Fetal magnesium homeostasis in the sheep

Abstract

SUMMARYThe fetal plasma magnesium concentration exceeds that of the mother but the difference is small compared to that of ionized calcium concentration. Although not fully independent of changes in maternal magnesaemia, fetal magnesaemia showed a high degree of autonomy during both hypermagnesaemic and hypomagnesaemic changes induced in the ewe. As with calcium, the placental gradient is reversed after fetal thyroparathyroidectomy (TXPTX) with thyroxine replacement. During perfusion in situ of the placenta from such TXPTX fetuses isolated from the fetus itself, a stable positive placental gradient of magnesium concentration could be re-established between the perfusing blood and the maternal circulation. As with calcium, this gradient could be increased by fetal calf parathyroid extract, parathyroid hormone-related protein (PTHrP 1-141), PTHrP (1-84) but not by PTHrP (1-34). It was concluded that a mid-molecule portion of PTHrP can stimulate a putative placental pump which is responsible for the gradients of both calcium ions and magnesium across the ovine placenta.

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

“…When PTHrP 38 -94 was later determined to be the structure of mid-regional PTHrP (490,517), these investigators repeated the experiments and confirmed that PTHrP 38 -94 also significantly increased the rate of placental calcium transport in perfused placentas from previously thyroparathyroidectomized fetal lambs (745). This approach has also been used to show that mid-molecular PTHrP stimulates placental magnesium transport, but not phosphate transport (43,47,96,653).…”

“…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).…”

“…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). Placental transport of these minerals has not been examined in Pthrp null fetuses.…”

“…Fetal parathyroids are also required to regulate magnesium and phosphorus, since thyroparathyroidectomized fetal lambs, and Hoxa3 null, Gcm2 null, and Pth null fetal mice have hypomagnesemia and hyperphosphatemia (47,341,397,398,623). Loss of PTHrP itself causes hyperphosphatemia with a normal serum magnesium concentration (341, and unpublished data).…”

“…A low level of FGF23 expression has also been demonstrated (395). 28 Mg has been used to study active placental magnesium transport in lambs (47,96), wherein the back-flow is ϳ30% of the forward flow (100). The factors that control magnesium transport are unknown.…”