SummaryCellular uptake mechanisms of 201T1+ were studied in Ehrlich mouse ascites tumor cells. 201Tl+ passes the cell membrane of tumor cells using three transport systems: the ATPase, the Tl+-Na+-2Cl−-cotransport, and the Ca++-dependent ion channel. In the case of 201T1+ the main route for entering the cells was the cotransport, its importance increasing with the age of the cells; in parallel, the ATPase activity was reduced. In contrast, the transport capacities of the ATPase and the cotransport were of the same magnitude in the case of 42K+ and 86Rb+. This change in ion distribution was not brought about by varying velocity relations but by changing the number of transport systems in the cell membrane. There was no relationship between transport rates and diameters of the ions. 201T1+ distribution is proportional to that of K+ with a higher intracellular concentration of about 30%. Under physiological conditions the cotransport was reversible suggesting the ability to regulate steady state during varying extracellular ion concentrations. Cells and medium were two compartments, kinetically seen. Due to the significant difference of transport capacities between the three systems with the respective ions the term “potassium-thallium-analogy” may be misleading as it erroneously assumes identical uptake conditions.

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