Is Ca2+ release from internal stores involved in membrane excitation in characean cells?

An action potential in characean cells is accompanied by an increase in the cytosolic Ca(2+) concentration ([Ca(2+)](c)) which subsequently causes cessation of cytoplasmic streaming. Two Ca(2+ )origins are postulated for the increase in [Ca(2+)](c), extracellular and intracellular ones. For the extracellular origin, a Ca(2+) influx through voltage-dependent Ca(2+)-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 (IP(3)) and IP(3)-dependent Ca(2+) release from internal stores [Biskup et al. (1999) FEBS Lett. 453: 72]. The hypothesis of the intracellular Ca(2+) origin was tested in three ways: injection of IP(3) into the streaming endoplasm, application of inhibitors of PI-PLC (U73122 and neomycin) and application of an inhibitor of IP(3)-receptor (2-aminoethoxydiphenyl borate; 2APB). Injection of 1 mM IP(3) into Chara cells did not change the rate of cytoplasmic streaming. Both U73122 (20 micro M) and neomycin (200 micro M) did not affect the generation of the action potential, cessation of cytoplasmic streaming and the increase in [Ca(2+)](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 Ca(2+) origins are discussed in the light of the present results.