Gestational changes in Ca²⁺ transport across rat placenta and mRNA for calbindin(9K) and Ca²⁺-ATPase

The unidirectional maternofetal clearance (K(mf)) of ⁴⁵Ca was measured across the rat placenta over the last one-third of gestation. K(mf) for ⁴⁵Ca normalized to its diffusion coefficient in water (K(mf)/D(w)) 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, K(mf)/D(w) for ⁴⁵Ca was similar to K(mf)/D(w) for the paracellular marker [¹⁴C]mannitol, but at 21 and 22 days of gestation, K(mf)/D(w) for ⁴⁵Ca was significantly higher than K(mf)/D(w) 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 calbindin(9K)-to-β-actin mRNA ratio increased 135-fold between 15 and 22 days of gestation and was temporally associated with the gestational increase in K(mf)/D(w) for ⁴⁵Ca. In contrast, rat placental Ca²⁺-ATPase-to-β-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 calbindin(9K), but not Ca²⁺-ATPase, may be rate limiting to placental calcium transport in the rat.