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.