Aspects of calcium transport by the ovine placenta: Studies based on the interplacentomal region of the chorion

An in vitro technique for the measurement of calcium uptake into the maternal-facing fetal chorionic membrane (apical trophoblast) was used to study the relationship between calcium uptake and stage of pregnancy in the sheep. The effects on calcium uptake of varying calcium concentration and temperature of the incubation medium, of adding calcium channel blockers or heavy metals (lanthanum and nickel) or calcium ionophore/agonist were also studied. The data indicate a saturable calcium uptake process, plateauing after 15 min incubation. This uptake remained constant throughout the last third of gestation until a significant fall in uptake was noted during the final week prior to parturition. This uptake was not due to extracellular cellular diffusion since there was no significant uptake of tritiated inulin over the same period in each case. Calcium uptake in this system was also shown to be a temperature dependent process which was abolished at temperatures of 0-4 degrees C. A decrease in calcium concentration to 0.12 mM in the incubation medium also caused a corresponding decrease in calcium uptake to 21 per cent of control (1.2 mM). The addition of the heavy metals lanthanum and nickel also significantly reduced calcium uptake as did the calcium channel blockers verapamil, metoprolol and diltiazem. The calcium channel ionophore A23187 increased calcium uptake into the material facing chorion. Although the interplacentomal chorion may not be representative of the whole of the placental unit, it clearly contains a specific calcium uptake process under local physiological control. The blocking of calcium uptake by the specific I-type calcium channel blocker verapamil may indicate the presence of I-type channels of unusually low sensitivity since the concentration needed to block them was much higher than would be required for excitable I-type channels in isolated cells. (C) 1997 W. B. Saunders Company Ltd.