Transport of 45Ca and of radioactively labelled inert saccharides across the intact or perfused placenta was measured in the rat on day 21 of pregnancy, the day after mating being day 1. The values of permeability--surface area product (PS) of the intact placenta to radioactive mannitol, sucrose, raffinose, and methoxyinulin were approximately proportional to their diffusion coefficients in water. This suggests that diffusion of inert hydrophilic molecules across the rat placenta takes place through wide aqueous channels. Net flux of Ca from mother to fetus, estimated from the increase of the fetal Ca content between day 20 and day 21 is 100 +/- 4 nmol min-1 (the limits here and below are S.E. of means). The unidirectional maternal-fetal flux of Ca (Jmf) in non-anaesthetized animals, estimated from the flux of 45Ca, is 100 +/- 7 nmol min-1. The similarity of the two values suggests that the fetal-maternal flux (Jfm) is small The umbilical vascular bed of the rat placenta was perfused in situ with Krebs-dextran fluid. Jmf estimated from the transfer of 45Ca from maternal plasma to perfusate was 81 +/- 4 nmol min-1. PS of the perfused placenta to radioactive sucrose was 2.6 +/- 0.3 microliter min-1. Jmf decreased reversibly when the placenta was perfused with 0.5 mM-dinitrophenol or 1 mM-CN-, which is consistent with the presumed active nature of the maternal-fetal transport of Ca. Jmf did not decrease when the placenta was perfused with Na-free fluids (substitution with Tris, Li or sucrose), indicating that Na-Ca exchange across the fetal border of the placental trophoblast is not involved in maternal-fetal transport of Ca. Transport of 45Ca to the perfusate was reduced to about 60% when maternal plasma concentration of Ca was doubled. This suggests that the affinity of the maternal-fetal transport system to Ca is high. Jmf did not change when the umbilical concentration of Ca was varied between 0.1 and 3 mM. There thus seems to be no rapid feed-back between umbilical concentration of Ca and transport of Ca from mother to fetus. Fetal-maternal transfer of Ca, estimated from the steady-state extraction of 45Ca from the umbilical perfusate, is only about 20% of the maternal-fetal transfer. Umbilical extraction of 45Ca changed only little when umbilical concentration of Ca was varied between 0.1 and 3 mM. This suggests that either most of the fetal-maternal transport of Ca is diffusional or the fetal-maternal transport system has a very low affinity to Ca.(ABSTRACT TRUNCATED AT 400 WORDS)
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