Kinetics of thallium exchange in cultured rat myocardial cells

The kinetics of thallium exchange in cultured rat myocardial cells were studied and compared to those of potassium in the same tissue. Studies were carried out using low concentrations (10 nM to 5 microM) of thallium-204, approximating those likely to be encountered during clinical myocardial scintigraphy. Both thallium uptake and release could be described by a single exponential with a half-time of exchange which was approximately half that of potassium and which was largely independent of extracellular thallium concentration. Some 60% of thallium uptake occurred via an "active" or ouabain-inhibitable mechanism which, in the absence of extracellular potassium, could be activated by low concentrations (10 nM to 5 microM) of thallium. The apparent Km for thallium on this active transport mechanism was 2-7 microM. Increasing extracellular potassium from 0-10 mM caused significant, concentration-dependent decreases in both the total and the active component of the thallium influx. Similarly nonradioactive thallium (0.10 microM to 0.10 mM) caused a concentration-dependent decrease in active potassium influx. Analysis of these results by both Lineweaver-Burk plots and Dixon plots confirmed competitive inhibition, potassium on thallium influx and vice versa, for the active component of the fluxes, and noncompetitive in the remainder. These findings indicate that active transport accounts for the greater portion of the influx of thallium and potassium, and that this active transport occurs via a common mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)