Free calcium concentrations in Krebs-Ringer bicarbonate buffer: Effects on 45Ca- and 32P-transport across the perfused guinea pig placenta

Calcium ion activity was measured in protein-free and protein-containing Krebs-Ringer bicarbonate buffer. Even in protein-free buffer calcium ion activity was 20% less than the total Ca concentration. In solutions containing albumin the calcium ion activity was dramatically reduced, an effect dependent on the albumin concentration and pH of the solution. Acutely changing calcium ion activity from 0.35 mM to 1.25 mM in a Krebs-Ringer bicarbonate buffer (5.4% albumin) perfusing the fetal side of the guinea pig placenta resulted in an immediate increase in transfer (mother to fetus) of ³²P but no change in ⁴⁵Ca transfer. The ratio of ³²P to ⁴⁵Ca clearance was positively related also to maternal plasma calcium ion activity.