Plasma kinetics of magnesium and calcium stable isotope tracers in a human subject after simultaneous oral ingestion of 25Mg and 44Ca determined by thermal ionization mass spectrometry

Protein abundance quantification in embryonic stem cells using incomplete metabolic labelling with 15N amino acids, matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry, and analysis of relative isotopologue abundances of peptides

An isotope dilution method for protein quantification is presented in the context of matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOFMS) and mass fingerprinting experiments, revealing an unappreciated high reproducibility and accuracy of relative peak intensity measurements. Labelled proteins were generated by growing cells in a medium containing (15)N-enriched amino acids, and were mixed with proteins of natural isotopic composition from control cells in ratios of approximately 0:1, 1:7, 1:2, 2:1, 7:1, and 1:0 (labelled/unlabelled). Mixtures were separated by two-dimensional gel electrophoresis and analysed by MALDI-TOFMS using typical experimental conditions. A linear relationship is demonstrated between the relative isotopologue abundances (RIA values) for particular peaks in the isotopic distribution of tryptic peptide fragments of the proteins, and the mole fractions of labelled proteins in the mixture. Analysis of RIA values (ARIA quantification) for peptides of six typical silver-stained protein spots for the various mixtures could reproduce the experimentally contrived ratios with approximate errors between 4% (2:1 mixture) and about 18% (1:7 mixture). A consideration of error and its propagation is discussed. ARIA does not require complete separation of the isotope patterns of labelled and unlabelled peptides, and is therefore advantageous in combination with all kinds of labelling experiments in biological systems, because it is compatible with minimal metabolic incorporation of labelling reagent. Simulations indicate that the minimum required (15)N enrichment of the total amino acid pool sufficient for ARIA is less than 4%. In an accompanying paper in this issue, we apply ARIA to proteins differentially labelled with isotope-coded alkylation reagents.

Determination of epithelial magnesium transport with stable isotopes

The inability to adequately determine Mg(2+) flux rates with radiotracer studies has stymied our efforts to understand how magnesium is transported by epithelial cells. To evaluate epithelial Mg(2+) transport, a stable 25Mg isotope was used to measure magnesium uptake into normal and Mg(2+)-depleted Madin-Darby canine kidney (MDCK) cells. 25Mg entry rates were significantly increased in Mg(2+)-depleted cells relative to those cultured in normal magnesium media, 0.5 mM. 25Mg uptake was inhibited by external La(3+) but not Ca(2+) in both normal and Mg(2+)-depleted cells suggesting a specific entry pathway. These results with 25Mg were the same as with microfluorescence determinations using mag-fura-2. We have shown that Mg(2+) entry into epithelial cells reflects transepithelial transport; accordingly, increased Mg(2+) uptake in Mg(2+)-depleted cells provides an important intrinsic control of renal magnesium absorption. Furthermore, these studies indicate that cellular Mg(2+) transport may be quantitated with the use of stable isotopes that may be successfully applied to cells other than epithelia.

Methodologies for using stable isotopes to assess magnesium absorption and secretion in children

OBJECTIVE

To provide methodological information regarding the absorption and excretion of dietary magnesium by children and adolescents.

METHODS

Recently, stable isotope techniques for assessing magnesium absorption and excretion have been developed which allow for these studies to be safely performed in subjects of all ages. In the report, we describe the dosing and sample requirements for such studies.

RESULTS

Our data demonstrate that, after oral and intravenous dosing of isotopes, a complete 72-hour urine collection will allow for determination of fractional magnesium absorption. In our study, urinary, but not endogenous fecal magnesium excretion, was closely correlated with magnesium intake (r = 0.47, p = 0.02 vs r = 0.08, p = 0.69). As endogenous fecal magnesium excretion is small relative to urinary magnesium excretion, measurement of endogenous fecal magnesium excretion is not needed to make a reasonable estimate of net magnesium retention for most studies. Using high-precision analytical techniques, an intravenous dose of 25Mg of approximately 0.2 to 0.3 mg/kg would be adequate for absorption measurements.

CONCLUSIONS

The cost and availability of isotopes and their analysis are such that it should be feasible for increasing numbers of investigators to make use of these techniques.

The relationship between magnesium and calcium kinetics in 9- to 14-year-old children

Few measurements of magnesium (Mg) kinetics have been performed, especially in children and adolescents. Simultaneous Mg and calcium (Ca) kinetics following intravenous administration of 42Ca and 25Mg were measured in 22 children (10 boys and 12 girls) 9-14 years of age. Kinetic values, including the body pool masses (MgComp and CaComp for the Mg and Ca compartment masses, respectively) and bone/tissue (hard and soft tissue) Ca and Mg deposition rates (Vo+Ca and Vo+Mg respectively) were calculated using the Simulation, Analysis, and Modeling (SAAM) program. No significant differences were found between males and females (p > 0.3 for each comparison) for pool masses or deposition rates. Vo+Ca and Vo+Mg were highly correlated (r = 0.78, p < 0.001). Vo+Mg but not Vo+Ca, was closely correlated with body weight (r = 0.55, p < 0.01 and r = 0.16, p = 0.47, respectively). Similarly, MgComp was more closely correlated with body weight (r = 0.76, p < 0.01) than CaComp (r = 037,p = 0.10). Neither Vo+Ca nor CaComp increased significantly over the age range studied. However, Vo+Mg and MgComp were significantly positively correlated with age. These findings demonstrate differences in regulation of Ca versus Mg kinetics by young adolescents. These differences may be related to the greater utilization of Mg for muscle and soft tissue growth. The close relationship between Vo+Mg and body weight suggests that the current practice of relating Mg intake requirements to body weight, as well as age, is appropriate for young adolescents.

Study of magnesium bioavailability using stable isotopes and the inductively-coupled plasma mass spectrometry technique in the rat: single and double labelling approaches

The present work aimed to investigate the feasibility of using stable isotopes and inductively-coupled plasma mass spectrometry (ICP/MS) to study Mg absorption in rats. Male Wistar rats, aged 7 weeks and weighing 180 g, were used. They were fed on a semi-purified diet containing 1070 mg Mg/kg, and had free access to feed and distilled water. In the first experiment, after a 16 d adaptation period, two doses of enriched 25Mg (6 and 12 mg) were administered by oral intubation, faeces and urine were collected daily and blood was sampled. Isotope ratios were determined by ICP/MS. 'True' absorption values, using the faecal isotope data, were 0.63 and 0.56 in rats receiving 6 and 12 mg 25Mg respectively, while apparent absorption was 0.50 for two successive periods of metabolic balance studies. Moreover, the oral isotope administration resulted in a measurable isotopic enrichment in plasma within hours which was still detectable on the third day following the isotope administration. In the second experiment, investigating the double labelling technique, similar rats were dosed simultaneously with 5 mg 26Mg orally (premixed with diet) and 0.29 mg 25Mg intravenously. The calculated Mg true absorption values were very similar when calculated from blood or urine data (0.38) but were lower than that obtained from faecal data (0.50). The possible causes of such an unexpected difference and limits of the double labelling technique for Mg absorption are discussed here. Together these results indicate that although 25Mg and 26Mg isotopes have high natural abundance, the described methodology permits meaningful investigations of Mg bioavailability and metabolism.

Effects of development on techniques for calcium stable isotope studies in children

We evaluated the effects of age and pubertal status on the tracer excess measured in urine samples collected during the 120 h after the oral and intravenous administration of calcium stable isotopes to 90 children (16 preterm infants and 74 females aged 4.9-16.8 years). Analysis of subjects shows differences in distribution volume and fractional absorption of calcium based on developmental status, with four separate groups being identified. These are: (i) premature infants; (ii) prepubertal girls (Tanner stage 1); (iii) early pubertal girls (Tanner states 2-3); and (iv) late pubertal girls (Tanner stages 4-5). Identification of groups in this fashion allows for adequate dosing in calcium stable isotope studies, with the minimum dose being given to achieve an increase in urinary isotopic content consistent with the precision of the analytical equipment.