Zinc ions block H+/OH- channels in Chara australis

Chara australis cells exposed to media of pH 10 and above exhibit high conductance, arising from the opening of H+/OH- channels in the plasma membrane. This high conductance can be totally inhibited by 1.0 mm ZnCl2 and restored by 0.5 mm 2-mercaptoethanol (ME). Important for carbon fixation, H+/OH- channels play a key role in cell pH banding. Banding was also shown to be abolished by 1.0 mm ZnCl2 and restored in some cells by ME. The proton pump is also involved in banding, but was little affected by ZnCl2 over the periods needed for the inhibition of H+/OH- channels. Previously, we postulated that H+/OH- channels open transiently at the onset of saline stress in salt-sensitive C. australis, causing membrane potential difference (PD) noise; and remain open in latter stages of saline stress, contributing to cell deterioration. ZnCl2 totally inhibited the saline noise and the upwardly concave I/V characteristics associated with the putative H+/OH- currents. Again, ME reversed both these effects. We discuss the mode of action of zinc ions and ME with reference to animal voltage-gated H+ channels and water channels.