Abstract
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.
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