Sucrose uptake by pinocytosis in Amoeba proteus and the influence of external calcium.

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Abstract
The relationship between Ca2+ and pinocytosis was investigated in A. proteus. Pinocytosis was induced with 0.01% alcian blue, a large MW dye which irreversibly binds to the cell surface. The time-course and intensity of pinocytosis was monitored by following the uptake of [3H]sucrose. When the cells are exposed to 0.01% alcian blue, there is an immediate sucrose uptake. The cells take up .apprx. 10% of their initial volume during the time-course of pinocytosis. The duration of pinocytosis in the amoeba is .apprx. 50 min, with maximum sucrose uptake occurring 15 min after the induction of pinocytosis. The pinocytotic uptake of sucrose is reversibly blocked at 3.degree. C and a decrease in pH increases the uptake of sucrose by pinocytosis. The process of pinocytosis is also dependent upon the concentration of the inducer in the external medium. The association between Ca2+ and pinocytosis in A. proteus was initially investigated by determining the effect of the external Ca2+ concentration on sucrose uptake induced by alcian blue. In Ca2+-free medium, no sucrose uptake is observed in the presence of 0.01% alcian blue. As the Ca2+ concentration is increased, up to a maximum of 0.1 mM, pinocytotic sucrose uptake is also increased. Increases in the external Ca2+ concentration above 0.1 mM facilitates a decrease in sucrose uptake. Further investigations into the association between Ca2+ and pinocytosis demonstrated that the inducer of pinocytosis displaces surface Ca2+ in the amoeba. Ca2+ may be involved in 2 separate stages in the process of pinocytosis; an initial displacement of surface Ca2+ by the inducer which may increase the permeability of the membrane to solutes, and a subsequent Ca2+ influx bringing about localized increases in cytoplasmic Ca2+ activity.