PERMEABILITY OF THE PLACENTA OF THE GUINEA PIG TO INORGANIC PHOSPHATE AND ITS RELATION TO FETAL GROWTH

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Abstract
The rate, of transfer of inorganic phosphate across the placenta of the guinea pig was measured with radioactive P32 as the tracer substance. Isotonic labelled sodium hydrogen phosphate at pH 7.4 was injected into a maternal vein. The amt. of PO4/hr. transferred across 1 g. of fresh placenta was estimated from the amt. of radioactivity accumulated in the fetus/unit of radioactivity in maternal plasma. As the fetus grows the. rate of transfer/unit wt. of placenta increases. During the 35 day interval between the 31st and 66th day (full term) of pregnancy, the rate increases about 10 times. While the inflow-outflow exchange of Na and water across the placenta is enormous, such that the inflow to the fetus greatly exceeds the net amt. incorporated in growth, the inflow of PO4 approximates only the rate of its net retention in growth. It is impossible to compare the transfer rate of Na and PO4, ion to ion, since phosphate ionizes into 2 or more anion spp. The phosphate anion spp. as a sum cross 2 to 3 times faster than Na.

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