Radioiodide transfer across sheep placenta.

Unidirectional transplacental clearances of radioiodide were calculated from the net radioiodide fluxes after injection into fetal and/or maternal circulations of 33 catheterized conscious sheep. Maternofetal potential difference (PD) was also recorded. Clearance reached a steady state 20 min after bolus injection. Fetomaternal clearance was related to PD. Bidirectional clearance ratios measured in five experiments showed a significant divergence from the value for passive flux predicted from the measured PD, and in four experiments these ratios were also significantly different from unity, this result being incompatible with passive flux even if the transplacental PD is assumed to be zero. Injection of thiocyanate or iodide reduced radioiodide clearance. Fetomaternal clearance of radioiodide was halved by an increase in fetal plasma iodide concentration of approximately 0.1 mM. There appears to be an inhibitable iodide-transporting site capable of active transport in either direction.