Cessation of Cytoplasmic Streaming Accompanying Osmosis-induced Membrane Depolarization in Nitella flexilis

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Abstract
The transcellular osmosis induced both slow membrane depolarization (< 50 mV s-1) and cessation of cytoplasmic streaming in the endosmotic cell half of an internodal cell of N. flexilis. Membrane depolarization and streaming cessation occurred in cells made inexcitable by bathing in 10 mM KCl, suggesting that Ca2+ in the external medium is not essential for streaming cessation. Depolarization preceded the streaming cessation, suggesting that some membrane event induced by endosmosis may directly induce the streaming cessation. The larger the rate of depolarization, the stronger the streaming inhibition and the shorter the time needed for the cessation. The maximum rate of depolarization of the action potential was more than 10 times that of osmosis-induced depolarization. The all-or-none nature of the streaming cessation accompanying the action potential is interpreted as indicating that the rate of depolarization is high enough to always cause instant cessation. Streaming cessation was observed even when the membrane potential was clamped at the same level as before the start of transcellular osmosis. The membrane electromotive force and membrane resistance decreased significantly as in the case where the membrane potential was not clamped.

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