GESTATIONAL CHANGES IN CA-2+ TRANSPORT ACROSS RAT PLACENTA AND MESSENGER-RNA FOR CALBINDIN9K AND CA2+-ATPASE

The unidirectional maternofetal clearance (K(mf)) of Ca-45 was measured across the rat placenta over the last one-third of gestation. K(mf) for Ca-45 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 Ca-45 was similar to K(mf)/D(w) for the paracellular marker [C-14]mannitol, but at 21 and 22 days of gestation, K(mf)/D(w) for Ca-45 was significantly higher than K(mf)/D(w) for [C-14]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 K(mf)/D(w) for Ca-45. In contrast, rat placental Ca2+-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 Ca2+-ATPase, may be rate limiting to placental calcium transport in the rat.