Problems in the assessment of magnesium depletion in the rat byin vivo³¹P NMR

Abstract

Prior in vitro studies, utilizing 31P nuclear magnetic resonance (31P NMR) to measure the chemical shift (sigma) of beta-ATP and lengthening of the phosphocreatine spin-spin (T2) relaxation time, suggested an assessment of their efficacy in measuring magnesium depletion in vivo. Dietary magnesium depletion (Mg2+ decreases) produced markedly lower magnesium in plasma (0.44 vs 1.13 mmol/liter) and bone (130 vs 190 mumol/g) but much smaller changes in muscle (41 vs 45 mumol/g, P less than 0.01), heart (42.5 vs 44.6 mumol/g), and brain (30 vs 32 mumol/g). NMR experiments in anesthetized rats in a Bruker 7-T vertical bore magnet showed that in Mg2+ decreases rats there was a significant change in brain beta-ATP shift (16.15 vs 16.03 ppm, P less than 0.05). These chemical shifts gave a calculated free [Mg2+] of 0.71 mM (control) and 0.48 mM (Mg2+ decreases). In muscle the change in beta-ATP shift was not significant (Mg2+ decreases 15.99 ppm, controls 15.96 ppm), corresponding to a calculated free Mg2+ of 0.83 and 0.95 mM, respectively. Phosphocreatine T2 (Carr-Purcell, spin-echo pulse sequence) was no different with Mg2+ decreases in muscle in vivo (surface coil) (Mg2+ decreases 136, control 142 ms) or in isolated perfused hearts (Helmholtz coil) (control 83, Mg2+ decreases 92 ms). 31P NMR is severely limited in its ability to detect dietary magnesium depletion in vivo. Measurement of beta-ATP shift in brain may allow studies of the effects of interaction in group studies but does not allow prediction of an individual magnesium status.

Free magnesium levels in normal human brain and brain tumors: 31P chemical-shift imaging measurements at 1.5 T.

“…The results presented here were based on analyses of uncorrected spectra. (4,6), whereas the mean shift for the brain voxels plotted for this data set was 18.92 ± 0.05 (n = 13). The y-P mean shifts were 4.80 ±0.06 ppm (n = 3) and 5.25 ± 0.030 (n = 13) for muscle and brain voxels, respectively.…”

“…In smooth muscle, however, ATPf percentages of 15 to 25% are observed, corresponding to [Mg2+], from 0.2 mM to 0.5 mM (5,9). [Mg2+]i has been measured as 0.4-0.7 mM in rat liver (10,11) and as -0.58-1.0 mM in rat brain (6,10,12); recently…”

“…) is a cofactor for many enzymes and signal-transduction proteins (1,2) (4)(5)(6). Measurements of [Mg2+i in perfused rat heart yield values of 0.8 mM (7,8).…”

EFFECT OF MAGNESIUM DEPLETION AND POTASSIUM DEPLETION AND CHLOROTHIAZIDE ON INTRACELLULAR pH IN THE RAT, STUDIED BY 31P NMR

“…These rats were compared with contr on the synthetic diet with a normal K+ and Mg2+ content (Adam et al 1988). Chlorothiazide v administered by adding 250 mg/kg to the synthetic diet for 1-2 months.…”

Fast imaging of phosphocreatine in the normal human myocardium using a three‐dimensional RARE pulse sequence at 4 Tesla

“…The acquisition of multiple echoes following a single excitation would cover k-space more efficiently and allow a longer TR, resulting in a higher signal-to-noise ratio (SNR) while reducing the total scan time (14). A transverse (T2) relaxation time of 190 msec for PCr has been measured in the rat heart (15). Thus, PCr is well suited to multi-echo techniques.…”