Impaired insulin stimulation of muscular ATP production in patients with type 1 diabetes

The Effects of Resistance Exercise Training on Skeletal Muscle Metabolism and Insulin Resistance Development in Female Rodents with Type 1 Diabetes

The etiology of insulin resistance (IR) development in type 1 diabetes mellitus (T1DM) remains unclear; however, impaired skeletal muscle metabolism may play a role. While IR development has been established in male T1DM rodents, female rodents have yet to be examined in this context. Resistance exercise training (RT) has been shown to improve IR and is associated with a lower risk of hypoglycemia onset in T1DM compared to aerobic exercise. The purpose of this study was to investigate the effects of RT on IR development in female T1DM rodents. Forty Sprague Dawley eight-week-old female rats were divided into four groups: control sedentary (CS; n = 10), control trained (CT; n = 10), T1DM sedentary (DS; n = 10), and T1DM trained (DT; n = 10). Multiple low-dose streptozotocin injections were used to induce T1DM. Blood glucose levels were maintained in the 4-9 mmol/l range with intensive insulin therapy. CT and DT underwent weighted ladder climbing 5 days/week for six weeks. Intravenous glucose tolerance tests (IVGTT) were conducted on all animals following the six-week period. Results demonstrate that DS animals exhibited significantly increased weekly blood glucose measures compared to all groups including DT (p < 0.0001), despite similar insulin dosage levels. This was concomitant with a significant increase in insulin-adjusted area under the curve following IVGTT in DS (p < 0.05), indicative of a reduction in insulin sensitivity. Both DT and DS exhibited greater serum insulin concentrations compared to CT and CS (p < 0.05). DS animals also exhibited significantly greater glycogen content in white gastrocnemius muscle compared to CS and DT (p < 0.05), whereas DT and DS animals exhibited greater p-Akt: Akt ratio in the white vastus lateralis muscle and citrate synthase activity in the red vastus lateralis muscle compared to CS and CT (p < 0.05). These results indicate that female rodents with T1DM develop poor glycemic control and IR which can be attenuated with RT, possibly related to differences in intramyocellular glycogen content.

Double diabetes-when type 1 diabetes meets type 2 diabetes: definition, pathogenesis and recognition

Currently, the differentiation between type 1 diabetes (T1D) and type 2 diabetes (T2D) is not straightforward, and the features of both types of diabetes coexist in one subject. This situation triggered the need to discriminate so-called double diabetes (DD), hybrid diabetes or type 1.5 diabetes, which is generally described as the presence of the insulin resistance characteristic of metabolic syndrome in individuals diagnosed with T1D. DD not only raises the question of proper classification of diabetes but is also associated with a significantly greater risk of developing micro- and macroangiopathic complications, which was independent of glycaemic control. When considering the global obesity pandemic and increasing incidence of T1D, the prevalence of DD may also presumably increase. Therefore, it is of the highest priority to discover the mechanisms underlying the development of DD and to identify appropriate methods to prevent or treat DD. In this article, we describe how the definition of double diabetes has changed over the years and how it is currently defined. We discuss the accuracy of including metabolic syndrome in the DD definition. We also present possible hypotheses connecting insulin resistance with T1D and propose possible methods to identify individuals with double diabetes based on indirect insulin resistance markers, which are easily assessed in everyday clinical practice. Moreover, we discuss adjuvant therapy which may be considered in double diabetic patients.

Intramyocellular Triglyceride Content During the Early Course of Type 1 and Type 2 Diabetes

Intramyocellular lipid content (IMCL) is elevated in insulin-resistant humans, but it changes over time, and relationships with comorbidities remain unclear. We examined IMCL during the initial course of diabetes and its associations with complications. Participants of the German Diabetes Study (GDS) with recent-onset type 1 (n = 132) or type 2 diabetes (n = 139) and glucose-tolerant control subjects (n = 128) underwent 1H-MRS to measure IMCL and muscle volume, whole-body insulin sensitivity (hyperinsulinemic-euglycemic clamps; M-value), and cycling spiroergometry (VO2max). Subgroups underwent the same measurements after 5 years. At baseline, IMCL was ∼30% higher in type 2 diabetes than in other groups independently of age, sex, BMI, and muscle volume. In type 2 diabetes, the M-value was ∼36% and ∼62% lower compared with type 1 diabetes and control subjects, respectively. After 5 years, the M-value decreased by ∼29% in type 1 and ∼13% in type 2 diabetes, whereas IMCL remained unchanged. The correlation between IMCL and M-value in type 2 diabetes at baseline was modulated by VO2max. IMCL also associated with microalbuminuria, the Framingham risk score for cardiovascular disease, and cardiac autonomic neuropathy. Changes in IMCL within 5 years after diagnosis do not mirror the progression of insulin resistance in type 2 diabetes but associate with early diabetes-related complications.

ARTICLE HIGHLIGHTS

Intramyocellular lipid content (IMCL) can be elevated in insulin-resistant humans, but its dynamics and association with comorbidities remain unclear. Independently of age, sex, body mass, and skeletal muscle volume, IMCL is higher in recent-onset type 2, but not type 1 diabetes, and remains unchanged within 5 years, despite worsening insulin resistance. A degree of physical fitness modulates the association between IMCL and insulin sensitivity in type 2 diabetes. Whereas higher IMCL associates with lower insulin sensitivity in people with lower physical fitness, there is no association between IMCL and insulin sensitivity in those with higher degree of physical fitness. IMCL associates with progression of microalbuminuria, cardiovascular disease risk, and cardiac autonomic neuropathy.

Normal to enhanced intrinsic mitochondrial respiration in skeletal muscle of middle- to older-aged women and men with uncomplicated type 1 diabetes

AIMS/HYPOTHESIS

This study interrogated mitochondrial respiratory function and content in skeletal muscle biopsies of healthy adults between 30 and 72 years old with and without uncomplicated type 1 diabetes.

METHODS

Participants (12 women/nine men) with type 1 diabetes (48 ± 11 years of age), without overt complications, were matched for age, sex, BMI and level of physical activity to participants without diabetes (control participants) (49 ± 12 years of age). Participants underwent a Bergström biopsy of the vastus lateralis to assess mitochondrial respiratory function using high-resolution respirometry and citrate synthase activity. Electron microscopy was used to quantify mitochondrial content and cristae (pixel) density.

RESULTS

Mean mitochondrial area density was 27% lower (p = 0.006) in participants with type 1 diabetes compared with control participants. This was largely due to smaller mitochondrial fragments in women with type 1 diabetes (-18%, p = 0.057), as opposed to a decrease in the total number of mitochondrial fragments in men with diabetes (-28%, p = 0.130). Mitochondrial respiratory measures, whether estimated per milligram of tissue (i.e. mass-specific) or normalised to area density (i.e. intrinsic mitochondrial function), differed between cohorts, and demonstrated sexual dimorphism. Mass-specific mitochondrial oxidative phosphorylation (OXPHOS) capacity with the substrates for complex I and complex II (C_I + II) was significantly lower (-24%, p = 0.033) in women with type 1 diabetes compared with control participants, whereas mass-specific OXPHOS capacities with substrates for complex I only (pyruvate [C_{I pyr}] or glutamate [C_{I glu}]) or complex II only (succinate [C_{II succ}]) were not different (p > 0.404). No statistical differences (p > 0.397) were found in mass-specific OXPHOS capacity in men with type 1 diabetes compared with control participants despite a 42% non-significant increase in C_{I glu} OXPHOS capacity (p = 0.218). In contrast, intrinsic C_I + II OXPHOS capacity was not different in women with type 1 diabetes (+5%, p = 0.378), whereas in men with type 1 diabetes it was 25% higher (p = 0.163) compared with control participants. Men with type 1 diabetes also demonstrated higher intrinsic OXPHOS capacity for C_{I pyr} (+50%, p = 0.159), C_{I glu} (+88%, p = 0.033) and C_{II succ} (+28%, p = 0.123), as well as higher intrinsic respiratory rates with low (more physiological) concentrations of either ADP, pyruvate, glutamate or succinate (p < 0.012). Women with type 1 diabetes had higher (p < 0.003) intrinsic respiratory rates with low concentrations of succinate only. Calculated aerobic fitness (Physical Working Capacity Test [PWC₁₃₀]) showed a strong relationship with mitochondrial respiratory function and content in the type 1 diabetes cohort.

CONCLUSIONS/INTERPRETATION

In middle- to older-aged adults with uncomplicated type 1 diabetes, we conclude that skeletal muscle mitochondria differentially adapt to type 1 diabetes and demonstrate sexual dimorphism. Importantly, these cellular alterations were significantly associated with our metric of aerobic fitness (PWC₁₃₀) and preceded notable impairments in skeletal mass and strength.

The Role of Mitochondrial Adaptation and Metabolic Flexibility in the Pathophysiology of Obesity and Insulin Resistance: an Updated Overview

PURPOSE OF REVIEW

The term "metabolic flexibility" denotes the dynamic responses of the cellular oxidative machinery in order to adapt to changes in energy substrate availability. A progressive loss of this adaptive capacity has been implicated in the development of obesity-related comorbidities. Mitochondria are dynamic intracellular organelles which play a fundamental role in energy metabolism, and the mitochondrial adaptation to environmental challenges may be viewed as the functional component of metabolic flexibility. Herein, we attempt to comprehensively review the available evidence regarding the role of mitochondrial adaptation and metabolic flexibility in the pathogenesis of obesity and related morbidities, namely insulin resistance states and non-alcoholic fatty liver disease (NAFLD).

RECENT FINDINGS

Overall, there is a concrete body of evidence to support the presence of impaired mitochondrial adaptation as a principal component of systemic metabolic inflexibility in conditions related to obesity. There are still many unresolved questions regarding the relationship between the gradual loss of mitochondrial adaptability and the progression of obesity-related complications, such as causality issues, the timely appearance and reversibility of the described disturbances, and the generalizability of the findings to the mitochondrial content of every affected tissue or organ. The evidence regarding the causality between the observed associations remains inconclusive, although most of the available data points towards a bidirectional, potentially mutually amplifying relationship. The spectrum of NAFLD is of particular interest, since functional and pathological changes in the course of its development closely mirror the progression of dysmetabolism, if not constituting a dynamic component of the latter.

Insulin Modulates the Bioenergetic and Thermogenic Capacity of Rat Brown Adipocytes In Vivo by Modulating Mitochondrial Mosaicism

The effects of insulin on the bioenergetic and thermogenic capacity of brown adipocyte mitochondria were investigated by focusing on key mitochondrial proteins. Two-month-old male Wistar rats were treated acutely or chronically with a low or high dose of insulin. Acute low insulin dose increased expression of all electron transport chain complexes and complex IV activity, whereas high dose increased complex II expression. Chronic low insulin dose decreased complex I and cyt c expression while increasing complex II and IV expression and complex IV activity. Chronic high insulin dose decreased complex II, III, cyt c, and increased complex IV expression. Uncoupling protein (UCP) 1 expression was decreased after acute high insulin but increased following chronic insulin treatment. ATP synthase expression was increased after acute and decreased after chronic insulin treatment. Only a high dose of insulin increased ATP synthase activity in acute and decreased it in chronic treatment. ATPase inhibitory factor protein expression was increased in all treated groups. Confocal microscopy showed that key mitochondrial proteins colocalize differently in different mitochondria within a single brown adipocyte, indicating mitochondrial mosaicism. These results suggest that insulin modulates the bioenergetic and thermogenic capacity of rat brown adipocytes in vivo by modulating mitochondrial mosaicism.

Muscle insulin resistance in type 1 diabetes with coronary artery disease

AIMS/HYPOTHESIS

The risk for coronary artery disease (CAD) is substantially increased in type 1 diabetes and it has been postulated that insulin resistance may contribute to this risk. The current study measured insulin resistance in type 1 diabetes with vs without CAD and with a focus upon skeletal muscle, to test the hypothesis that insulin resistance is more severe in participants who have type 1 diabetes and CAD. Additionally, in type 1 diabetes, we examined the hypothesis that insulin resistance is more severe in soleus (an oxidative type muscle) vs tibialis anterior (a more glycolytic type of muscle).

METHODS

Insulin resistance was measured in participants with type 1 diabetes with (n = 9, CAD+) and without CAD (n = 10, CAD-) using euglycaemic insulin infusions combined with positron emission tomography (PET) imaging of [¹⁸F]fluorodeoxyglucose (FDG) uptake into soleus and tibialis anterior skeletal muscles. Coronary artery calcium (CAC) score was quantified by electron beam tomography.

RESULTS

CAD+ participants with type 1 diabetes had a >100-fold higher CAC score than did CAD- participants with type 1 diabetes but groups did not differ in HbA_1c or insulin dose. During clamp studies, CAD+ and CAD- groups had similar glucose disposal but were insulin resistant compared with historical non-diabetic participants (n = 13). FDG uptake by soleus muscle was similarly reduced, overall, in individuals with type 1 diabetes with or without CAD compared with non-diabetic individuals. However, FDG uptake by tibialis anterior muscle was not reduced in CAD- participants with type 1 diabetes while in CAD+ participants with type 1 diabetes it was 75% greater (p < 0.01). Across all participants with type 1 diabetes, FDG uptake by tibialis anterior muscle correlated positively with CAC severity.

CONCLUSIONS/INTERPRETATION

Our study confirms that systemic and skeletal muscle-specific insulin resistance is seen in type 1 diabetes but found that it does not appear to be more severe in the presence of CAD. There were, however, sharp differences between soleus and tibialis anterior muscles in type 1 diabetes: while insulin resistance was clearly manifest in soleus muscle, and was of equal severity in CAD+ and CAD- participants, tibialis anterior did not suggest insulin resistance in participants with type 1 diabetes, as FDG uptake by tibialis anterior correlated positively with CAC severity and was significantly increased in participants with type 1 diabetes and clinical CAD. Graphical abstract.

A Human Randomized Controlled Trial Comparing Metabolic Responses to Single and Repeated Hypoglycemia in Type 1 Diabetes

AIMS

Hypoglycemia hinders optimal glycemic management in type 1 diabetes (T1D). Long diabetes duration and hypoglycemia impair hormonal counter-regulatory responses to hypoglycemia. Our study was designed to test whether (1) the metabolic responses and insulin sensitivity are impaired, and (2) whether they are affected by short-lived antecedent hypoglycemia in participants with T1D.

MATERIALS AND METHODS

In a randomized, crossover, 2x2 factorial design, 9 male participants with T1D and 9 comparable control participants underwent 30 minutes of hypoglycemia (p-glucose < 2.9 mmol/L) followed by a euglycemic clamp on 2 separate interventions: with and without 30 minutes of hypoglycemia the day before the study day.

RESULTS

During both interventions insulin sensitivity was consistently lower, while counter-regulatory hormones were reduced, with 75% lower glucagon and 50% lower epinephrine during hypoglycemia in participants with T1D, who also displayed 40% lower lactate and 5- to 10-fold increased ketone body concentrations following hypoglycemia, whereas palmitate and glucose turnover, forearm glucose uptake, and substrate oxidation did not differ between the groups. In participants with T1D, adipose tissue phosphatase and tensin homolog (PTEN) content, hormone-sensitive lipase (HSL) phosphorylation, and muscle glucose transporter type 4 (GLUT4) content were decreased compared with controls. And antecedent hypoglycemic episodes lasting 30 minutes did not affect counter-regulation or insulin sensitivity.

CONCLUSIONS

Participants with T1D displayed insulin resistance and impaired hormonal counter-regulation during hypoglycemia, whereas glucose and fatty acid fluxes were intact and ketogenic responses were amplified. We observed subtle alterations of intracellular signaling and no effect of short-lived antecedent hypoglycemia on subsequent counter-regulation. This plausibly reflects the presence of insulin resistance and implies that T1D is a condition with defective hormonal but preserved metabolic responsiveness to short-lived hypoglycemia.

Limitations to exercise tolerance in type 1 diabetes: the role of pulmonary oxygen uptake kinetics and priming exercise

We compared the time constant (τV̇O2) of the fundamental phase of pulmonary oxygen uptake (V̇o₂) kinetics between young adult men with type 1 diabetes and healthy control subjects. We also assessed the impact of priming exercise on τV̇O2, critical power, and muscle deoxygenation in a subset of participants with type 1 diabetes. Seventeen men with type 1 diabetes and 17 healthy male control subjects performed moderate-intensity exercise to determine τV̇O2. A subset of seven participants with type 1 diabetes performed an additional eight visits, in which critical power, τV̇O2, and muscle deoxyhemoglobin + myoglobin ([HHb+Mb], via near-infrared spectroscopy) kinetics (described by a time constant, τ_[HHb+Mb]) were determined with (PRI) and without (CON) a prior 6-min bout of heavy exercise. τV̇O2 was greater in participants with type 1 diabetes compared with control subjects (type 1 diabetes 50 ± 13 vs. control 32 ± 12 s; P < 0.001). Critical power was greater in PRI compared with CON (PRI 161 ± 25 vs. CON 149 ± 22 W; P < 0.001), whereas τV̇O2 (PRI 36 ± 15 vs. CON 50 ± 21 s; P = 0.006) and τ_[HHb+Mb] (PRI 10 ± 5 vs. CON 17 ± 11 s; P = 0.037) were reduced in PRI compared with CON. Type 1 diabetes patients showed slower pulmonary V̇o₂ kinetics compared with control subjects; priming exercise speeded V̇o₂ and [HHb + Mb] kinetics and increased critical power in a subgroup with type 1 diabetes. These data therefore represent the first characterization of the power-duration relationship in type 1 diabetes and the first experimental evidence that τV̇O2 is an independent determinant of critical power in this population.NEW & NOTEWORTHY Patients with type 1 diabetes demonstrated slower oxygen uptake (V̇o₂) kinetics compared with healthy control subjects. Furthermore, a prior bout of high-intensity exercise speeded V̇o₂ kinetics and increased critical power in people with type 1 diabetes. Prior exercise speeded muscle deoxygenation kinetics, indicating that V̇o₂ kinetics in type 1 diabetes are limited primarily by oxygen extraction and/or intracellular factors. These findings highlight the potential for interventions that decrease metabolic inertia for enhancing exercise tolerance in this condition.

The Peripheral Peril: Injected Insulin Induces Insulin Insensitivity in Type 1 Diabetes

Insulin resistance is an underappreciated facet of type 1 diabetes that occurs with remarkable consistency and considerable magnitude. Although therapeutic innovations are continuing to normalize dysglycemia, a sizable body of data suggests a second metabolic abnormality-iatrogenic hyperinsulinemia-principally drives insulin resistance and its consequences in this population and has not been addressed. We review this evidence to show that injecting insulin into the peripheral circulation bypasses first-pass hepatic insulin clearance, which leads to the unintended metabolic consequence of whole-body insulin resistance. We propose restructuring insulin therapy to restore the physiological insulin balance between the hepatic portal and peripheral circulations and thereby avoid the complications of life-long insulin resistance. As technology rapidly advances and our ability to ensure euglycemia improves, iatrogenic insulin resistance will become the final barrier to overcome to restore normal physiology, health, and life in type 1 diabetes.

Distinct alterations of gut morphology and microbiota characterize accelerated diabetes onset in nonobese diabetic mice

The rising prevalence of type 1 diabetes (T1D) over the past decades has been linked to lifestyle changes, but the underlying mechanisms are largely unknown. Recent findings point to gut-associated mechanisms in the control of T1D pathogenesis. In nonobese diabetic (NOD) mice, a model of T1D, diabetes development accelerates after deletion of the Toll-like receptor 4 (TLR4). We hypothesized that altered intestinal functions contribute to metabolic alterations, which favor accelerated diabetes development in TLR4-deficient (TLR4^-/-) NOD mice. In 70-90-day-old normoglycemic (prediabetic) female NOD TLR4^+/+ and NOD TLR4^-/- mice, gut morphology and microbiome composition were analyzed. Parameters of lipid metabolism, glucose homeostasis, and mitochondrial respiratory activity were measured in vivo and ex vivo Compared with NOD TLR4^+/+ mice, NOD TLR4^-/- animals showed lower muscle mass of the small intestine, higher abundance of Bacteroidetes, and lower Firmicutes in the large intestine, along with lower levels of circulating short-chain fatty acids (SCFA). These changes are associated with higher body weight, hyperlipidemia, and severe insulin and glucose intolerance, all occurring before the onset of diabetes. These mice also exhibited insulin resistance-related abnormalities of energy metabolism, such as lower total respiratory exchange rates and higher hepatic oxidative capacity. Distinct alterations of gut morphology and microbiota composition associated with reduction of circulating SCFA may contribute to metabolic disorders promoting the progression of insulin-deficient diabetes/T1D development.

Mitochondrial dysfunction is the cause of one of the earliest changes seen on magnetic resonance imaging in Charcot neuroarthopathy - Oedema of the small muscles in the foot

The hypothesis laid out in this thesis states that the early changes seen on an MR imaging in those with early Charcot neuroarthopathy may be due to mitochondrial dysfunction. In a Charcot foot, there is movement between bones. In an attempt to prevent this movement, the small muscles of the foot contract continuously when the foot is weight bearing. This contraction takes energy in the form of ATP. However, the reduction of glucose transport into the muscle cells due to insulin resistance / insufficiency, leads to reduction in the ATP producing capacity of the mitochondria. The ATP depletion affects the cell membrane gradient leading to mitochondrial and cellular swelling. These early cellular changes could then be picked up with MR imaging as muscle oedema.

MAXIMAL FAT OXIDATION DURING AEROBIC EXERCISE IN ADOLESCENTS WITH TYPE 1 DIABETES

ABSTRACT Objective: To compare maximal fat oxidation rates (FATMAX) and analyze their association with cardiorespiratory fitness in adolescents with type 1 diabetes mellitus (T1DM). Methods: Twenty-two male and female adolescents aged between 11 to 17 years, following clinical and anthropometric evaluations, were assigned to the diabetic group (DG; n = 10) or control group (CG; n = 12). Cardiorespiratory fitness was determined by maximal oxygen uptake (VO2max) during a maximal aerobic test on a cycle ergometer using the Balke protocol. Maximal fat oxidation (FATMAX) was determined by the respiratory exchange ratio proposed in the Lusk table. Results: Adolescents in the DG had lower mean FATMAX (p<0.01) and %VO2FATMAX (p=0.001) values when compared with those in the CG. FATMAX values were inversely correlated with serum glycosylated hemoglobin (HbA1c) levels (r= −0.77) and directly correlated with BMI z-scores (r=0.76), while %VO2FATMAX results were correlated with age (r=0.81), BMI z-scores (r=0.65), and VO2max values (r=0.81). On multiple linear regression, HbA1c values explained 54% (adjusted r²=0.54, p=0.009) and BMI z-scores explained 3.1% (adjusted r²=-0.031, p=0.009) of the variation in FATMAX in the DG. Adolescents with T1DM had similar cardiorespiratory fitness and lower FATMAX rates (35±11 VO2max) when compared with controls (60±12 VO2max). Conclusion: These results suggest lower fat oxidation rates and greater use of glucose as an energy substrate during exercise and worse control in T1DM. Therefore, results may contribute to appropriate exercise prescription in T1DM, after verifying exercise intensity to reduce hypoglycemia risk. Level of evidence III; Case-control study.

Considering Type 1 Diabetes as a Form of Accelerated Muscle Aging

Recent evidence reveals impairments to skeletal muscle health in adolescent/young adults with type 1 diabetes (T1D). Interestingly, the observed changes in T1D are not unlike aged muscle, particularly, the alterations to mitochondria. Thus, we put forth the novel hypothesis that T1D may be considered a condition of accelerated muscle aging and that, similar to aging, mitochondrial dysfunction is a primary contributor to this complication.

Reduced hepatocellular lipid accumulation and energy metabolism in patients with long standing type 1 diabetes mellitus

The prevalence of obesity and metabolic syndrome increases in patients with type 1 diabetes mellitus (T1DM). In the general population this is linked with ectopic lipid accumulation in liver (HCL) and skeletal muscle (IMCL), representing hallmarks in the development of insulin resistance. Moreover, hepatic mitochondrial activity is lower in newly diagnosed patients with T1DM. If this precedes later development of diabetes related fatty liver disease is currently not known. This study aims to investigate energy metabolism in liver (k_ATP) and skeletal muscle (k_CK) and its impact on HCL, IMCL, cardiac fat depots and heart function in 10 patients with long standing T1DM compared to 11 well-matched controls by ³¹P/¹H magnetic resonance spectroscopy. HCL was almost 70% lower in T1DM compared to controls (6.9 ± 5% vs 2.1 ± 1.3%; p = 0.030). Also k_ATP was significantly reduced (0.33 ± 0.1 s^-1 vs 0.17 ± 0.1 s^-1; p = 0.018). In T1DM, dose of basal insulin strongly correlated with BMI (r = 0.676, p = 0.032) and HCL (r = 0.643, p = 0.045), but not with k_ATP. In the whole cohort, HCL was significantly associated with BMI (r = 0.615, p = 0.005). In skeletal muscle k_CK was lower in patients with T1DM (0.25 ± 0.05 s^-1 vs 0.31 ± 0-04 s^-1; p = 0.039). No significant differences were found in IMCL. Cardiac fat depots as well as heart function were not different. Our results in patients with long standing T1DM show that HCL is lower compared to matched controls, despite reduced energy metabolism in liver and skeletal muscle.

Preadipocytes of obese humans display gender-specific bioenergetic responses to glucose and insulin

BACKGROUND/OBJECTIVES

Although the prevalence of obesity and its associated metabolic disorders is increasing in both sexes, the clinical phenotype differs between men and women, highlighting the need for individual treatment options. Mitochondrial dysfunction in various tissues, including white adipose tissue (WAT), has been accepted as a key factor for obesity-associated comorbidities such as diabetes. Given higher expression of mitochondria-related genes in the WAT of women, we hypothesized that gender differences in the bioenergetic profile of white (pre-) adipocytes from obese (age- and BMI-matched) donors must exist.

SUBJECTS/METHODS

Using Seahorse technology, we measured oxygen consumption rates (OCR) and extracellular acidification rates (ECAR) of (pre-)adipocytes from male (n = 10) and female (n = 10) deeply-phenotyped obese donors under hypo-, normo- and hyperglycemic (0, 5 and 25 mM glucose) and insulin-stimulated conditions. Additionally, expression levels (mRNA/protein) of mitochondria-related genes (e.g. UQCRC2) and glycolytic enzymes (e.g. PKM2) were determined.

RESULTS

Dissecting cellular OCR and ECAR into different functional modules revealed that preadipocytes from female donors show significantly higher mitochondrial to glycolytic activity (higher OCR/ECAR ratio, p = 0.036), which is supported by a higher ratio of UQCRC2 to PKM2 mRNA levels (p = 0.021). However, no major gender differences are detectable in in vitro differentiated adipocytes (e.g. OCR/ECAR, p = 0.248). Importantly, glucose and insulin suppress mitochondrial activity (i.e. ATP-linked respiration) significantly only in preadipocytes of female donors, reflecting their trends towards higher insulin sensitivity.

CONCLUSIONS

Collectively, we show that preadipocytes, but not in vitro differentiated adipocytes, represent a model system to reveal gender differences with clinical importance for metabolic disease status. In particular preadipocytes of females maintain enhanced mitochondrial flexibility, as demonstrated by pronounced responses of ATP-linked respiration to glucose.

Altered mitochondrial bioenergetics and ultrastructure in the skeletal muscle of young adults with type 1 diabetes

AIMS/HYPOTHESIS

A comprehensive assessment of skeletal muscle ultrastructure and mitochondrial bioenergetics has not been undertaken in individuals with type 1 diabetes. This study aimed to systematically assess skeletal muscle mitochondrial phenotype in young adults with type 1 diabetes.

METHODS

Physically active, young adults (men and women) with type 1 diabetes (HbA_1c 63.0 ± 16.0 mmol/mol [7.9% ± 1.5%]) and without type 1 diabetes (control), matched for sex, age, BMI and level of physical activity, were recruited (n = 12/group) to undergo vastus lateralis muscle microbiopsies. Mitochondrial respiration (high-resolution respirometry), site-specific mitochondrial H₂O₂ emission and Ca²⁺ retention capacity (CRC) (spectrofluorometry) were assessed using permeabilised myofibre bundles. Electron microscopy and tomography were used to quantify mitochondrial content and investigate muscle ultrastructure. Skeletal muscle microvasculature was assessed by immunofluorescence.

RESULTS

Mitochondrial oxidative capacity was significantly lower in participants with type 1 diabetes vs the control group, specifically at Complex II of the electron transport chain, without differences in mitochondrial content between groups. Muscles of those with type 1 diabetes also exhibited increased mitochondrial H₂O₂ emission at Complex III and decreased CRC relative to control individuals. Electron tomography revealed an increase in the size and number of autophagic remnants in the muscles of participants with type 1 diabetes. Despite this, levels of the autophagic regulatory protein, phosphorylated AMP-activated protein kinase (p-AMPKα^Thr172), and its downstream targets, phosphorylated Unc-51 like autophagy activating kinase 1 (p-ULK1^Ser555) and p62, was similar between groups. In addition, no differences in muscle capillary density or platelet aggregation were observed between the groups.

CONCLUSIONS/INTERPRETATION

Alterations in mitochondrial ultrastructure and bioenergetics are evident within the skeletal muscle of active young adults with type 1 diabetes. It is yet to be elucidated whether more rigorous exercise may help to prevent skeletal muscle metabolic deficiencies in both active and inactive individuals with type 1 diabetes.

Inverse association of insulin antibody levels with insulin sensitivity in adults with Type 1 diabetes

AIMS

Insulin resistance may contribute to the pathogenesis of autoimmune-mediated diabetes. Antibodies against β-cell-associated molecules, comprising islet cell antigen (ICA), glutamic acid decarboxylase (GAD) and insulin, characterize the autoimmune process. Because the link between insulin resistance and autoimmunity might be relevant for disease progression and treatment, we hypothesized that insulin resistance associates positively with β-cell-directed antibodies in newly diagnosed Type 1 diabetes.

METHODS

Within the German Diabetes Study, an observational study including adults with newly diagnosed diabetes, 142 adults [84 men, 58 women; age 33.1 (26.4, 41.9) years; diabetes duration 6.3 (4.2, 9.1) months] positive for at least one antibody against ICA, GAD or insulin underwent hyperinsulinaemic-euglycaemic clamp tests to assess insulin sensitivity (M-value) in a cross-sectional setting.

RESULTS

Insulin-directed antibodies were inversely correlated with M-values (β = -0.039). Albeit not strong, the association persisted after adjustment for age, sex and BMI, and even after further adjustment for confounders reflecting exposure to exogenous insulin and residual β-cell secretory capacity. Correlation network-based analyses revealed a complex interaction between levels of fasting insulin and of insulin antibodies with respect to their relationship with the M-value. GAD- or ICA-directed antibodies did not correlate with insulin sensitivity.

CONCLUSIONS

In adults with recent-onset Type 1 diabetes expressing at least one β-cell-directed antibody, insulin sensitivity is inversely related to insulin antibody titres, but not to other autoantibodies. Our finding may allow for the identification of insulin resistance in adults with high levels of insulin antibodies.

Variants in Genes Controlling Oxidative Metabolism Contribute to Lower Hepatic ATP Independent of Liver Fat Content in Type 1 Diabetes

Type 1 diabetes has been recently linked to nonalcoholic fatty liver disease (NAFLD), which is known to associate with insulin resistance, obesity, and type 2 diabetes. However, the role of insulin resistance and hyperglycemia for hepatic energy metabolism is yet unclear. To analyze early abnormalities in hepatic energy metabolism, we examined 55 patients with recently diagnosed type 1 diabetes. They underwent hyperinsulinemic-normoglycemic clamps with [6,6-(2)H2]glucose to assess whole-body and hepatic insulin sensitivity. Hepatic γATP, inorganic phosphate (Pi), and triglyceride concentrations (hepatocellular lipid content [HCL]) were measured with multinuclei magnetic resonance spectroscopy ((31)P/(1)H-MRS). Glucose-tolerant humans served as control (CON) (n = 57). Whole-body insulin sensitivity was 44% lower in patients than in age- and BMI-matched CON. Hepatic γATP was 15% reduced (2.3 ± 0.6 vs. 2.7 ± 0.6 mmol/L, P < 0.001), whereas hepatic Pi and HCL were similar in patients when compared with CON. Across all participants, hepatic γATP correlated negatively with glycemia and oxidized LDL. Carriers of the PPARG G allele (rs1801282) and noncarriers of PPARGC1A A allele (rs8192678) had 21 and 13% lower hepatic ATP concentrations. Variations in genes controlling oxidative metabolism contribute to a reduction in hepatic ATP in the absence of NAFLD, suggesting that alterations in hepatic mitochondrial function may precede diabetes-related liver diseases.

Cohort profile: the German Diabetes Study (GDS)

Background

The German Diabetes Study (GDS) is a prospective longitudinal cohort study describing the impact of subphenotypes on the course of the disease. GDS aims at identifying prognostic factors and mechanisms underlying the development of related comorbidities.

Study design and methods

The study comprises intensive phenotyping within 12 months after clinical diagnosis, at 5-year intervals for 20 years and annual telephone interviews in between. Dynamic tests, including glucagon, mixed meal, intravenous glucose tolerance and hyperinsulinemic clamp tests, serve to assess beta-cell function and tissue-specific insulin sensitivity. Magnetic resonance imaging and multinuclei spectroscopy allow quantifying whole-body fat distribution, tissue-specific lipid deposition and energy metabolism. Comprehensive analyses of microvascular (nerve, eye, kidney) and macrovascular (endothelial, cardiorespiratory) morphology and function enable identification and monitoring of comorbidities. The GDS biobank stores specimens from blood, stool, skeletal muscle, subcutaneous adipose tissue and skin for future analyses including multiomics, expression profiles and histology. Repeated questionnaires on socioeconomic conditions, patient-reported outcomes as quality of life, health-related behavior as physical activity and nutritional habits are a specific asset of GDS. This study will recruit 3000 patients and a group of humans without familiy history of diabetes. 237 type 1 and 456 type 2 diabetes patients have been already included.

Electronic supplementary material

The online version of this article (doi:10.1186/s12933-016-0374-9) contains supplementary material, which is available to authorized users.

31P magnetization transfer measurements of Pi→ATP flux in exercising human muscle

Fundamental criticisms have been made over the use of (31)P magnetic resonance spectroscopy (MRS) magnetization transfer estimates of inorganic phosphate (Pi)→ATP flux (VPi-ATP) in human resting skeletal muscle for assessing mitochondrial function. Although the discrepancy in the magnitude of VPi-ATP is now acknowledged, little is known about its metabolic determinants. Here we use a novel protocol to measure VPi-ATP in human exercising muscle for the first time. Steady-state VPi-ATP was measured at rest and over a range of exercise intensities and compared with suprabasal oxidative ATP synthesis rates estimated from the initial rates of postexercise phosphocreatine resynthesis (VATP). We define a surplus Pi→ATP flux as the difference between VPi-ATP and VATP. The coupled reactions catalyzed by the glycolytic enzymes GAPDH and phosphoglycerate kinase (PGK) have been shown to catalyze measurable exchange between ATP and Pi in some systems and have been suggested to be responsible for this surplus flux. Surplus VPi-ATP did not change between rest and exercise, even though the concentrations of Pi and ADP, which are substrates for GAPDH and PGK, respectively, increased as expected. However, involvement of these enzymes is suggested by correlations between absolute and surplus Pi→ATP flux, both at rest and during exercise, and the intensity of the phosphomonoester peak in the (31)P NMR spectrum. This peak includes contributions from sugar phosphates in the glycolytic pathway, and changes in its intensity may indicate changes in downstream glycolytic intermediates, including 3-phosphoglycerate, which has been shown to influence the exchange between ATP and Pi catalyzed by GAPDH and PGK.

Insulin resistance in type 1 diabetes mellitus

For long the presence of insulin resistance in type 1 diabetes has been questioned. Detailed metabolic analyses revealed 12%-61% and up to 20% lower whole-body (skeletal muscle) and hepatic insulin sensitivity in type 1 diabetes, depending on the population studied. Type 1 diabetes patients feature impaired muscle adenosine triphosphate (ATP) synthesis and enhanced oxidative stress, predominantly relating to hyperglycemia. They may also exhibit abnormal fasting and postprandial glycogen metabolism in liver, while the role of hepatic energy metabolism for insulin resistance remains uncertain. Recent rodent studies point to tissue-specific differences in the mechanisms underlying insulin resistance. In non-obese diabetic mice, increased lipid availability contributes to muscle insulin resistance via diacylglycerol/protein kinase C isoforms. Furthermore, humans with type 1 diabetes respond to lifestyle modifications or metformin by 20%-60% increased whole-body insulin sensitivity, likely through improvement in both glycemic control and oxidative phosphorylation. Intensive insulin treatment and islet transplantation also increase but fail to completely restore whole-body and hepatic insulin sensitivity. In conclusion, insulin resistance is a feature of type 1 diabetes, but more controlled trials are needed to address its contribution to disease progression, which might help to optimize treatment and reduce comorbidities.

Insulin resistance in patients with type 1 diabetes assessed by glucose clamp studies: systematic review and meta-analysis

OBJECTIVE

The aim of this study was to perform a systematic review and meta-analysis on insulin resistance in adult patients with type 1 diabetes mellitus compared to healthy controls, assessed by hyperinsulinemic euglycemic clamp studies.

DESIGN AND METHODS

We conducted a systematic search of publications using PubMed, EMBASE, Web of Science and COCHRANE Library. Hyperinsulinemic euglycemic clamp studies comparing adult patients with type 1 diabetes mellitus to healthy controls were eligible. Primary outcome measures were pooled mean differences of insulin sensitivity of endogenous glucose production (EGP), of glucose uptake and of lipolysis. We estimated mean (standardized) differences and 95% CIs using random effects meta-analysis.

RESULTS

We included 38 publications in this meta-analysis. The weighed mean differences in EGP during hyperinsulinemia between patients and controls was 0.88 (95% CI: 0.47, 1.29) in the basal state and 0.52 (95% CI: 0.09, 0.95) in insulin stimulated conditions, indicating decreased hepatic insulin sensitivity in patients. Insulin sensitivity of glucose uptake was either reported as M value (M), glucose infusion rate (GIR), glucose disposal rate (GDR) or metabolic clearance rate (MCR). Weighed mean differences were similar for M -3.98 (95% CI: -4.68, -3.29) and GIR -4.61 (95% CI: -5.86, -3.53). Weighed mean difference for GDR was -2.43 (95% CI: -3.03, -1.83) and -3.29 (95% CI: -5.37, -1.22) for MCR, indicating decreased peripheral insulin sensitivity in patients. Insulin mediated inhibition of lipolysis was decreased in patients, reflected by increased non-esterified fatty acid levels.

CONCLUSIONS

Insulin resistance is a prominent feature of patients with type 1 diabetes mellitus and involves hepatic, peripheral and adipose tissues.

Delayed skeletal muscle mitochondrial ADP recovery in youth with type 1 diabetes relates to muscle insulin resistance

Insulin resistance (IR) increases cardiovascular morbidity and is associated with mitochondrial dysfunction. IR is now recognized to be present in type 1 diabetes; however, its relationship with mitochondrial function is unknown. We determined the relationship between IR and muscle mitochondrial function in type 1 diabetes using the hyperinsulinemic-euglycemic clamp and (31)P-MRS before, during, and after near-maximal isometric calf exercise. Volunteers included 21 nonobese adolescents with type 1 diabetes and 17 nondiabetic control subjects with similar age, sex, BMI, Tanner stage, and activity levels. We found that youths with type 1 diabetes were more insulin resistant (median glucose infusion rate 10.1 vs. 18.9 mg/kglean/min; P < 0.0001) and had a longer time constant of the curve of ADP conversion to ATP (23.4 ± 5.3 vs. 18.8 ± 3.9 s, P < 0.001) and a lower rate of oxidative phosphorylation (median 0.09 vs. 0.21 mmol/L/s, P < 0.001). The ADP time constant (β = -0.36, P = 0.026) and oxidative phosphorylation (β = 0.02, P < 0.038) were related to IR but not HbA1c. Normal-weight youths with type 1 diabetes demonstrated slowed postexercise ATP resynthesis and were more insulin resistant than control subjects. The correlation between skeletal muscle mitochondrial dysfunction in type 1 diabetes and IR suggests a relationship between mitochondrial dysfunction and IR in type 1 diabetes.

Ectopic lipid storage in non-alcoholic fatty liver disease is not mediated by impaired mitochondrial oxidative capacity in skeletal muscle

Non-alcoholic fatty liver disease (NAFLD), characterized by lipid deposition within the liver [intrahepatocellular lipid (IHCL)], is associated with insulin resistance and the metabolic syndrome (MS). It has been suggested that impaired skeletal muscle mitochondrial function may contribute to ectopic lipid deposition, and the associated MS, by altering post-prandial energy storage. To test this hypothesis, we performed a cross-sectional study of 17 patients with NAFLD [mean±S.D.; age, 45±11 years; body mass index (BMI), 31.6±3.4 kg/m2] and 18 age- and BMI-matched healthy controls (age, 44±11 years; BMI, 30.5±5.2 kg/m2). We determined body composition by MRI, IHCL and intramyocellular (soleus and tibialis anterior) lipids (IMCLs) by proton magnetic resonance spectroscopy (1H-MRS) and skeletal muscle mitochondrial function by dynamic phosphorus magnetic resonance spectroscopy (31P-MRS) of quadriceps muscle. Although matched for BMI and total adiposity, after statistical adjustment for gender, patients with NAFLD (defined by IHCL ≥ 5.5%) had higher IHCLs (25±16% compared with 2±2%; P<0.0005) and a higher prevalence of the MS (76% compared with 28%) compared with healthy controls. Despite this, the visceral fat/subcutaneous fat ratio, IMCLs and muscle mitochondrial function were similar between the NAFLD and control groups, with no significant difference in the rate constants of post-exercise phosphocreatine (PCr) recovery (1.55±0.4 compared with 1.51±0.4 min-1), a measure of muscle mitochondrial function. In conclusion, impaired muscle mitochondrial function does not seem to underlie ectopic lipid deposition, or the accompanying features of the MS, in patients with NAFLD.

Tissue-specific differences in the development of insulin resistance in a mouse model for type 1 diabetes

Although insulin resistance is known to underlie type 2 diabetes, its role in the development of type 1 diabetes has been gaining increasing interest. In a model of type 1 diabetes, the nonobese diabetic (NOD) mouse, we found that insulin resistance driven by lipid- and glucose-independent mechanisms is already present in the liver of prediabetic mice. Hepatic insulin resistance is associated with a transient rise in mitochondrial respiration followed by increased production of lipid peroxides and c-Jun N-terminal kinase activity. At the onset of diabetes, increased adipose tissue lipolysis promotes myocellular diacylglycerol accumulation. This is paralleled by increased myocellular protein kinase C θ activity and serum fetuin A levels. Muscle mitochondrial oxidative capacity is unchanged at the onset but decreases at later stages of diabetes. In conclusion, hepatic and muscle insulin resistance manifest at different stages and involve distinct cellular mechanisms during the development of diabetes in the NOD mouse.

Leukocyte profiles differ between type 1 and type 2 diabetes and are associated with metabolic phenotypes: results from the German Diabetes Study (GDS)

OBJECTIVE

Altered immune reactivity precedes and accompanies type 1 and type 2 diabetes. We hypothesized that the metabolic phenotype relates to the systemic cellular immune status.

RESEARCH DESIGN AND METHODS

A total of 194 metabolically well-controlled patients with type 1 diabetes (n = 62, mean diabetes duration 1.29 years) or type 2 diabetes (n = 132, 1.98 years) and 60 normoglycemic persons underwent blood sampling for automated white blood cell counting (WBC) and flow cytometry. Whole-body insulin sensitivity was measured with hyperinsulinemic-euglycemic clamp tests.

RESULTS

Patients with type 2 diabetes had higher WBC counts than control subjects along with a higher percentage of T cells and activated T helper (Th) and cytotoxic T (Tc) cells but lower proportions of natural killer (NK) cells. In type 1 diabetes, the percentage of activated Th and Tc cells was also higher compared with control subjects, whereas the ratio of regulatory T (Treg) cells to activated Th cells was lower, suggesting diminished regulatory capacity. Parameters of glycemic control related positively to Treg cells only in type 2 diabetes. Upon age, sex, and body mass adjustments, insulin sensitivity correlated positively with monocytes, while circulating lipids correlated positively with T cell subsets in type 1 diabetes.

CONCLUSIONS

Immune cell phenotypes showed distinct frequencies of occurrence in both diabetes types and associate with insulin sensitivity, glycemia, and lipidemia.

Reduction of non-esterified fatty acids improves insulin sensitivity and lowers oxidative stress, but fails to restore oxidative capacity in type 2 diabetes: a randomised clinical trial

AIMS/HYPOTHESIS

Muscle mitochondrial function can vary during fasting, but is lower during hyperinsulinaemia in insulin-resistant humans. Ageing and hyperlipidaemia may be the culprits, but the mechanisms remain unclear. We hypothesised that (1) insulin would fail to increase mitochondrial oxidative capacity in non-diabetic insulin-resistant young obese humans and in elderly patients with type 2 diabetes and (2) reducing NEFA levels would improve insulin sensitivity by raising oxidative capacity and lowering oxidative stress.

METHODS

Before and after insulin (4, 40, 100 nmol/l) stimulation, mitochondrial oxidative capacity was measured in permeabilised fibres and isolated mitochondria using high-resolution respirometry, and H2O2 production was assessed fluorimetrically. Tissue-specific insulin sensitivity was measured with hyperinsulinaemic-euglycaemic clamps combined with stable isotopes. To test the second hypothesis, in a 1-day randomised, crossover study, 15 patients with type 2 diabetes recruited via local advertisement were assessed for eligibility. Nine patients fulfilled the inclusion criteria (BMI <35 kg/m(2); age <65 years) and were allocated to and completed the intervention, including oral administration of 750 mg placebo or acipimox. Blinded randomisation was performed by the pharmacy; all participants, researchers performing the measurements and those assessing study outcomes were blinded. The main outcome measures were insulin sensitivity, oxidative capacity and oxidative stress.

RESULTS

Insulin sensitivity and mitochondrial oxidative capacity were ~31% and ~21% lower in the obese groups than in the lean group. The obese participants also exhibited blunted substrate oxidation upon insulin stimulation. In the patients with type 2 diabetes, acipimox improved insulin sensitivity by ~27% and reduced H2O2 production by ~45%, but did not improve basal or insulin-stimulated mitochondrial oxidative capacity. No harmful treatment side effects occurred.

CONCLUSIONS/INTERPRETATION

Decreased mitochondrial oxidative capacity can also occur independently of age in insulin-resistant young obese humans. Insulin resistance is present at the muscle mitochondrial level, and is not affected by reducing circulating NEFAs in type 2 diabetes. Thus, impaired plasticity of mitochondrial function is an intrinsic phenomenon that probably occurs independently of lipotoxicity and reduced glucose uptake.

TRIAL REGISTRATION

Clinical Trials NCT00943059 FUNDING: This study was funded in part by a grant from the German Federal Ministry of Education and Research to the German Center for Diabetes Research (DZD e.V.).

Lower fasting muscle mitochondrial activity relates to hepatic steatosis in humans

OBJECTIVE

Muscle insulin resistance has been implicated in the development of steatosis and dyslipidemia by changing the partitioning of postprandial substrate fluxes. Also, insulin resistance may be due to reduced mitochondrial function. We examined the association between mitochondrial activity, insulin sensitivity, and steatosis in a larger human population.

RESEARCH DESIGN AND METHODS

We analyzed muscle mitochondrial activity from ATP synthase flux (fATP) and ectopic lipids by multinuclei magnetic resonance spectroscopy from 113 volunteers with and without diabetes. Insulin sensitivity was assessed from M values using euglycemic-hyperinsulinemic clamps and/or from oral glucose insulin sensitivity (OGIS) using oral glucose tolerance tests.

RESULTS

Muscle fATP correlated negatively with hepatic lipid content and HbA1c. After model adjustment for study effects and other confounders, fATP showed a strong negative correlation with hepatic lipid content and a positive correlation with insulin sensitivity and fasting C-peptide. The negative correlation of muscle fATP with age, HbA1c, and plasma free fatty acids was weakened after adjustment. Body mass, muscle lipid contents, plasma lipoproteins, and triglycerides did not associate with fATP.

CONCLUSIONS

The association of impaired muscle mitochondrial activity with hepatic steatosis supports the concept of a close link between altered muscle and liver energy metabolism as early abnormalities promoting insulin resistance.

Mitochondrial plasticity in obesity and diabetes mellitus

SIGNIFICANCE

Insulin resistance and its related diseases, obesity and type 2 diabetes mellitus (T2DM), have been linked to changes in aerobic metabolism, pointing to a possible role of mitochondria in the development of insulin resistance.

RECENT ADVANCES

Refined methodology of ex vivo high-resolution respirometry and in vivo magnetic resonance spectroscopy now allows describing several features of mitochondria in humans. In addition to measuring mitochondrial function at baseline and after exercise-induced submaximal energy depletion, the response of mitochondria to endocrine and metabolic challenges, termed mitochondrial plasticity, can be assessed using hyperinsulinemic clamp tests. While insulin resistant states do not uniformly relate to baseline and post-exercise mitochondrial function, mitochondrial plasticity is typically impaired in insulin resistant relatives of T2DM, in overt T2DM and even in type 1 diabetes mellitus (T1DM).

CRITICAL ISSUES

The variability of baseline mitochondrial function in the main target tissue of insulin action, skeletal muscle and liver, may be attributed to inherited and acquired changes in either mitochondrial quantity or quality. In addition to certain gene polymorphisms and aging, circulating glucose and lipid concentrations correlate with both mitochondrial function and plasticity.

FUTURE DIRECTIONS

Despite the associations between features of mitochondrial function and insulin sensitivity, the question of a causal relationship between compromised mitochondrial plasticity and insulin resistance in the development of obesity and T2DM remains to be resolved.

Insulin resistance in type 1 diabetes: what is 'double diabetes' and what are the risks?

In this review, we explore the concept of 'double diabetes', a combination of type 1 diabetes with features of insulin resistance and type 2 diabetes. After considering whether double diabetes is a useful concept, we discuss potential mechanisms of increased insulin resistance in type 1 diabetes before examining the extent to which double diabetes might increase the risk of cardiovascular disease (CVD). We then go on to consider the proposal that weight gain from intensive insulin regimens may be associated with increased CV risk factors in some patients with type 1 diabetes, and explore the complex relationships between weight gain, insulin resistance, glycaemic control and CV outcome. Important comparisons and contrasts between type 1 diabetes and type 2 diabetes are highlighted in terms of hepatic fat, fat partitioning and lipid profile, and how these may differ between type 1 diabetic patients with and without double diabetes. In so doing, we hope this work will stimulate much-needed research in this area and an improvement in clinical practice.

Mitochondrial function in diabetes: novel methodology and new insight

Interpreting mitochondrial function as affected by comparative physiologic conditions is confounding because individual functional parameters are interdependent. Here, we studied muscle mitochondrial function in insulin-deficient diabetes using a novel, highly sensitive, and specific method to quantify ATP production simultaneously with reactive oxygen species (ROS) at clamped levels of inner mitochondrial membrane potential (ΔΨ), enabling more detailed study. We used a 2-deoxyglucose (2DOG) energy clamp to set ΔΨ at fixed levels and to quantify ATP production as 2DOG conversion to 2DOG-phosphate measured by one-dimensional (1)H and two-dimensional (1)H/(13)C heteronuclear single quantum coherence nuclear magnetic resonance spectroscopy. These techniques proved far more sensitive than conventional (31)P nuclear magnetic resonance and allowed high-throughput study of small mitochondrial isolates. Over conditions ranging from state 4 to state 3 respiration, ATP production was lower and ROS per unit of ATP generated was greater in mitochondria isolated from diabetic muscle. Moreover, ROS began to increase at a lower threshold for inner membrane potential in diabetic mitochondria. Further, ATP production in diabetic mitochondria is limited not only by respiration but also by limited capacity to use ΔΨ for ATP synthesis. In summary, we describe novel methodology for measuring ATP and provide new mechanistic insight into the dysregulation of ATP production and ROS in mitochondria of insulin-deficient rodents.

Insulin resistance in multiple tissues in patients with type 1 diabetes mellitus on long-term continuous subcutaneous insulin infusion therapy

BACKGROUND

The aim of this study was to determine whether insulin resistance is present in lean patients with uncomplicated type 1 diabetes mellitus on long-term continuous subcutaneous insulin infusion (CSII), compared with matched healthy controls.

METHODS

We studied eight patients (four men and four women) with type 1 diabetes mellitus on continuous subcutaneous insulin infusion and eight healthy controls, matched for age, gender and body mass index. Insulin sensitivity was measured by hyperinsulinemic euglycemic clamp studies with infusion of [6,6-(2) H(2)] glucose.

RESULTS

Endogenous glucose production did not differ in the basal state between patients and controls. However, endogenous glucose production was less suppressed during clamp conditions in patients compared with controls (64% vs 79%, p = 0.01), indicating decreased hepatic insulin sensitivity. During the clamp study, glucose disposal rate was ~38% lower in patients compared with controls (24.4 ± 2.5 vs 39.7 ± 5.6 µmol/kgLBM/min, p = 0.04). Accordingly, the rate of infusion of glucose was ~51% lower in patients (17.7 ± 2.8 vs 39.7 ± 5.7 µmol/kgLBM/min, p = 0.02). Finally, non-esterified fatty acids levels were ~2.5 times higher in patients during steady state clamp conditions (150 ± 26 vs 58 ± 4 pmol/L, p = 0.01), reflecting decreased insulin sensitivity of lipolysis.

CONCLUSIONS

Insulin resistance is a prominent feature of lean patients with type 1 diabetes mellitus, despite long term and stable treatment with continuous subcutaneous insulin infusion. Insulin resistance in type 1 diabetes involves both lipolysis, hepatic and peripheral glucose metabolism.

What do magnetic resonance-based measurements of Pi→ATP flux tell us about skeletal muscle metabolism?

Magnetic resonance spectroscopy (MRS) methods offer a potentially valuable window into cellular metabolism. Measurement of flux between inorganic phosphate (Pi) and ATP using (31)P MRS magnetization transfer has been used in resting muscle to assess what is claimed to be mitochondrial ATP synthesis and has been particularly popular in the study of insulin effects and insulin resistance. However, the measured Pi→ATP flux in resting skeletal muscle is far higher than the true rate of oxidative ATP synthesis, being dominated by a glycolytically mediated Pi↔ATP exchange reaction that is unrelated to mitochondrial function. Furthermore, even if measured accurately, the ATP production rate in resting muscle has no simple relationship to mitochondrial capacity as measured either ex vivo or in vivo. We summarize the published measurements of Pi→ATP flux, concentrating on work relevant to diabetes and insulin, relate it to current understanding of the physiology of mitochondrial ATP synthesis and glycolytic Pi↔ATP exchange, and discuss some possible implications of recently reported correlations between Pi→ATP flux and other physiological measures.

Features of hepatic and skeletal muscle insulin resistance unique to type 1 diabetes

CONTEXT

Type 1 diabetes is known to be a state of insulin resistance; however, the tissues involved in whole-body insulin resistance are less well known. It is unclear whether insulin resistance is due to glucose toxicity in the post-Diabetes Control and Complications Trial era of tighter glucose control.

OBJECTIVE

We performed this study to determine muscle and liver insulin sensitivity individuals with type 1 diabetes after overnight insulin infusion to lower fasting glucose concentration.

DESIGN, PATIENTS, AND METHODS

Fifty subjects [25 controls without and 25 individuals with type 1 diabetes (diabetes duration 22.9 ± 1.7 yr, without known end organ damage] were frequency matched on age and body mass index by group and studied. After 3 d of dietary control and overnight insulin infusion to normalize glucose, we performed a three-stage hyperinsulinemic/euglycemic clamp infusing insulin at 4, 8, and 40 mU/m(2) · min. Glucose metabolism was quantified using an infusion of [6,6-(2)H(2)]glucose. Hepatic insulin sensitivity was measured using the insulin IC(50) for glucose rate of appearance (Ra), whereas muscle insulin sensitivity was measured using the glucose rate of disappearance during the highest insulin dose.

RESULTS

Throughout the study, glucose Ra was significantly greater in individuals compared with those without type 1 diabetes. The concentration of insulin required for 50% suppression of glucose Ra was 2-fold higher in subjects with type 1 diabetes. Glucose rate of disappearance was significantly lower in individuals with type 1 diabetes during the 8- and 40-mU/m(2) · min stages.

CONCLUSION

Insulin resistance in liver and skeletal muscle was a significant feature in type 1 diabetes. Nevertheless, the etiology of insulin resistance was not explained by body mass index, percentage fat, plasma lipids, visceral fat, and physical activity and was also not fully explained by hyperglycemia.

The role of mitochondria in insulin resistance and type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) has been related to alterations of oxidative metabolism in insulin-responsive tissues. Overt T2DM can present with acquired or inherited reductions of mitochondrial oxidative phosphorylation capacity, submaximal ADP-stimulated oxidative phosphorylation and plasticity of mitochondria and/or lower mitochondrial content in skeletal muscle cells and potentially also in hepatocytes. Acquired insulin resistance is associated with reduced insulin-stimulated mitochondrial activity as the result of blunted mitochondrial plasticity. Hereditary insulin resistance is frequently associated with reduced mitochondrial activity at rest, probably due to diminished mitochondrial content. Lifestyle and pharmacological interventions can enhance the capacity for oxidative phosphorylation and mitochondrial content and improve insulin resistance in some (pre)diabetic cases. Various mitochondrial features can be abnormal but are not necessarily responsible for all forms of insulin resistance. Nevertheless, mitochondrial abnormalities might accelerate progression of insulin resistance and subsequent organ dysfunction via increased production of reactive oxygen species. This Review discusses the association between mitochondrial function and insulin sensitivity in various tissues, such as skeletal muscle, liver and heart, with a main focus on studies in humans, and addresses the effects of therapeutic strategies that affect mitochondrial function and insulin sensitivity.