Membrane Potential and Resistance in Relation to Cytoplasmic pH in Nitellopsis

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The responses of membrane potential and membrane resistance of tonoplast-free Nitellopsis cells to step changes of internal pH (pH_i) from 7 were studied during continuous perfusion with media containing either 1 mM ATP or no ATP. Whether ATP was present or not, the time course of E_m responses was composed of an initial rapid change (initial phase) and a subsequent slow change (second phase). At the end of the second phase, E_m attained nearly stable values. When E_m values of ATP-containing cells obtained at the peak of the initial phase were plotted against pH_i, E_m was found to hyperpolarize most at pH_i 6.5. This was also found for steady E_m values measured at the end of the second phase. The E_m values ofATP-lacking cells were almost insensitive to pHi changes between 4 and 8, but more positive than those of' ATP-containing cells at pH_i 4-7.5. Above pH_i 8, no difference in E_m was observed between the two types of cells. In this range of pH_i, the E_m change in the initial phase amounted to about 60 mV per unit of pH_i change. The light-induced hyperpolarization still occurred at pH_i 6.5 where the electrogenic potential was maximal, and over the wide pH_i range from 6.0 to 7.5 even when pH_i was strongly buffered. Thus, we concluded that the pH_i change may not be the cause of light-induced hyperpolarization.
日本植物生理学会の論文