Internodal cells of Nitellopsis obtusa were made tonoplast-free by intracellular perfusion of the vacuole with medium containing 5 mM EGTA. Next, the cells were perfused with media of various Ca2+ concentrations and the plasma membrane potential (Em) and resistance (Rm) were measured by the open-vacuole method. In the presence of ATP, 10-7 M Ca2+ had no effect on Em and Rm but 10-6 M Ca2+ depolarized Em slightly and decreased Rm a little. Ca2+ at more than 10-5 M depolarized Em drastically (Em: about -10 to - 20 mV) and decreased Rm to almost zero. Even when the electrogenic pump was stopped by removing ATP, the Ca2+ effect was almost the same as that observed in its presence. The maximum depolarization level measured in the presence of more than 10-5 M Ca2+ was more positive than the peak of the action potential. Therefore, the intracellular concentration of Ca2+ ([Ca2+]i) should not become more than 10-5 M during excitation. The tonoplast-free cell did not release Cl- to the external medium during depolarization induced by high [Ca2+]i. Since no Cl- efflux was observed on membrane excitation in the tonoplast-free cell, the question remains of whether or not the depolarization accompanying excitation is caused by a transient increase in [Ca2+]i which was demonstrated by Williamson and Ashley (1982).
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