Ratiometric analysis in hyperpolarized NMR (I): test of the two‐site exchange model and the quantification of reaction rate constants

Conventional methods for the analysis of in vivo hyperpolarized ¹³C NMR data from the lactate dehydrogenase (LDH) reaction usually make assumptions on the stability of rate constants and/or the validity of the two‐site exchange model. In this study, we developed a framework to test the validity of the assumption of stable reaction rate constants and the two‐site exchange model in vivo via ratiometric fitting of the time courses of the signal ratio L(t)/P(t). Our analysis provided evidence that the LDH enzymatic kinetics observed by hyperpolarized NMR are in near‐equilibrium and satisfy the two‐site exchange model for only a specific time window. In addition, we quantified both the forward and reverse exchange rate constants of the LDH reaction for the transgenic and mouse xenograft models of breast cancer using the ratio fitting method developed, which includes only two modeling parameters and is less sensitive to the influence of instrument settings/protocols, such as flip angles, degree of polarization and tracer dosage. We further compared the ratio fitting method with a conventional two‐site exchange modeling method, i.e. the differential equation fitting method, using both the experimental and simulated hyperpolarized NMR data. The ratio fitting method appeared to fit better than the differential equation fitting method for the reverse rate constant on the mouse tumor data, with less relative errors on average, whereas the differential equation fitting method also resulted in a negative reverse rate constant for one tumor. The simulation results indicated that the accuracy of both methods depends on the width of the transport function, noise level and rate constant ratio; one method may be more accurate than the other based on the experimental/biological conditions aforementioned. We were able to categorize our tumor models into specific conditions of the computer simulation and to estimate the errors of rate quantification. We also discussed possible approaches to the development of more accurate rate quantification methods for hyperpolarized NMR. Copyright © 2013 John Wiley & Sons, Ltd.     

Effect of Vitamin D and Docosahexaenoic Acid Co-Supplementation on Vitamin D Status,Body Composition,and Metabolic Markers in Obese Children: A Randomized,Double Blind,Controlled Study

Obese children are at high risk of developing vitamin D deficiency. Omega-3 polyunsaturated fatty acids and their derivatives might have a beneficial effect on vitamin D status of obese children, due to their anti-inflammatory action, and increasing its absorption. This multicenter, randomized, double-blind controlled study aims to investigate the effect of vitamin D and docosahexaenoic acid (DHA) co-supplementation for six months on vitamin D status, body composition, and metabolic markers of obese children with vitamin D deficiency. A total of 108 children were enrolled and 73 children completed the study: 33 were supplemented with an oral dose of 500 mg of DHA and 1200 IU/day of vitamin D3 and 41 were supplemented with 1200 IU/day of vitamin D3 + wheat germ oil. At the end of the study, more than 50% of the subjects improved their vitamin D status. However, co-supplementation was not more effective than vitamin D plus wheat germ oil. Fat mass percentage was significantly reduced, and body mass index improved in both groups, even if all the subjects were still obese at the end of the study. Children receiving both vitamin D and DHA presented a higher increase of DHA levels that could be relevant to prevent inflammatory-associated complications of obesity, but they had no effect on vitamin D levels.     

Regional determination of oxygen uptake in rodent lungs using hyperpolarized gas and an analytical treatment of intrapulmonary gas redistribution

A method is presented which allows for the accurate extraction of regional functional metrics in rodent lungs using hyperpolarized gas. The technique is based on the combination of measured T(1) decay, an independent measure of specific ventilation and mass balance considerations to extract the regional oxygen levels and uptake. In phantom and animal experiments, it is demonstrated that the redistribution of gas during the measurement is a significant confounding factor, and this effect is addressed analytically. The resulting parameterization of gas flow increases the accuracy of oxygen-sensitive MRI, and may also be used independently to assess air trapping and airway constriction. Limitations of the technique with respect to spatial resolution and robustness are also discussed.     

Metabolism of hyperpolarized [1-(13) C]pyruvate in the isolated perfused rat lung - an ischemia study

We report studies of the effect of ischemia on the metabolic activity of the intact perfused lung and its restoration after a period of reperfusion. Two groups of rat lungs were studied using hyperpolarized 1‐¹³C pyruvate to compare the rate of lactate labeling differing only in the temporal ordering of ischemic and normoxic acquisitions. In both cases, a several‐fold increase in lactate labeling was observed immediately after a 25‐min ischemia event as was its reversal back to the baseline after 30–40 min of resumed perfusion (n = 5, p < 0.025 for both comparisons). These results were corroborated by ³¹P spectroscopy and correspond well to measured changes in lactate pool size determined by ¹H spectroscopy of freeze‐clamped specimens. Copyright © 2012 John Wiley & Sons, Ltd.     

Epicardial fat thickness significantly decreases after short-term growth hormone (GH) replacement therapy in adults with GH deficiency

Background and AimGrowth Hormone Deficiency (GHD) is characterized by increased visceral fat accumulation. Echocardiographic epicardial fat thickness is a new marker of visceral adiposity. Aim of the present study was to evaluate whether epicardial fat thickness can significantly change and therefore serve as a marker of visceral fat reduction after short-term rhGH replacement therapy in patients with adult-onset GHD.Methods and ResultsEchocardiographic epicardial fat thickness was measured in 18 patients (10 M, 8 F, age 48 ± 11.8 yrs, BMI 29 ± 5.9 kg/m²) with adult-onset GHD, at baseline and after 6 and 12 months of rhGH therapy and in 18 healthy matched controls, at baseline. Echocardiographic epicardial fat thickness, conventional anthropometric and metabolic parameters, body fat percentage and quality of life were also evaluated. Epicardial fat thickness in adult GHD patients was higher than in controls (9.8 ± 2.8 vs 8 ± 3 mm, p < 0.05). Epicardial fat thickness significantly decreased after 6-months of rhGH replacement therapy (from 9.8 ± 2.8 to 7.0 ± 2.3 mm, P < 0.01, i.e. ?29% from baseline). After 12 months of rhGH replacement therapy, epicardial fat thickness showed a further significant decrease (from 7.0 ± 2.3 to 5.9 ± 3.1 mm, P < 0.01, i.e. ?40% from baseline). No significant changes in BMI or waist circumference after 6 or 12 months of rhGH therapy were observed.ConclusionsEchocardiographic epicardial fat thickness may represent a valuable and easy marker of visceral fat and visceral fat changes during rhGH replacement treatment in patients with adult-onset growth hormone deficiency.     

Metabolic spectroscopy of inflammation in a bleomycin‐induced lung injury model using hyperpolarized 1‐13C pyruvate

Metabolic activity in the lung is known to change in response to external insults, inflammation, and cancer. We report measurements of metabolism in the isolated, perfused rat lung of healthy controls and in diseased lungs undergoing acute inflammation using hyperpolarized 1‐¹³C‐labeled pyruvate. The overall apparent activity of lactate dehydrogenase is shown to increase significantly (on average by a factor of 3.3) at the 7 day acute stage and to revert substantially to baseline at 21 days, while other markers indicating monocarboxylate uptake and transamination rate are unchanged. Elevated lung lactate signal levels correlate well with phosphodiester levels as determined with ³¹P spectroscopy and with the presence of neutrophils as determined by histology, consistent with a relationship between intracellular lactate pool labeling and the density and type of inflammatory cells present. We discuss several alternate hypotheses, and conclude that the most probable source of the observed signal increase is direct uptake and metabolism of pyruvate by inflammatory cells and primarily neutrophils. This signal is seen in high contrast to the low baseline activity of the lung. Copyright © 2014 John Wiley & Sons, Ltd.