The Influence of H⁺ on the Membrane Potential and Ion Fluxes of Nitella

The resting membrane potential of the Nitella cell is relatively insensitive to [K]_o, but behaves like a hydrogen electrode. K⁺ and Cl^- effluxes from the cell were measured continuously, while the membrane potential was changed either by means of a negative feedback circuit or by external pH changes. The experiments indicate that P_K and P_Cl are independent of pH but are a function of membrane potential. Slope ion conductances, G_K, G_Cl, and G_Na were calculated from efflux measurements, and their sum was found to be negligible compared to membrane conductance. The possibility that a boundary potential change might be responsible for the membrane potential change was considered but was ruled out by the fact that the peak of the action potential remained at a constant level regardless of pH changes in the external solution. The conductance for H⁺ was estimated by measuring the membrane current change during an external pH change while the membrane potential was clamped at K⁺ equilibrium potential. In the range of external pH 5 to 6, H⁺ chord conductance was substantially equal to the membrane conductance. However, the [H]_i measured by various methods was not such as would be predicted from the [H]_o and the membrane potential using the Nernst equation. In artificial pond water containing DNP, the resting membrane potential decreased; this suggested that some energy-consuming mechanism maintains the membrane potential at the resting level. It is probable that there is a H⁺ extrusion mechanism in the Nitella cell, because the potential difference between the resting potential and the H⁺ equilibrium potential is always maintained notwithstanding a continuous H⁺ inward current which should result from the potential difference.