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Plant and Cell Physiology, 2003, Vol. 44, No. 5 518-526
© 2003 Oxford University Press

Is Ca2+ Release from Internal Stores Involved in Membrane Excitation in Characean Cells?

Masashi Tazawa1,3 and Munehiro Kikuyama2

1 Department of Applied Physics and Chemistry, Fukui University of Technology, Fukui, 910-8505 Japan
2 Department of Biology, Faculty of Science, Niigata University, Niigata, 950-2181 Japan

An action potential in characean cells is accompanied by an increase in the cytosolic Ca2+ concentration ([Ca2+]c) which subsequently causes cessation of cytoplasmic streaming. Two Ca2+ origins are postulated for the increase in [Ca2+]c, extracellular and intracellular ones. For the extracellular origin, a Ca2+ influx through voltage-dependent Ca2+-permeable channels is postulated. For the intracellular origin, a chain of reactions is assumed to occur, involving phosphoinositide-specific phospholipase C (PI-PLC) activation, production of inositol 1,4,5-trisphosphate (IP3) and IP3-dependent Ca2+ release from internal stores [Biskup et al. (1999) FEBS Lett. 453: 72]. The hypothesis of the intracellular Ca2+ origin was tested in three ways: injection of IP3 into the streaming endoplasm, application of inhibitors of PI-PLC (U73122 and neomycin) and application of an inhibitor of IP3-receptor (2-aminoethoxydiphenyl borate; 2APB). Injection of 1 mM IP3 into Chara cells did not change the rate of cytoplasmic streaming. Both U73122 (20 µM) and neomycin (200 µM) did not affect the generation of the action potential, cessation of cytoplasmic streaming and the increase in [Ca2+]c caused by electric stimulus even 20–30 min after application. 2APB depolarized the membrane and inhibited the excitability of the plasma membrane. The results are not consistent with the data obtained by Biskup et al. (1999) who found inhibition of the excitatory inward current by neomycin and U73122. The hypotheses of internal and external Ca2+ origins are discussed in the light of the present results.

3 Corresponding author: E-mail, masashi.tazawa{at}mb5.seikyou.ne.jp; Fax, +81-77-524-9221.


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 Plant Cell PhysiolHome page
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