Metformin is used as an anti-diabetic drug. Metformin ameliorates insulin resistance by improving insulin sensitivity in liver and skeletal muscle. Reduced mitochondrial content has been reported in type 2 diabetic muscles and it may contribute to decreased insulin sensitivity characteristic for diabetic muscles. The molecular mechanism behind the effect of metformin is not fully clarified but inhibition of complex I in the mitochondria and also activation of the 5′AMP activated protein kinase (AMPK) has been reported in muscle. Furthermore, both AMPK activation and metformin treatment have been associated with stimulation of mitochondrial function and biogenesis. However, a causal relationship in skeletal muscle has not been investigated. We hypothesized that potential effects of in vivo metformin treatment on mitochondrial function and protein expressions in skeletal muscle are dependent upon AMPK signaling. We investigated this by two weeks of oral metformin treatment of muscle specific kinase dead α2 (KD) AMPK mice and wild type (WT) littermates. We measured mitochondrial respiration and protein activity and expressions of key enzymes involved in mitochondrial carbohydrate and fat metabolism and oxidative phosphorylation. Mitochondrial respiration, HAD and CS activity, PDH and complex I-V and cytochrome c protein expression were all reduced in AMPK KD compared to WT tibialis anterior muscles. Surprisingly, metformin treatment only enhanced respiration in AMPK KD mice and thereby rescued the respiration defect compared to the WT mice. Metformin did not influence protein activities or expressions in either WT or AMPK KD mice.

We conclude that two weeks of in vivo metformin treatment enhances mitochondrial respiration in the mitochondrial deficient AMPK KD but not WT mice. The improvement seems to be unrelated to AMPK, and does not involve changes in key mitochondrial proteins.

OBJECTIVE

First-degree relatives (FDRs) of patients with type 2 diabetes may exhibit a disproportionately elevated risk of developing insulin resistance, obesity, and type 2 diabetes when exposed to physical inactivity, which to some unknown extent may involve low-grade inflammation. We investigated whether subjects who are nonobese FDRs show signs of low-grade inflammation before or after exposure to short-term physical inactivity.

RESEARCH DESIGN AND METHODS

We studied 13 healthy FDR subjects and 20 control (CON) subjects matched for age, sex, and BMI before and after 10 days of bed rest (BR). Insulin sensitivity was measured by the hyperinsulinemic euglycemic clamp. Key low-grade inflammation mediators were measured in arterial blood and microdialysate from subcutaneous abdominal (SCAAT) and femoral adipose tissue. Adipokine mRNA expression was determined in SCAAT.

RESULTS

Before BR, FDR subjects displayed insulin resistance, elevated plasma C-reactive protein, leptin, and monocyte chemoattractant protein (MCP)-1, high interleukin (IL)-6, and MCP-1 expressions, as well as low adiponectin and leptin expressions. FDR subjects responded to BR by decreasing plasma adiponectin and IL-10 expression and increasing plasma expression of IL-10 and tumor necrosis factor-α. In contrast, CON subjects responded to BR by increasing plasma adiponectin and adiponectin expression and by decreasing SCAAT microdialysate leptin.

CONCLUSIONS

Young and nonobese FDR of patients with type 2 diabetes exhibit low-grade inflammation, which is further and disproportionately aggravated when exposed to physical inactivity. The study provides support for the notion that people at increased risk of type 2 diabetes should avoid even short periods of physical inactivity.

Background

Insulin resistance has been linked to exercise intolerance in heart failure patients. The aim of this study was to assess the potential role of coronary flow reserve (CFR), endothelial function and arterial stiffness in explaining this linkage.

Methods

39 patients with LVEF < 35% (median LV ejection fraction (LVEF) 31 (interquartile range (IQ) 26–34), 23/39 of ischemic origin) underwent echocardiography with measurement of CFR. Peak coronary flow velocity (CFV) was measured in the LAD and coronary flow reserve was calculated as the ratio between CFV at rest and during a 2 minutes adenosine infusion. All patients performed a maximal symptom limited exercise test with measurement of peak oxygen uptake (VO2peak), digital measurement of endothelial function and arterial stiffness (augmentation index), dual X-ray absorptiometry scan (DEXA) for body composition and insulin sensitivity by a 2 hr hyperinsulinemic (40 mU/min/m2) isoglycemic clamp.

Results

Fat free mass adjusted insulin sensitivity was significantly correlated to VO2peak (r = 0.43, p = 0.007). Median CFR was 1.77 (IQ 1.26-2.42) and was correlated to insulin sensitivity (r 0.43, p = 0.008). CFR (r = 0.48, p = 0.002), and arterial stiffness (r = −0.35, p = 0.04) were correlated to VO2peak whereas endothelial function and LVEF were not (all p > 0.15). In multivariable linear regression adjusting for age, CFR remained independently associated with VO2peak (standardized coefficient (SC) 1.98, p = 0.05) whereas insulin sensitivity (SC 1.75, p = 0.09) and arterial stiffness (SC −1.17, p = 0.29) were no longer associated with VO2peak.

Conclusions

The study confirms that insulin resistance is associated with exercise intolerance in heart failure patients and suggests that this is partly through reduced CFR. This is the first study to our knowledge that shows an association between CFR and exercise capacity in heart failure patients and links the relationship between insulin resistance and exercise capacity to CFR.

Erythropoietin (Epo) treatment has been shown to induce mitochondrial biogenesis in cardiac muscle along with enhanced mitochondrial capacity in mice. We hypothesized that recombinant human Epo (rhEpo) treatment enhances skeletal muscle mitochondrial oxidative phosphorylation (OXPHOS) capacity in humans. In six healthy volunteers rhEpo was administered by sub-cutaneous injection over 8 weeks with oral iron (100 mg) supplementation taken daily. Mitochondrial OXPHOS was quantified by high-resolution respirometry in saponin-permeabilized muscle fibers obtained from biopsies of the vastus lateralis before and after rhEpo treatment. OXPHOS was determined with the mitochondrial complex I substrates malate, glutamate, pyruvate, and complex II substrate succinate in the presence of saturating ADP concentrations, while maximal electron transport capacity (ETS) was assessed by addition of an uncoupler. rhEpo treatment increased OXPHOS (from 92 ± 5 to 113 ± 7 pmol·s−1·mg−1) and ETS (107 ± 4 to 143 ± 14 pmol·s−1·mg−1, p < 0.05), demonstrating that Epo treatment induces an upregulation of OXPHOS and ETS in human skeletal muscle.

5′-adenosine monophosphate-activated protein kinase (AMPK) is considered central in regulation of energy status and substrate utilization within cells. In heart failure the energetic state is compromised and substrate metabolism is altered. We hypothesized that this could be linked to changes in AMPK activity and we therefore investigated mitochondrial oxidative phosphorylation capacity from the oxidation of long- and medium-chain fatty acids (LCFA and MCFA) in cardiomyocytes from young and old mice expressing a dominant negative AMPKα2 (AMPKα2-KD) construct and their wildtype (WT) littermates. We found a 35–45% (P < 0.05) lower mitochondrial capacity for oxidizing MCFA in AMPKα2-KD of both age-groups, compared to WT. This coincided with marked decreases in protein expression (19/29%, P < 0.05) and activity (14/21%, P < 0.05) of 3-hydroxyacyl-CoA-dehydrogenase (HAD), in young and old AMPKα2-KD mice, respectively, compared to WT. Maximal LCFA oxidation capacity was similar in AMPKα2-KD and WT mice independently of age implying that LCFA-transport into the mitochondria was unaffected by loss of AMPK activity or progressing age. Expression of regulatory proteins of glycolysis and glycogen breakdown showed equivocal effects of age and genotype. These results illustrate that AMPK is necessary for normal mitochondrial function in the heart and that decreased AMPK activity may lead to an altered energetic state as a consequence of reduced capacity to oxidize MCFA. We did not identify any clear aging effects on mitochondrial function.

OBJECTIVE

Physical inactivity is a risk factor for type 2 diabetes and may be more detrimental in first-degree relative (FDR) subjects, unmasking underlying defects of metabolism. Using a positive family history of type 2 diabetes as a marker of increased genetic risk, the aim of this study was to investigate the impact of physical inactivity on adipose tissue (AT) metabolism in FDR subjects.

RESEARCH DESIGN AND METHODS

A total of 13 FDR and 20 control (CON) subjects participated in the study. All were studied before and after 10 days of bed rest using the glucose clamp technique combined with measurements of glucose uptake, lipolysis, and lactate release from subcutaneous abdominal (SCAAT) and femoral (SCFAT) adipose tissue by the microdialysis technique. Additionally, mRNA expression of lipases was determined in biopsies from SCAAT.

RESULTS

Before bed rest, the FDR subjects revealed significantly increased glucose uptake in SCAAT. Furthermore, mRNA expression of lipases was significantly decreased in the SCAAT of FDR subjects. Bed rest significantly decreased lipolysis and tended to increase glucose uptake in the SCFAT of both CON and FDR subjects. In response to bed rest, SCAAT glucose uptake significantly increased in CON subjects but not in FDR subjects.

CONCLUSIONS

FDR subjects exhibit an abnormal AT metabolism including increased glucose uptake prior to bed rest. However, the differences between FDR and CON subjects in AT metabolism were attenuated during bed rest due to relatively more adverse changes in CON subjects compared with FDR subjects. Physical inactivity per se is not more deleterious in FDR subjects as compared with CON subjects with respect to derangements in AT metabolism.

OBJECTIVE

The aim of this study was to determine whether the type 2 diabetes–associated T-allele of transcription factor 7-like 2 (TCF7L2) rs7903146 associates with impaired insulin secretion to compensate for insulin resistance induced by bed rest.

RESEARCH DESIGN AND METHODS

A total of 38 healthy young Caucasian men were studied before and after bed rest using the hyperinsulinemic-euglycemic clamp technique combined with indirect calorimetry preceded by an intravenous glucose tolerance test. The TCF7L2 rs7903146 was genotyped using allelic discrimination performed with an ABI 7900 system. The genetic analyses were done assuming a dominant model of inheritance.

RESULTS

The first-phase insulin response (FPIR) was significantly lower in carriers of the T-allele compared with carriers of the CC genotype before bed rest, with and without correction for insulin resistance. The incremental rise of FPIR in response to insulin resistance induced by bed rest was lower in carriers of the T-allele (P < 0.001). Fasting plasma glucagon levels were significantly lower in carriers of the T-allele before and after bed rest. While carriers of the CC genotype developed increased hepatic insulin resistance, the TCF7L2 rs7903146 did not influence peripheral insulin action or the rate of lipolysis before or after bed rest.

CONCLUSIONS

Healthy carriers of the T-allele of TCF7L2 rs7903146 exhibit a diminished increase of insulin secretion in response to intravenous glucose to compensate for insulin resistance as induced by bed rest. Reduced paracrine glucagon stimulation may contribute to the impairment of β-cell function in the carriers TCF7L2 rs7903146 T-allele associated with increased risk of type 2 diabetes.

OBJECTIVE

The aim of this study was to investigate the impact of 9 days of bed rest on insulin secretion, insulin action, and whole-body glucose and fat metabolism in first-degree relative (FDR) and matched control (CON) subjects.

RESEARCH DESIGN AND METHODS

A total of 13 FDR and 20 CON subjects participated in the study. All were studied before and after 9 days of bed rest using the clamp technique combined with indirect calorimetry preceded by an intravenous glucose tolerance test. Glucose and glycerol turnover rates were studied using stable isotope kinetics.

RESULTS

Bed rest caused a significant decrease in whole-body insulin sensitivity in both groups. Hepatic insulin resistance was elevated in FDR subjects prior to bed rest and was significantly augmented by bed rest in FDR (P < 0.01) but not in CON (P = NS) subjects. The rate of whole-body lipolysis decreased during bed rest in both FDR and CON subjects, with no significant differences between the groups. Insulin resistance induced by bed rest was fully accounted for by the impairment of nonoxidative glucose metabolism in both groups (overall P < 0.001).

CONCLUSIONS

Whole-body insulin action in both insulin-resistant FDR and healthy CON subjects deteriorates with 9 days of bed rest, converging toward similar degrees of whole-body insulin resistance. FDR subjects exhibit hepatic insulin resistance (HIR), which, in contrast to CON subjects, deteriorates in response to physical inactivity. FDR subjects exhibit reduced insulin secretion when seen in relation to their degree of HIR but not peripheral insulin resistance.