[14C]chloroquine, added in concentrations between 1 to 10μM to the medium of normal cultured fibroblasts, is rapidly adsorbed and taken up by the cells. At pH 7.40 about 15 per cent of the total uptake can be removed by washing the cells with Hank's solution, up to 24 per cent can be detached from the surface by trypsin and about 50 per cent appears to be intracellular. At an extracellular pH of 7.4 the adsorption of chloroquine to the cell surface is the highest; it approaches saturation at concentrations in the medium exceeding to 10 μM. Both at pH 6.7 and 8.1 less chloroquine is adsorbed. On the other hand the apparent intracellular uptake of chloroquine rises with increasing pH. Incubation at 6° greatly reduces both surface binding and even more intracellular uptake. The data suggest adsorption and accumulation of chloroquine at the cell membrane before entering the cells.
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