Studies on the transport of calcium across the dually perfused lobule of the human term placenta
Movements of calcium (Ca) across the maternal and fetal aspects of the human placenta were investigated using the isolated placental lobule dually perfused in vitro. Tissues uptakes and releases of calcium were measured and the effects on calcium movements by calcium-protein binding in the perfusion fluids, (associated with extracellular pathways and non-uniform perfusion), evaluated. The effects of ouabain, dinitrophenol (DNP), and cooling on calcium movements were measured and compared to movements of Na+ and K+. These indicated the presence of active transport of calcium but no evidence was obtained for Ca2+/Na+ exchange. Cyclic adenosine 3', 5' -monophosphate (cAMP) levels in dually perfused tissue were measured following microwave fixation. This technique was used to measure changes in tissue cAMP production following exposure to forskolin, 3-iso-butyl-l-ethyl-xanthine (IBMX), and various fragments of both bovine parathyroid hormone (bPTH) and human parathyroid hormone-related peptide (hPTHrP). Rises in cAMP were produced by exposure to bPTH(1-34) but not hPTHrP(1-34), hPTHrP(67-86) or hPTHrP (107-138). It is concluded that calcium is actively transported across the placenta but there is no major contribution via a Ca2+/Na+ exchanger. The patterns of calcium uptake as a function of perfusate calcium concentration support the evidence of other workers that extracellular pathways are present in the syncytiotrophoblast. A significant amount of passive movement of calcium may therefore take place across the perfused placenta. The 1 to 34 region of the PTH molecule stimulates the production of cAMP by the trophoblast, but there is no indication that this has any effect on transplacental Ca transport.