Marek Chmelik

Muscle mitochondrial ATP synthesis and glucose transport/phosphorylation in type 2 diabetes.

Background Muscular insulin resistance is frequently characterized by blunted increases in glucose-6-phosphate (G-6-P) reflecting impaired glucose transport/phosphorylation. These abnormalities likely relate to excessive intramyocellular lipids and mitochondrial dysfunction. We hypothesized that alterations in insulin action and mitochondrial function should be present even in nonobese patients with well-controlled type 2 diabetes mellitus (T2DM). Methods and Findings We measured G-6-P, ATP synthetic flux (i.e., synthesis) and lipid contents of skeletal muscle with 31P/1H magnetic resonance spectroscopy in ten patients with T2DM and in two control groups: ten sex-, age-, and body mass-matched elderly people; and 11 younger healthy individuals. Although insulin sensitivity was lower in patients with T2DM, muscle lipid contents were comparable and hyperinsulinemia increased G-6-P by 50% (95% confidence interval [CI] 39%–99%) in all groups. Patients with diabetes had 27% lower fasting ATP synthetic flux compared to younger controls (p = 0.031). Insulin stimulation increased ATP synthetic flux only in controls (younger: 26%, 95% CI 13%–42%; older: 11%, 95% CI 2%–25%), but failed to increase even during hyperglycemic hyperinsulinemia in patients with T2DM. Fasting free fatty acids and waist-to-hip ratios explained 44% of basal ATP synthetic flux. Insulin sensitivity explained 30% of insulin-stimulated ATP synthetic flux. Conclusions Patients with well-controlled T2DM feature slightly lower flux through muscle ATP synthesis, which occurs independently of glucose transport /phosphorylation and lipid deposition but is determined by lipid availability and insulin sensitivity. Furthermore, the reduction in insulin-stimulated glucose disposal despite normal glucose transport/phosphorylation suggests further abnormalities mainly in glycogen synthesis in these patients.

Abnormal hepatic energy homeostasis in type 2 diabetes

Increased hepatocellular lipids relate to insulin resistance and are typical for individuals with type 2 diabetes mellitus (T2DM). Steatosis and T2DM have been further associated with impaired muscular adenosine triphosphate (ATP) turnover indicating reduced mitochondrial fitness. Thus, we tested the hypothesis that hepatic energy metabolism could be impaired even in metabolically well‐controlled T2DM. We measured hepatic lipid volume fraction (HLVF) and absolute concentrations of γATP, inorganic phosphate (Pi), phosphomonoesters and phosphodiesters using noninvasive 1H/ 31P magnetic resonance spectroscopy in individuals with T2DM (58 ± 6 years, 27 ± 3 kg/m 2), and age‐matched and body mass index–matched (mCON; 61 ± 4 years, 26 ± 4 kg/m 2) and young lean humans (yCON; 25 ± 3 years, 22 ± 2 kg/m 2, P < 0.005, P < 0.05 versus T2DM and mCON). Insulin‐mediated whole‐body glucose disposal (M) and endogenous glucose production (iEGP) were assessed during euglycemic‐hyperinsulinemic clamps. Individuals with T2DM had 26% and 23% lower γATP (1.68 ± 0.11; 2.26 ± 0.20; 2.20 ± 0.09 mmol/L; P < 0.05) than mCON and yCON individuals, respectively. Further, they had 28% and 31% lower Pi than did individuals from the mCON and yCON groups (0.96 ± 0.06; 1.33 ± 0.13; 1.41 ± 0.07 mmol/L; P < 0.05). Phosphomonoesters, phosphodiesters, and liver aminotransferases did not differ between groups. HLVF was not different between those from the T2DM and mCON groups, but higher (P = 0.002) than in those from the yCON group. T2DM had 13‐fold higher iEGP than mCON (P < 0.05). Even after adjustment for HLVF, hepatic ATP and Pi related negatively to hepatic insulin sensitivity (iEGP) (r =‐0.665, P = 0.010, r =‐0.680, P = 0.007) but not to whole‐body insulin sensitivity. Conclusion: These data suggest that impaired hepatic energy metabolism and insulin resistance could precede the development of steatosis in individuals with T2DM. (HEPATOLOGY 2009.)

Liver ATP Synthesis Is Lower and Relates to Insulin Sensitivity in Patients With Type 2 Diabetes

OBJECTIVE Steatosis associates with insulin resistance and may even predict type 2 diabetes and cardiovascular complications. Because muscular insulin resistance relates to myocellular fat deposition and disturbed energy metabolism, we hypothesized that reduced hepatic ATP turnover (fATP) underlies insulin resistance and elevated hepatocellular lipid (HCL) contents. RESEARCH DESIGN AND METHODS We measured hepatic fATP using 31P magnetic resonance spectroscopy in patients with type 2 diabetes and age- and body mass–matched controls. Peripheral (M and M/I) and hepatic (suppression of endogenous glucose production) insulin sensitivity were assessed with euglycemic-hyperinsulinemic clamps. RESULTS Diabetic individuals had 29% and 28% lower peripheral and hepatic insulin sensitivity as well as 42% reduced fATP than controls. After adjusting for HCL, fATP correlated positively with peripheral and hepatic insulin sensitivity but negatively with waist circumference, BMI, and fasting plasma glucose. Multiple regression analysis identified waist circumference as an independent predictor of fATP and inorganic phosphate (PI) concentrations, explaining 65% (P = 0.001) and 56% (P = 0.003) of the variations. Hepatocellular PI primarily determined the alterations in fATP. CONCLUSIONS In patients with type 2 diabetes, insulin resistance relates to perturbed hepatic energy metabolism, which is at least partly accounted for by fat depots.

Assessment of 31P relaxation times in the human calf muscle: A comparison between 3 T and 7 T in vivo

Phosphorus (31P) T1 and T2 relaxation times in the resting human calf muscle were assessed by interleaved, surface coil localized inversion recovery and frequency‐selective spin‐echo at 3 and 7 T. The obtained T1 (mean ± SD) decreased significantly (P < 0.05) from 3 to 7 T for phosphomonoesters (PME) (8.1 ± 1.7 s to 3.1 ± 0.9 s), phosphodiesters (PDE) (8.6 ± 1.2 s to 6.0 ± 1.1 s), phosphocreatine (PCr) (6.7 ± 0.4 s to 4.0 ± 0.2 s), γ‐NTP (nucleotide triphosphate) (5.5 ± 0.4 s to 3.3 ± 0.2 s), α‐NTP (3.4 ± 0.3 s to 1.8 ± 0.1 s), and β‐NTP (3.9 ± 0.4 s to 1.8 ± 0.1 s), but not for inorganic phosphate (Pi) (6.9 ± 0.6 s to 6.3 ± 1.0 s). The decrease in T2 was significant for Pi (153 ± 9 ms to 109 ± 17 ms), PDE (414 ± 128 ms to 314 ± 35 ms), PCr (354 ± 16 ms to 217 ± 14 ms), and γ‐NTP (61.9 ± 8.6 ms to 29.0 ± 3.3 ms). This decrease in T1 with increasing field strength of up to 62% can be explained by the increasing influence of chemical shift anisotropy on relaxation mechanisms and may allow shorter measurements at higher field strengths or up to 62% additional signal‐to‐noise ratio (SNR) per unit time. The fully relaxed SNR increased by +96%, while the linewidth increased from 6.5 ± 1.2 Hz to 11.2 ± 1.9 Hz or +72%. At 7 T 31P‐MRS in the human calf muscle offers more than twice as much SNR per unit time in reduced measurement time compared to 3 T. This will facilitate in vivo 31P‐MRS of the human muscle at 7 T. Magn Reson Med, 2009.

Readout-segmented Echo-planar Imaging Improves the Diagnostic Performance of Diffusion-weighted MR Breast Examinations at 3.0 T

PURPOSE To qualitatively and quantitatively compare the diagnostic value of diffusion-weighted (DW) magnetic resonance (MR) imaging based on standard single-shot echo-planar imaging and readout-segmented echo-planar imaging in patients with breast cancer at 3.0 T. MATERIALS AND METHODS Institutional review board approval and written informed consent were obtained. Forty-seven patients with 49 histopathologically verified lesions were included in this study. In all patients, DW imaging, with single-shot echo-planar imaging and readout-segmented echo-planar imaging with comparable imaging parameters, was performed with a 3.0-T MR imager. Two independent readers visually assessed image quality and lesion conspicuity, and image properties (ie, signal-to-noise ratio, contrast, geometric distortions) were quantified. Regions of interest were drawn in all lesions (28 malignant, 21 benign) and in the normal breast parenchyma to investigate differences in apparent diffusion coefficient (ADC). Diagnostic accuracy was calculated on the basis of an ADC threshold of 1.25 × 10(-3) mm(2)/sec. RESULTS Each reader found a higher diagnostic accuracy for readout-segmented (96%) than for single-shot (90%) echo-planar imaging. The area under the curve for readout-segmented echo-planar imaging (0.981) was significantly larger than for single-shot echo-planar imaging (0.867) (P = .026). There was no significant difference in the ADC obtained by using either DW imaging method. Lesion conspicuity and image quality of readout-segmented echo-planar imaging were rated superior to those of single-shot echo-planar imaging (P < .001). Readout-segmented echo-planar imaging reduced geometric distortions by a factor of three. CONCLUSION DW imaging based on readout-segmented echo-planar imaging provided significantly higher image quality and lesion conspicuity than single-shot echo-planar imaging by reducing geometric distortions, image blurring, and artifact level with a clinical high-field-strength MR imager. Thereby, readout-segmented echo-planar imaging reached a higher diagnostic accuracy for the differentiation of benign and malignant breast lesions.

Three-dimensional Proton MR Spectroscopic Imaging at 3 T for the Differentiation of Benign and Malignant Breast Lesions

PURPOSE To evaluate the diagnostic accuracy of quantitative, three-dimensional (3D) magnetic resonance (MR) spectroscopic imaging at 3 T for the differentiation of benign and malignant breast lesions, on the basis of choline (Cho) signal-to-noise ratio (SNR) threshold levels, in a clinically feasible measurement time. MATERIALS AND METHODS Institutional review board approval and written informed consent were obtained from all subjects. Fifty female patients (mean age, 50 years; age range, 25-82 years) with mammographic or ultrasonographic (US) abnormalities were successfully examined in the prone position with a 3-T MR system by using a dedicated breast coil. Lesions were verified by either histopathologic examination or follow-up of at least 24 months. For 3D MR spectroscopic imaging, a point-resolved spectroscopic sequence (repetition time msec/echo time msec, 750/145; field of view, 12 × 12 × 12 cm(3); matrix size, 12 × 12 × 12, interpolated to 16 × 16 × 16; acquisition time, 11 minutes 17 seconds) was used. The maximum Cho SNR was assessed in all lesions and correlated with the histopathologic results. RESULTS Thirty-two malignant and 12 benign lesions were confirmed in 43 patients with histopathologic examination. Seven patients without biopsy underwent imaging follow-up. In 31 of 32 (97%) malignant and 10 of 19 (53%) benign lesions, Cho was detected. The median Cho SNR in malignant lesions was 5.7, compared with 2.0 in benign lesions. With a Cho SNR threshold level of 2.6, 3D MR spectroscopic imaging provided a sensitivity of 97% and a specificity of 84% for the differentiation of benign and malignant breast lesions. CONCLUSION At 3T, 3D MR spectroscopic imaging yields high diagnostic sensitivity and specificity for discrimination of benign and malignant breast lesions within reasonable measurement times. This technique allows the study of heterogeneous and multicentric breast tumors and simplifies acquisition planning.

High‐resolution mapping of human brain metabolites by free induction decay 1H MRSI at 7 T

This work describes a new approach for high‐spatial‐resolution 1H MRSI of the human brain at 7 T. 1H MRSI at 7 T using conventional approaches, such as point‐resolved spectroscopy and stimulated echo acquisition mode with volume head coils, is limited by technical difficulties, including chemical shift displacement errors, B0/B1 inhomogeneities, a high specific absorption rate and decreased T2 relaxation times. The method presented here is based on free induction decay acquisition with an ultrashort acquisition delay (TE*) of 1.3 ms. This allows full signal detection with negligible T2 decay or J‐modulation. Chemical shift displacement errors were reduced to below 5% per part per million in the in‐slice direction and were eliminated in‐plane. The B1 sensitivity was reduced significantly and further corrected using flip angle maps. Specific absorption rate requirements were well below the limit (~20 % = 0.7 W/kg). The suppression of subcutaneous lipid signals was achieved by substantially improving the point‐spread function. High‐quality metabolic mapping of five important brain metabolites was achieved with high in‐plane resolution (64 × 64 matrix with a 3.4 × 3.4 × 12 mm3 nominal voxel size) in four healthy subjects. The ultrashort TE* increased the signal‐to‐noise ratio of J‐coupled resonances, such as glutamate and myo‐inositol, several‐fold to enable the mapping of even these metabolites with high resolution. Four measurement repetitions in one healthy volunteer provided proof of the good reproducibility of this method. The high spatial resolution allowed the visualization of several anatomical structures on metabolic maps. Free induction decay MRSI is insensitive to T2 decay, J‐modulation, B1 inhomogeneities and chemical shift displacement errors, and overcomes specific absorption rate restrictions at ultrahigh magnetic fields. This makes it a promising method for high‐resolution 1H MRSI at 7 T and above. Copyright

Short-Term Exercise Training Does Not Stimulate Skeletal Muscle ATP Synthesis in Relatives of Humans With Type 2 Diabetes

OBJECTIVE We tested the hypothesis that short-term exercise training improves hereditary insulin resistance by stimulating ATP synthesis and investigated associations with gene polymorphisms. RESEARCH DESIGN AND METHODS We studied 24 nonobese first-degree relatives of type 2 diabetic patients and 12 control subjects at rest and 48 h after three bouts of exercise. In addition to measurements of oxygen uptake and insulin sensitivity (oral glucose tolerance test), ectopic lipids and mitochondrial ATP synthesis were assessed using1H and31P magnetic resonance spectroscopy, respectively. They were genotyped for polymorphisms in genes regulating mitochondrial function, PPARGC1A (rs8192678) and NDUFB6 (rs540467). RESULTS Relatives had slightly lower (P = 0.012) insulin sensitivity than control subjects. In control subjects, ATP synthase flux rose by 18% (P = 0.0001), being 23% higher (P = 0.002) than that in relatives after exercise training. Relatives responding to exercise training with increased ATP synthesis (+19%, P = 0.009) showed improved insulin sensitivity (P = 0.009) compared with those whose insulin sensitivity did not improve. A polymorphism in the NDUFB6 gene from respiratory chain complex I related to ATP synthesis (P = 0.02) and insulin sensitivity response to exercise training (P = 0.05). ATP synthase flux correlated with O2uptake and insulin sensitivity. CONCLUSIONS The ability of short-term exercise to stimulate ATP production distinguished individuals with improved insulin sensitivity from those whose insulin sensitivity did not improve. In addition, the NDUFB6 gene polymorphism appeared to modulate this adaptation. This finding suggests that genes involved in mitochondrial function contribute to the response of ATP synthesis to exercise training.

Body and liver fat mass rather than muscle mitochondrial function determine glucose metabolism in women with a history of gestational diabetes mellitus.

OBJECTIVE Ectopic lipid storage in muscle (intramyocellular lipids [IMCL]) and liver (hepatocellular lipids [HCL]) coexists with impaired myocellular flux through ATP synthase (fATPase) in certain cohorts with increased risk of type 2 diabetes. Because women with a history of gestational diabetes mellitus (pGDM) have elevated ectopic lipids and diabetes risk, we tested whether deteriorated energy metabolism contributes to these abnormalities. RESEARCH DESIGN AND METHODS A total of 23 glucose-tolerant nonobese pGDM and eight women with normal glucose metabolism during pregnancy with similar age, body mass, and physical activity underwent oral glucose tolerance tests (OGTT) and intravenous glucose tolerance tests at 4–5 years after delivery. OGTT values <463 mL ⋅ min−1 ⋅ m−2 were considered to indicate insulin resistance. pGDM were further stratified into insulin-resistant (pGDM-IR) and insulin-sensitive (pGDM-IS) groups. IMCL, HCL, and fATPase were measured with 1H/31P magnetic resonance spectroscopy. RESULTS pGDM had 36% higher fat mass and 12% lower insulin sensitivity. Log-transformed fATPase was lower in pGDM (10.6 ± 3.8 µmol ⋅ mL muscle−1 ⋅ min−1 vs. 12.1 ± 1.4 µmol ⋅ mL muscle−1 ⋅ min−1, P < 0.03) and related to plasma adiponectin after adjustment for body fat (r = 0.44, P < 0.04). IMCL were 61% and 69% higher in pGDM-IR (P < 0.05 vs. pGDM-IS) and insulin resistant women (P < 0.003 vs. insulin sensitive), respectively. HCL were doubled (P < 0.05) in pGDM and insulin resistant women, and correlated positively with body fat mass (r = 0.50, P < 0.01) and inversely with insulin sensitivity (r = −0.46, P < 0.05). CONCLUSIONS Glucose-tolerant pGDM show increased liver fat but only slightly lower muscular insulin sensitivity and ATP synthesis. This suggests that alteration of hepatic lipid storage represents an early and predominant abnormality in this cohort.

Three-dimensional high-resolution magnetic resonance spectroscopic imaging for absolute quantification of 31P metabolites in human liver.

Liver dysfunction correlates with alterations of intracellular concentrations of 31P metabolites. Localization and absolute quantification should help to trace regional hepatic metabolism. An improved protocol for the absolute quantification of 31P metabolites in vivo in human liver was developed by employing three‐dimensional (3D) k‐space weighted spectroscopic imaging (MRSI) with B1‐insensitive adiabatic excitation. The protocol allowed for high spatial resolution of 17.8 ± 0.22 cm3 in 34 min at 3 T. No pulse adjustment prior to MRSI measurement was necessary due to adiabatic excitation. The protocol geometry was identical for all measurements so that one calibration data set, acquired from phantom replacement measurement, was applied for all quantifications. The protocol was tested in 10 young, healthy volunteers, for whom 57 ± 7 spectra were quantified. Concentrations per liter of liver volume (reproducibilities) were 2.24 ± 0.10 mmol/L (1.8%) for phosphomonoesters (PME), 1.37 ± 0.07 mmol/L (7.9%) for inorganic phosphate (Pi), 11.40 ± 0.96 mmol/L (2.9%) for phosphodiesters (PDE), and 2.14 ± 0.10 mmol/L (1.6%) for adenosine triphosphate (ATP), respectively. Taken together, this approach provides fast, simple, and reproducible high‐resolution absolute quantification and detailed mapping of the spatial distribution of hepatic 31P metabolites. This method allows for examination of regional deviations of energy metabolism in human liver diseases. Magn Reson Med 60:796–802, 2008.