Multiple Tracer Experiments Research Articles

Simultaneous tracers and a unified model of positional and mass isotopomers for quantification of metabolic flux in liver

Computational models based on the metabolism of stable isotope tracers can yield valuable insight into the metabolic basis of disease. The complexity of these models is limited by the number of tracers and the ability to characterize tracer labeling in downstream metabolites. NMR spectroscopy is ideal for multiple tracer experiments since it precisely detects the position of tracer nuclei in molecules, but it lacks sensitivity for detecting low-concentration metabolites. GC-MS detects stable isotope mass enrichment in low-concentration metabolites, but lacks nuclei and positional specificity. We performed liver perfusions and in vivo infusions of 2H and 13C tracers, yielding complex glucose isotopomers that were assigned by NMR and fit to a newly developed metabolic model. Fluxes regressed from 2H and 13C NMR positional isotopomer enrichments served to validate GC-MS-based flux estimates obtained from the same experimental samples. NMR-derived fluxes were largely recapitulated by modeling the mass isotopomer distributions of six glucose fragment ions measured by GC-MS. Modest differences related to limited fragmentation coverage of glucose C1–C3 were identified, but fluxes such as gluconeogenesis, glycogenolysis, cataplerosis and TCA cycle flux were tightly correlated between the methods. Most importantly, modeling of GC-MS data could assign fluxes in primary mouse hepatocytes, an experiment that is impractical by 2H or 13C NMR.

A Long-Term WRF Meteorological Archive for Dispersion Simulations: Application to Controlled Tracer Experiments

AbstractA long-term archive of meteorological data using the Weather Research and Forecasting (WRF) Model was created to provide data that are compatible with the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) dispersion model and to serve as initial and boundary conditions for simulations at a finer resolution. On the basis of these WRF data, generated with a variety of planetary boundary layer (PBL) schemes and nudging options, the HYSPLIT model was run to simulate four controlled tracer experiments—the Cross Appalachian Tracer Experiment (CAPTEX), the Across North America Tracer Experiment (ANATEX), a 1980 release in Oklahoma City, Oklahoma (OKC80), and the Metropolitan Tracer Experiment (METREX)—covering different time periods with diverse durations, including a summer day, several days in autumn, three months during winter, and one full year, respectively. The evaluation of the WRF results utilizing conventional observations showed a similar statistical performance for the different PBL schemes. Given the limited information the meteorological evaluation alone can provide, the authors used the dispersion evaluation with measurements from multiple tracer experiments to gain further insight into the most appropriate WRF configuration to generate reasonable data for dispersion applications. The dispersion simulations that were based on WRF data generated equal or slightly better statistical performance than did those driven by the North American Regional Reanalysis (NARR) dataset. The statistical comparison showed a mixed impact for the dispersion results driven by the nonnudged and nudged WRF data. The main advantage of the WRF data is the availability of hourly meteorological data from 1980 to the present and the inclusion of additional variables that are relevant to atmospheric dispersion and are not available from NARR. This WRF dataset will be accessible online, providing additional capabilities for using different meteorological inputs and a variety of options to compute the HYSPLIT mixing parameters.

Comprehensive metabolic modeling of multiple 13C-isotopomer data sets to study metabolism in perfused working hearts.

In many forms of cardiomyopathy, alterations in energy substrate metabolism play a key role in disease pathogenesis. Stable isotope tracing in rodent heart perfusion systems can be used to determine cardiac metabolic fluxes, namely those relative fluxes that contribute to pyruvate, the acetyl-CoA pool, and pyruvate anaplerosis, which are critical to cardiac homeostasis. Methods have previously been developed to interrogate these relative fluxes using isotopomer enrichments of measured metabolites and algebraic equations to determine a predefined metabolic flux model. However, this approach is exquisitely sensitive to measurement error, thus precluding accurate relative flux parameter determination. In this study, we applied a novel mathematical approach to determine relative cardiac metabolic fluxes using 13C-metabolic flux analysis (13C-MFA) aided by multiple tracer experiments and integrated data analysis. Using 13C-MFA, we validated a metabolic network model to explain myocardial energy substrate metabolism. Four different 13C-labeled substrates were queried (i.e., glucose, lactate, pyruvate, and oleate) based on a previously published study. We integrated the analysis of the complete set of isotopomer data gathered from these mouse heart perfusion experiments into a single comprehensive network model that delineates substrate contributions to both pyruvate and acetyl-CoA pools at a greater resolution than that offered by traditional methods using algebraic equations. To our knowledge, this is the first rigorous application of 13C-MFA to interrogate data from multiple tracer experiments in the perfused heart. We anticipate that this approach can be used widely to study energy substrate metabolism in this and other similar biological systems.

Characterizing Heterogeneous Processes of Water Flow and Solute Transport in Soils Using Multiple Tracer Experiments

It is important yet challenging to adequately describe heterogeneous flow processes in soils. The objectives of this study were to “visualize” heterogeneous processes of water flow and solute transport in soils by using multiple tracers and to quantify heterogeneities of soil water flow and solute transport by using a discrete random cascade model. Field tracer experiments on two soils were conducted with sequences of iodine–nitrate and iodine–bromide mixtures. On the basis of the experimental data, a random cascade model, an annular histogram, and a similarity index were applied to characterize the heterogeneous flow and transport processes in the soils. Results showed that water and solute penetrated into deep soil preferentially mainly because of the macropore flow. At the beginning of infiltration, the solute transport heterogeneity was mainly related to the saturation process of soil macropores. As more water infiltrated into the soil, the solute transport heterogeneity was mainly attributable to the change of flow paths. After the macropores were saturated, unstable infiltration fronts resulted in the transport heterogeneity. The experimental data and analyses of the random cascade model demonstrated that the solute transport process included more heterogeneous information than the heterogeneous water flow, which was related to the soil heterogeneity, water flow heterogeneity, and the heterogeneity of the solute transport itself.

Production of 13C-labelled β-carotenefrom Dunaliella salina

A 4.5 litre photobioreactor was developed and used to produce 13 C-labelled b-carotene by feeding 13 CO2 to growing ells of the marine alga Dunaliella salina. Three runs were carried out using different feeding regimes producing b-carotene containing 40, 56 and 76 Atom% 13 C. Approximately 145 mg b-carotene was recovered from each run. Incorporation of 13 C into b-carotene ranged from 1.4 to 2.8%. The labelled b-carotene produced would allow multiple tracer experiments for the study of human carotenoid metabolism.

Interpretation and evaluation of multiple tracer experiments

Tracer experiments for studying transport processes in multiphase systems are discussed. Such experiments are useful in in vivo physiological studies of transport processes across membranes, in engineering studies of porous packed columns, and in chromatography. In all these processes, the main forward flow is in one continuous phase from which fluid diffuses into a stagnant outer phase (and back). A nondiffusible tracer is used to characterize the flow in the main phase (inside the capillaries of an organ or between the particles of a column), and diffusible tracers are used to study the transport through the interface (membrane or film) and in the outer phase (extravascular space or inside a porous particle). From the concentration history of the different tracers at the outlet, we can reconstruct sojourn time distributions in the different phases. The statistical properties and the relations between the distributions and their moments are discussed. Methods are given for estimating interfacial transport coefficients (or permeabilities) as well as the diffusion coefficients in the outer phase from the moments of the measured distributions. It is also shown that these relations simplify considerably the mathematical modelling of such systems.