Evidence for active maternofetal transfer of magnesium across the in situ perfused rat placenta

Mechanisms of maternofetal Mg transfer have been investigated across the in situ perfused rat placenta at 21 d gestation (term = 23 d). The fetal placental circulation was perfused with Mg-free Krebs-Ringer solution and clearance of Mg from maternal plasma across the placenta [unidirectional maternofetal clearance (Kmf) Mg] compared with that for 45Ca and 51Cr-EDTA, the latter being used as a diffusional marker. Because diffusion coefficients determined for these solutes were similar (6.8-7.6 X 10(-6) cm2.sec-1), greater Kmf values determined for Mg and 45Ca (mean +/- SD: 26.7 +/- 9.2 and 93.1 +/- 29.8 microL.min-1.g-1 placenta, respectively) compared to 51Cr-EDTA (3.2 +/- 0.9 microL.min-1.g-1) suggest that maternofetal transfer of these cations occurs by mechanisms in addition to diffusion. Kmf Mg was also greater than Kmf 51Cr-EDTA when measured across the dually perfused rat placenta, in which the maternal uterine artery was additionally perfused with Mg-containing (0.5 mmol.L-1) Krebs-Ringer solution. Decreasing the Mg concentration in the maternal perfusate by 90% reduced Mg appearance in the fetal perfusate by 87% within 8 min; this suggests that Kmf Mg across the in situ perfused placenta largely reflects Mg transfer from maternal plasma and not simply elution of a placental Mg pool. Addition of KCN (1 mmol.L-1) to the fetal perfusate or lowering perfusate temperature from 37 to 26 degrees C significantly reduced Kmf Mg and Kmf 45Ca across the in situ perfused placenta. In contrast, Kmf 51Cr-EDTA was increased by KCN and unaffected by temperature.(ABSTRACT TRUNCATED AT 250 WORDS)