STIMULATION BY CALCITRIOL OF THE UPTAKE OF CALCIUM AND OF ITS TRANSPLACENTAL FLUX IN THE OVINE INTERPLACENTOMAL CHORION - WORK-IN-PROGRESS

Our past results have shown that the placenta of an intact twin lamb f etus transports calcium more rapidly in vivo than that of its twin tha t had been totally nephrectomized 8 to 19 d earlier. Nephrectomy reduc es the calcitriol concentration in the fetal circulation. The addition of calcitriol to the medium used to perfuse the placenta of a previou sly nephrectomized fetus rapidly increased the rate of calcium transpo rt. We are now focusing more narrowly on potential individual componen ts of this calcium transport process, which is fundamental to successf ul long bone growth and production of a viable neonate. Initially, we assessed the effects of calcitriol on the entry of calcium ions into t he maternal-facing surface of discs of interplacentomal chorion in vit ro, and our preliminary results clearly indicate the value of this new methodology in dissecting the basic mechanism of placental calcium tr ansport. The chorion was obtained from fetuses between 120 and 145 d o f gestation that had been nephrectomized 6 to 21 d earlier. These fetu ses included 5 pairs of twins, 1 member of each pair of which had been nephrectomized. Our results demonstrate that previous nephrectomy, wi th subsequent reduction in calcitriol and development of fetal hypocal caemia, was associated with a significant reduction in the rate of upt ake of calcium ions into the maternal-facing aspect of the chorion. Th e addition of 600 pM calcitriol to the fluid bathing this chorionic su rface led to a rapid, significant increase in the rate of uptake of ca lcium ions by the chorionic trophoblast epithelia cells. No such effec t was noted with chorion from the placentas of fetuses with intact kid neys. The addition of diltiazem (50 mu M) to the incubation medium cau sed a reduction in the uptake of calcium ions by chorionic discs from either intact or nephrectomized fetuses. These results indicate for th e first time that the ovine placental trophectodermal barrier is at le ast partly under calcitriol control and demonstrate the value of separ ation of a complex process into its constituent parts. (C) 1997 by Els evier Science inc.