Mechanisms involved in the transfer of calcium across the isolated guinea pigs placenta.

The mechanisms of calcium transfer across the trophoblast cells of the guinea pig placenta were investigated. The isolated guinea pig placenta was perfused on both sides with an artificial medium. Uptake and release of 45Ca was studied using different compositions of the perfusion medium. It was found, that calcium enters the trophoblast as a positive ion. Addition of calcium to the perfusion medium caused depolarisation of the cell potential. As a result the uptake of 45Ca decreased when excess calcium was added to the medium. When the NaCl in the medium was replaced by choline chloride or KCl, the uptake of 45Ca was increased, despite depolarisation of the cell potential (van Dijk and van Kreel, 1982). This increased net uptake is probably due to a decrease in the release of 45Ca from the cell. The same effect was observed when KCN or Mg was added to the medium. By inhibiting calcium extrusion at one side of the trophoblast, a net transfer of 45Ca from one side of the placenta to the other side was demonstrated. This transfer occurred against the electrochemical potential gradient and must therefore involve a free energy yielding process.