Evidence for Active Maternofetal Transfer of Magnesium across the in Situ Perfused Rat Placenta

Abstract

ABSTRACT. Mechanisms of maternofetal Mg transfer gradient does not reflect greater protein binding in fetal plasma, have been investigated across the in situ perfused rat ultrafiltrable Mg concentrations also being higher in fetal complacenta at 21 d gestation (term = 23 d). The fetal placental pared with maternal plasma (1). These observations indicate that circulation was perfused with Mg-free Krebs-Ringer solu-placental transfer of Mg from mother to fetus must occur against tion and clearance of Mg from maternal plasma across the a chemical gradient. Using '*Mg, Care et al. (4) determined placenta [unidirectional maternofetal clearance (K,3 Mg] maternofetal and fetomaternal Mg fluxes of 0.042 and 0.012 compared with that for 4sCa and "Cr-EDTA, the latter mg. h-I. kg-' fetus, respectively, in sheep at 13 d preterm. These being used as a diffusional marker. Because diffusion coef-fluxes suggest the presence of an active mechanism for the ficients determined for these solutes were similar (6.8-7.6 maternofetal transfer of Mg. x cm2.sec-I), greater Kmf values determined for Mg In our study, maternal and fetal plasma Mg concentrations and 45Ca (mean f SD: 26.7 f 9.2 and 93.1 f 29.8 pL. (total and ultrafiltrable) were measured in the near term rat and min-' .g-' placenta, respectively) compared to "Cr-EDTA maternofetal Mg transfer was investigated using the in situ (um-(3.2 f 0.9 pL . min-' . g-I) suggest that maternofetal trans-bilically) perfused rat placenta preparation (7). K,f of 24Mg was fer of these cations occurs by mechanisms in addition to compared with that of "Cr-EDTA, used as a diffusional marker diffusion. Kmf Mg was also greater than Kmf "Cr-EDTA (7), and also with that of 45Ca, a divalent cation with which Mg when measured across the dually perfused rat placenta, in may share transcellular transport systems (8). Previous studies which the maternal uterine artery was additionally perfused (9) suggest that Ca is actively transferred across the in situ with Mg-containing (0.5 mmol. L-') Krebs-Ringer solu-perfused rat placenta. The effects of adding KCN to the perfusion tion. Decreasing the Mg concentration in the maternal fluid or decreasing perfusate temperature were therefore examperfusate by 90% reduced Mg appearance in the fetal ined for effects on possible active placental transfer of Mg as well perfusate by 87% within 8 min; this suggests that Kmf Mg as of 45Ca. Experiments were also performed using a dually across the in situ perfused placenta largely reflects Mg perfused rat placenta in which the maternal uterine artery was transfer from maternal plasma and not simply elution of a additionally perfused with Krebs Ringer solution with or without placental Mg pool. Addition of KCN (1 mrno1.L-I) to the Mg (10). These were performed to confirm that Mg transfer (as fetal perfusate or lowering perfusate temperature from 37 measured using the in situ perfused placenta) reflects maternoto 26°C significantly reduced Kmf Mg and Kmf 45Ca across fetal transfer and not simply elution of a placental Mg poo...

Effect of chronic maternal dietary magnesium deficiency on placental calcium transport.

“…Furthermore, the rat placenta is a hemochorial system, as is the human placenta. In the present study, evidence for uphill transport of Mg from maternal to fetal circulation, similar to the uphill gradient present in maternofetal Ca transport, was also verified [11].…”

Nutrient Transport Pathways Across the Epithelium of the Placenta

“…MAGNESIUM The paucity of data conceming magnesium transport--not only in the placenta but in general--is primarily due to the lack of a long half-lived radioisotope. Two recent studies using the perfused rat placenta indicate that maternal-fetal Mg 2+ transfer is transcellular, Na+-dependent, and independent of Ca 2+ transport (118,119). Perfusion with the carbonic anhydrase inhibitor acetazolamide did not affect maternal-fetal Mg 2+ clearance, which suggests that intracellular HCO~-or H + concentrations do not alter Mg 2÷ transport (119).…”

Bone Development and Mineral Homeostasis in the Fetus and Neonate: Roles of the Calciotropic and Phosphotropic Hormones

“…Magnesium and phosphorus are each actively transported across the in situ perfused placentas of rats (612,653) and lambs (47,96,397). Mid-molecular PTHrP stimulated magnesium transport across in situ perfused ovine placentas (47,96,397) but not rodent placentas (611), and it did not affect placental phosphorus transfer (43,397,653).…”