 5-min bouts of MPA. Similarly, REE was positively associated with TPA, MPA, and 5-min bouts of MPA.

CONCLUSIONS

In this population, physical activity was positively related to both glucose tolerance and REE. These results suggest that moderate activity may be beneficial in the prevention of diabetes in adolescent populations both through promoting efficient glucose disposal and through increasing energy expenditure.

Acute exercise-induced glucose change during an exercise program in type 2 diabetes

PURPOSE

Supervised exercise programs have been demonstrated to improve overall glycemic control but less well characterized is the evolution of glucose response to exercise during an exercise program. We addressed this issue, using an observational cohort design, among overweight adults with type 2 diabetes. We hypothesized that during the course of the program, glucose levels during exercise would become more stable, as insulin sensitivity improved. Among adults with type 2 diabetes, glucose levels often decline acutely during exercise.

METHODS

Thirty-five adults with type 2 diabetes underwent capillary blood glucose (CBG) testing before and after supervised exercise during a 24-week program (48 sessions). After-exercise CBG values were subtracted from before-exercise values (CBG difference). Through repeated measures analysis, we examined CBG difference, before-exercise values, and after-exercise values during the program. Assuming that some initial period of exercise training is necessary to impact CBG difference, in exploratory analyses, we varied the time period analyzed (eg. Weeks 2-24, Weeks 3-24, etc).

RESULTS

CBG difference appeared stable throughout the program when all available data were considered. In models that examined periods following Week 11, however, the magnitude of CBG difference declined progressively, as did before-exercise values. After-exercise values remained stable for all time periods examined.

CONCLUSIONS

Our exploratory analyses suggest that following 11 weeks of exercise supervision, before-exercise CBG values decline progressively but after-exercise values remain stable, resulting in a progressive decline in CBG difference.

Exercise training for cardiometabolic adaptation after stroke

Patients with stroke are severely deconditioned, leading to metabolic abnormalities that significantly increase risk for myocardial infarction and recurrent stroke. This review characterizes the nature of the metabolic decline, the underlying causes, and the potential for progressive aerobic exercise to address metabolic impairment following disabling stroke. Although exercise training has previously been shown to improve peak aerobic capacity and sensorimotor function after stroke, establishing safe and effective exercise programs in this population presents unique challenges stemming from neurological deficit complexities and comorbid conditions. Thus, recommendations for application to practice are provided that include proper preexercise evaluation, guidelines for symptom-limited maximal effort exercise testing, as well as evidence-based suggestions for initiation and progression of an exercise program. Implementing regular, progressive exercise therapy is critical on the basis of the devastating impact of physical inactivity on overall metabolic heath. Prevalence of impaired or diabetic glucose metabolism may be as high as 80% in chronic stroke, predicting 2- and 3-fold increased risk for recurrent stroke, respectively. Tragically, nearly one third of patients with stroke experience recurrent stroke within 5 years, and comorbid cardiovascular conditions represent the leading cause of death in this population. Recent evidence showing the positive impact of exercise training on hyperinsulinemia and glucose tolerance in survivors of stroke is presented, given the central importance of these factors to overall cardiovascular risk. On the basis of these and other findings, structured exercise programs should be considered for all survivors of stroke.

Adrenaline potentiates insulin-stimulated PKB activation in the rat fast-twitch epitrochlearis muscle without affecting IRS-1-associated PI 3-kinase activity

We have previously shown in the rat slow-twitch soleus muscle that adrenaline greatly potentiates insulin-stimulated protein kinase B (PKB) phosphorylation without having an effect alone. However, insulin signalling capacity through the PKB pathway is higher in soleus than in fast-twitch muscles, whereas adrenaline activates phosphorylase more strongly in epitrochlearis. Therefore, the aim of the present study was to investigate the interaction between adrenaline and insulin signalling in the fast-twitch epitrochlearis muscle. Insulin increased insulin receptor substrate-1 (IRS-1)-associated phosphoinositide (PI) 3-kinase activity threefold, and adrenaline did not influence basal or insulin-stimulated PI 3-kinase activity. Insulin but not adrenaline increased PKB activity and phosphorylation of Ser(473) and Thr(308). It is interesting to note that adrenaline potentiated insulin-stimulated PKB activity and PKB Ser(473) and Thr(308) phosphorylation. These effects were mimicked by dibutyryl-cyclic adenosine monophosphate (db-cAMP). Adrenaline and db-cAMP increased glycogen synthase kinase (GSK)-3beta Ser(9) phosphorylation independently of PKB activation and enhanced insulin-stimulated GSK-3beta Ser(9) phosphorylation. Although adrenaline increased GSK-3 phosphorylation (inhibiting activity), phosphorylation of its target sites on glycogen synthase was increased, and adrenaline blocked insulin-stimulated glycogen synthase dephosphorylation of Ser(641) and Ser(645,649,653,657), glycogen synthase activation and glycogen synthesis. Insulin-stimulated glucose transport was not influenced by adrenaline despite the increased PKB activation. In conclusion, as in the slow-twitch soleus muscle, adrenaline potentiates insulin-stimulated PKB activation in the fast-twitch glycolytic epitrochlearis muscle without increasing IRS-1-associated PI 3-kinase activity. Furthermore, adrenaline induces phosphorylation of a pool of GSK-3 that is not involved in the regulation of glycogen metabolism. These results indicate that the combination of adrenaline and insulin may activate novel signalling molecules rather than just summing up their effects on linear pathways.

Fighting fat with muscle: bulking up to slim down

Akt1 is a well-characterized mediator of muscle hypertrophy. In this issue of Cell Metabolism, Izumiya et al. (2008) reveal a striking link between Akt1 signaling, fast muscle fiber size, and whole-body metabolism. These results provide new insights into the ability of muscle to combat diet-induced obesity and metabolic dysfunction.

Abnormalities of glucose homeostasis and the hypothalamic-pituitary-adrenal axis in mice lacking hexose-6-phosphate dehydrogenase

Hexose-6-phosphate dehydrogenase (EC 1.1.1.47) catalyzes the conversion of glucose 6-phosphate to 6-phosphogluconolactone within the lumen of the endoplasmic reticulum, thereby generating reduced nicotinamide adenine dinucleotide phosphate. Reduced nicotinamide adenine dinucleotide phosphate is a necessary cofactor for the reductase activity of 11beta-hydroxysteroid dehydrogenase type 1 (EC 1.1.1.146), which converts hormonally inactive cortisone to active cortisol (in rodents, 11-dehydrocorticosterone to corticosterone). Mice with targeted inactivation of hexose-6-phosphate dehydrogenase lack 11beta-hydroxysteroid dehydrogenase type 1 reductase activity, whereas dehydrogenase activity (corticosterone to 11-dehydrocorticosterone) is increased. We now report that both glucose output and glucose use are abnormal in these mice. Mutant mice have fasting hypoglycemia. In mutant primary hepatocytes, glucose output does not increase normally in response to glucagon. Mutant animals have lower hepatic glycogen content when fed and cannot mobilize it normally when fasting. As assessed by RT-PCR, responses of hepatic enzymes to fasting are blunted; enzymes involved in gluconeogenesis (phosphoenolpyruvate carboxykinase, tyrosine aminotransferase) are not appropriately up-regulated, and expression of glucokinase, an enzyme required for glycolysis, is not suppressed. Corticosterone has attenuated effects on expression of these enzymes in cultured mutant primary hepatocytes. Mutant mice have increased sensitivity to insulin, as assessed by homeostatic model assessment values and by increased glucose uptake by the muscle. The hypothalamic-pituitary-adrenal axis is also abnormal. Circulating ACTH, deoxycorticosterone, and corticosterone levels are increased in mutant animals, suggesting decreased negative feedback on the hypothalamic-pituitary-adrenal axis. Comparison with other animal models of adrenal insufficiency suggests that many of the observed abnormalities can be explained by blunted intracellular corticosterone actions, despite elevated circulating levels of this hormone.

Fatness, fitness, and insulin sensitivity among 7- to 9-year-old children

OBJECTIVE

The purpose of this study was to examine the relationships among fatness and aerobic fitness on indices of insulin resistance and sensitivity in children.

RESEARCH DESIGN AND METHODS

A total of 375 children (193 girls and 182 boys) 7 to 9 years of age were categorized by weight as normal-weight, overweight, or obese and by aerobic fitness based on a submaximal physical working capacity test (PWC). Fasting blood glucose (GLU) and insulin (INS) were used to calculate various indices of insulin sensitivity (GLU/INS), the homeostasis model assessment (HOMA), and the quantitative insulin sensitivity check index (QUICKI). Surrogate measures of pancreatic beta cell function included the insulinogenic index (INS/GLU) and the HOMA estimate of pancreatic beta-cell function (HOMA %B).

RESULTS

Insulin sensitivity and secretion variables were significantly different between the normal-weight children and the overweight and obese subjects. Fasting insulin (FI), HOMA, QUICKI, and INS/GLU were significantly different between the overweight and obese subjects. Likewise, the high fitness group possessed a better insulin sensitivity profile. In general, the normal-weight-high fit group possessed the best insulin sensitivity profile and the obese-unfit group possessed the worst insulin sensitivity profile. Several significant differences existed among the six fat-fit groups. Of particular note are the differences within BMI groups by fitness level and the comparison of values between the normal-weight-unfit subjects and the overweight and obese subjects with high fitness.

CONCLUSIONS

The results indicate that aerobic fitness attenuates the difference in insulin sensitivity within BMI categories, thus emphasizing the role of fitness even among overweight and obese children.

Effects of acute exercise and training on insulin action and sensitivity: focus on molecular mechanisms in muscle

A single bout of exercise increases insulin sensitivity for several hours and the effect is mainly restricted to the muscles recruited during exercise. When exercise is repeated over time, adaptations to physical training occur that include more long-lasting increases in insulin sensitivity. The present review explores the molecular mechanisms involved in both the acute and chronic effects of exercise on insulin sensitivity in skeletal muscle.