Abstract
When Concanavalin A, 1-20 micrograms ml-1 binds to the surface of Amoeba proteus the cell's response to Ca2+ and to cationic inducers of pinocytosis is strikingly altered. Separately, Concanavalin A and Ca2+ are weak inducers but their combined effects are intense pinocytosis and suppression of the normal inhibitory effect of Ca2+ on cation-induced pinocytosis. At high concentrations (greater than 25 micrograms ml-1) the lectin increases cellular uptake and binding of 45Ca in the cell surface but the ionic permeability and the potential of the cell membrane are little changed by treatment with Concanavalin A. Subsequent addition of Ca2+ starts the pinocytic cycle and causes a fivefold increase of the membrane resistance without depolarizing the cell. Neither a rise of free intracellular calcium nor an influx of 45Ca precedes formation of pinocytotic channels when Con A is applied to cells in the presence of calcium. All effects of Concanavalin A are blocked by alpha-methylmannoside while succinyl-Concanavalin A is without effects on pinocytosis. These findings suggest that crosslinking of cell surface carbohydrates reverses calcium inhibition of pinocytosis and increases the cell's pinocytotic response to cations without modifying either the bioelectrical effects or the intracellular free concentration of calcium.
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