The uptake of mepacrine by horse polymorphonuclear leucocytes in vitro

The uptake of mepacrine by isolated horse polymorphonuclear leucocytes (PMN) was measured using spectrophotofluorimetry. Two phases of uptake were observed, the first, rapid fraction, essentially complete by 10 min, and a second, slow fraction, which was still proceeding after 60 min. The appearance of mepacrine within the PMN was also visualized by fluorescence microscopy. Discrete yellow points of fluorescence were observed in the cytoplasm of PMN within 30 s. These discrete points corresponded both in size and number to the PMN granules. After 5 min, the nuclei showed faint fluorescence which increased in intensity with time. It was concluded that the accumulation of mepacrine by horse PMN consists of at least two phases, an initial rapid component which represents uptake of the drug through the plasma membrane into the cytoplasmic granules, and a subsequent slower component, possibly representing nuclear binding.