Calcium and phosphate fluxes across the fetal membranes of the guinea pig: in vitro measurement.

Bidirectional calcium (45Ca) and phosphate (32P) fluxes across the amnion and visceral yolk sac of the guinea pig were measured in vitro in modified Ussing chambers. The net flux of these ions across both fetal membranes was in the maternal-to-fetal direction. The net flux of 45Ca and 32P across the yolk sac was significantly greater than that across the amnion. This difference was due to a greater maternal-to-fetal flux across the yolk sac. In addition, net 32P flux was greater than net 45Ca flux across the yolk sac, while in the amnion, there was no significant difference in the net flux of the two ions. It is suggested that the fetal membranes, especially the visceral yolk sac, contribute significantly to fetal acquisition of calcium and phosphate in mammals possessing a functional yolk sac placenta.