Abstract
The mechanism for 201Tl + uptake was studied in cultured human glioma cell lines. Ouabain (1 mM) decreased the uptake at steady-state to 60%, but the rate of uptake was faster in the presence of ouabain. Addition of non-radioactive Tl + (to a K +-free medium) decreased the uptake, but much less than expected for a system limited by the number of transport sites. Changes in K + concentration of incubation medium affected the 201Tl + uptake as predicted by the electrochemical equilibrium (Nernst equation). Using the uptake in isotonic KCl as a reference for membrane potential (0 mV), the calculated membrane potential was −75 mV in a medium with 3.0 mM K +. The Tl +-flux constants and the membrane permeabilities for Tl + and K + were calculated from the rate of uptake and from wash-out experiments. This is a new method for membrane potential and permeability studies in cell populations. The mechanism for 201Tl + uptake is relevant for the clinical interpretation of 201Tl + scintigraphy.
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