The electric membrane potential as functions of position and time of Characean internode has been studied using a modified water-film electrode technique. Between the low-conductance hyperpolarized region (called the H-region or acidic region) and the high-conductance depolarized region ( D-region), there is a difference in the direction of responses to light-off and -on stimulations. In darkness the membrane potential becomes hyperpolarized in the D-region, whereas it is depolarized in the H-region at the steady state. The potential difference between D- and H- regions, ΔVm, is increased by exposure to pure O2, N2, or CO2-free air. When the amount of water surrounding the internode is limited, the formation of an electric pattern occurs rapidly. In contrast, the recovery is delayed. The membrane potential of the D-region is sometimes hyperpolarized significantly with lowering of the extracellular pH to 7.5, while the potential of the H-region is slightly depolarized. This seems to be an all-or-none type response. However, the electric profile is always homogenized with the pH of 6.8. Thus, the pH around 7.5 may be a threshold level to open/close putative OH- (or H +) channels of the D-region.
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