Placenta potassium

Thallium and thallium salts are readily absorbed by virtually all routes, with gastrointestinal exposure being the most common route to produce toxicity. Thallium also crosses the placenta freely. Thallium enters cells by a unique process governed by its similarity in charge and ionic radius to potassium. Unlike potassium, however, once thallium enters the cells, it is released slowly. It can concentrate in the liver and kidneys. Since it is soluble at physiological pH, it does not form complexes with bone. Most thallium is excreted in the urine, but it is excreted slowly and can be detected months after exposure. 

Twardock AR, Downey HF, Kirk ES, et al. 1969. Comparative transfer of calcium and strontium and of potassium and cesium in the guinea pig placenta. AEC Symp Ser 17 97-104. 

The transfer of zinc across perfused placentas is slow only - 3% of maternal zinc reached the fetal compartment in 2 hours (Beer et al. 1992). The in vitro transfer of zinc between mother and fetus is bidirectional, with binding in the placenta (Beer et al. 1992). It is proposed that zinc uptake in the placenta involves a potassium/zinc transport system (Aslam and McArdle 1992). Newborns may also be exposed to zinc from their mothers by milk transfer of zinc during lactation (Rossowska and Nakamoto 1992). 

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