| Summary: | When transcellular osmosis was induced in internodal cells of Nitella flexilis that had been rendered inexcitable by treatment with KCl or EGTA, the rate of cytoplasmic streaming was reduced and the membrane was depolarized. In both KCl- and EGTA-treated cells, the endoosmosis induced a significant increase in the concentration of Ca2+ in the cytoplasm, 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 intracellular Ca2+ concentration had been stabilized at a very low level by treatment with the Ca2+-chelating agent EGTA, no change in the rate of cytoplasmic streaming on the endoosmosis side was observed. Hydration of the cytoplasm in the absence of endoosmosis was induced by direct introduction of a hypotonic medium into the vacuole by intracellular perfusion. The results mimicked the inhibition of streaming induced by transcellular osmosis. During transcellular osmosis, chloroplasts on the endoosmosis side swelled as a result of dilution of the cell sap. Swelling of chloroplasts was demonstrated to be unrelated to the inhibition of streaming, since streaming was retarded at sites from which chloroplasts had been removed. It is suggested that hydration of the cytoplasm 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.
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