Phagocytosis and pinocytosis in Acanthamoeba castellanii.

Endocytotic activity of Acanthamoeba trophozoites attenuates once the cells enter stationary phase in liquid culture. Phagocytosis, monitored by the ingestion of polystyrene latex beads, essentially ceases and the uptake of [3H]inulin, known to be mediated by pinocytosis, is reduced by about half. The reduced pinocytotic activity of stationary-phase cells remains sensitive to respiratory inhibitors. Preincubation of stationary-phase cells in fresh growth medium for 1-5 h before the initiation of endocytosis has no effect on phagocytosis and only marginally increases pinocytosis. This impairment of ingestion, particularly of pinocytosis, may account for the reduced contractile vacuole activity known to characterize stationary-phase cells of this organism. The unequal responses of phagocytosis and pinocytosis to the onset of stationary-phase growth suggest that they are independent processes subject to different controls.