Transplacental differences in Ca, Na, K, and electropotential in the guinea pig

The fetal side of hemochorial placentas from guinea pigs between 58 and 62 days gestation were perfused in situ. Concurrent measurements were made of the clearances of radiocalcium and tritiated water from maternal to fetal circulation of the placenta, transplacental potential difference (TPD), maternal plasma and perfusate Ca, Na, and K concentrations, maternal blood pressure, gross permeability of albumin in the placenta, and net water movements. Movement of Ca from dam to fetus appeared to occur against an electrochemical gradient and was not associated with the movements of Na and K across the placenta. A negative correlation between perfusate Na and K concentrations, not dependent on maternal plasma Na and K concentrations and abolished by high concentrations of Ca in the fetal circulation of the placenta, strongly supports the concept of a Na-K exchange mechanism in the placenta directed so that K is moved against a concentration gradient towards the dam. There was no evidence that the TPD existed at the site of maternal-fetal exchange for Ca, Na, or K.