Concentrative Accumulation of Choline by Human Erythrocytes

Influx and efflux of choline in human erythrocytes were studied using ¹⁴C-choline. When incubated at 37°C with physiological concentrations of choline erythrocytes concentrate choline; the steady-state ratio is 2.08 ± 0.23 when the external choline is 2.5 µM and falls to 0.94 ± 0.13 as the external concentration is raised to 50 µM. During the steady state the influx of choline is consistent with a carrier system with an apparent Michaelis constant of 30 x 10^-6 and a maximum flux of 1.1 µmoles per liter cells per min. For the influx into cells preequilibrated with a choline-free buffer the apparent Michaelis constant is about 6.5 x 10^-6 M and the maximum flux is 0.22 µmole per liter cells per min. At intracellular concentrations below 50 µmole per liter cells the efflux in the steady state approximates first order kinetics; however, it is not flux through a leak because it is inhibited by hemicholinium. Influx and efflux show a pronounced exchange flux phenomenon. The ability to concentrate choline is lost when external sodium is replaced by lithium or potassium. However, the uphill movement of choline is probably not coupled directly to the Na⁺ electrochemical gradient.