CLEARANCE OF INERT MOLECULES NA AND CL IONS ACROSS PRIMATE PLACENTA

Transplacental clearances of antipyrine, tritiated water, urea, Na, and Cl ions were studied in the rhesus monkey. In order of decreasing placental permeability determined in vivo, these experiments showed the following order: tritiated water and antipyrine, urea, chloride, and sodium ions. Clearances of tritiated water and antipyrine were equal. The placental clearances of tritiated water and antipyrine were significantly different when determined across the chorion laeve in vitro. These observations suggest that the transplacental diffusion of these molecules is flow limited. The diffusion rates of urea, Na, and Cl ions were limited primarily by the permeability of the placental membrane. The placenta of the rhesus monkey is more permeable to Na and Cl ions than the sheep placenta. Permeability to urea per kilogram of fetal body weight is of the same order of magnitude in both species. © 1968.