Age-dependent differences in the relationship between plasma and brain extracellular fluid concentrations of magnesium after MgSO4 infusions in miniswine

Magnesium is a potential neuroprotective agent in the treatment of head inj ury and ischemia whose efficacy is likely determined by increases in brain extracellular fluid (ECF) magnesium, which in turn depends on its concentra tion in plasma. The objectives of this study were to: I) examine the effect s of increasing plasma magnesium concentration ([Mg](plasma)) to 4-6 mM on brain ECF magnesium concentration ([Mg](ECF)) and 2) determine whether matu rational changes occur in the transfer of magnesium into brain ECF for newb orn and more mature (approximately 1 month old) miniswine. Increases in [Mg ](plasma) by systemic administration of MgSO4, resulted in similar maximal elevations in brain [Mg](ECF) for both age groups (193 +/- 76% versus 253 /- 106% of control for newborn and 1-month-old miniswine, respectively). Ca lculations of half-lives (t(1/2)) for the increase and decrease in magnesiu m concentration (t(1/2) uptake and t(1/2) clearance) were used to character ize magnesium kinetics in plasma and brain ECF. Plasma magnesium uptake was shorter in 1-month-old (t(1/2) = 11.1 +/- 0.9 min) compared with newborns (12.9 +/- 1.7 min, p < 0.05). The faster increase in [Mg](plasma) probably contributed to a faster uptake of brain [Mg](ECF) in 1-month-old compared w ith newborn swine (t(1/2) uptake = 27.9 +/- 12.8 versus 46.0 +/- 20.9 min, respectively, p < 0.05). Although plasma magnesium clearance was shorter in 1-month-old swine compared with newborn (t(1/2) = 34.3 +/- 7.0 versus 74.7 +/- 33.7 min, respectively, p < 0.05), the clearance of magnesium from the brain ECF was similar for each age group. Reductions in blood pressure and heart rate occurred during hypermagnesemia and were similar in each age gr oup. This study shows that acute elevations in [Mg](plasma) to 4-6 mh? resu lt in similar relative increases in brain [Mg](ECF) for both newborn and 1- month-old miniswine. However, there are maturational differences, as demons trated by the faster rate of magnesium uptake into the ECF observed in the older miniswine.