Acetaminophen glucuronide and plasma glucose report identical estimates of gluconeogenesis and glycogenolysis for healthy and prediabetic subjects using the deuterated water method

Plasma glucose ²H‐enrichment in positions 5 (²H5) and 2 (²H2) from deuterated water (²H₂O) provides a measure of the gluconeogenic contribution to endogenous glucose production. Urinary glucuronide analysis can circumvent blood sampling but it is not known if glucuronide and glucose enrichments are equal. Thirteen subjects with impaired fasting glucose/impaired glucose tolerance and 11 subjects with normal fasting glucose and normal glucose tolerance ingested ²H₂O to ～0.5% body water and acetaminophen. Glucose and glucuronide ²H5 and ²H2 were measured by ²H NMR spectroscopy of monoacetone glucose. For normal fasting glucose/normal glucose tolerance, ²H5 was 0.23 ± 0.02% and 0.25 ± 0.02% for glucose and glucuronide, respectively, whereas ²H2 was 0.47 ± 0.01% and 0.49 ± 0.02%, respectively. For impaired fasting glucose/impaired glucose tolerance, ²H5 was 0.22 ± 0.01% and 0.26 ± 0.02% for glucose and glucuronide, respectively, whereas ²H2 was 0.46 ± 0.01% and 0.49 ± 0.02%, respectively. The gluconeogenic contribution to endogenous glucose production measured from glucose and glucuronide were identical for both normal fasting glucose/normal glucose tolerance (48 ± 4 vs. 51 ± 3%) and impaired fasting glucose/impaired glucose tolerance (48 ± 2 vs. 53 ± 3%). Magn Reson Med 70:315–319, 2013. © 2012 Wiley Periodicals, Inc.     

Quantification of hepatic transaldolase exchange activity and its effects on tracer measurements of indirect pathway flux in humans.

Exchange of hepatic glucose-6-phosphate (G6P) and glyceraldehyde-3-phosphate via transaldolase modifies hepatic G6P enrichment from glucose or gluconeogenic tracers. Transaldolase exchange was quantified in five healthy, fed subjects following an oral bolus of [1,2,3-(13)C(3)]glycerol (25-30 mg/kg) and paracetamol (10-12 mg/kg). (13)C Isotopomers of hepatic G6P were quantified by (13)C NMR spectroscopy of urinary glucuronide. [1,2,3-(13)C(3)]- and [4,5,6-(13)C(3)]glucuronide isotopomers, representing the conversion of [1,2,3-(13)C(3)]glycerol to G6P via dihydroxyacetone phosphate, were resolved from [1,2-(13)C(2)]- and [5,6-(13)C(2)]glucuronide (13)C-isotopomers, derived from metabolism of [1,2,3-(13)C(3)]glycerol via pyruvate and phosphoenolpyruvate. Enrichment of [1,2,3-(13)C(3)]glucuronide was significantly less than that of [4,5,6-(13)C(3)]glucuronide (1.30 +/- 0.57% versus 1.67 +/- 0.42%, P < 0.05). Also, [1,2-(13)C(2)]glucuronide enrichment was significantly less than that of [5,6-(13)C(2)]glucuronide (0.28 +/- 0.08% versus 0.36 +/- 0.03%, P < 0.05). Transaldolase and triose phosphate isomerase exchange activities were estimated by applying the (13)C-isotopomer data to a model of hepatic sugar phosphate metabolism. Triose phosphate isomerase exchange was approximately 99% complete and did not contribute significantly to the unequal (13)C-isotopomer distributions of the glucuronide triose halves. Instead, this was attributable to 25 +/- 23% of hepatic G6P flux undergoing transaldolase exchange. This results in substantial overestimates of indirect pathway contributions to hepatic glycogen synthesis with tracers such as [5-(3)H]glucose and (2)H(2)O.     

Quantifying hepatic glycogen synthesis by direct and indirect pathways in rats under normal ad libitum feeding conditions

Hepatic glycogen synthesis from intact hexose (direct pathway) relative to that from gluconeogenic precursors (indirect pathway) was quantified in ad libitum‐fed rats. Following ²H₂O administration and overnight feeding, the livers were removed and glycogen ²H‐enrichment was measured by ²H NMR. Six controls and six rats rendered hyperglycemic by streptozotocin (STZ; fasting blood glucose = 385 ± 31 mg/dl) were studied. The indirect pathway contribution, estimated as glycogen hydrogen 5 relative to hydrogen 2 enrichment, was 54% ± 4% for control rats—similar to values from healthy, meal‐fed humans. In STZ‐treated rats, the indirect pathway contribution was significantly higher (68% ± 4%, P < 0.05 vs. controls), similar to that of Type 1 diabetic (T1D) patients. In conclusion, sources of hepatic glycogen synthesis in rats during ad libitum nocturnal feeding were quantified by analysis of glycogen enrichment from ²H₂O. STZ caused alterations resembling the pathophysiology of hepatic glycogen synthesis in T1D patients. Magn Reson Med 61:1–5, 2009. © 2008 Wiley‐Liss, Inc.     

Sources of hepatic glucose production by 2H2O ingestion and Bayesian analysis of 2H glucuronide enrichment

The contribution of gluconeogenesis to hepatic glucose production (GP) was quantified after ²H₂O ingestion by Bayesian analysis of the position 2 and 5 ²H‐NMR signals (H2 and H5) of monoacetone glucose (MAG) derived from urinary acetaminophen glucuronide. Six controls and 10 kidney transplant (KTx) patients with cyclosporine A (CsA) immunosuppressant therapy were studied. Seven KTx patients were lean and euglycemic (BMI = 24.3 ± 1.0 kg/m²; fasting glucose = 4.7 ± 0.1 mM) while three were obese and hyperglycemic (BMI = 30.5 ± 0.7 kg/m²; fasting glucose = 7.1 ± 0.5 mM). For the 16 spectra analyzed, the mean coefficient of variation for the gluconeogenesis contribution was 10% ± 5%. This uncertainty was associated with a mean signal‐to‐noise ratio (SNR) of 79:1 and 45:1 for the MAG H2 and H5 signals, respectively. For control subjects, gluconeogenesis contributed 54% ± 7% of GP as determined by the mean and standard deviation (SD) of individual Bayesian analyses. For the lean/normoglycemic KTx subjects, the gluconeogenic contribution to GP was 62% ± 7% (P = 0.06 vs. controls), while hyperglycemic/obese KTx patients had a gluconeogenic contribution of 68% ± 3% (P < 0.005 vs. controls). These data suggest that in KTx patients, an increased gluconeogenic contribution to GP is strongly associated with obesity and hyperglycemia. Magn Reson Med 60:517–523, 2008. © 2008 Wiley‐Liss, Inc.     

Metabolic loss of deuterium from isotopically labeled glucose

The isotopically substituted molecule (6-¹³C, 1, 6, 6-²H₃)glucose was evaluated to determine whether metabolic ²H loss would prevent its use in quantitating pentose phosphate pathway (PPP) activity. PPP activity causes the C1 of glucose to be lost as CO₂, while C6 can appear in lactate. ²H NMR analysis of the lactate produced from this glucose can distinguish (3-²H)-lactate (from C1 of glucose) from (3-¹³C, 3, 3-²H_Z)lactate (from C6 of glucose). ²H NMR spectroscopic analysis of medium containing (4-¹³C, 1,6,6^?2H₃) glucose after incubation with cultured rat 9L glioma cells suggested a 30.8 ± 2.1% PPP activity as compared with 6.0 ± 0.8% from separate, parallel incubations with (1-¹³C)glucose and (6-¹³C)glucose. Subsequent experiments with other isotopically labeled glucose molecules suggest that this discrepancy is due to selective loss of ²H from the C1 position of glucose, catalyzed by phosphoman-nose isomerase. Failure to consider ²H exchange from the C1 and C6 positions of glucose can lead to incorrect conclusions in metabolic studies utilizing this and other deuterated or tritiated glucose molecules.     

2H2O incorporation into hepatic acetyl‐CoA and de novo lipogenesis as measured by Krebs cycle‐mediated 2H‐enrichment of glutamate and glutamine

Deuterated water is widely used for measuring de novo lipogenesis on the basis of quantifying lipid ²H‐enrichment relative to that of body water. However, incorporation of ²H‐enrichment from body water into newly synthesized lipid molecules is incomplete therefore the true lipid precursor enrichment differs from that of body water. We describe a novel measurement of de novo lipogenesis that is based on a true precursor‐product analysis of hepatic acetyl‐CoA and triglyceride methyl enrichments from deuterated water. After deuterated water administration to seven in situ and seven perfused livers, acetyl‐CoA methyl enrichment was inferred from ²H nuclear magnetic resonance analysis of hepatic glutamate/glutamine (Glx) enrichment and triglyceride methyl enrichment was directly determined by ²H nuclear magnetic resonance of triglycerides. Acetyl‐CoA ²H‐enrichment was 71% ± 1% that of body water for in situ livers and 53% ± 2% of perfusate water for perfused livers. From the ratio of triglyceride‐methyl/acetyl‐CoA enrichments, fractional de novo lipogenesis rates of 0.97% ± 0.09%/2 hr and 7.92% ± 1.47%/48 hr were obtained for perfused and in situ liver triglycerides, respectively. Our method reveals that acetyl‐CoA enrichment is significantly less than body water both for in situ and perfused livers. Furthermore, the difference between acetyl‐CoA and body water enrichments is sensitive to the experimental setting. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Quantifying endogenous glucose production and contributing source fluxes from a single 2H NMR spectrum

Endogenous glucose production (EGP), gluconeogenic and glycogenolytic fluxes by analysis of a single ²H‐NMR spectrum is demonstrated with 6‐hr and 24‐hr fasted rats. Animals were administered [1‐²H, 1‐¹³C]glucose, a novel tracer of glucose turnover, and ²H₂O. Plasma glucose enrichment from both tracers was quantified by ²H‐NMR analysis of monoacetone glucose. The 6‐hr fasted group (n = 7) had EGP rates of 95.6 ± 13.3 μmol/kg/min, where 56.2 ± 7.9 μmol/kg/min were derived from PEP; 12.1 ± 2.1 μmol/kg/min from glycerol, and 32.1 ± 4.9 μmol/kg/min from glycogen. The 24‐hr fasted group (n = 7) had significantly lower EGP rates (52.8 ± 7.2 μmol/kg/min, P = 0.004 vs. 6 hr) mediated by a significantly reduced contribution from glycogen (4.7 ± 5.9 μmol/kg/min, P = 0.02 vs. 6 hr) while PEP and glycerol contributions were not significantly different (39.5 ± 3.9 and 8.5 ± 1.2 μmol/kg/min, respectively). These estimates agree with previous assays of EGP fluxes in fasted rats obtained by multinuclear NMR analyses of plasma glucose enrichment from ²H₂O and ¹³C‐glucose tracers. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Hepatic UDP-glucose 13C isotopomers from [U-13C]glucose: a simple analysis by 13C NMR of urinary menthol glucuronide.

Menthol glucuronide was isolated from the urine of a healthy 70-kg female subject following ingestion of 400 mg of peppermint oil and 6 g of 99% [U-(13)C]glucose. Glucuronide (13)C-excess enrichment levels were 4-6% and thus provided high signal-to-noise ratios (SNRs) for confident assignment of (13)C-(13)C spin-coupled multiplet components within each (13)C resonance by (13)C NMR. The [U-(13)C]glucuronide isotopomer derived via direct pathway conversion of [U-(13)C]glucose to [U-(13)C]UDP-glucose was resolved from [1,2,3-(13)C(3)]- and [1,2-(13)C(2)]glucuronide isotopomers derived via Cori cycle or indirect pathway metabolism of [U-(13)C]glucose. In a second study, a group of four overnight-fasted patients (63 +/- 10 kg) with severe heart failure were given peppermint oil and infused with [U-(13)C]glucose for 4 hr (14 mg/kg prime, 0.12 mg/kg/min constant infusion) resulting in a steady-state plasma [U-(13)C]glucose enrichment of 4.6% +/- 0.6%. Menthol glucuronide was harvested and glucuronide (13)C-isotopomers were analyzed by (13)C NMR. [U-(13)C]glucuronide enrichment was 0.6% +/- 0.1%, and the sum of [1,2,3-(13)C(3)] and [1,2-(13)C(2)]glucuronide enrichments was 0.9% +/- 0.2%. From these data, flux of plasma glucose to hepatic UDPG was estimated to be 15% +/- 4% that of endogenous glucose production (EGP), and the Cori cycle accounted for at least 32% +/- 10% of GP.     

Comparison of [3,4‐13C2]glucose to [6,6‐2H2]glucose as a tracer for glucose turnover by nuclear magnetic resonance

A recently introduced tracer, [3,4‐¹³C₂]glucose, was compared to the widely used tracer, [6,6‐²H₂]glucose, for measurement of whole‐body glucose turnover. The rate of glucose production (GP) was measured in rats after primed infusions of [3,4‐¹³C₂]glucose, [6,6‐²H₂]glucose, or both tracers simultaneously followed by a constant infusion of tracer(s) over 90 min. Blood glucose was purified and converted into monoacetone glucose for analysis by ¹³C NMR (for [3,4‐¹³C₂]glucose) or ¹H and ²H NMR (for [6,6‐²H₂]glucose). The values of GP measured during infusion of each single tracer were not significantly different. In rats infused with both tracers simultaneously, GP was identical as reported by each tracer, 42 ± 4 μmol/kg/min. Since ²H and ¹³C enrichment in glucose is typically much less than 2% for in vivo studies, [3,4‐¹³C₂]glucose does not interfere with measurements of ¹³C or ²H enrichment patterns and therefore is valuable when multiple metabolic pathways are being evaluated simultaneously. Magn Reson Med 53:1479–1483, 2005. © 2005 Wiley‐Liss, Inc.     

Methods for metabolic evaluation of prostate cancer cells using proton and 13C HR‐MAS spectroscopy and [3‐13C] pyruvate as a metabolic substrate

Prostate cancer has been shown to undergo unique metabolic changes associated with neoplastic transformation, with associated changes in citrate, alanine, and lactate concentrations. ¹³C high resolution‐magic angle spinning (HR‐MAS) spectroscopy provides an opportunity to simultaneously investigate the metabolic pathways implicated in these changes by using ¹³C‐labeled substrates as metabolic probes. In this work, a method to reproducibly interrogate metabolism in prostate cancer cells in primary culture was developed using HR‐MAS spectroscopy. Optimization of cell culture protocols, labeling parameters, harvesting, storage, and transfer was performed. Using [3‐¹³C] pyruvate as a metabolic probe, ¹H and ¹³C HR‐MAS spectroscopy was used to quantify the net amount and fractional enrichment of several labeled metabolites that evolved in multiple cell samples from each of five different prostate cancers. Average enrichment across all cancers was 32.4 ± 5.4% for [3‐¹³C] alanine, 24.5 ± 5.4% for [4‐¹³C] glutamate, 9.1 ± 2.5% for [3‐¹³C] glutamate, 25.2 ± 5.7% for [3‐¹³C] aspartate, and 4.2 ± 1.0% for [3‐¹³C] lactate. Cell samples from the same parent population demonstrated reproducible fractional enrichments of alanine, glutamate, and aspartate to within 12%, 10%, and 10%, respectively. Furthermore, the cells produced a significant amount of [4‐¹³C] glutamate, which supports the bioenergetic theory for prostate cancer. These methods will allow further characterization of metabolic properties of prostate cancer cells in the future. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Cellular‐interstitial water exchange and its effect on the determination of contrast agent concentration in vivo: Dynamic contrast‐enhanced MRI of human internal obturator muscle

The purpose of this study was to assess the effects of cellular‐interstitial water exchange on estimates of tracer kinetics parameters obtained using rapid dynamic contrast‐enhanced (DCE) MRI. Data from the internal obturator muscle of six patients were examined using three models of water exchange: no exchange (NX), fast exchange limit (FXL), and intermediate rate (shutter‐speed [SS]). In combination with additional multiple flip angle (FA) data, a full two‐pool exchange model was also used. The results obtained using the NX model (transfer constant, K^trans = 0.049 ± 0.027 min^–1, apparent interstitial volume, v_e = 0.14 ± 0.04) were marginally higher than those obtained using the FXL model (K^trans = 0.045 ± 0.025 min^–1, v_e = 0.13 ± 0.04), but the error bars overlapped in two‐thirds of these parameter estimate pairs. Estimates of K^trans and v_e obtained using the SS model exceeded those obtained using the NX model in half the patients, and many estimates, including all those of intracellular residence time of water, t_i, were imprecise. Results obtained using the full two‐pool model fell between those obtained using FXL and NX models, and estimates of t_i were also imprecise. The results suggest that data obtained using clinically relevant DCE‐MRI are exchange‐insensitive and unsuitable for the assessment of cellular‐interstitial water exchange. Magn Reson Med 60:1011–1019, 2008. © 2008 Wiley‐Liss, Inc.     

Intravoxel incoherent motion imaging of focal hepatic lesions

Purpose:

To compare diffusivity values between malignant and benign focal hepatic lesions using the intravoxel incoherent motion model.

Materials and Methods:

This study included 84 focal hepatic lesions in 84 patients. Final diagnoses were as follows: hepatocellular carcinoma (n = 45), cholangiocarcinoma (n = 6), metastatic liver tumor (n = 3), cyst (n = 20), hemangioma (n = 5), inflammatory pseudotumor (n = 2), abscess (n = 2), and focal nodular hyperplasia (n = 1). Diffusion‐weighted images at 12 b‐values were used to obtain the diffusion coefficient of pure molecular diffusion (D), diffusion coefficient of microcirculation or perfusion‐related diffusion (D*), and perfusion‐related diffusion fraction (f). Parameters of malignant and benign focal hepatic lesions were compared using the Wilcoxon test. The diagnostic performance for distinguishing between malignant and benign hepatic lesions was also analyzed.

Results:

Both the D value (1.15 ± 0.21 × 10^?3 mm²/s [mean ± standard deviation]) and D* value (62.7 ±12.7 × 10^?3 mm²/s) in malignant lesions was significantly lower than that in benign lesions (D value [2.46 ± 0.45× 10^?3 mm²/s], P < 0.0001; D* value [87.6 ± 35.3 × 10^?3 mm²/s], P = 0.0008). The f value did not differ significantly between malignant (25.0 15.1 ± 15.1%) and benign lesions (30.1 ± 16.3%).

Conclusion:

D* and D values were suppressed in malignant lesions. However, the D value was more reliable for distinguishing between malignant and benign focal hepatic lesions. J. Magn. Reson. Imaging 2013;37:1371–1376. © 2012 Wiley Periodicals, Inc.

     

Caffeine ingestion and antigen‐stimulated human lymphocyte activation after prolonged cycling

This study investigated the effect of caffeine ingestion on antigen‐stimulated T‐ (CD4⁺ and CD8⁺) and natural killer (NK)‐ (CD3^?CD56⁺) cell activation after prolonged, strenuous cycling. In a randomized cross‐over design, nine male endurance cyclists (age: 22 ± 3 years, V?O_2peak: 62 ± 4 mL/kg/min, mean ± SD) cycled for 90 min at 70% V?O_2peak 60 min after ingesting 6 mg/kg body mass of caffeine (CAF) or placebo (PLA). Venous blood samples were obtained before supplementation, pre‐exercise, immediately post‐exercise and 1 h post‐exercise. Whole blood was stimulated with Pediacel (five in one) vaccine. At 1 h post‐exercise the number of antigen‐stimulated CD4⁺ cells expressing CD69 decreased on CAF compared with PLA [15 (17) × 10⁶ vs 23 (22) × 10⁶ cells/L, P<0.05]. In addition, the geometric mean fluorescence intensity (GMFI) of CD69 expression on antigen‐stimulated CD8⁺ cells decreased on CAF compared with PLA 1 h post‐exercise [78 (10)% vs 102 (24)%, P<0.05]. At the same time‐point GMFI of CD69 expression on antigen‐stimulated CD3^?CD56⁺ cells was increased on CAF compared with PLA [103 (9)% vs 87 (8)%, P<0.05]. These findings suggest that caffeine reduces antigen‐stimulated CD69 expression on T cells while at the same time increases NK‐cell activation 1 h after intensive cycling.     

Multitissue assessment of in vivo postprandial intracellular lipid partitioning in rats using localized (1) H-[(13) C] magnetic resonance spectroscopy

Excess accumulation of lipids in nonadipose tissues such as skeletal muscle and liver has been implicated in the development of obesity‐related disorders, but the cause of this ectopic lipid overload remains unknown. The aim of this study was to determine in vivo postprandial lipid partitioning in rat skeletal muscle and liver, using localized ¹H‐[¹³C] magnetic resonance spectroscopy in combination with the oral administration of ¹³C‐labeled lipids. Six rats were measured at baseline and 5 and 24 h after administration of 400 mg [U‐¹³C]‐labeled algal lipids. Five hours after administration, fractional ¹³C enrichments of the lipid pools in muscle and liver were increased 3.9‐fold and 4.6‐fold (P < 0.05), respectively, indicating that part of the ingested lipids had been taken up by muscle and liver tissue. At 24 h, fractional ¹³C enrichments of muscle and liver lipids were decreased 1.6‐fold and 2.2‐fold (P < 0.05), respectively, compared with the 5 h values. This can be interpreted as a depletion of ¹³C‐labeled lipids from the intracellular lipid pools as a consequence of lipid turnover. In conclusion, the novel application of ¹H‐[¹³C] magnetic resonance spectroscopy in combination with the oral administration of ¹³C‐labeled lipids is applicable for the longitudinal assessment of in vivo lipid partitioning between multiple tissues. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Improved synthesis of [18F]FLETT via a fully automated vacuum distillation method for [18F]2-fluoroethyl azide purification

The synthesis of [¹⁸F]2-fluoroethyl azide and its subsequent click reaction with 5-ethynyl-2′-deoxyuridine (EDU) to form [¹⁸F]FLETT was performed using an iPhase FlexLab module. The implementation of a vacuum distillation method afforded [¹⁸F]2-fluoroethyl azide in 87±5.3% radiochemical yield. The use of Cu(CH₃CN)₄PF₆ and TBTA as catalyst enabled us to fully automate the [¹⁸F]FLETT synthesis without the need for the operator to enter the radiation field. [¹⁸F]FLETT was produced in higher overall yield (41.3±6.5%) and shorter synthesis time (67 min) than with our previously reported manual method (32.5±2.5% in 130 min).     

Metabolite kinetics in C6 rat glioma model using magnetic resonance spectroscopic imaging of hyperpolarized [1‐13C]pyruvate

In addition to an increased lactate‐to‐pyruvate ratio, altered metabolism of a malignant glioma can be further characterized by its kinetics. Spatially resolved dynamic data of pyruvate and lactate from C6‐implanted female Sprague–Dawley rat brain were acquired using a spiral chemical shift imaging sequence after a bolus injection of a hyperpolarized [1‐¹³C]pyruvate. Apparent rate constants for the conversion of pyruvate to lactate in three different regions (glioma, normal appearing brain, and vasculature) were estimated based on a two‐site exchange model. The apparent conversion rate constant was 0.018 ± 0.004 s^?1 (mean ± standard deviation, n = 6) for glioma, 0.009 ± 0.003 s^?1 for normal brain, and 0.005 ± 0.001 s^?1 for vasculature, whereas the lactate‐to‐pyruvate ratio, the metabolic marker used to date to identify tumor regions, was 0.36 ± 0.07 (mean ± SD), 0.24 ± 0.07, and 0.12 ± 0.02 for glioma, normal brain, and vasculature, respectively. The data suggest that the apparent conversion rate better differentiate glioma from normal brain (P = 0.001, n = 6) than the lactate‐to‐pyruvate ratio (P = 0.02). Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

In vivo diffusion tensor MRI of the human heart: Reproducibility of breath‐hold and navigator‐based approaches

The aim of this study was to implement a quantitative in vivo cardiac diffusion tensor imaging (DTI) technique that was robust, reproducible, and feasible to perform in patients with cardiovascular disease. A stimulated‐echo single‐shot echo‐planar imaging (EPI) sequence with zonal excitation and parallel imaging was implemented, together with a novel modification of the prospective navigator (NAV) technique combined with a biofeedback mechanism. Ten volunteers were scanned on two different days, each time with both multiple breath‐hold (MBH) and NAV multislice protocols. Fractional anisotropy (FA), mean diffusivity (MD), and helix angle (HA) fiber maps were created. Comparison of initial and repeat scans showed good reproducibility for both MBH and NAV techniques for FA (P > 0.22), MD (P > 0.15), and HA (P > 0.28). Comparison of MBH and NAV FA (FA_MBHday1 = 0.60 ± 0.04, FA_NAVday1 = 0.60 ± 0.03, P = 0.57) and MD (MD_MBHday1 = 0.8 ± 0.2 × 10^?3 mm²/s, MD_NAVday1 = 0.9 ± 0.2 × 10^?3 mm²/s, P = 0.07) values showed no significant differences, while HA values (HA_MBHday1Endo = 22 ± 10°, HA_{MBHday1Mid‐Endo} = 20 ± 6°, HA_{MBHday1Mid‐Epi} = ?1 ± 6°, HA_MBHday1Epi = ?17 ± 6°, HA_NAVday1Endo = 7 ± 7°, HA_{NAVday1Mid‐Endo} = 13 ± 8°, HA_{NAVday1Mid‐Epi} = ?2 ± 7°, HA_NAVday1Epi = ?14 ± 6°) were significantly different. The scan duration was 20% longer with the NAV approach. Currently, the MBH approach is the more robust in normal volunteers. While the NAV technique still requires resolution of some bulk motion sensitivity issues, these preliminary experiments show its potential for in vivo clinical cardiac diffusion tensor imaging and for delivering high‐resolution in vivo 3D DTI tractography of the heart. Magn Reson Med 70:454–465, 2013. © 2012 Wiley Periodicals, Inc.     

Test‐retest variability of VO2max using total‐capture indirect calorimetry reveals linear relationship of VO2 and Power

This study aimed to analyze the intra‐individual variation in VO_2max of human subjects using total‐capture and free‐flow indirect calorimetry. Twenty‐seven men (27 ± 5 year; VO_2max 49‐79 mL?kg^?1?min^?1) performed two maximal exertion tests (CPETs) on a cycle ergometer, separated by a 7 ± 2 day interval. VO₂ and VCO₂ were assessed using an indirect calorimeter (Omnical) with total capture of exhalation in a free‐flow airstream. Thirteen subjects performed a third maximal exertion test using a breath‐by‐breath calorimeter (Oxycon Pro). On‐site validation was deemed a requirement. For the Omnical, the mean within‐subject CV for VO_2max was 1.2 ± 0.9% (0.0%‐4.4%) and for ergometer workload P _max 1.3 ± 1.3% (0%‐4.6%). VO_2max values with the Oxycon Pro were significantly lower in comparison with Omnical (P < 0.001; t test) with mean 3570 vs 4061 and difference SD 361 mL?min^?1. Validation results for the Omnical with methanol combustion were ?0.05 ± 0.70% (mean ± SD; n = 31) at the 225 mL?min^?1 VO₂ level and ?0.23 ± 0.80% (n = 31) at the 150 mL?min^?1 VCO₂ level. Results using gas infusion were 0.04 ± 0.75% (n = 34) and ?0.99 ± 1.05% (n = 24) over the respective 500‐6000 mL?min^?1 VO₂ and VCO₂ ranges. Validation results for the Oxycon Pro in breath‐by‐breath mode were ‐ 2.2 ± 1.6% (n = 12) for VO₂ and 5.7 ± 3.3% (n = 12) for VCO₂ over the 1000‐4000 mL?min^?1 range. On a Visual analog scale, participants reported improved breathing using the free‐flow indirect calorimetry (score 7.6 ± 1.2 vs 5.1 ± 2.7, P = 0.008). We conclude that total capturing free‐flow indirect calorimetry is suitable for measuring VO₂ even with the highest range. VO_2max was linear with the incline in P _max over the full range.     

Renal oxygenation changes during water loading as evaluated by BOLD MRI: effect of NOS inhibition

Purpose:

To demonstrate a possible role for endogenous release of nitric oxide in determining the response of water loading on intrarenal oxygenation as evaluated by blood oxygenation level‐dependent (BOLD) magnetic resonance imaging (MRI).

Materials and Methods:

Twelve Sprague Dawley rats (weight 344.9 ± 40.6 g) were equally divided into two groups, A and B. Water loading was implemented by continuous infusion of hypotonic saline containing glucose (0.25% NaCl, 0.5% glucose). Rats in group A were subject to water loading alone, while group B rats were dosed with N‐nitro‐L‐arginine methyl ester, (L‐NAME) (10.0 mg/kg) prior to water loading. T₂*‐weighted images of the kidneys were obtained on a Siemens 3T Verio MRI scanner using a multiple gradient recalled echo (mGRE) sequence.

Results:

Consistent with previous reports, group A exhibited a significant decrease in medullary R₂* during water loading (40.64 ± 1.10 s^?1 to 34.68 ± 1.49 s^?1, P < 0.05). On the other hand, in group B there was no decrease in R₂* during water loading (48.11 ± 2.38 s^?1 to 51.06 ± 2.18 s^?1). The increased prewater loading R₂* is due to the pretreatment with L‐NAME (40.82 ± 3.23 s^?1 to 48.11 ± 2.38 s^?1, P < 0.05).

Conclusion:

Our data suggest for the first time a role for endogenous nitric oxide in determining the response of renal medullary oxygenation to water loading. J. Magn. Reson. Imaging 2011;33:898–901. © 2011 Wiley‐Liss, Inc.

     

Evidence for 100% 13C NMR visibility of glucose in human skeletal muscle

The accuracy of the measurement of total muscle glucose by in vivo ¹³C NMR spectroscopy was tested in five normal volunteers during a euglycemic [1-¹³C]glucose infusion. The NMR visible concentration calibrated using an external reference was compared with that calculated from plasma glucose concentration, assuming that glucose remained extracellular. The NMR measurement always provided higher values than the calculation from plasma glucose: 0.51 ± 0.035 (mean ± SE) versus 0.38 ± 0.005 mmol/liter of muscle on average. This systematic difference was interpreted as reflecting the presence of muscle glucose-6-phospnate, co-resonating with free glucose. Thus, glucose appeared to be virtually 100% NMR visible in human skeletal muscle.