Intracellular Mg+2 and PCr/Pi Values are a Function of Gender and Excercise Status
K.M. Ward, S.S. Rajan, D. Clauw, D. Radulovic Georgetown University Medical Center, Washington D.C.
Introduction:
Magnesium, the second most abundant intracellular cation, is a critical cofactor in over 300 enzymatic reactions within normal human cells. These reactions enable the cell to produce and use energy and to synthesize cellular proteins and nucleic acids (1). Clinically, magnesium deficiency has been linked to a wide variety of clinical problems such as atherosclerosis, myocardial infarction, hypertension, cancer, kidney stones, psychiatric problems, migraine, and premenstrual syndrome. 31P NMR spectroscopy is known to be able to measure free intracellular magnesium, the biologically active form of this ion.
Exercise appears to provide a protective effect on thepool of total magnesium retained in skeletal muscle (2). Magnesium, in turn, may enhance the ability of skeletal muscle to increase its strength during progressive weight training (3).
We used the magnesium sensitive chemical shift of the Beta-nucleotide triphosphate signal in 31P MRS to measure [Mg 2+] in the skeletal muscle of normal male and female controls. PCr/Pi, Pi/ATP, and PCr/ATP ratios were also determined from the spectra.
Sample phosphorus spectrum of a human calf muscle
Methods:
31P Nuclear Magnetic Resonance spectroscopy was performed on the right calf muscle of 32 normal female controls (ages 23-48, mean 33.2 +/- 6.4 yrs) and 33 normal male controls (ages 22-48, mean 34.0 +/- 7.2 yrs). The examinations were performed on a Siemens clinical spectrometer at 1.5T using a dual-tuned surface coil. A twin loop (10,12cm) concave coil design was employed, and placed adjacent to the calf muscle. Fully relaxed spectra were recorded using a rectangular RF pulse, TR 2.5 sec, 2000 Hz spectral width, 128 acquisitions, and 4096 data points. Following zero filling to 8196 data points, 2 Hz line broadening, phasing , and baseline flattening, the muscle spectra were fitted to lorentzian curves utilizing NMR1 (Tripos Associates, Inc., St. Louis, MO) software. Each individual spectrum was analyzed three times from the raw data through to the final lorentzian curve fit. The free intracellular magnesium [Mg 2+] levels were calculated using Henderson-Hasselbach equation with the parameters of Mosher, et al (5).
Normals studied for this research were questioned and scored for their excercise status using the Minnesota Health Activity Questionaire (MHHQ) (4).
Results:
Males had significantly lower muscle intracellular free magnesium levels (499.8 microM +/- 26.3 microM vs 530.7 microM +/- 36.5 microM, p=0.001, d.f.=63, ANOVA) and higher pH levels than females (7.015 +/- 0.021 vs 7.001 +/- 0.024, p=0.026, d.f.=63, ANOVA). There was a significant negative relationship between exercise levels and free magnesium in males (Rs = -0.4102, p=0.018) and females (Rs = -0.4759, p=0.009) fell with increased exercise.
Discussion:
This study is the largest we are aware of examining the factors influencing skeletal muscle metabolism in healthy individuals. Further experiments are planned to investigate the factors causing the gender difference, both in the level of and the variance in the intracellular free magnesium values. It is interesting to note that while recent reviews have emphasized the use of 31P MRS as a tool to study intracellular free magnesium and cellular bioenergetics in skeletal muscle, our findings indicate that these results must be interpreted in light of the gender and exercise status of the individuals studied.
References:
1. Elin, R.J. Dis. Mon. 34:4, 165, 1988.
2. Brilla, L.R., et al. Metabolism. 38:8, 797, 1989.
3. Brilla, L.R., et al. Jrnl. Mer. Coll. Nutr. 11:3, 326, 1992.