Effects of treadmill exercise to exhaustion on the insulin response to hyperglycemia in untrained men

GDF15 Mediates the Effect of Skeletal Muscle Contraction on Glucose-Stimulated Insulin Secretion

Exercise is a first-line treatment for type 2 diabetes and preserves β-cell function by hitherto unknown mechanisms. We postulated that proteins from contracting skeletal muscle may act as cellular signals to regulate pancreatic β-cell function. We used electric pulse stimulation (EPS) to induce contraction in C2C12 myotubes and found that treatment of β-cells with EPS-conditioned medium enhanced glucose-stimulated insulin secretion (GSIS). Transcriptomics and subsequent targeted validation revealed growth differentiation factor 15 (GDF15) as a central component of the skeletal muscle secretome. Exposure to recombinant GDF15 enhanced GSIS in cells, islets, and mice. GDF15 enhanced GSIS by upregulating the insulin secretion pathway in β-cells, which was abrogated in the presence of a GDF15 neutralizing antibody. The effect of GDF15 on GSIS was also observed in islets from GFRAL-deficient mice. Circulating GDF15 was incrementally elevated in patients with pre- and type 2 diabetes and positively associated with C-peptide in humans with overweight or obesity. Six weeks of high-intensity exercise training increased circulating GDF15 concentrations, which positively correlated with improvements in β-cell function in patients with type 2 diabetes. Taken together, GDF15 can function as a contraction-induced protein that enhances GSIS through activating the canonical signaling pathway in a GFRAL-independent manner.

ARTICLE HIGHLIGHTS

Exercise improves glucose-stimulated insulin secretion through direct interorgan communication. Contracting skeletal muscle releases growth differentiation factor 15 (GDF15), which is required to synergistically enhance glucose-stimulated insulin secretion. GDF15 enhances glucose-stimulated insulin secretion by activating the canonical insulin release pathway. Increased levels of circulating GDF15 after exercise training are related to improvements in β-cell function in patients with type 2 diabetes.

Exercise and inactivity as modifiers of β cell function and type 2 diabetes risk

Exercise and regular physical activity are beneficial for the prevention and management of metabolic diseases such as obesity and type 2 diabetes, whereas exercise cessation, defined as deconditioning from regular exercise or physical activity that has lasted for a period of months to years, can lead to metabolic derangements that drive disease. Adaptations to the insulin-secreting pancreatic β-cells are an important benefit of exercise, whereas less is known about how exercise cessation affects these cells. Our aim is to review the impact that exercise and exercise cessation have on β-cell function, with a focus on the evidence from studies examining glucose-stimulated insulin secretion (GSIS) using gold-standard techniques. Potential mechanisms by which the β-cell adapts to exercise, including exerkine and incretin signaling, autonomic nervous system signaling, and changes in insulin clearance, will also be explored. We will highlight areas for future research.

Exercise Training Rapidly Increases Hepatic Insulin Extraction in NAFLD

PURPOSE

We aimed to determine the immediacy of exercise intervention on liver-specific metabolic processes in nonalcoholic fatty liver disease.

METHODS

We undertook a short-term (7-d) exercise training study (60 min·d treadmill walking at 80%-85% of maximal heart rate) in obese adults (N = 13, 58 ± 3 yr, 34.3 ± 1.1 kg·m, >5% hepatic lipid by H-magnetic resonance spectroscopy). Insulin sensitivity index was estimated by oral glucose tolerance test using the Soonthorpun model. Hepatic insulin extraction (HIE) was calculated as the molar difference in area under the curve (AUC) for insulin and C-peptide (HIE = 1 - (AUCInsulin/AUCC-Pep)).

RESULTS

The increases in HIE, V˙O2max, and insulin sensitivity index after the intervention were 9.8%, 9.8%, and 34%, respectively (all, P < 0.05). Basal fat oxidation increased (pre: 47 ± 6 mg·min vs post: 65 ± 6 mg·min, P < 0.05) and carbohydrate oxidation decreased (pre: 160 ± 20 mg·min vs post: 112 ± 15 mg·min, P < 0.05) with exercise training. After the intervention, HIE correlated positively with adiponectin (r = 0.56, P < 0.05) and negatively with TNF-α (r = -0.78, P < 0.001).

CONCLUSIONS

By increasing HIE along with peripheral insulin sensitivity, aerobic exercise training rapidly reverses some of the underlying physiological mechanisms associated with nonalcoholic fatty liver disease, in a weight loss-independent manner. This reversal could potentially act through adipokine-related pathways.

The historical context and scientific legacy of John O. Holloszy

John O. Holloszy, as perhaps the world's preeminent exercise biochemist/physiologist, published >400 papers over his 50+ year career, and they have been cited >41,000 times. In 1965 Holloszy showed for the first time that exercise training in rodents resulted in a doubling of skeletal muscle mitochondria, ushering in a very active era of skeletal muscle plasticity research. He subsequently went on to describe the consequences of and the mechanisms underlying these adaptations. Holloszy was first to show that muscle contractions increase muscle glucose transport independent of insulin, and he studied the mechanisms underlying this response throughout his career. He published important papers assessing the impact of training on glucose and insulin metabolism in healthy and diseased humans. Holloszy was at the forefront of rodent studies of caloric restriction and longevity in the 1980s, following these studies with important cross-sectional and longitudinal caloric restriction studies in humans. Holloszy was influential in the discipline of cardiovascular physiology, showing that older healthy and diseased populations could still elicit beneficial cardiovascular adaptations with exercise training. Holloszy and his group made important contributions to exercise physiology on the effects of training on numerous metabolic, hormonal, and cardiovascular adaptations. Holloszy's outstanding productivity was made possible by his mentoring of ~100 postdoctoral fellows and substantial NIH grant funding over his entire career. Many of these fellows have also played critical roles in the exercise physiology/biochemistry discipline. Thus it is clear that exercise biochemistry and physiology will be influenced by John Holloszy for numerous years to come.

Improvement of obesity-linked skeletal muscle insulin resistance by strength and endurance training

Obesity-linked insulin resistance is mainly due to fatty acid overload in non-adipose tissues, particularly skeletal muscle and liver, where it results in high production of reactive oxygen species and mitochondrial dysfunction. Accumulating evidence indicates that resistance and endurance training alone and in combination can counteract the harmful effects of obesity increasing insulin sensitivity, thus preventing diabetes. This review focuses the mechanisms underlying the exercise role in opposing skeletal muscle insulin resistance-linked metabolic dysfunction. It is apparent that exercise acts through two mechanisms: (1) it stimulates glucose transport by activating an insulin-independent pathway and (2) it protects against mitochondrial dysfunction-induced insulin resistance by increasing muscle antioxidant defenses and mitochondrial biogenesis. However, antioxidant supplementation combined with endurance training increases glucose transport in insulin-resistant skeletal muscle in an additive fashion only when antioxidants that are able to increase the expression of antioxidant enzymes and/or the activity of components of the insulin signaling pathway are used.

Acute Exercise Improves Insulin Clearance and Increases the Expression of Insulin-Degrading Enzyme in the Liver and Skeletal Muscle of Swiss Mice

The effects of exercise on insulin clearance and IDE expression are not yet fully elucidated. Here, we have explored the effect of acute exercise on insulin clearance and IDE expression in lean mice. Male Swiss mice were subjected to a single bout of exercise on a speed/angle controlled treadmill for 3-h at approximately 60-70% of maximum oxygen consumption. As expected, acute exercise reduced glycemia and insulinemia, and increased insulin tolerance. The activity of AMPK-ACC, but not of IR-Akt, pathway was increased in the liver and skeletal muscle of trained mice. In an apparent contrast to the reduced insulinemia, glucose-stimulated insulin secretion was increased in isolated islets of these mice. However, insulin clearance was increased after acute exercise and was accompanied by increased expression of the insulin-degrading enzyme (IDE), in the liver and skeletal muscle. Finally, C2C12, but not HEPG2 cells, incubated at different concentrations of 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) for 3-h, showed increased expression of IDE. In conclusion, acute exercise increases insulin clearance, probably due to an augmentation of IDE expression in the liver and skeletal muscle. The elevated IDE expression, in the skeletal muscle, seems to be mediated by activation of AMPK-ACC pathway, in response to exercise. We believe that the increase in the IDE expression, comprise a safety measure to maintain glycemia at or close to physiological levels, turning physical exercise more effective and safe.

Genetic variation and exercise-induced muscle damage: implications for athletic performance, injury and ageing

Prolonged unaccustomed exercise involving muscle lengthening (eccentric) actions can result in ultrastructural muscle disruption, impaired excitation-contraction coupling, inflammation and muscle protein degradation. This process is associated with delayed onset muscle soreness and is referred to as exercise-induced muscle damage. Although a certain amount of muscle damage may be necessary for adaptation to occur, excessive damage or inadequate recovery from exercise-induced muscle damage can increase injury risk, particularly in older individuals, who experience more damage and require longer to recover from muscle damaging exercise than younger adults. Furthermore, it is apparent that inter-individual variation exists in the response to exercise-induced muscle damage, and there is evidence that genetic variability may play a key role. Although this area of research is in its infancy, certain gene variations, or polymorphisms have been associated with exercise-induced muscle damage (i.e. individuals with certain genotypes experience greater muscle damage, and require longer recovery, following strenuous exercise). These polymorphisms include ACTN3 (R577X, rs1815739), TNF (-308 G>A, rs1800629), IL6 (-174 G>C, rs1800795), and IGF2 (ApaI, 17200 G>A, rs680). Knowing how someone is likely to respond to a particular type of exercise could help coaches/practitioners individualise the exercise training of their athletes/patients, thus maximising recovery and adaptation, while reducing overload-associated injury risk. The purpose of this review is to provide a critical analysis of the literature concerning gene polymorphisms associated with exercise-induced muscle damage, both in young and older individuals, and to highlight the potential mechanisms underpinning these associations, thus providing a better understanding of exercise-induced muscle damage.

Role of oxidative stress in impaired insulin signaling associated with exercise-induced muscle damage

Skeletal muscle is a major tissue that utilizes blood glucose. A single bout of exercise improves glucose uptake in skeletal muscle through insulin-dependent and insulin-independent signal transduction mechanisms. However, glucose utilization is decreased in muscle damage induced by acute, unaccustomed, or eccentric exercise. The decrease in glucose utilization is caused by decreased insulin-stimulated glucose uptake in damaged muscles with inhibition of the membrane translocation of glucose transporter 4 through phosphatidyl 3-kinase/Akt signaling. In addition to inflammatory cytokines, reactive oxygen species including 4-hydroxy-2-nonenal and peroxynitrate can induce degradation or inactivation of signaling proteins through posttranslational modification, thereby resulting in a disturbance in insulin signal transduction. In contrast, treatment with factors that attenuate oxidative stress in damaged muscle suppresses the impairment of insulin sensitivity. Muscle-damaging exercise may thus lead to decreased endurance capacity and muscle fatigue in exercise, and it may decrease the efficiency of exercise therapy for metabolic improvement.

Study in Parkinson disease of exercise (SPARX): translating high-intensity exercise from animals to humans

A burgeoning literature suggests that exercise has a therapeutic benefit in persons with Parkinson disease (PD) and in animal models of PD, especially when animals exercise at high intensity. If exercise is to be prescribed as "first-line" or "add-on" therapy in patients with PD, we must demonstrate its efficacy and dose-response effects through testing phases similar to those used in the testing of pharmacologic agents. The SPARX Trial is a multicenter, randomized, controlled, single-blinded, Phase II study that we designed to test the feasibility of using high-intensity exercise to modify symptoms of PD and to simultaneously test the nonfutility of achieving a prespecified change in patients' motor scores on the Unified Parkinson Disease Rating Scale (UPDRS). The trial began in May 2102 and is in the process of screening, enrolling, and randomly assigning 126 patients with early-stage PD to 1 of 3 groups: usual care (wait-listed controls), moderate-intensity exercise (4 days/week at 60%-65% maximal heart rate [HRmax]), or high-intensity exercise (4 days/week at 80%-85% HRmax). At 6-month follow-up, the trial is randomly reassigning usual care participants to a moderate-intensity or high-intensity exercise group for the remaining 6 months. The goals of the Phase II trial are to determine if participants can exercise at moderate and high intensities; to determine if either exercise yields benefits consistent with meaningful clinical change (nonfutility); and to document safety and attrition. The advantage of using a non-futility approach allows us to efficiently determine if moderate- or high-intensity exercise warrants further large-scale investigation in PD.

Metabolic consequences of exercise-induced muscle damage

Exercise-induced muscle damage (EIMD) is commonly experienced following either a bout of unaccustomed physical activity or following physical activity of greater than normal duration or intensity. The mechanistic factor responsible for the initiation of EIMD is not known; however, it is hypothesised to be either mechanical or metabolic in nature. The mechanical stress hypothesis states that EIMD is the result of physical stress upon the muscle fibre. In contrast, the metabolic stress model predicts that EIMD is the result of metabolic deficiencies, possibly through the decreased action of Ca(2+)-adenosine triphosphatase. Irrespective of the cause of the damage, EIMD has a number of profound metabolic effects. The most notable metabolic effects of EIMD are decreased insulin sensitivity, prolonged glycogen depletion and an increase in metabolic rate both at rest and during exercise. Based on current knowledge regarding the effects that various types of damaging exercise have on muscle metabolism, a new model for the initiation of EIMD is proposed. This model states that damage initiation may be either metabolic or mechanical, or a combination of both, depending on the mode, intensity and duration of exercise and the training status of the individual.

Effects of a single bout of exercise on resting heart rate variability

PURPOSE

Chronic exercise training has been shown to have a positive influence on cardiac autonomic function as assessed by measures of heart rate variability (HRV). Recent evidence indicates that several benefits associated with exercise training (e.g., improved insulin action, reduced blood pressure, improved blood lipid profile) may be realized transiently after a single bout of exercise. As many of these effects of recent exercise are linked to cardiovascular control systems, the purpose of this investigation was to test the hypothesis that a single bout of exercise would result in favorable changes in cardiac autonomic function as assessed by frequency-domain measures of HRV.

METHODS

Subjects were 11 healthy male volunteers ages 18-35 yr. Resting HRV measures were obtained during 5 min of paced breathing before and 1, 3, 6, and 22 h after a 60-min bout of cycling exercise at approximately 65% of peak oxygen uptake. Identical measures were obtained in a nonexercise condition based on a randomized crossover design.

RESULTS

Exercise resulted in increased high-frequency HRV, decreased low-frequency HRV, and consequently a decrease in the ratio of low-frequency to high-frequency HRV compared with the nonexercise condition. Additionally, a time-domain measure of HRV (pNN50) was markedly higher in the exercise condition as compared to nonexercise.

CONCLUSION

The changes in cardiac autonomic function observed are similar to those seen in investigations of long-term training. These changes indicate a shift in autonomic function toward increased parasympathetic nervous system activity and decreased sympathetic nervous system activity, suggesting a more stable autonomic environment for the heart. These results may provide further evidence of the cardioprotective effects of a single bout of submaximal exercise.

Effect of insulin and contraction up on glucose transport in skeletal muscle

The major glucose transporter protein expressed in skeletal muscle is GLUT4. Both muscle contraction and insulin induce translocation of GLUT4 from the intracellular pool to the plasma membrane. The intracellular pathways that lead to contraction- and insulin-stimulated GLUT4 translocation seem to be different, allowing the attainment of a maximal effect when acting together. Insulin utilizes a phosphatidylinositol 3-kinase-dependent mechanism, whereas the exercise signal may be initiated by calcium release from the sarcoplasmic reticulum or from autocrine- or paracrine-mediated activation of glucose transport. During exercise skeletal muscle utilizes more glucose than when at rest. However, endurance training leads to decreased glucose utilization during sub-maximal exercise, in spite of a large increase in the total GLUT4 content associated with training. The mechanisms involved in this reduction have not been totally elucidated, but appear to cause the decrease of the amount of GLUT4 translocated to the plasma membrane by altering the exercise-induced enhancement of glucose transport capacity. On the other hand, the effect of resistance training is controversial. Recent studies, however, demonstrated the improvement in insulin sensitivity correlated with increasing muscle mass. New studies should be designed to define the molecular basis for these important adaptations to skeletal muscle. Since during exercise the muscle may utilize insulin-independent mechanisms to increase glucose uptake, the mechanisms involved should provide important knowledge to the understanding and managing peripheral insulin resistance.

Impaired substrate oxidation in healthy elderly men after eccentric exercise

The metabolic response to eccentric exercise in healthy older adults is unknown. Therefore, substrate metabolism was examined in the basal state and after sustained hyperglycemia (180 min, 10 mM) in eight healthy, sedentary older [66 +/- 2 yr; body mass index (BMI) of 25.5 +/- 1.2 kg/m] and nine younger (23 +/- 1 yr; BMI of 23.6 +/- 1.7 kg/m) men, under control conditions and 48 h after eccentric exercise. Indirect calorimetry was performed to evaluate carbohydrate and lipid oxidation (C(ox) and L(ox), respectively). Eccentric exercise caused muscle soreness and increased plasma creatine kinase in both groups of men (P < 0.02). Although a similar level of hyperglycemia was maintained in the two groups, glucose infusion rates were lower (P < 0.001) in the older men. Compared with basal levels, hyperglycemia stimulated an increase in C(ox) and a decrease in L(ox) during the control and exercise trials in the younger group (P < 0.03), but only during the control trial in the older subjects (P < 0.007). C(ox) was unchanged after eccentric exercise in the younger men [4.00 +/- 0.30 vs. 3.54 +/- 0.44 mg x kg fat-free mass (FFM)(-1) x min(-1); exercise vs. control] but was suppressed by 20% in the older group (3.37 +/- 0.37 vs. 4.21 +/- 0.23 mg x kg FFM(-1) x min(-1); P < 0.04). Moreover, L(ox) was reduced by 38% in the younger subjects (0.47 +/- 0.09 vs. 0.76 +/- 0.10 mg x kg FFM(-1) x min(-1); P< 0.03) but was augmented by 89% in the older group (0.68 +/- 0.11 vs. 0.36 +/- 0.08 mg x kg FFM(-1) x min(-1); P < 0.04). In addition, hyperglycemia-stimulated C(ox), L(ox), and respiratory exchange ratio responses to eccentric exercise were related to abdominal adiposity (r = -0.57, P < 0.04, r = 0.68, P < 0.02 and r = -0.60, P < 0.02, respectively). Despite normal glucose tolerance and the absence of obesity per se, older men experience a reduction in carbohydrate oxidation in response to hyperglycemia after eccentric exercise.

Sequential hyperglycemic-euglycemic clamp to assess beta-cell and peripheral tissue: studies in female athletes

Insulin secretion and rate of utilization (R(d)) of glucose were tested during a newly developed sequential clamp in 42 highly trained female athletes (A; 18-69 yr old) and 14 sedentary control women (C; 18--50 yr old; body mass index <25 kg/m(2)). The A women were categorized into four age groups: 18--29, 30--39, 40--49, and 50--69 yr old. The C women were also grouped by age (18--29 and 40--50 yr old). During the three-step clamp (hyperglycemia, return to euglycemia, and hyperinsulinemia), glucose turnover was assessed with [3-(3)H]glucose. Among the A, the youngest group had the largest first- and second-phase insulin response, which was significantly different from the oldest A (P < 0.05). Among the two C groups, first-phase response of both groups and second-phase response of the older group was higher than respective age-matched A (P < 0.05). During the hyperglycemic period, glucose R(d) was similar among A groups and between A and C. Despite similar levels of insulin between groups during the hyperinsulinemic period (approximately 400 pmol/l), A utilized 36% more glucose than C (P < 0.001). Glucose R(d) was not different across the age groups of A. This newly developed sequential clamp procedure allows assessment of both beta-cell sensitivity to glucose and peripheral tissue sensitivity to insulin in a single session. We have shown that physical activity improves beta-cell efficiency across the age span in women and ameliorates the effect of age on the decline of peripheral tissue sensitivity to insulin.

Muscle damage impairs insulin stimulation of IRS-1, PI 3-kinase, and Akt-kinase in human skeletal muscle

Physiological stress associated with muscle damage results in systemic insulin resistance. However, the mechanisms responsible for the insulin resistance are not known; therefore, the present study was conducted to elucidate the molecular mechanisms associated with insulin resistance after muscle damage. Muscle biopsies were obtained before (base) and at 1 h during a hyperinsulinemic-euglycemic clamp (40 mU x kg(-1) x min(-1)) in eight young (age 24+/-1 yr) healthy sedentary (maximal O(2) consumption, 49.7+/-2.4 ml x kg(-1) x min(-1)) males before and 24 h after eccentric exercise (ECC)-induced muscle damage. To determine the role of cytokines in ECC-induced insulin resistance, venous blood samples were obtained before (control) and 24 h after ECC to evaluate ex vivo endotoxin-induced mononuclear cell secretion of tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, and IL-1beta. Glucose disposal was 19% lower after ECC (P<0.05). Insulin-stimulated insulin receptor substrate (IRS)-1 tyrosine phosphorylation was 45% lower after ECC (P<0.05). Insulin-stimulated phosphatidylinositol (PI) 3-kinase, Akt (protein kinase B) serine phosphorylation, and Akt activity were reduced 34, 65, and 20%, respectively, after ECC (P < 0.05). TNF-alpha, but not IL-6 or IL-1beta production, increased 2.4-fold 24 h after ECC (P<0.05). TNF-alpha production was positively correlated with reduced insulin action on PI 3-kinase (r = 0.77, P = 0.04). In summary, the physiological stress associated with muscle damage impairs insulin stimulation of IRS-1, PI 3-kinase, and Akt-kinase, presumably leading to decreased insulin-mediated glucose uptake. Although more research is needed on the potential role for TNF-alpha inhibition of insulin action, elevated TNF-alpha production after muscle damage may impair insulin signal transduction.

TNF-alpha impairs insulin signaling and insulin stimulation of glucose uptake in C2C12 muscle cells

Physiological stressors such as sepsis and tissue damage initiate an acute immune response and cause transient systemic insulin resistance. This study was conducted to determine whether tumor necrosis factor-alpha (TNF-alpha), a cytokine produced by immune cells during skeletal muscle damage, decreases insulin responsiveness at the cellular level. To examine the molecular mechanisms associated with TNF-alpha and insulin action, we measured insulin receptor substrate (IRS)-1- and IRS-2-mediated phosphatidylinositol 3-kinase (PI 3-kinase) activation, IRS-1-PI 3-kinase binding, IRS-1 tyrosine phosphorylation, and the phosphorylation of two mitogen-activated protein kinases (MAPK, known as p42(MAPK) and p44(MAPK)) in cultured C2C12 myotubes. Furthermore, we determined the effects of TNF-alpha on insulin-stimulated 2-deoxyglucose (2-DG) uptake. We observed that TNF-alpha impaired insulin stimulation of IRS-1- and IRS-2-mediated PI 3-kinase activation by 54 and 55% (P < 0.05), respectively. In addition, TNF-alpha decreased insulin-stimulated IRS-1 tyrosine phosphorylation by 40% (P < 0.05). Furthermore, TNF-alpha repressed insulin-induced p42(MAPK) and p44(MAPK) tyrosine phosphorylation by 81% (P < 0.01). TNF-alpha impairment of insulin signaling activation was accompanied by a decrease (P < 0.05) in 2-DG uptake in the muscle cells (60 +/- 4 vs. 44 +/- 6 pmol. min-1. mg-1). These data suggest that increases in TNF-alpha may cause insulin resistance in skeletal muscle by inhibiting IRS-1- and IRS-2-mediated PI 3-kinase activation as well as p42(MAPK) and p44(MAPK) tyrosine phosphorylation, leading to impaired insulin-stimulated glucose uptake.

Age-related differences in the pancreatic beta-cell response to hyperglycemia after eccentric exercise

Eccentric exercise (ECC) causes muscle damage, insulin resistance, and increased pancreatic beta-cell secretion in young individuals. However, the effects of age on the pancreatic beta-cell response to glucose after ECC are unknown. Hyperglycemic clamps (180 min, 10.0 mM) were performed on eight young (age 22 +/- 1 yr) and eight older (age 66 +/- 2 yr) healthy sedentary males without exercise (CONT) and 48 h after ECC. ECC increased (P < 0.02) muscle soreness ratings and plasma creatine kinase concentrations in both groups. Insulin and C-peptide secretions were similar between young and older subjects during CONT clamps. ECC increased (P < 0.05) first-phase (0-10 min) C-peptide area under the curve in young (4.2 +/- 0.4 vs. 3.7 +/- 0.6 nM . min; ECC vs. CONT, respectively) but not in older subjects (3.2 +/- 0.7 vs. 3.5 +/- 0.7 nM . min; ECC vs. CONT), with significant group differences (P < 0.02). Indeed, ECC repressed (P < 0.05) first-phase peak C-peptide concentrations in older subjects (0. 93 +/- 0.16 vs. 1.12 +/- 0.11 nM; ECC vs. CONT). Moreover, first-phase C-peptide-to-insulin molar ratios suggest age-related differences (P < 0.05) in insulin/C-peptide clearance after ECC. Furthermore, the observed C-peptide response after ECC was related to abdominal adiposity [r = -0.62, P < 0.02, and r = -0.66, P < 0. 006, for first and second (10-180 min) phases, respectively]. In conclusion, older individuals did not exhibit the compensatory increase in beta-cell secretion observed among young individuals after ECC. Thus, with increasing age, the pancreatic beta-cell may be less responsive to the physiological stress associated with ECC.

Eccentric exercise decreases maximal insulin action in humans: muscle and systemic effects

1. Unaccustomed eccentric exercise decreases whole-body insulin action in humans. To study the effects of one-legged eccentric exercise on insulin action in muscle and systemically, the euglycaemic clamp technique combined with arterial and bilateral femoral venous catheterization was used. Seven subjects participated in two euglycaemic clamps, performed in random order. One clamp was preceded 2 days earlier by one-legged eccentric exercise (post-eccentric exercise clamp (PEC)) and one was without the prior exercise (control clamp (CC)). 2. During PEC the maximal insulin-stimulated glucose uptake over the eccentric thigh was marginally lower when compared with the control thigh, (11.9%, 64.6 +/- 10.3 vs. 73.3 +/- 10.2 mumol kg-1 min-1, P = 0.08), whereas no inter-thigh difference was observed at a submaximal insulin concentration. The glycogen concentration was lower in the eccentric thigh for all three clamp steps used (P < 0.05). The glucose transporter GLUT4 protein content was on average 39% lower (P < 0.05) in the eccentric thigh in the basal state, whereas the maximal activity of glycogen synthase was identical in the two thighs for all clamp steps. 3. The glucose infusion rate (GIR) necessary to maintain euglycaemia during maximal insulin stimulation was lower during PEC compared with CC (15.7%, 81.3 +/- 3.2 vs. 96.4 +/- 8.8 mumol kg-1 min-1, P < 0.05). 4. Our data show that 2 days after unaccustomed eccentric exercise, muscle and whole-body insulin action is impaired at maximal but not submaximal concentrations. The local effect cannot account for the whole-body effect, suggesting the release of a factor which decreases insulin responsiveness systemically.