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31P NMR spectra were acquired at I = 0.25, pH 5.75 -8.5 containing 5 mM each of K2ATP, Na2PCr, K2Glu 6-P and KH2PO4 and a calculated free [Mg2+] from 0 to 5 mM. Within the range of intracellular pH from 7 to 7.3 and free [Mg2+] from 0.25 to 1 mM, the chemical shift of the α-β peak of ATP, δATP was only 0.57 PPM giving low free [Mg2+]. In contrast, the β relative to a peak height of ATP at intracellular conditions increased 2.3 fold. Comparison of the estimation of free [Mg2+] in the working rat heart perfused with lOmM glucose was 0.48 ±0.22 mM from δαβ, 1.23 ±0.03 mM from the measured [citrate]/[isocitrate] ratio, and 0.95 ±0.17 mM from pYct peak height ratio. These results suggest that the free [Mg2+] estimated by the convention method of measuring oβ underestimates free [Mg2+] in tissue by about a factor of 2.


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FASEB Journal

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