Pioglitazone-induced improvements in insulin sensitivity occur without concomitant changes in muscle mitochondrial function.

AIMS: Pioglitazone (Pio) is known to improve insulin sensitivity in skeletal muscle. However, the role of Pio in skeletal muscle lipid metabolism and skeletal muscle oxidative capacity is not clear. The aim of this study was to determine the effects of chronic Pio treatment on skeletal muscle mitochondrial activity in individuals with type 2 diabetes (T2D). MATERIALS AND METHODS: Twenty-four participants with T2D (13M/11F 53.38±2.1years; BMI 36.47±1.1kg/m2) were randomized to either a placebo (CON, n=8) or a pioglitazone (PIO, n=16) group. Following 12weeks of treatment, we measured insulin sensitivity by hyperinsulinemic-euglycemic clamp (clamp), metabolic flexibility by calculating the change in respiratory quotient (ΔRQ) during the steady state of the clamp, intra- and extra-myocellular lipid content (IMCL and EMCL, respectively) by 1H magnetic resonance spectroscopy (1H-MRS) and muscle maximal ATP synthetic capacity (ATPmax) by 31P-MRS. RESULTS: Following 12weeks of PIO treatment, insulin sensitivity (p<0.0005 vs. baseline) and metabolic flexibility (p<0.05 vs. CON) significantly increased. PIO treatment significantly decreased IMCL content and increased EMCL content in gastrocnemius, soleus and tibialis anterior muscles. ATPmax was unaffected by PIO treatment. CONCLUSIONS: These results suggest that 12weeks of pioglitazone treatment improves insulin sensitivity, metabolic flexibility and myocellular lipid distribution without any effect on maximal ATP synthetic capacity in skeletal muscle. Consequently, pioglitazone-induced enhancements in insulin responsiveness and fuel utilization are independent of mitochondrial function.

Skeletal muscle mitochondrial capacity and insulin resistance in type 2 diabetes.

OBJECTIVE: The objective of this study was to determine the role of maximum mitochondrial capacity on the variation in insulin sensitivity within a population of patients with type 2 diabetes mellitus (T2DM). RESEARCH DESIGN AND METHODS: Fifty-eight participants enrolled in a cross-sectional design: eight active controls [maximum aerobic capacity (VO(2max)) > 40 ml/kg · min], 17 healthy sedentary controls without a family history (FH-) and seven with a family history (FH+) of diabetes, four obese participants, and 21 patients with T2DM. Mitochondrial capacity was measured noninvasively using (31)P magnetic resonance spectroscopy of the vastus lateralis. Maximal ATP synthetic rate (ATP(max)) was determined from the rate of phosphocreatine (PCr) recovery after short-term isometric exercise. RESULTS: ATP(max) was lower (P < 0.001) in T2DM and higher (P < 0.001) in active as compared with healthy sedentary FH- (active, 1.01 ± 0.2; FH-, 0.7 ± 0.2; FH+, 0.6 ± 0.1; obese, 0.6 ± 0.1; T2DM, 0.5 ± 0.2 mm ATP/sec; ANOVA P < 0.0001). Insulin sensitivity, measured by euglycemic-hyperinsulinemic (80 mIU/m(2) · min) clamp was also reduced in T2DM (P < 0.001) (active, 12.0 ± 3.2; FH-, 7.8 ± 2.2; FH+, 6.8 ± 3.5; obese, 3.1 ± 1.0; T2DM, 3.4 ± 1.6; mg/kg estimated metabolic body size · min; ANOVA P < 0.0001). Unexpectedly, there was a broad range of ATP(max) within the T2DM population where 52% of subjects with T2DM had ATP(max) values that were within the range observed in healthy sedentary controls. In addition, 24% of the T2DM subjects overlapped with the active control group (range, 0.65-1.27 mm ATP/sec). In contrast to the positive correlation between ATP(max) and M-value in the whole population (r(2) = 0.35; P < 0.0001), there was no correlation between ATP(max) and M-value in the patients with T2DM (r(2) = 0.004; P = 0.79). CONCLUSIONS: Mitochondrial capacity is not associated with insulin action in T2DM.

Characterization of the metabolic and physiologic response to chromium supplementation in subjects with type 2 diabetes mellitus.

The objective of the study was to provide a comprehensive evaluation of chromium (Cr) supplementation on metabolic parameters in a cohort of type 2 diabetes mellitus subjects representing a wide phenotype range and to evaluate changes in "responders" and "nonresponders." After preintervention testing to assess glycemia, insulin sensitivity (assessed by euglycemic clamps), Cr status, and body composition, subjects were randomized in a double-blind fashion to placebo or 1000 microg Cr. A substudy was performed to evaluate 24-hour energy balance/substrate oxidation and myocellular/intrahepatic lipid content. There was not a consistent effect of Cr supplementation to improve insulin action across all phenotypes. Insulin sensitivity was negatively correlated to soleus and tibialis muscle intramyocellular lipids and intrahepatic lipid content. Myocellular lipids were significantly lower in subjects randomized to Cr. At preintervention, responders, defined as insulin sensitivity change from baseline of at least 10% or greater, had significantly lower insulin sensitivity and higher fasting glucose and A(1c) when compared with placebo and nonresponders, that is, insulin sensitivity change from baseline of less than 10%. Clinical response was significantly correlated (P < .001) to the baseline insulin sensitivity, fasting glucose, and A(1c). There was no difference in Cr status between responder and nonresponders. Clinical response to Cr is more likely in insulin-resistant subjects who have more elevated fasting glucose and A(1c) levels. Chromium may reduce myocellular lipids and enhance insulin sensitivity in subjects with type 2 diabetes mellitus who do respond clinically independent of effects on weight or hepatic glucose production. Thus, modulation of lipid metabolism by Cr in peripheral tissues may represent a novel mechanism of action.

Effect of adipose tissue on the sexual dimorphism in metabolic flexibility.

Metabolic flexibility is the ability to transition between fat oxidation (fasting state) and glucose oxidation (fed state). We hypothesized that adipose tissue inflammation and lipid metabolism contribute to sexual dimorphism in metabolic flexibility. Respiratory quotient (DeltaRQ, metabolic flexibility) and nonesterified fatty acids (NEFAs) before and during euglycemic-hyperinsulinemic clamp were measured in healthy young women (n = 22) and men (n = 56). Adiponectin levels were measured in plasma. Abdominal subcutaneous adipose tissue gene expression was measured by quantitative reverse transcriptase polymerase chain reaction. As compared with men, women had higher DeltaRQ (0.14 +/- 0.04 vs 0.09 +/- 0.04, P < .01). Fasting RQ and fat cell size were not different between sexes. As compared with men, women had lower insulin-suppressed NEFAs (P < .05); greater adiponectin levels; and higher expression of adipogenesis, fatty acid storage, and oxidation genes (PPARgamma2, PCK1, SCD1, and PPARalpha; P < .05). There were no sex differences in messenger RNA of macrophage markers or chemokines. Stepwise regression analysis revealed that the only adipose tissue characteristics that influenced metabolic flexibility in women were SCD1 and PCK1 messenger RNA (model R(2) = 0.49, P < .05); in men, these were serum adiponectin and insulin-suppressed NEFAs (model R(2) = 0.34, P < .05). Healthy young women are more metabolically flexible than men, driven by an increase in insulin-stimulated glucose oxidation rather than differences in fasting fat oxidation. Women have greater capacity for insulin suppression of NEFAs despite similar chemokine and macrophage content in adipose tissue. Combined, these results provide evidence for a role of adipose tissue characteristics in the sexual dimorphism of metabolic flexibility.

Reduced adipose tissue oxygenation in human obesity: evidence for rarefaction, macrophage chemotaxis, and inflammation without an angiogenic response.

OBJECTIVE: Based on rodent studies, we examined the hypothesis that increased adipose tissue (AT) mass in obesity without an adequate support of vascularization might lead to hypoxia, macrophage infiltration, and inflammation. RESEARCH DESIGN AND METHODS: Oxygen partial pressure (AT pO2) and AT temperature in abdominal AT (9 lean and 12 overweight/obese men and women) was measured by direct insertion of a polarographic Clark electrode. Body composition was measured by dual-energy X-ray absorptiometry, and insulin sensitivity was measured by hyperinsulinemic-euglycemic clamp. Abdominal subcutaneous tissue was used for staining, quantitative RT-PCR, and chemokine secretion assay. RESULTS: AT pO2 was lower in overweight/obese subjects than lean subjects (47 +/- 10.6 vs. 55 +/- 9.1 mmHg); however, this level of pO2 did not activate the classic hypoxia targets (pyruvate dehydrogenase kinase and vascular endothelial growth factor [VEGF]). AT pO2 was negatively correlated with percent body fat (R = -0.50, P < 0.05). Compared with lean subjects, overweight/obese subjects had 44% lower capillary density and 58% lower VEGF, suggesting AT rarefaction (capillary drop out). This might be due to lower peroxisome proliferator-activated receptor gamma1 and higher collagen VI mRNA expression, which correlated with AT pO2 (P < 0.05). Of clinical importance, AT pO2 negatively correlated with CD68 mRNA and macrophage inflammatory protein 1alpha secretion (R = -0.58, R = -0.79, P < 0.05), suggesting that lower AT pO2 could drive AT inflammation in obesity. CONCLUSIONS: Adipose tissue rarefaction might lie upstream of both low AT pO2 and inflammation in obesity. These results suggest novel approaches to treat the dysfunctional AT found in obesity.

Phenotype of subjects with type 2 diabetes mellitus may determine clinical response to chromium supplementation.

Considerable controversy exists regarding the use of chromium (Cr) supplementation to modulate carbohydrate metabolism in subjects with diabetes. Recently, we reported that Cr supplementation, provided as 1000 microg/d as Cr picolinate, enhanced insulin sensitivity in subjects with type 2 diabetes mellitus. Our data agreed with some, but not all, studies that evaluated a similar dose and formulation in type 2 diabetes mellitus and suggested that subject selection and characteristics may be important considerations when assessing the clinical response. Thus, the goal of this study was to assess which metabolic or clinical characteristics, when obtained at baseline, best determine a clinical response to Cr when assessing changes in insulin sensitivity. Seventy-three subjects with type 2 diabetes mellitus were assessed in a double-blinded, randomized, placebo-controlled study. Subjects were assessed at baseline for glycemic control with glycated hemoglobin measures, oral glucose tolerance tests, and body weight and body fat measures (dual-energy x-ray absorptiometry). After baseline, insulin sensitivity in vivo was assessed with the use of hyperinsulinemic-euglycemic clamps. After the baseline clamp, subjects were randomized to receive Cr supplementation (1000 microg Cr/d provided as Cr picolinate) or placebo daily for 6 months. All study parameters were repeated after 6 months. The relationship of the baseline characteristics of the study subjects to the change in insulin sensitivity was determined. Sixty-three percent of the subjects with type 2 diabetes mellitus responded to the Cr treatment as compared with 30% with placebo. The only subject variable significantly associated with the clinical response to Cr was the baseline insulin sensitivity, as assessed with the hyperinsulinemic-euglycemic clamp (partial R(2) = .4038) (P = .0004). Subject phenotype appears to be very important when assessing the clinical response to Cr because baseline insulin sensitivity was found to account for nearly 40% of the variance in the clinical response to Cr.

Family history of diabetes links impaired substrate switching and reduced mitochondrial content in skeletal muscle.

Insulin resistance is associated with metabolic inflexibility, impaired switching of substrate oxidation from fatty acids to glucose in response to insulin. Impaired switching to fat oxidation in response to a high-fat diet (HFD) is hypothesized to contribute to insulin resistance. The objective of this study was to test the hypothesis that defects in substrate switching in response to insulin and a HFD are linked to reduced mitochondrial biogenesis and occur before the development of diabetes. Metabolic flexibility was measured in young sedentary men with (n = 16) or without (n = 34) a family history of diabetes by euglycemic-hyperinsulinemic clamp. Flexibility correlated with fat oxidation measured in a respiratory chamber after a 3-day HFD. Muscle mitochondrial content was higher in flexible subjects with high fat oxidation after a HFD and contributed 49% of the variance. Subjects with a family history of diabetes were inflexible and had reduced HFD-induced fat oxidation and muscle mitochondrial content but did not differ in the amount of body or visceral fat. Metabolic inflexibility, lower adaptation to a HFD, and reduced muscle mitochondrial mass cluster together in subjects with a family history of diabetes, supporting the role of an intrinsic metabolic defect of skeletal muscle in the pathogenesis of insulin resistance.

Dynamic changes in fat oxidation in human primary myocytes mirror metabolic characteristics of the donor.

Metabolic flexibility of skeletal muscle, that is, the preference for fat oxidation (FOx) during fasting and for carbohydrate oxidation in response to insulin, is decreased during insulin resistance. The aim of this study was to test the hypothesis that the capacity of myotubes to oxidize fat in vitro reflects the donor's metabolic characteristics. Insulin sensitivity (IS) and metabolic flexibility of 16 healthy, young male subjects was determined by euglycemic hyperinsulinemic clamp. Muscle samples were obtained from vastus lateralis, cultured, and differentiated into myotubes. In human myotubes in vitro, we measured suppressibility (glucose suppression of FOx) and adaptability (an increase in FOx in the presence of high palmitate concentration). We termed these dynamic changes in FOx metabolic switching. In vivo, metabolic flexibility was positively correlated with IS and maximal oxygen uptake and inversely correlated with percent body fat. In vitro suppressibility was inversely correlated with IS and metabolic flexibility and positively correlated with body fat and fasting FFA levels. Adaptability was negatively associated with percent body fat and fasting insulin and positively correlated with IS and metabolic flexibility. The interindividual variability in metabolic phenotypes was preserved in human myotubes separated from their neuroendocrine environment, which supports the hypothesis that metabolic switching is an intrinsic property of skeletal muscle.