Gestational changes in Ca2+ transport across rat placenta and mRNA for calbindin9K and Ca(2+)-ATPase.

The unidirectional maternofetal clearance (Kmf) of 45Ca was measured across the rat placenta over the last one-third of gestation. Kmf for 45Ca normalized to its diffusion coefficient in water (Kmf/Dw) increased 72-fold between days 15 and 22 of gestation from 3.5 +/- 0.3 to 253.1 +/- 22.0 cm/g placenta, respectively. At 15 and 18 days of gestation, Kmf/Dw for 45Ca was similar to Kmf/Dw for the paracellular marker [14C]mannitol, but at 21 and 22 days of gestation, Kmf/Dw for 45Ca was significantly higher than Kmf/Dw for [14C]mannitol, indicating that an additional route of transfer, other than diffusion, becomes available to calcium during this period. Northern hybridization analysis demonstrated that rat placental calbindin9K-to-beta-actin mRNA ratio increased 135-fold between 15 and 22 days of gestation and was temporally associated with the gestational increase in Kmf/Dw for 45Ca. In contrast, rat placental Ca(2+)-ATPase-to-beta-actin mRNA ratio increased only two- to threefold over the same gestational period and did not mirror the gestational changes in calcium clearance. These trends suggest that the expression of placental calbindin9K, but not Ca(2+)-ATPase, may be rate limiting to placental calcium transport in the rat.