THE EFFECT OF CALCIUM-REGULATING HORMONES ON TRANSPORT OF CALCIUM ACROSS THE CHORIOALLANTOIC MEMBRANE OF THE CHICKEN-EMBRYO

The hormonal form of vitamin D-3 (1,25(OH)(2)D-3), parathyroid hormone (PTH), or appropriate vehicle were injected into the yolk sac of eggs of domestic fowl on days 16 and 17 of incubation. The chorioallantoic membrane (CAM) and overlying inner shell membrane were removed from e ggs on day 18 and mounted in a Ussing-type apparatus. Transport of cal cium was assessed by monitoring movements of radiolabeled calcium. Tra nsport of calcium from the chorionic aspect of the CAM to the allantoi c aspect increased considerably with time for all treatment groups exc ept the one receiving PTH. ''Back-flux'' of calcium (movement of calci um from the allantoic aspect to the chorionic) was negligible for all treatment groups at all sampling periods. PTH treatment did not affect flux of calcium from allantois to chorion but reduced flux from chori on to allantois considerably. The underlying cause of this effect has not been identified. The hormonal form of vitamin D, did not affect fl ux of calcium in either direction. These data raise the possibility th at control of calcium transport by the CAM may not be the primary func tion of the vitamin D hormone. (C) 1998 Elsevier Science Inc.