MAGNESIUM HOMEOSTASIS IN PREMATURE AND FULL-TERM NEONATES

Renal handling of magnesium (Mg) has not been comprehensively studied in the newborn period due to the difficulty, until recently, of measur ing the diffusible fraction of plasma Mg (UfMg). In the present study this methodology was used to assess Mg homeostasis in 84 newborn infan ts of different postconceptional age (26-42 weeks), weight (720-4,830 g) and postnatal age (1-72 days). Very premature infants (postconcepti onal age less than 35 weeks) had significantly higher values of plasma Mg than mature newborn infants, Plasma Mg related inversely to postco nceptional age, weight, plasma total protein and plasma calcium, and d irectly to plasma potassium. Stepwise multiple regression analysis rev ealed that postconceptional age was the unique factor contributing to variations in plasma Mg. Plasma values of UfMg were the same in preter m as in term infants but, when expressed as a fraction of total plasma Mg (UfMg/Mg), they were significantly lower in very preterm infants. Fractional excretion of Mg and the ratio of urine Mg to urine creatini ne did not vary as a function of postconceptional age. These results i ndicate that plasma UfMg is kept constant at different gestational age s despite variations in total plasma Mg; furthermore, no functional im maturity is present for renal tubular reabsorption of Mg, even in very low birth weight infants.