Effect of intracellular pH on the light-induced potential change and electrogenic activity in tonoplast-free cells of Chora australis

Effects of the intracellular H+ concentration on the membranepotential and membrane resistance of tonoplast-free cells ofChara australis were examined under light and dark conditions.Cells were made tonoplast-free by perfusing the vacuole withmedia containing 5 mM EGTA and 30 mM buffers of various pH values.The electrogenic pump was stopped by removing the intracellularATP. The ATP-dependent part of the membrane potential was largeat pH₁ 6.2 and 6.9, but decreased in both the acidic (pH₁ 5.1,5.7) and alkaline (pH₁ 7.9, 8.7) ranges. Assuming that the putativeelectrogenic H+ pump acts solely as a current source, we estimatedthe current and the work done by the pump. Both the currentand the work were at a maximum at pH₁ 6.9, but decreased tosome extent at pH₁ 7.9–8.7 and decreased greatly at pH₁5.7–5.1. Light caused a significant membrane hyperpolarization at pH₁6.2–7.9, but only a slight hyperpolarization at pH₁ 5.1,5.7 and 8.7. The presence of light-induced hyperpolarizationat pH₁ lower than 6.2 suggests that acidification of the cytoplasmupon illumination does not cause activation of the putativeH+ pump. Membrane resistance was very high at pH₁ 5.1 and 5.7 and verylow at pH₁ 8.7. The very high R_m values at pH₁ 5.1 and 5.6 arebelieved to be related to inhibition of the activity of thepump. (Received June 20, 1979; )