Uptake of inorganic phosphate by the maternal border of the guinea pig placenta.

Mechanism of uptake of inorganic phosphate (Pi) by the maternal border of the dually perfused guinea pig placenta was studied using the paired-tracer dilution technique with 32P-phosphate and 14C-sucrose being the tracers. Placental uptake of radioactive phosphate increased when the concentration of Pi in the perfusion fluid was reduced, and it decreased during anoxia, in presence of CN or during perfusion with low-Na or Na-free fluids. Iodoacetate was without effect. These observations are consistent with placental uptake of Pi being effected by a carrier mediated process dependent on external Na and, partly, on placental metabolism. Unidirectional flux of Pi from the maternal vascular space into the cell compartment of the placenta, estimated from the values of instantaneous extraction of 32P, correlated significantly with foetal weight. The flux per unit weight of the foetus was 17.0 +/- 1.0 nmol X min-1 g-1.