RELATIONSHIP BETWEEN SKELETAL-MUSCLE INTRACELLULAR IONIZED MAGNESIUM AND MEASUREMENTS OF BLOOD MAGNESIUM

The current laboratory approach to assessing magnesium status is based on determining the concentration of total Mg ((Mg)) in serum or plasm a. This strategy is problematic in that the amount of Mg in blood is l ess than 1% of total body Mg and does not accurately reflect (Mg) in o ther tissues, Furthermore, the (Mg) of blood does not distinguish biol ogically active, ionized Mg from the bound fraction, The goal of this study was to determine intracellular ionized Mg ((MS(++))(i)) of skele tal muscle in vivo and to compare results with the (Mg) of blood const ituents, (Mg++)(i) was determined in resting skeletal muscle by using phosphorus 31 magnetic resonance (P-31-MR) spectroscopy. (Mg) was meas ured in serum (S(Mg)), serum ultrafiltrate (UF(Mg)), mononuclear blood cells (MBC(Mg)), and red blood cells (RBC(Mg)) by using atomic absorp tion spectroscopy or a colorimetric assay, In a sample of 60 healthy a dult subjects, skeletal muscle (Mg++), = 557 +/- 97 mu mol/L (mean a S D); S(Mg) = 0.78 +/- 0.09 mmol/L; UF(Mg) = 0.60 +/- 0.12 mmol/L; MBC(M g) = 13.8 +/- 2.3 mmol/L; and, RBC(Mg) = 1.92 +/- 0.33 mmol/L, A signi ficant negative correlation was found between (Mg++)(i) and S(Mg) (r = -0.43, p < 0.05). S(Mg) was significantly lower (p < 0.05) and (Mg++) (i) significantly higher (p < 0.05) in women than in men, but neither was related to age. These findings provide new insight into the relati onship between blood Mg measures and (Mg++)(i) of the largest soft tis sue mass of the human body.