Abstract
The origins of the two peaks of the action potential inNitella flexilis were analyzed by inserting two microelectrodes. one into the vacuole and the other into the cytoplasm. It was unequivocally demonstrated that the rapid first peak was generated at the plasmalemma and the slow second peak at the tonoplast. MnCl2 applied in the external medium abolished the second, tonoplast, peak but not the first, plasmalemma, peak, MnCl2 also inhibited the cessation of the cytoplasmic streaming accompanying the action potential. CaCl2 added in MnCl2-containing medium recovered generation of the tonoplast action potential and the streaming cessation. Since it has been established that the cessation of cytoplasmic streaming on membrane excitation is caused by an increase in cytoplasmic free Ca2− (Williamson, R.E., Ashley, C.C., 1982.Nature (London) 296:647–651: Tominaga, Y., Shimmen, T., Tazawa, M., 1983,Protoplasma 116:75–77), it is suggested that the tonoplast action potential is also induced by an increase in cytoplasmic Ca2+ resulting from the plasmalemma excitation. When vacuolar Cl was replaced with SO 4 2 by vacuolar perfusion, the polarity of the second, slow peak was reversed from vacuolar positive to vacuolar negative with respect to the cytoplasm, supporting the previous report that the tonoplast action potential is caused by increase in Cl permeability (Kikuyama, M., Tazawa, M., 1976.J. Membrane Biol.29:95–110).
Published Version
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