[Hormonal regulation and metabolic inter-relations of magnesium]. / Régulation hormonale et interrelations métaboliques du magnésium.

Magnesium ion is of great importance in physiology by its intervention in 300 enzymatic systems, its role in membrane structure and its function in neuromuscular excitability. The skeleton is the first pool of magnesium in the body. Intestinal absorption, renal metabolism, bone accretion and resorption of magnesium are very similar to those of calcium. Magnesium metabolism is accurately controlled, in particular by parathyroid hormone, 25 - dihydroxy vitamin D3, calcitonin, catecholamine and estrogens. The main regulation mechanisms of magnesium metabolism are located in the kidney which is the principal excretory organ.

Comparison of the feedback effect of magnesium and calcium on parathyroid hormone secretion in man.

Administration of high doses of magnesium is known to produce a decrease in parathyroid hormone (PTH) secretion in human patients but the effect of magnesium on the secretion of PTH in healthy man is not known. We have looked at the effect of a relatively moderate i.v. dose of magnesium (7.08 mmol) in seven healthy men. In addition and for comparison the effect of calcium (4.25 mmol) was studied. Two magnesium salts were considered, magnesium sulphate (MgSO4) and magnesium pyrrolidone carboxylate (MgPC). Four i.v. injections were given at 08.00 h (MgPC, NaCl (control), MgSO4 and Ca gluconate), with an interval of 1 week between each injection. Whatever the magnesium salt the variations in plasma concentrations of magnesium were the same whereas no change in erythrocyte magnesium was observed. Plasma concentration of C-terminal PTH did not show significant variations after MgPC or saline injection. Both MgSO4 and Ca gluconate produced a statistically significant 30% decrease in plasma PTH levels 45 min after the injection. The effect was more sustained with calcium (2 h) than with magnesium (45 min). The urinary excretion of magnesium was significantly higher after injection of MgSO4 than after MgPC. These results suggest that magnesium was, on a molar basis, less potent than calcium in regulating PTH secretion in vivo, that the nature of the magnesium salt used must be kept in mind for the interpretation of the effect of magnesium on PTH secretion in vivo and that the decrease in plasma PTH can partly explain the larger urinary excretion of magnesium after MgSO4 than after MgPC.

Prevalence of magnesium and potassium deficiencies in the elderly.

Concentrations of magnesium and potassium in erythrocytes and plasma were determined in a population of 381 unselected elderly men and women, most of them in their eighties. The effects of biological factors (age, sex, weight) and a large set of pathological conditions, malignant or not, were examined. Analyses of variance showed a relation between age and concentrations of plasma potassium and between weight and concentrations of plasma magnesium. The chi-square test showed correlations between low concentrations of plasma magnesium and diabetes, abuse of alcohol and tobacco, and also between low values for erythrocyte magnesium and hypertension. Low values for plasma potassium were correlated with hypertension whereas high values were correlated with cardiovascular disease. Although some of the differences in the mean concentrations observed were statistically significant, these differences were always small. Most interesting was the distribution of the concentrations of the cations. This study shows that assays of both of these cations in erythrocytes were better than assays in plasma to evidence a deficiency. Indeed, about 20% of the studied population had low concentrations of both erythrocyte potassium and magnesium, whereas 2 and 10% had low values for plasma potassium and magnesium, respectively. This study underlines the large prevalence of magnesium and potassium deficiencies in the elderly, an observation we could not attribute to pathology or treatment. Routine electrolyte studies therefore appear to be justified in aged human subjects.

[Feedback of the magnesium ion on the parathyroid hormone: differences in the action of magnesium sulfate and pyrrolidone carboxylate]. / Rétrocontrôle de l'ion magnésium sur la parathormone: différences d'action du sulfate et du pyrrolidone carboxylate de magnésium.

Fourty-five minutes after an intravenous injection of Mg SO4 (170 mg of element Mg), in 7 young and healthy men, a significant decrease in circulating 53-84 PTH has been observed. An injection of magnesium pyrrolidone carboxylate (170 mg of Mg) failed to induce changes in plasma levels of PTH. The urinary excretion of Mg was 2-fold higher after the injection of Mg SO4 than after the injection of magnesium pyrrolidone carboxylate. For both magnesium salts used, the time patterns of plasma magnesium concentrations were the same and red blood cells magnesium was not increased. These results suggest that, in our experimental conditions, the retention of magnesium was higher after magnesium pyrrolidone carboxylate than after Mg SO4 and that the drop in plasma PTH could partly explain the larger urinary excretion of magnesium after Mg SO4.

Magnesium: metabolism and hormonal regulation in different species.

1. The magnesium ion is of great importance in physiology by its intervention in 300 enzymatic systems, its membrane role and its function in neuromuscular excitability. 2. The skeleton is the first pool of magnesium in the animal body. 3. For intestinal absorption, renal metabolism, bone accretion and resorption, magnesium shows analogies with calcium. 4. Magnesium exchange between extracellular, cellular and skeletal compartments are very precisely controlled. 5. Parathyroid hormone, 1 alpha, 25-dihydroxy-vitamin D3, calcitonin and estrogens are the principal hormone systems implicated in magnesium metabolism. 6. The kidney is the principal site of magnesium excretion and shows important magnesium regulation mechanisms.