ASPECTS OF CALCIUM-TRANSPORT BY THE OVINE PLACENTA - STUDIES BASED ONTHE INTERPLACENTOMAL REGION OF THE CHORION

An in vitro technique for the measurement of calcium uptake into the m aternal-facing fetal chorionic membrane (apical trophoblast) was used to study the relationship between calcium uptake and stage of pregnanc y in the sheep. The effects on calcium uptake of varying calcium conce ntration and temperature of the incubation medium, of adding calcium c hannel blockers or heavy metals (lanthanum and nickel) or calcium iono phore/agonist were also studied. The data indicate a saturable calcium uptake process, plateauing after 15 min incubation. This uptake remai ned constant throughout the last third of gestation until a significan t fall in uptake was noted during the final week prior to parturition. This uptake was not due to extracellular cellular diffusion since the re 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 tem perature dependent process which was abolished at temperatures of 0-4 degrees C. A decrease in calcium concentration to 0.12 mM in the incub ation medium also caused a corresponding decrease in calcium uptake to 21 per cent of control (1.2 mM). The addition of the heavy metals lan thanum and nickel also significantly reduced calcium uptake as did the calcium channel blockers verapamil, metoprolol and diltiazem. The cal cium channel ionophore A23187 increased calcium uptake into the materi al facing chorion. Although the interplacentomal chorion may not be re presentative of the whole of the placental unit, it clearly contains a specific calcium uptake process under local physiological control. Th e blocking of calcium uptake by the specific I-type calcium channel bl ocker verapamil may indicate the presence of I-type channels of unusua lly low sensitivity since the concentration needed to block them was m uch higher than would be required for excitable I-type channels in iso lated cells. (C) 1997 W. B. Saunders Company Ltd.