Magnesium metabolism in blood and the whole body in man using 28magnesium.

Studies of magnesium kinetics in plasma, red cells, and the whole-body with ²⁸Mg are described. The observations in plasma were analyzed with a three-compartment model and those in red cells with a two-compartment model. An integral approach, using the occupancy principle, was used in the analysis of the whole-body observations. At 120 hr after intravenous administration of ²⁸Mg, exchangeable magnesium calculated by the dilution principle was only 23% of total body magnesium. There was good agreement between the estimates of exchangeable magnesium based on plasma and “spot” urine ²⁸Mg activity. Exchangeable magnesium, as estimated by compartmental analysis and the occupancy principle, was only 12.5%–15% of total body magnesium. These results suggest the existence of magnesium compartments in the body with turnover half-periods of at least 63–181 days that cannot be investigated adequately with the short-lived tracer, ²⁸Mg. A possible mechanism for the observed pattern of red cell uptake of ²⁸Mg in vivo is suggested in which magnesium enters these cells only during erythropoiesis and is then lost progressively from circulating red cells during the aging process. Present results suggest that the concentration of magnesium in reticulocytes may be 3.9 times greater than the mean red cell concentration. Magnesium may leave red cells exponentially with age; a half-period of 22.4 days is suggested.