Skeletal muscle GLUT4 protein concentration and aging in humans

Metabolic dysfunction and the development of physical frailty: an aging war of attrition

The World Health Organization recently declared 2021-2030 the decade of healthy aging. Such emphasis on healthy aging requires an understanding of the biologic challenges aging populations face. Physical frailty is a syndrome of vulnerability that puts a subset of older adults at high risk for adverse health outcomes including functional and cognitive decline, falls, hospitalization, and mortality. The physiology driving physical frailty is complex with age-related biological changes, dysregulated stress response systems, chronic inflammatory pathway activation, and altered energy metabolism all likely contributing. Indeed, a series of recent studies suggests circulating metabolomic distinctions can be made between frail and non-frail older adults. For example, marked restrictions on glycolytic and mitochondrial energy production have been independently observed in frail older adults and collectively appear to yield a reliance on the highly fatigable ATP-phosphocreatine (PCr) energy system. Further, there is evidence that age-associated impairments in the primary ATP generating systems (glycolysis, TCA cycle, electron transport) yield cumulative deficits and fail to adequately support the ATP-PCr system. This in turn may acutely contribute to several major components of the physical frailty phenotype including muscular fatigue, weakness, slow walking speed and, over time, result in low physical activity and accelerate reductions in lean body mass. This review describes specific age-associated metabolic declines and how they can collectively lead to metabolic inflexibility, ATP-PCr reliance, and the development of physical frailty. Further investigation remains necessary to understand the etiology of age-associated metabolic deficits and develop targeted preventive strategies that maintain robust metabolic health in older adults.

New-Onset Diabetes Mellitus after Kidney Transplantation

New-Onset Diabetes Mellitus after Transplantation (NODAT) emerges as a prevalent complication post-kidney transplantation, with its incidence influenced by variations in NODAT definitions and follow-up periods. The condition's pathophysiology is marked by impaired insulin sensitivity and β-cell dysfunction. Significant risk factors encompass age, gender, obesity, and genetics, among others, with the use of post-transplant immunosuppressants intensifying the condition. NODAT's significant impact on patient survival and graft durability underscores the need for its prevention, early detection, and treatment. This review addresses the complexities of managing NODAT, including the challenges posed by various immunosuppressive regimens crucial for transplant success yet harmful to glucose metabolism. It discusses management strategies involving adjustments in immunosuppressive protocols, lifestyle modifications, and pharmacological interventions to minimize diabetes risk while maintaining transplant longevity. The importance of early detection and proactive, personalized intervention strategies to modify NODAT's trajectory is also emphasized, advocating for a shift towards more anticipatory post-transplant care.

Molecular Background and Clinical Implications of Glucose Disorders in Patients with Psoriatic Arthritis

Psoriatic arthritis (PsA) is an inflammatory musculoskeletal disease characterized by joint and entheses involvement. This condition is often associated with an increased prevalence of obesity, encompassing more than one-third of all patients. Given the presence of metabolic disorders, it becomes crucial to enhance clinical oversight of metabolic parameters. An early diagnosis of glucose irregularities in PsA allows for the assessment of an effective treatment strategy. The approach proves valuable in preventing the development of insulin resistance (IR) or diabetes mellitus type 2 (DMt2). Similar pathways characterize the pathomechanism of PsA and DMt2, offering an innovative perspective on treatment management. The cytokines and adipokines synthesized in the course of PsA significantly impact the development process of IR and DMt2 in different mechanisms of action. Conversely, glucose disorders influence the activity of PsA and therapy outcomes. Given the chronic inflammatory background shared by PsA, obesity, and DMt2, it is evident that inadequate management of any of the mentioned conditions can exacerbate the others. Thus, when PsA coincides with DMt2, a comprehensive multidimensional approach is necessary. This includes an effective immunosuppressive regimen complemented by appropriate anti-diabetic and insulin therapies. Moreover, often overlooked recommendations concerning overall well-being and lifestyle adjustments hold significance. This manuscript explores the connections and the relationship between the molecular background of PsA and glucose disorders. It provides a detailed exposition of specific therapeutic approaches for both conditions.

Mulberry (Morus alba L.) Leaf Extract and 1-Deoxynojirimycin Improve Skeletal Muscle Insulin Resistance via the Activation of IRS-1/PI3K/Akt Pathway in db/db Mice

Mulberry (Morus alba L.) leaves have been used to lower blood glucose in patients with diabetes. We evaluated the effects of mulberry leaves extract (MLE) and 1-deoxynojirimycin (1-DNJ) in improving insulin resistance through the activation of the IRS-1/PI3K/Akt pathway in the skeletal muscle of db/db mice. Histological analysis revealed an amelioration of muscle deformation and increased muscle fiber size. MLE and 1-DNJ positively raised the protein expression of related glucose uptake and increased the translocation of glucose transporter type 4 (GLUT4) to the membrane. Furthermore, MLE and 1-DNJ activated the IRS-1/PI3K/Akt pathway in the skeletal muscle and, subsequently, modulated the protein levels of glycogen synthase kinase-3beta (GSK-3β) and glycogen synthase (GS), leading to elevated muscle glycogen content. These findings suggest that MLE and 1-DNJ supplementation improves insulin resistance by modulating the insulin signaling pathway in the skeletal muscle of db/db mice.

Low Skeletal Muscle Mass Is Associated With the Presence, Incidence, and Progression of Coronary Artery Calcification

BACKGROUND

Low skeletal muscle mass (SMM) is an emerging risk factor of cardiovascular disease (CVD). We investigated the association between SMM and coronary artery calcification (CAC).

METHODS

We enrolled 19,728 adults free of CVD who underwent computed tomographic estimation of Agatston CAC scores for cross-sectional analysis. Among them, 5,401 subjects who had at least 2 follow-up CAC scores were included in longitudinal analysis. Relative SMM is presented as the skeletal muscle mass index [SMI (%) = total appendicular muscle mass (kg)/body weight (kg) × 100]. CAC presence and incidence were defined as CAC score > 0, and CAC progression was defined as √CAC score (follow-up) - √CAC score (baseline) > 2.5.

RESULTS

Among all of the subjects (mean age 53.4 years, 80.8% male), the prevalence of CAC was 36.7%. The incidence of CAC was 17.4% during a mean of 3.6 years, and the progression of CAC was 49.9% during a mean of 2.3 years. The lowest SMI quartile was significantly associated with an increased risk of CAC presence (adjusted odds ratio 2.75, 95% confidence interval [CI] 2.45-3.05; P < 0.001), incidence (adjusted hazard ratio [AHR] 1.99, 95% CI 1.36-2.91; P < 0.001), and progression (AHR 1.48, 95% CI 1.25-1.77; P < 0.001) compared with the highest quartile. SMI as a continuous value was also significantly inversely associated with CAC. SMI was the best parameter to be related to CAC among other quantitative indices such as height or body mass index adjusted.

CONCLUSIONS

Low SMM is significantly associated with an elevated risk of CAC, independently from other cardiometabolic parameters.

Age-related changes in muscle architecture and metabolism in humans: The likely contribution of physical inactivity to age-related functional decline

In the United Kingdom (UK), it is projected that by 2035 people aged >65 years will make up 23 % of the population, with those aged >85 years accounting for 5% of the total population. Ageing is associated with progressive changes in muscle metabolism and a decline in functional capacity, leading to a loss of independence. Muscle metabolic changes associated with ageing have been linked to alterations in muscle architecture and declines in muscle mass and insulin sensitivity. However, the biological features often attributed to muscle ageing are also seen in controlled studies of physical inactivity (e.g. reduced step-count and bed-rest), and it is currently unclear how many of these ageing features are due to ageing per se or sedentarism. This is particularly relevant at a time of home confinements reducing physical activity levels during the Covid-19 pandemic. Current knowledge gaps include the relative contribution that physical inactivity plays in the development of many of the negative features associated with muscle decline in older age. Similarly, data demonstrating positive effects of government recommended physical activity guidelines on muscle health are largely non-existent. It is imperative therefore that research examining interactions between ageing, physical activity and muscle mass and metabolic health is prioritised so that it can inform on the "normal" muscle ageing process and on strategies for improving health span and well-being. This review will focus on important changes in muscle architecture and metabolism that accompany ageing and highlight the likely contribution of physical inactivity to these changes.

Sirtuin 1 is not required for contraction-stimulated glucose uptake in mouse skeletal muscle

While it has long been known that contraction robustly stimulates skeletal muscle glucose uptake, the molecular steps regulating this increase remain incompletely defined. The mammalian ortholog of Sir2, sirtuin 1 (SIRT1), is an NAD⁺-dependent protein deacetylase that is thought to link perturbations in energy flux associated with exercise to subsequent cellular adaptations. Nevertheless, its role in contraction-stimulated glucose uptake has not been described. The objective of this study was to determine the importance of SIRT1 to contraction-stimulated glucose uptake in mouse skeletal muscle. Using a radioactive 2-deoxyglucose uptake (2DOGU) approach, we measured ex vivo glucose uptake in unstimulated (rested) and electrically stimulated (100 Hz contraction every 15 s for 10 min; contracted) extensor digitorum longus (EDL) and soleus from ∼15-wk-old male and female mice with muscle-specific knockout of SIRT1 deacetylase activity and their wild-type littermates. Skeletal muscle force decreased over the contraction protocol, although there were no differences in the rate of fatigue between genotypes. In EDL and soleus, loss of SIRT1 deacetylase activity did not affect contraction-induced increase in glucose uptake in either sex. Interestingly, the absolute rate of contraction-stimulated 2DOGU was ∼1.4-fold higher in female compared with male mice, regardless of muscle type. Taken together, our findings demonstrate that SIRT1 is not required for contraction-stimulated glucose uptake in mouse skeletal muscle. Moreover, to our knowledge, this is the first demonstration of sex-based differences in contraction-stimulated glucose uptake in mouse skeletal muscle.NEW & NOTEWORTHY Here, we demonstrate that glucose uptake in response to ex vivo contractions is not affected by the loss of sirtuin 1 (SIRT1) deacetylase function in muscle, regardless of sex or muscle type. Interestingly, however, similar to studies on insulin-stimulated glucose uptake, we demonstrate that contraction-stimulated glucose uptake is robustly higher in female compared with the male skeletal muscle. To our knowledge, this is the first demonstration of sex-based differences in contraction-stimulated glucose uptake in skeletal muscle.

The p21-activated kinase 2 (PAK2), but not PAK1, regulates contraction-stimulated skeletal muscle glucose transport

AIM

Muscle contraction stimulates skeletal muscle glucose transport. Since it occurs independently of insulin, it is an important alternative pathway to increase glucose transport in insulin-resistant states, but the intracellular signaling mechanisms are not fully understood. Muscle contraction activates group I p21-activated kinases (PAKs) in mouse and human skeletal muscle. PAK1 and PAK2 are downstream targets of Rac1, which is a key regulator of contraction-stimulated glucose transport. Thus, PAK1 and PAK2 could be downstream effectors of Rac1 in contraction-stimulated glucose transport. The current study aimed to test the hypothesis that PAK1 and/or PAK2 regulate contraction-induced glucose transport.

METHODS

Glucose transport was measured in isolated soleus and extensor digitorum longus (EDL) mouse skeletal muscle incubated either in the presence or absence of a pharmacological inhibitor (IPA-3) of group I PAKs or originating from whole-body PAK1 knockout, muscle-specific PAK2 knockout or double whole-body PAK1 and muscle-specific PAK2 knockout mice.

RESULTS

IPA-3 attenuated (-22%) the increase in glucose transport in response to electrically stimulated contractions in soleus and EDL muscle. PAK1 was dispensable for contraction-stimulated glucose transport in both soleus and EDL muscle. Lack of PAK2, either alone (-13%) or in combination with PAK1 (-14%), partly reduced contraction-stimulated glucose transport compared to control littermates in EDL, but not soleus muscle.

CONCLUSION

Contraction-stimulated glucose transport in isolated glycolytic mouse EDL muscle is partly dependent on PAK2, but not PAK1.

Impact of Endurance and Resistance Training on Skeletal Muscle Glucose Metabolism in Older Adults

Aging is associated with insulin resistance and the development of type 2 diabetes. While this process is multifaceted, age-related changes to skeletal muscle are expected to contribute to impaired glucose metabolism. Some of these changes include sarcopenia, impaired insulin signaling, and imbalances in glucose utilization. Endurance and resistance exercise training have been endorsed as interventions to improve glucose tolerance and whole-body insulin sensitivity in the elderly. While both types of exercise generally increase insulin sensitivity in older adults, the metabolic pathways through which this occurs can differ and can be dependent on preexisting conditions including obesity and type 2 diabetes. In this review, we will first highlight age-related changes to skeletal muscle which can contribute to insulin resistance, followed by a comparison of endurance and resistance training adaptations to insulin-stimulated glucose metabolism in older adults.

An aPPARent Functional Consequence in Skeletal Muscle Physiology via Peroxisome Proliferator-Activated Receptors

Skeletal muscle comprises 30–40% of the total body mass and plays a central role in energy homeostasis in the body. The deregulation of energy homeostasis is a common underlying characteristic of metabolic syndrome. Over the past decades, peroxisome proliferator-activated receptors (PPARs) have been shown to play critical regulatory roles in skeletal muscle. The three family members of PPAR have overlapping roles that contribute to the myriad of processes in skeletal muscle. This review aims to provide an overview of the functions of different PPAR members in energy homeostasis as well as during skeletal muscle metabolic disorders, with a particular focus on human and relevant mouse model studies.

Diminished stress resistance and defective adaptive homeostasis in age-related diseases

Adaptive homeostasis is defined as the transient expansion or contraction of the homeostatic range following exposure to subtoxic, non-damaging, signaling molecules or events, or the removal or cessation of such molecules or events (Mol. Aspects Med. (2016) 49, 1-7). Adaptive homeostasis allows us to transiently adapt (and then de-adapt) to fluctuating levels of internal and external stressors. The ability to cope with transient changes in internal and external environmental stress, however, diminishes with age. Declining adaptive homeostasis may make older people more susceptible to many diseases. Chronic oxidative stress and defective protein homeostasis (proteostasis) are two major factors associated with the etiology of age-related disorders. In the present paper, we review the contribution of impaired responses to oxidative stress and defective adaptive homeostasis in the development of age-associated diseases.

Similarities and interactions between the ageing process and high chronic intake of added sugars

In our societies, the proportions of elderly people and of obese individuals are increasing. Both factors are associated with high health-related costs. During obesity, many authors suggest that it is a high chronic intake of added sugars (HCIAS) that triggers the shift towards pathology. However, the majority of studies were performed in young subjects and only a few were interested in the interaction with the ageing process. Our purpose was to discuss the metabolic effects of HCIAS, compare with the effects of ageing, and evaluate how deleterious the combined action of HCIAS and ageing could be. This effect of HCIAS seems mediated by fructose, targeting the liver first, which may lead to all subsequent metabolic alterations. The first basic alterations induced by fructose are increased oxidative stress, protein glycation, inflammation, dyslipidaemia and insulin resistance. These alterations are also present during the ageing process, and are closely related to each other, one leading to the other. These basic alterations are also involved in more complex syndromes, which are also favoured by HCIAS, and present during ageing. These include non-alcoholic fatty liver disease, hypertension, neurodegenerative diseases, sarcopenia and osteoporosis. Cumulative effects of ageing and HCIAS have been seldom tested and may not always be strictly additive. Data also suggest that some of the metabolic alterations that are more prevalent during ageing could be related more with nutritional habits than to intrinsic ageing. In conclusion, it is clear that HCIAS interacts with the ageing process, accelerates the accumulation of metabolic alterations, and that it should be avoided.

Exercise Promotes Healthy Aging of Skeletal Muscle

Primary aging is the progressive and inevitable process of bodily deterioration during adulthood. In skeletal muscle, primary aging causes defective mitochondrial energetics and reduced muscle mass. Secondary aging refers to additional deleterious structural and functional age-related changes caused by diseases and lifestyle factors. Secondary aging can exacerbate deficits in mitochondrial function and muscle mass, concomitant with the development of skeletal muscle insulin resistance. Exercise opposes deleterious effects of secondary aging by preventing the decline in mitochondrial respiration, mitigating aging-related loss of muscle mass and enhancing insulin sensitivity. This review focuses on mechanisms by which exercise promotes "healthy aging" by inducing modifications in skeletal muscle.

Predictors of Whole-Body Insulin Sensitivity Across Ages and Adiposity in Adult Humans

CONTEXT

Numerous factors are purported to influence insulin sensitivity including age, adiposity, mitochondrial function, and physical fitness. Univariate associations cannot address the complexity of insulin resistance or the interrelationship among potential determinants.

OBJECTIVE

The objective of the study was to identify significant independent predictors of insulin sensitivity across a range of age and adiposity in humans.

DESIGN, SETTING, AND PARTICIPANTS

Peripheral and hepatic insulin sensitivity were measured by two stage hyperinsulinemic-euglycemic clamps in 116 men and women (aged 19-78 y). Insulin-stimulated glucose disposal, the suppression of endogenous glucose production during hyperinsulinemia, and homeostatic model assessment of insulin resistance were tested for associations with 11 potential predictors. Abdominal subcutaneous fat, visceral fat (AFVISC), intrahepatic lipid, and intramyocellular lipid (IMCL) were quantified by magnetic resonance imaging and spectroscopy. Skeletal muscle mitochondrial respiratory capacity (state 3), coupling efficiency, and reactive oxygen species production were evaluated from muscle biopsies. Aerobic fitness was measured from whole-body maximum oxygen uptake (VO2 peak), and metabolic flexibility was determined using indirect calorimetry.

RESULTS

Multiple regression analysis revealed that AFVISC (P < .0001) and intrahepatic lipid (P = .002) were independent negative predictors of peripheral insulin sensitivity, whereas VO2 peak (P = .0007) and IMCL (P = .023) were positive predictors. Mitochondrial capacity and efficiency were not independent determinants of peripheral insulin sensitivity. The suppression of endogenous glucose production during hyperinsulinemia model of hepatic insulin sensitivity revealed percentage fat (P < .0001) and AFVISC (P = .001) as significant negative predictors. Modeling homeostatic model assessment of insulin resistance identified AFVISC (P < .0001), VO2 peak (P = .001), and IMCL (P = .01) as independent predictors.

CONCLUSION

The reduction in insulin sensitivity observed with aging is driven primarily by age-related changes in the content and distribution of adipose tissue and is independent of muscle mitochondrial function or chronological age.

Alternating bouts of sitting and standing attenuate postprandial glucose responses

PURPOSE

This study aimed to examine whether reductions in sitting time through alternating 30-min bouts of sitting and standing can reduce postprandial glucose, insulin, and triglyceride responses.

METHODS

Twenty-three overweight/obese sedentary office workers (17 males and six females; mean ± SD: age, 48.2 ± 7.9 yr; body mass index, 29.6 ± 4.0 kg · m(-2)) undertook two short-term (5 d) experimental conditions in an equal, randomized (1:1) order. In a simulated office environment, participants performed typical occupational tasks for 8 h · d(-1) while in a 1) seated work posture (control condition) or 2) interchanging between a seated and standing work posture every 30 min using an electric, height-adjustable workstation (intervention condition). Fasting and postprandial blood samples after a mixed test drink were collected hourly for 4 h on days 1 and 5 of each condition to assess serum insulin, plasma glucose, and triglycerides. Dietary intake (kJ · d(-1)) and physical activity were standardized during each condition. The trial was registered with the Australian New Zealand Clinical Trials Registry (ACTRN12611000632998).

RESULTS

After adjustment for time (days 1 and 5), incremental area under the analyte time curve differed significantly between conditions for plasma glucose (P = 0.007) but not for serum insulin or plasma triglycerides. Adjusted mean glucose incremental area under the analyte time curve was lowered by 11.1% after the intervention condition (6.38 mM · h(-1) (confidence interval, 5.04-7.71)) relative to the control condition (7.18 mM · h(-1) (confidence interval, 5.85-8.52)). No temporal changes (days 1 vs 5) between conditions were observed.

CONCLUSIONS

Alternating standing and sitting in 30-min bouts results in modest beneficial effects on postprandial glucose responses in overweight/obese office workers.

The emergence of sedentary behaviour physiology and its effects on the cardiometabolic profile in young and older adults

It has recently emerged that sedentary behaviour is independent of a lack of physical activity as individuals can be sufficiently active, based on the recommended physical activity guidelines, but also spend the majority of their waking hours engaging in sedentary behaviour. Individuals who follow this pattern of physical activity and sedentary behaviour are known as 'active couch potatoes'. Sedentary behaviour has been found to have detrimental effects on cardiometabolic markers associated with cardiovascular disease. Since the positive effects of moderate-to-vigorous intensity physical activity do not necessarily negate the deleterious effects of sedentary behaviour on cardiometabolic markers, it is postulated that engaging in light physical activity is an intervention that will successfully reduce levels of sedentary behaviour and may hence improve health markers of quality of life. We propose that such lifestyle changes may be particularly relevant to older populations as these engage in sedentary behaviour for the majority of their waking hours, thereby adding to the negative aging effect on cardiometabolic markers.

Dietary (-)-Epigallocatechin-3-gallate Supplementation Counteracts Aging-Associated Skeletal Muscle Insulin Resistance and Fatty Liver in Senescence-Accelerated Mouse

Aging is accompanied by pathophysiological changes including insulin resistance and fatty liver. Dietary supplementation with (-)-epigallocatechin-3-gallate (EGCG) improves insulin sensitivity and attenuates fatty liver disease. We hypothesized that EGCG could effectively modulate aging-associated changes in glucose and lipid metabolism in senescence-accelerated mice (SAM) prone 8 (SAMP8). Higher levels of glucose, insulin, and free fatty acid, inhibited Akt activity, and decreased glucose transporter 4 (GLUT4) expression were observed in SAMP8 mice compared to the normal aging group, SAM resistant 1 mice. EGCG supplementation for 12 weeks successfully decreased blood glucose and insulin levels via restoring Akt activity and GLUT4 expression and stimulating AMPKα activation in skeletal muscle. EGCG up-regulated genes involved in mitochondrial biogenesis and subsequently restored mitochondrial DNA copy number in skeletal muscle of SAMP8 mice. Decreased adipose triglyceride lipase and increased sterol regulatory element binding proteins-1c (SREBP-1c) and carbohydrate responsive element binding protein at mRNA levels were observed in SAMP8 mice in accordance with hepatocellular ballooning and excess lipid accumulation. The pevention of hepatic lipid accumulation by EGCG was mainly attributed to down-regulation of mTOR and SREBP-1c-mediated lipid biosynthesis via suppression of the positive regulator, Akt, and activation of the negative regulator, AMPKα, in the liver. EGCG beneficially modulates glucose and lipid homeostasis in skeletal muscle and liver, leading to alleviation of aging-associated metabolic disorders.

Modifications of the homeostasis model assessment of insulin resistance index with age

The aim of the study was to analyze the association between aging and insulin resistance estimated by the homeostasis model assessment of insulin resistance (HOMA-IR). This work involved two studies: (1) the Di@bet.es study is a cross-sectional study including 4,948 subjects, comprising a representative sample of the adult Spanish population; (2) the Pizarra study is a population-based cohort study undertaken in Pizarra (Spain), in which 1,051 subjects were evaluated at baseline and 714 completed the 6-year follow-up study. Study variables included a clinical and demographic structured survey, a lifestyle survey, a physical examination, and an oral glucose tolerance test in subjects without diabetes. In the Di@bet.es study overall, an increase occurred in blood glucose until the age of 50, after which it remained stable (data adjusted for gender, body mass index, abnormal glucose regulation [AGR]). The HOMA-IR increased significantly with age (p = 0.01), due to a higher prevalence of obesity (p < 0.0001) and AGR (p < 0.001). In non-obese subjects without AGR, HOMA-IR values were not modified with age (p = 0.30), but they were with body mass index (p < 0.001). In the Pizarra study, the HOMA-IR was significantly lower after 6-year follow-up in the whole study population. Subjects with a HOMA-IR level higher than the 75th percentile at baseline were more likely to develop diabetes (OR 2.2, 95 % CI 1.2-3.9; p = 0.007) than subjects with a lower HOMA-IR. We concluded that age per se did not increase HOMA-IR levels, changes that might be related to higher rates of obesity and AGR in older subjects. The HOMA-IR was associated with an increased risk of developing type 2 diabetes 6 years later.

Ageing, adipose tissue, fatty acids and inflammation

A common feature of ageing is the alteration in tissue distribution and composition, with a shift in fat away from lower body and subcutaneous depots to visceral and ectopic sites. Redistribution of adipose tissue towards an ectopic site can have dramatic effects on metabolic function. In skeletal muscle, increased ectopic adiposity is linked to insulin resistance through lipid mediators such as ceramide or DAG, inhibiting the insulin receptor signalling pathway. Additionally, the risk of developing cardiovascular disease is increased with elevated visceral adipose distribution. In ageing, adipose tissue becomes dysfunctional, with the pathway of differentiation of preadipocytes to mature adipocytes becoming impaired; this results in dysfunctional adipocytes less able to store fat and subsequent fat redistribution to ectopic sites. Low grade systemic inflammation is commonly observed in ageing, and may drive the adipose tissue dysfunction, as proinflammatory cytokines are capable of inhibiting adipocyte differentiation. Beyond increased ectopic adiposity, the effect of impaired adipose tissue function is an elevation in systemic free fatty acids (FFA), a common feature of many metabolic disorders. Saturated fatty acids can be regarded as the most detrimental of FFA, being capable of inducing insulin resistance and inflammation through lipid mediators such as ceramide, which can increase risk of developing atherosclerosis. Elevated FFA, in particular saturated fatty acids, maybe a driving factor for both the increased insulin resistance, cardiovascular disease risk and inflammation in older adults.

[Gender-specific differences in age-associated endocrinology]

The endocrine system is intimately involved in modulating lifespan and quality of life. Facing an ever increasing proportion of aged people in the western society, there is great interest in understanding the complex interrelations between increasing age and hormonal regulation. Age-associated endocrinological changes comprise the decline of basal hormonal levels, pulsatile hormone distribution, and activity of hormonal axis, which result in changes in body composition. Men and women experience different age-associated alterations of the hormonal system. Aging per se is a risk factor for diseases like diabetes mellitus type 2, thyroid disorders, osteoporosis, frailty, and sarcopenia. Gender-specific differences with respect to symptoms, interactions, diagnosis, and therapy must be taken into consideration. Current data do not allow a general recommendation for hormonal substitution, neither for women nor for men. New research approaches following a multifactorial pathway are required to elucidate the complexity of age-associated endocrinological changes and to develop gender-specific therapies for endocrinological diseases.

Impairments in site-specific AS160 phosphorylation and effects of exercise training

The purpose of this study was to determine if site-specific phosphorylation at the level of Akt substrate of 160 kDa (AS160) is altered in skeletal muscle from sedentary humans across a wide range of the adult life span (18-84 years of age) and if endurance- and/or strength-oriented exercise training could rescue decrements in insulin action and skeletal muscle AS160 phosphorylation. A euglycemic-hyperinsulinemic clamp and skeletal muscle biopsies were performed in 73 individuals encompassing a wide age range (18-84 years of age), and insulin-stimulated AS160 phosphorylation was determined. Decrements in whole-body insulin action were associated with impairments in insulin-induced phosphorylation of skeletal muscle AS160 on sites Ser-588, Thr-642, Ser-666, and phospho-Akt substrate, but not Ser-318 or Ser-751. Twelve weeks of endurance- or strength-oriented exercise training increased whole-body insulin action and reversed impairments in AS160 phosphorylation evident in insulin-resistant aged individuals. These findings suggest that a dampening of insulin-induced phosphorylation of AS160 on specific sites in skeletal muscle contributes to the insulin resistance evident in a sedentary aging population and that exercise training is an effective intervention for treating these impairments.

Clinical features, diagnostic criteria and pathogenesis of diabetes mellitus

Diabetes mellitus is a metabolic disorder of glucose homeostasis and associated with long-term vascular complications leading to morbidity and mortality. It is the fastest growing non-communicable disease throughout the world. The pathophysiology of diabetes is complex and multifactorial. Understanding pathological mechanisms of disease can help clinicians to identify and treat the factors involved effectively, and design preventive strategies so as to halt the pandemic of this deadly disorder.

Pancreatic function, type 2 diabetes, and metabolism in aging

Aging is a risk factor for impaired glucose tolerance and diabetes. Of the reported 25.8 million Americans estimated to have diabetes, 26.9% are over the age of 65. In certain ethnic groups, the proportion is even higher; almost 1 in 3 older Hispanics and African Americans and 3 out of 4 Pima Indian elders have diabetes. As per the NHANES III (Third National Health and Nutrition Examination) survey, the percentage of physician-diagnosed diabetes increased from 3.9% in middle-aged adults (40-49 years) to 13.2% in elderly adults (≥75 years). The higher incidence of diabetes is especially alarming considering that diabetes in itself increases the risk for multiple other age-related diseases such as cancer, stroke, cardiovascular diseases, Parkinson's disease, and Alzheimer's disease (AD). In this review, we summarize the current evidence on how aging affects pancreatic β cell function, β cell mass, insulin secretion and insulin sensitivity. We also review the effects of aging on the relationship between insulin sensitivity and insulin secretion. Understanding the mechanisms that lead to impaired glucose homeostasis and T2D in the elderly will lead to development of novel treatments that will prevent or delay diabetes, substantially improve quality of life and ultimately increase overall life span.

Acetaminophen: beyond pain and Fever-relieving

Acetaminophen, also known as APAP or paracetamol, is one of the most widely used analgesics (pain reliever) and antipyretics (fever reducer). According to the U.S. Food and Drug Administration, currently there are 235 approved prescription and over-the-counter drug products containing acetaminophen as an active ingredient. When used as directed, acetaminophen is very safe and effective; however when taken in excess or ingested with alcohol hepatotoxicity and irreversible liver damage can arise. In addition to well known use pain relief and fever reduction, recent laboratory and pre-clinical studies have demonstrated that acetaminophen may also have beneficial effects on blood glucose levels, skeletal muscle function, and potential use as cardioprotective and neuroprotective agents. Extensive laboratory and pre-clinical studies have revealed that these off-label applications may be derived from the ability of acetaminophen to function as an antioxidant. Herein, we will highlight these novel applications of acetaminophen, and attempt, where possible, to highlight how these findings may lead to new directions of inquiry and clinical relevance of other disorders.

Effect of macronutrients, age, and obesity on 6- and 24-h postprandial glucose metabolism in cats

Obesity and age are risk factors for feline diabetes. This study aimed to test the hypothesis that age, long-term obesity, and dietary composition would lead to peripheral and hepatorenal insulin resistance, indicated by higher endogenous glucose production (EGP) in the fasted and postprandial state, higher blood glucose and insulin, and higher leptin, free thyroxine, and lower adiponectin concentrations. Using triple tracer-(2)H(2)O, [U-(13)C(3)] propionate, and [3,4-(13)C(2)] glucose infusion, and indirect calorimetry-we investigated carbohydrate and fat metabolic pathways in overnight-fasted neutered cats (13 young lean, 12 old lean, and 12 old obese), each fed three different diets (high protein with and without polyunsaturated fatty acids, and high carbohydrate) in a crossover design. EGP was lowest in fasted and postprandial obese cats despite peripheral insulin resistance, indicated by hyperinsulinemia. Gluconeogenesis was the most important pathway for EGP in all groups, but glycogen contributed significantly. Insulin and leptin concentrations were higher in old than in young lean cats; adiponectin was lowest in obese cats but surprisingly highest in lean old cats. Diet had little effect on metabolic parameters. We conclude that hepatorenal insulin resistance does not develop in the fasted or postprandial state, even in long-term obese cats, allowing the maintenance of euglycemia through lowering EGP. Glycogen plays a major role in EGP, especially in lean fasted cats, and in the postprandial state. Aging may predispose to insulin resistance, which is a risk factor for diabetes in cats. Mechanisms underlying the high adiponectin of healthy old lean cats need to be further explored.

Management of dyslipidemia in the elderly population

The elderly population is increasing worldwide. Despite a major decrease in deaths from coronary heart disease (CHD), this malady remains the major cause of death in elderly men and women. In this paper, we review the role of dyslipidemia as a major known risk factor in the pathogenesis of CHD, age-related changes in lipoprotein metabolism, and differences in changes in lipids that occur in men and women during aging. Next we provide an overview of the available studies and recommendations from ATP III. Finally, we comment on the screening and management, cost and side effects of therapy as it applies to an aging population.

Regulation of skeletal muscle mitochondrial function: genes to proteins

The impact of ageing on mitochondrial function and the deterministic role of mitochondria on senescence continue to be topics of vigorous debate. Many studies report that skeletal muscle mitochondrial content and function are reduced with ageing and metabolic diseases associated with insulin resistance. However, an accumulating body of literature suggests that physical inactivity typical of ageing may be a more important determinant of mitochondrial function than chronological age, per se. Reports of age-related declines in mitochondrial function have spawned a vast body of literature devoted to understanding the underlying mechanisms. These mechanisms include decreased abundance of mtDNA, reduced mRNA levels, as well as decreased synthesis and expression of mitochondrial proteins, ultimately resulting in decreased function of the whole organelle. Effective therapies to prevent, reverse or delay the onset of the aforementioned mitochondrial changes, regardless of their inevitability or precise underlying causes, require an intimate understanding of the processes that regulate mitochondrial biogenesis, which necessitates the coordinated regulation of nuclear and mitochondrial genomes. Herein we review the current thinking on regulation of mitochondrial biogenesis by transcription factors and transcriptional co-activators and the role of hormones and exercise in initiating this process. We review how exercise may help preserve mitochondrial content and functionality across the lifespan, and how physical inactivity is emerging as a major determinant of many age-associated changes at the level of the mitochondrion. We also review evidence that some mitochondrial changes with ageing are independent of exercise or physical activity and appear to be inevitable consequences of old age.

Age, obesity, and sex effects on insulin sensitivity and skeletal muscle mitochondrial function

OBJECTIVE

Reductions in insulin sensitivity in conjunction with muscle mitochondrial dysfunction have been reported to occur in many conditions including aging. The objective was to determine whether insulin resistance and mitochondrial dysfunction are directly related to chronological age or are related to age-related changes in body composition.

RESEARCH DESIGN AND METHODS

Twelve young lean, 12 young obese, 12 elderly lean, and 12 elderly obese sedentary adults were studied. Insulin sensitivity was measured by a hyperinsulinemic-euglycemic clamp, and skeletal muscle mitochondrial ATP production rates (MAPRs) were measured in freshly isolated mitochondria obtained from vastus lateralis biopsy samples using the luciferase reaction.

RESULTS

Obese participants, independent of age, had reduced insulin sensitivity based on lower rates of glucose infusion during a hyperinsulinemic-euglycemic clamp. In contrast, age had no independent effect on insulin sensitivity. However, the elderly participants had lower muscle MAPRs than the young participants, independent of obesity. Elderly participants also had higher levels inflammatory cytokines and total adiponectin. In addition, higher muscle MAPRs were also noted in men than in women, whereas glucose infusion rates were higher in women.

CONCLUSIONS

The results demonstrate that age-related reductions in insulin sensitivity are likely due to an age-related increase in adiposity rather than a consequence of advanced chronological age. The results also indicate that an age-related decrease in muscle mitochondrial function is neither related to adiposity nor insulin sensitivity. Of interest, a higher mitochondrial ATP production capacity was noted in the men, whereas the women were more insulin sensitive, demonstrating further dissociation between insulin sensitivity and muscle mitochondrial function.

Regulation of GLUT4 expression in denervated skeletal muscle

Denervation by sciatic nerve resection causes decreased muscle glucose transporter 4 (GLUT4) expression, but little is known about the signaling events that cause this decrease. Experiments were designed to test the hypothesis that decreased GLUT4 expression in denervated muscle occurs because of decreased calcium/CaMK activity, which would then lead to decreased activation of the transcription factors myocyte enhancer factor 2 (MEF2) and GLUT4 enhancer factor (GEF), which are required for normal GLUT4 expression. GLUT4 mRNA was elevated in mice expressing constitutively active CaMK isoform IV (CaMKIV) and decreased by denervation. Denervation decreased GEF binding to the promoter and the content of GEF in the nucleus, but there was no change in either MEF2 binding or MEF2 protein content. Expression of a MEF2-dependent reporter gene did not change in denervated skeletal muscle. To determine the domains of the GLUT4 promoter that respond to denervation, transgenic mice expressing the chloramphenicol acetyl transferase (CAT) reporter gene driven by different lengths of the human GLUT4 promoter were denervated. Using several different promoter/reporter gene constructs, we found that all areas of the GLUT4 promoter were truncated or missing, except for the MEF2 binding domain and the basal promoter. All of the GLUT4 promoter/CAT reporter constructs evaluated responded normally to denervation. Our data lead us to conclude that decreased CaMK activity is not the reason for decreased GLUT4 content in denervated muscle and that negative control of GLUT4 expression is not mediated through the MEF2 or GEF-binding domains. These findings indicate that withdrawal of a GEF- or MEF2-dependent signal is not likely a major determinant of the denervation effect on GLUT4 expression. Thus, the response to denervation may be mediated by other elements present in the basal promoter of the GLUT4 gene.

Acetaminophen prevents aging-associated hyperglycemia in aged rats: effect of aging-associated hyperactivation of p38-MAPK and ERK1/2

BACKGROUND

Aging-related hyperglycemia is associated with increased oxidative stress and diminished muscle glucose transporter-4 (Glut4) that may be regulated, at least in part, by the mitogen-activated protein kinases (MAPK).

METHODS

To test the possibility that aging-related hyperglycemia can be prevented by pharmacological manipulation of MAPK hyperactivation, aged (27-month old) Fischer 344/NNiaHSD x Brown Norway/BiNia F1 (F344BN) rats were administered acetaminophen (30 mg/kg body weight/day) for 6 months in drinking water.

RESULTS

Hepatic histopathology, serum aspartate aminotransferase and alanine aminotransferase analyses suggested that chronic acetaminophen did not cause hepatotoxicity. Compared with adult (6-month) and aged (27-month) rats, very aged rats (33-month) had higher levels of blood glucose, phosphorylation of soleus p38-MAPK and extracellular-regulated kinase 1/2 (ERK1/2), superoxide and oxidatively modified proteins (p<0.05), and these changes were associated with decreased soleus Glut4 protein abundance (p<0.05). Chronic acetaminophen treatment attenuated age-associated increase in blood glucose by 61.3% (p<0.05) and increased soleus Glut4 protein by 157.2% (p<0.05). These changes were accompanied by diminished superoxide levels, decrease in oxidatively modified proteins (-60.8%; p<0.05) and reduced p38-MAPK and ERK1/2 hyperactivation (-50.4% and -35.4%, respectively; p<0.05).

CONCLUSIONS

These results suggest that acetaminophen may be useful for the treatment of age-associated hyperglycemia.

Muscle mitochondrial changes with aging and exercise

Aging has been reported to be accompanied by reduced mitochondrial function and insulin sensitivity. Whether these deleterious effects result from chronological age or lifestyle-related factors such as adiposity and physical inactivity remains debatable. The beneficial effects of exercise on mitochondrial function and insulin sensitivity are well documented; however, it is unclear whether exercise can effectively prevent, reverse, or delay the onset of these age-related dysfunctions. Other investigators and we have found that endurance exercise enhances mitochondrial function across the life span, highlighting the beneficial role of exercise in combating age-related mitochondrial dysfunction. The literature is mixed concerning the ability of endurance exercise to normalize age-related insulin resistance; however, emerging evidence points to adiposity rather than age per se as a primary determinant of age-related declines in insulin sensitivity. Recent data from our laboratory also shed some light on the controversial relation between mitochondrial function and insulin sensitivity. Although some investigators purport a causal role of mitochondrial dysfunction in the etiology of insulin resistance, we provide evidence that the reverse may be true based on the fact that insulin stimulates mitochondrial function in healthy control subjects but not in patients with type 2 diabetes. Furthermore, we find that these 2 variables are completely dissociated in some populations, such as Asian Indians, who exhibit elevated mitochondrial capacity despite marked insulin resistance compared with European Americans. Our data not only point to regular endurance exercise as a viable strategy to delay the onset of age-related dysfunctions but they suggest that mitochondrial function and insulin resistance may be linked by additional factors such as physical activity.

Mechanism for improved insulin sensitivity after gastric bypass surgery

CONTEXT

Surgical treatments of obesity have been shown to induce rapid and prolonged improvements in insulin sensitivity.

OBJECTIVE

The aim of the study was to investigate the effects of gastric bypass surgery and the mechanisms that explain the improvement in insulin sensitivity.

DESIGN

We performed a cross-sectional, nonrandomized, controlled study.

SETTING

This study was conducted jointly between the Departments of Exercise Science and Physiology at East Carolina University in Greenville, North Carolina.

SUBJECTS

Subjects were recruited into four groups: 1) lean [body mass index (BMI) < 25 kg/m(2); n = 93]; 2) weight-matched (BMI = 25 to 35 kg/m(2); n = 310); 3) morbidly obese (BMI > 35 kg/m(2); n = 43); and 4) postsurgery patients (BMI approximately 30 kg/m(2); n = 40). Postsurgery patients were weight stable 1 yr after surgery.

MAIN OUTCOME MEASURES

Whole-body insulin sensitivity, muscle glucose transport, and muscle insulin signaling were assessed.

RESULTS

Postsurgery subjects had insulin sensitivity index values that were similar to the lean and higher than morbidly obese and weight-matched control subjects. Glucose transport was higher in the postsurgery vs. morbidly obese and weight-matched groups. IRS1-pSer(312) in the postsurgery group was lower than morbidly obese and weight-matched groups. Inhibitor kappaBalpha was higher in the postsurgery vs. the morbidly obese and weight-matched controls, indicating reduced inhibitor of kappaB kinase beta activity.

CONCLUSIONS

Insulin sensitivity and glucose transport are greater in the postsurgery patients than predicted from the weight-matched group, suggesting that improved insulin sensitivity after bypass is due to something other than, or in addition to, weight loss. Improved insulin sensitivity is related to reduced inhibitor of kappaB kinase beta activity and enhanced insulin signaling in muscle.

Endurance exercise as a countermeasure for aging

OBJECTIVE

We determined whether reduced insulin sensitivity, mitochondrial dysfunction, and other age-related dysfunctions are inevitable consequences of aging or secondary to physical inactivity.

RESEARCH DESIGN AND METHODS

Insulin sensitivity was measured by hyperinsulinemic-euglycemic clamp and ATP production in mitochondria isolated from vastus lateralis biopsies of 42 healthy sedentary and endurance-trained young (18-30 years old) and older (59-76 years old) subjects. Expression of proteins involved in fuel metabolism was measured by mass spectrometry. Citrate synthase activity, mitochondrial DNA (mtDNA) abundance, and expression of nuclear-encoded transcription factors for mitochondrial biogenesis were measured. SIRT3, a mitochondrial sirtuin linked to lifespan-enhancing effects of caloric restriction, was measured by immunoblot.

RESULTS

Insulin-induced glucose disposal and suppression of endogenous glucose production were higher in the trained young and older subjects, but no age effect was noted. Age-related decline in mitochondrial oxidative capacity was absent in endurance-trained individuals. Although endurance-trained individuals exhibited higher expression of mitochondrial proteins, mtDNA, and mitochondrial transcription factors, there were persisting effects of age. SIRT3 expression was lower with age in sedentary but equally elevated regardless of age in endurance-trained individuals.

CONCLUSIONS

The results demonstrate that reduced insulin sensitivity is likely related to changes in adiposity and to physical inactivity rather than being an inevitable consequence of aging. The results also show that regular endurance exercise partly normalizes age-related mitochondrial dysfunction, although there are persisting effects of age on mtDNA abundance and expression of nuclear transcription factors and mitochondrial protein. Furthermore, exercise may promote longevity through pathways common to effects of caloric restriction.

Transportadores de glicose na síndrome metabólica

A regulação da homeostasia intra e extra-celular da glicose está diretamente relacionada ao controle preciso da expressão dos genes que codificam as diferentes isoformas de proteínas transportadoras de glicose, as quais se expressam de maneira tecido-específica, em conseqüência do padrão de ativação dos fatores transcricionais reguladores de cada gene, em cada tipo celular. A síndrome metabólica (SM) abrange uma grande variedade de alterações fisiopatológicas, todas de repercussões sistêmicas, acometendo os mais distintos territórios do organismo, nos quais alterações nos transportadores de glicose presentes são observadas em maior ou menor grau. A presente revisão abordará as alterações na expressão de transportadores de glicose claramente demonstradas na literatura, cujas repercussões nos fluxos territoriais de glicose auxiliam na compreensão de mecanismos fisiopatológicos da SM, assim como dos tratamentos propostos para esta entidade.

Aerobic training improves insulin sensitivity 72–120 h after the last exercise session in younger but not in older women

The regular practice of aerobic training (AT) induces an improvement in insulin sensitivity (IS) in healthy younger women that persists until 96-120 h after the last exercise bout. Due to the dearth of research data, it still remains unclear whether a regular AT program can improve IS for such a period of time after the last training bout in healthy older women. To address this issue, we trained 14 younger and 8 older women 3 days per week during 6 months, and measured IS 3-5 days after the last training bout. AT consisted of 25-60 min sessions of running at 60-95% of maximal heart rate. Fat mass decreased (8%) in older women only. VO(2max) and fat-free mass increased in both groups. Only older women decreased bodyweight (4%) and subcutaneous adipose tissue (19%). Visceral adipose tissue decreased in none of the groups. The IS improved only in younger women (relative: 22%; absolute: 24%). The present findings suggest that in older women the improvement in IS following AT is short-lived and results mainly from the acute effect of the last training bout, whereas in younger women the chronic adaptations induced by AT are implicated, as the improvement in IS is maintained beyond the residual effect of the last training bout. From a clinical standpoint, our findings suggest that in older women AT should be performed every day to improve glucose metabolism, whereas in younger women an AT frequency (three times per week) allowing to induce and maintain chronic minimal physiological adaptations would be required.

No Sustained Effect of Aerobic or Resistance Training on Insulin Sensitivity in Nonobese, Healthy Older Women

It is unclear whether long-term aerobic (AT) or resistance (RT) training can improve insulin sensitivity (IS) beyond the residual effect of the last training bout in older women (54–78 years). Therefore, a group of nonobese, healthy older women underwent 6 months of AT (n= 8) or RT (n= 10), and the authors measured IS 4 days after the last training bouts using the hyperinsulinemic-euglycemic clamp technique. Women trained 3 days/week. AT consisted of 25- to 60-min sessions of walking/jogging at 60–95% of maximal heart rate. RT consisted of three sets of nine exercises repeated 10 times at 80% of 1 repetition maximum. AT decreased fat mass, whereas both AT and RT increased fat-free mass. Neither training program, however, improved absolute or relative rates of glucose disposal. The authors therefore concluded that nonobese, healthy older women should perform AT or RT on a daily basis in order to improve IS and maintain the improvement.

Effect of the volume and intensity of exercise training on insulin sensitivity

Physical activity enhances insulin action in obese/overweight individuals. However, the exercise prescription required for the optimal enhancement is not known. The purpose of this study was to test the hypothesis that exercise training consisting of vigorous-intensity activity would enhance insulin sensitivity more substantially than moderate-intensity activity. Sedentary, overweight/obese subjects (n = 154) were randomly assigned to either control or an exercise group for 6 mo: 1) low-volume/moderate-intensity group [ approximately 12 miles walking/wk at 40-55% peak O2 consumption (Vo2 peak)], 2) low-volume/high-intensity group ( approximately 12 miles jogging/wk at 65-80% Vo2 peak), and 3) high-volume/high-intensity group ( approximately 20 miles jogging/wk at 65-80% Vo2 peak). Training volume (miles/wk) was achieved by exercising approximately 115 min/wk (low-volume/high-intensity group) or approximately 170 min/wk (low-volume/moderate-intensity and high-volume/high-intensity groups). Insulin action was measured with an insulin sensitivity index (SI) from an intravenous glucose tolerance test. In the control group, there was a decrement (P < 0.05) in SI. In contrast, all the exercise groups significantly (P < 0.05) increased SI; the relative increment in the low-volume/moderate-intensity and high-volume/high-intensity groups ( approximately 85%) were greater than in the low-volume/high-intensity group ( approximately 40%). In conclusion, physical activity encompassing a wide range of intensity and volume minimizes the insulin resistance that develops with a sedentary lifestyle. However, an exercise prescription that incorporated approximately 170 min of exercise/wk improved insulin sensitivity more substantially than a program utilizing approximately 115 min of exercise/wk, regardless of exercise intensity and volume. Total exercise duration should thus be considered when designing training programs with the intent of improving insulin action.

Skeletal muscle mitochondrial protein metabolism and function in ageing and type 2 diabetes

PURPOSE OF REVIEW

Mitochondria are the site of oxidative substrate utilization to produce adenosine triphosphate for normal tissue function. Tissue substrate utilization is impaired in ageing and type 2 diabetes. Defects in mitochondrial gene expression, protein synthesis and function occur with ageing in various tissues including skeletal muscle, and are emerging in individuals with type 2 diabetes. The current review will discuss advances in the understanding of skeletal muscle mitochondrial alterations associated with age and type 2 diabetes.

RECENT FINDINGS

Insulin acutely stimulates skeletal muscle mitochondrial protein synthesis and adenosine triphosphate production. These insulin effects are impaired in insulin-resistant patients with type 2 diabetes who also exhibit defective basal muscle mitochondrial function. The age-related reduction in mitochondrial adenosine triphosphate production has been confirmed in vivo in skeletal muscle in humans and rodents.

SUMMARY

The emerging concept that insulin stimulates mitochondrial protein synthesis and function indicates potential novel molecular mechanisms of metabolic defects in type 2 diabetes, particularly in the post-prandial period characterized by acute increments of plasma insulin concentrations. The potential relationship between insulin resistance and basal post-absorptive muscle mitochondrial defects should be further investigated. As ageing is characterized by insulin resistance, the hypothesis that impaired insulin action could contribute to age-related muscle mitochondrial dysfunction, and metabolic alterations should be addressed.

Serum levels of insulin-like growth factor-I (IGF-I), and IGF-binding proteins-1 and -3 in middle-aged and young athletes versus sedentary men: relationship with glucose disposal

The goal of this study was to characterize the respective effects of aging and endurance training on serum insulin-like growth factor I (IGF-I), as well as IGF-binding proteins (IGFBP)-1 and -3 in relationship with glucose disposal. Thirty-two subjects (16 middle-aged men: 8 cyclists and 8 sedentary men; and 16 young men: 8 cyclists and 8 sedentary men) were compared in this study. Insulin sensitivity (SI) and glucose effectiveness (Sg) were assessed by the minimal model. Endurance training increased SI, Sg, and IGFBP-1 and -3 in both age groups (P<.05), but the older group showed a greater increase in SI and IGFBP-1 than the younger group (P<.05). IGF-I was increased only in the middle-aged trained men (P<.05). An effect of aging was found in the sedentary subjects, who presented lower IGF-I and SI (P<.05) when older. This effect disappeared with training since IGF-I and SI were nearly identical in young and middle-aged trained subjects. SI was correlated with IGFBP-1 (P<.01). These data suggest that (1) endurance training increases SI, Sg, and IGFBP-1 and -3 in men and, for SI and IGFBP-1, this increase becomes more pronounced with age; (2) endurance training may attenuate the aged-related decline in SI and IGF-I; and (3) IGFBP-1 may protect against the risk of hypoglycemia by counteracting the hypoglycemic effect of IGF-I in such situations of high SI.

Decline in insulin action with age in endurance-trained humans

We tested the hypothesis that regular endurance exercise prevents the age-related decline in insulin action typically observed in healthy, sedentary adults. An index of whole body insulin sensitivity (ISI), obtained from minimal model analysis of insulin and glucose concentrations during a frequently sampled intravenous glucose tolerance test, was determined in 126 healthy adults: 25 young [27 +/- 1 (SE) yr; 13 men/12 women] and 43 older (59 +/- 1 yr; 20/13) sedentary and 25 young (29 +/- 1 yr; 12/13) and 33 older (60 +/- 1 yr; 20/13) endurance trained. ISI values were lower in the older vs. young adults in both sedentary (-53%; 3.9 +/- 0.3 vs. 7.0 +/- 0.7 x10(-4) x min(-1) x microU(-1) x ml(-1); P < 0.01) and endurance-trained (-36%; 7.9 +/- 0.6 vs. 12.4 +/- 1.0 x 10(-4) min(-1) x microU(-1) x ml(-1); P < 0.01) groups, but the value was 72-102% higher in the trained subjects at either age (P < 0.01). In subgroup analysis of sedentary and endurance-trained adults with similar body fat levels (n = 62), the age-related reduction in ISI persisted only in the endurance-trained subjects (12.9 +/- 1.9 vs. 8.7 +/- 1.2 x 10(-4) x min(-1) x microU(-1) x ml(-1); P < 0.01). The results of the present study suggest that habitual endurance exercise does not prevent the age-associated decline insulin action. Moreover, the age-related reduction in ISI in endurance-trained adults appears to be independent of adiposity.

Exercise-induced transcription of the muscle glucose transporter (GLUT 4) gene

We studied the effects of exercise on GLUT4 gene transcription in several lines of transgenic mice expressing the chloramphenicol acyltransferase (CAT) reporter gene, driven by various lengths of the human GLUT4 promoter (2400, 1600, 895, and 730 bp). In all transgenic lines examined, endogenous GLUT4 mRNA increased in response to exercise (19-90%, P < 0.05). Exercise increased CAT mRNA (51-83%, P < 0.05) in mice when the transgene was driven by at least 895 bp of the promoter but showed no effect in mice in which the transgene was driven by only 730 bp. These results suggest that the exercise-induced increase in the transcriptional activity of the human GLUT4 gene is mediated, at least in part, by element(s) within -895 bp of the promoter. These observations reveal a striking similarity to the time course and regional promoter requirements of AMPK-induced GLUT4 gene expression, providing further evidence that AMPK may be mediating the effects of exercise on GLUT4 expression.

Fuel oxidation during exercise in middle-aged men: role of training and glucose disposal

PURPOSE

The purpose of this study was to test the hypothesis that carbohydrate (CHO) utilization in middle-aged trained men is increased during hard-intensity exercise and decreased during moderate-intensity exercise in comparison with age-matched sedentary men. We also investigated whether a relationship between CHO utilization and glucose disposal exists.

METHODS

Seven trained cyclists (Tr) and seven age-matched sedentary men (Sed) underwent an intravenous glucose tolerance test after an overnight fast (minimal model method) to determine their glucose disposal; they also performed two 1-h trials on a cycle ergometer below and above their individual ventilatory threshold (VT). Substrate oxidation was evaluated by indirect calorimetry. Hormonal responses were investigated during exercise.

RESULTS

Insulin sensitivity (SI) and glucose effectiveness (Sg) were significantly higher in the Tr group than in the Sed group (P < 0.001, P < 0.03). CHO oxidation was significantly higher in the Tr group than in the Sed group when exercise was performed above VT, whereas CHO oxidation was higher in the Sed group when exercise was performed below VT (P < 0.05). Epinephrine (Epi) response during hard-intensity exercise was higher in the Tr group than in the Sed group (P < 0.01). SI was negatively correlated to CHO oxidation in the Tr group (r = -0.743, P < 0.05).

CONCLUSION

Endurance training results in increased CHO utilization during hard-intensity exercise and reduced CHO oxidation during moderate-intensity exercise in middle-aged men. During hard-intensity exercise, the increased CHO utilization in middle-aged trained men is associated with a greater response in Epi and is inversely related with SI.

Effect of aging and obesity on insulin responsiveness and glut-4 glucose transporter content in skeletal muscle of Fischer 344 x Brown Norway rats

This study investigated the metabolic changes with age in the Fischer 344 x Brown Norway rat and its suitability as an animal model of postmaturational insulin resistance. Specifically, we determined whether an age-associated decrease in glucose disposal is associated with diminished whole body insulin responsiveness and/or a decrease in glucose transporter (GLUT-4) protein and mRNA content in medial gastrocnemius muscle of male Fischer 344 x Brown Norway rats of ages 8, 18, and 28 months. Fasting plasma glucose was unchanged with age. There was a significant age effect on visceral adiposity, fasting plasma insulin levels, insulin responsiveness, and GLUT-4 protein content. Insulin responsiveness and GLUT-4 protein were lower in the 18-month-old rats than in the 8-month-old rats. The findings of age-associated increases in visceral adiposity and insulin resistance, and decreases in GLUT-4 in the Fisher 344 x Brown Norway rat, suggest that this rat strain may be an appropriate model for studying the effects of aging on glucose homeostasis.

GLUT4 protein expression in obese and lean 12-month-old rats: insights from different types of data analysis

GLUT4 protein expression in white adipose tissue (WAT) and skeletal muscle (SM) was investigated in 2-month-old, 12-month-old spontaneously obese or 12-month-old calorie-restricted lean Wistar rats, by considering different parameters of analysis, such as tissue and body weight, and total protein yield of the tissue. In WAT, an approximately 70% decrease was observed in plasma membrane and microsomal GLUT4 protein, expressed as microg protein or g tissue, in both 12-month-old obese and 12-month-old lean rats compared to 2-month-old rats. However, when plasma membrane and microsomal GLUT4 tissue contents were expressed as g body weight, they were the same. In SM, GLUT4 protein content, expressed as microg protein, was similar in 2-month-old and 12-month-old obese rats, whereas it was reduced in 12-month-old obese rats, when expressed as g tissue or g body weight, which may play an important role in insulin resistance. Weight loss did not change the SM GLUT4 content. These results show that altered insulin sensitivity is accompanied by modulation of GLUT4 protein expression. However, the true role of WAT and SM GLUT4 contents in whole-body or tissue insulin sensitivity should be determined considering not only GLUT4 protein expression, but also the strong morphostructural changes in these tissues, which require different types of data analysis.

Regulation of muscle GLUT-4 transcription by AMP-activated protein kinase

Skeletal muscle GLUT-4 transcription in response to treatment with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR), a known activator of AMP-activated protein kinase (AMPK), was studied in rats and mice. The increase in GLUT-4 mRNA levels in response to a single subcutaneous injection of AICAR, peaked at 13 h in white and red quadriceps muscles but not in the soleus muscle. The mRNA level of chloramphenicol acyltransferase reporter gene which is driven by 1,154 or 895 bp of the human GLUT-4 proximal promoter was increased in AICAR-treated transgenic mice, demonstrating the transcriptional upregulation of the GLUT-4 gene by AICAR. However, this induction of transcription was not apparent with 730 bp of the promoter. In addition, nuclear extracts from AICAR-treated mice bound to the consensus sequence of myocyte enhancer factor-2 (from -473 to -464) to a greater extent than from saline-injected mice. Thus AMP-activated protein kinase activation by AICAR increases GLUT-4 transcription by a mechanism that requires response elements within 895 bp of human GLUT-4 proximal promoter and that may be cooperatively mediated by myocyte enhancer factor-2.

Insulin resistance with aging: effects of diet and exercise

Insulin resistance, a reduction in the rate of glucose disposal elicited by a given insulin concentration, is present in individuals who are obese, and those with diabetes mellitus, and may develop with aging. Methods which are utilised to measure insulin sensitivity include the hyperinsulinaemic-euglycaemic and hyperglycaemic clamps and the intravenous glucose tolerance tests. Several hormones and regulatory factors affect insulin action and may contribute to the insulin resistance observed in obesity. In addition, abnormal free fatty acid metabolism plays an important role in insulin resistance and the abnormal carbohydrate metabolism seen in individuals who are obese or diabetic. Thus, the mechanisms underlying the development of insulin resistance are multifactorial, and also involve alterations of the insulin signalling pathway. Aging is associated with an increase in bodyweight and fat mass. Not only is abdominal fat associated with hyperinsulinaemia but visceral adiposity is correlated with insulin resistance as well. Modifications of the changes in body composition with aging by diet and exercise training could delay the onset of insulin resistance. Weight loss and aerobic and resistive exercise training result in losses of total body fat and abdominal fat. Several studies report that bodyweight loss increases insulin sensitivity and improves glucose tolerance. In addition, the insulin resistance observed in aged persons can be modified by physical training. Longitudinal studies indicate significant improvements in glucose metabolism with aerobic exercise training in middle-aged and older men and women. Moreover, the improvements in insulin sensitivity with resistive training are similar in magnitude to those achieved with aerobic exercise. The improvements in glucose metabolism after bodyweight loss and exercise training may in some cases be partially attributed to changes in body composition, including reductions in total and central body fat. Yet, additional changes in skeletal muscle, blood flow and other mechanisms likely interact to modify insulin resistance with exercise training. Lifestyle modifications including bodyweight loss and physical activity provide health benefits and functional gains and should be promoted to increase insulin sensitivity and prevent glucose intolerance and type 2 diabetes mellitus in older adults.