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

Abstract

ABSTRACT. The transcellular osmosis induced both slow membrane depolarization (<50 mV sec-1) and cessation of cytoplasmic streaming in the endosmotic cell half of an internodal cell of Nitella 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.In Characeae internodal cells, cytoplasmic streaming stops suddenly at the moment an action potential (A.P.) is generated (cf 6). When electric stimulus is applied to the internodal cell, the so-called all-or-none law is valid not only for the generation of A.P. but also for the cessation of streaming (16). The threshold for cessation is the same as that for the A.P. This suggests that the streaming cessation is not a direct consequence of the electrical stimulus itself but is caused by the A.P. However, Kishimoto and Akabori (9) found cases where the streaming does not stop but decelerates temporarily when the A.P. is not large enough.In Nitella flexilis, transcellular-osmosis (t-osmosis) (7) induces a very slow depolarization compared with A.P. only in the endosmotic cell half (19,5). The membrane depolarization induced by t-osmosis is often accompanied by A.P. However, the membrane is depolarized slowly by t-osmosis without eliciting A.P. when the concentration of KCl in the external medium is high. This type of membrane depolariza-*Present address:

Calcium and plant action potentials

“…Experiments with '''^Ca'^''^ have shown that in normal conditions Ca^"^ influx at the time of excitation is too small to stop the streaming, even considering the permeabilized plasmalemtna results. Further, Hayama & Tazawa (1978) observed cessation of cytoplasmic streaming in cells depolarized by transcellular osmosis. There was no Ca^^ in the outside medium and the cells showed no action potentials.…”