Extracellular binding of calcium in the guinea-pig placenta

Extraction of 45Ca from the perfused umbilical vascular bed of the guinea-pig placenta was measured using 3H-sucrose or 24Na as reference substances at a constant inflow concentration of the tracers. La3+ (5mM) depressed extraction of 45Ca below zero for several minutes. This suggests that there is a significant amount of Ca bound extracellularly on the umbilical side of the placental barrier.