Uptake of Some Quaternary Ammonium Ions by Human Erythrocytes

In many biophysical studies on erythrocytes some quaternary ammonium ions are used asreplacements for Na⁺ and K⁺ of the physiologicalsolutions. The object of this work was to study the possible uptake of quaternary ammoniumions by erythrocytes. Uptake of C¹⁴–choline chloride andC¹⁴–tetramethylammonium chloride by human erythrocytes was proved. Itwas shown that the compounds were neither incorporated into phospholipids of the cell norconverted to any other metabolites. Studies of uptake as a function of time, at severalexternal concentrations of choline and tetramethylammonium, showed that within the first 4hours uptake was a linear function of time regardless of the external concentration of thequaternary ammonium ions. The effects of various external concentrations of choline andtetramethylammonium ions on the rate of uptake by the cells were studied. The resultsshowed the presence of two distinct mechanisms for the uptake of choline: one, afacilitated uptake mechanism which becomes saturated at low external concentrations of theion; the other, a simple diffusion mechanism in which the rate of uptake is proportionalto concentration. For the facilitated part of the uptake the external cholineconcentration at which half-maximum rate was obtained was found to be 0.02 mm.Although the kinetic studies with tetramethylammonium ion were not as extensive as thosewith choline, they did suggest the presence of similar mechanisms for the uptake of bothions. Tetramethylammonium and tetraethylammonium ions were shown to be competitiveinhibitors of the facilitated choline uptake.