Inversion of extracellular current and axial voltage profile in Chara and Nitella

Reducing the pH of the bathing solution from 8.2 to pH 6 can induce an inversion of the extracellular current pattern that develops at the surface of Chara corallina internodal cells. A similar result can be obtained on some cells by changing the medium to a pH value of 10. In noninverting Chara cells the currents were strongly reduced when the pH value of the medium was changed between 3 and 11. Simultaneous measurements of the Chara transmembrane potential in the acid and alkaline regions revealed that a light-induced electrical potential gradient of approximately 24 mV was present in the axial (or longitudinal) direction. Correlated to the external current pattern inversion was an inversion of this internal longitudinal voltage gradient. Reillumination of Nitella cells, after a period of darkness, often resulted in a complete inversion of the extracellular current pattern. These results are discussed in terms of spatial and temporal control of membrane transport processes, and in particular the control of current loops that pass through these cells.