Regulation of Pinocytosis in Amoeba Proteus by the Calcium Ion

Abstract

Pinocytosis in Amoeba proteus involves the uptake of surface bound solute and a portion of the bulk-phase medium by surface membrane infolding and vesiculation. The complex process of pinocytosis in the cell is initiated by the binding of a cationic inducer (Chapman-Andresen, 1962) to the cell surface and is terminated by the internalization of the membrane bound inducer into the cytoplasm. The calcium ion plays a major role in pinocytosis, as it does in a number of other physiological processes, participating in several distinct phases of the pinocytotic cycle. It was observed some time ago (Brandt and Freeman, 1967) that the induction of pinocytosis in the amoeba was correlated with a decrease in membrane resistance and an increase in membrane permeability. This change in membrane resistance associated with the initiation of pinocytosis in the amoeba could be reversed by increasing the level of Ca⁺⁺ in the external medium. It has now been established that external Ca⁺⁺ controls the overall solute permeability of the amoeba surface (Prusch and Dunham, 1972). Consequently, it has been suggested by a number of investigators (Cooper, 1968, Josefsson, 1976) that pinocytosis in the amoeba is initiated by the displacement of surface associated calcium upon the addition of inducer to the external medium. More recently, it has been demonstrated that pinocytotic intensity is dependent upon the level of Ca in the external medium (Josefsson, 1976; Prusch and Hannafin, 1979b).