Tonoplast action potential in Nitella in relation to vacuolar chloride concentration

The action potential of Nitella internode was studied in relation to K+ and C1- concentrations in the vacuole. When the vacuole of Nitella pulchella was filled with an artificial solution with extremely low C1- concentration, a diphasic action potential (DAP) was observed. T he first phase consists of a rapid depolarization followed by a relatively rapid repolarization, and the second one consists of a strong hyperpolarization followed by a gradual return to the resting potential. When the cell was stimulated immediately after the generation of DAP, a monophasic action potential which resembles an action potential of the natural cell was observed, indicating that the DAP consists of two components with different refractory periods. The refractory period of the component responsible for the depolarizing is shorter than that of a component responsible for the hyperpolarizing phase. Measuring the plasmalemma potential and vascuolar potential separately, it was demonstrated that the hyperpolarizing component of DAP originates from the tonoplast. The action potential of the tonoplast, in contrast with that of the plasmalemma, could be generated independently of concentration of K+ in the vasuole. Since the maximum amplitude of hyperpolarization decreased significantly by increasing C1- concentration of the vacuole, it is concluded that the tonoplast is very sensitive to C1- during excitation.