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 m m ZnCl 2 and restored by 0.5 m m 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 m m ZnCl 2 and restored in some cells by ME. The proton pump is also involved in banding, but was little affected by ZnCl 2 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. ZnCl 2 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.