INTRACELLULAR MOBILIZATION OF CA2-FLEXILIS( AND INHIBITION OF CYTOPLASMIC STREAMING INDUCED BY TRANSCELLULAR OSMOSIS IN INTERNODAL CELLS OFNITELLA)

When transcellular osmosis was induced in internodal cells of Nitella flexilis that had been rendered inexcitable by treatment with KCl or E GTA, the rate of cytoplasmic streaming was reduced and the membrane wa s depolarized. In both KCl- and EGTA-treated cells, the endoosmosis in duced a significant increase in the concentration of Ca2+ in the cytop lasm, which was demonstrated by monitoring the emission of light from aequorin that had been injected into the cytoplasm. When transcellular osmosis was induced in tonoplast-free cells, in which the intracellul ar Ca2+ concentration had been stabilized at a very low level by treat ment with the Ca2+-chelating agent EGTA, no change in the rate of cyto plasmic streaming on the endoosmosis side was observed. Hydration of t he cytoplasm in the absence of endoosmosis was induced by direct intro duction of a hypotonic medium into the vacuole by intracellular perfus ion. The results mimicked the inhibition of streaming induced by trans cellular osmosis. During transcellular osmosis, chloroplasts on the en doosmosis side swelled as a result of dilution of the cell sap. Swelli ng of chloroplasts was demonstrated to be unrelated to the inhibition of streaming, since streaming was retarded at sites from which chlorop lasts had been removed. It is suggested that hydration of the cytoplas m triggers the mobilization of Ca2+ from internal stores and causes an increase in the level of cytoplasmic Ca2+ that is responsible for the inhibition of streaming. Possible mechanisms for the osmosis-induced increases in the level of Ca2+ in the cytoplasm are discussed.