| Abstract: | Acanthamoeba, with high rates of phagocytosis and pinocytosis of the non-concentrative type, offers favorable experimental material for investigation of similarities and possible differences in these two modes of uptake. Phagocytosis was measured by the rate of uptake of latex beads and pinocytosis by the rate of uptake of radioactive inulin and albumin. The effects of the metabolic inhibitors NaN3, NaCN, NaF, iodoacetate, 2,4-dinitrophenol and cold were found to be identical on both forms of endocytosis. Both endocytic processes were suppressed by inhibitors of aerobic metabolism and low temperature and were not appreciably affected by inhibitors of glycolysis. The cells recovered capacity to endocytose after exposure to all these compounds except 2,4-dinitrophenol, which was irreversibly toxic. Endocytosis and O2 consumption were measured as a function of temperature. Below 5 °C both phagocytosis and pinocytosis ceased; between 9 and 15 °C uptake was less than 10% that at 29 °C. From 16 to 29 °C uptake was a linear function of temperature for both pinocytosis and phagocytosis. Curves for O2 consumption and endocytosis both showed breaks at about 16 °C. Concanavalin A (ConA) inhibited both types of endocytosis more than 50% at concentrations as low as 5 μg/2 × 105 cells/ml. Pinocytosis and phagocytosis were also measured simultaneously in the same cells. Increasing the rate of phagocytosis suppressed pinocytosis, but the combined volume of the two forms of uptake was essentially constant. In contrast, the estimated combined surface intake varied over a two-fold range. These data show no differences between phagocytosis and pinocytosis of the non-concentrative type, and suggest that control of the rate of endocytosis is determined by the volume of an internal compartment. The volume of this compartment, estimated by measuring the volume of latex beads that “saturate” the phagocytic mechanism, amounted to about 500 μm3/cell or roughly 15% of the cell volume. |
