The role of H+/OH- channels in saline pathology of Chara australis: brief history

摘要

The pH banding pattern, observed in Chara and Nitella early last century, suggests that there are H~+ or OH~- conducting channels in the alkaline bands of the characean plasma membrane. If the cells are exposed to high pH medium, pHo, these channels become the dominant membrane conductance: High pH state. Goldman-Hodgkin-Katz (GHK) equation can model the currentvoltage (I/V) characteristics of this state with either H~+ or OH~- as permeant ion. However, H~+ requires unrealistically large increases in channel conductance/number parameter as pHo increases. Recently we discovered a typical spiky noise in the membrane potential difference (PD) of salt sensitive Chara australis, appearing immediately upon exposure to saline. The H~+/ OH~- channels are thought to open transiently in small patches all over the cell, while negative membrane PD does not support their open state/s. As the membrane PD depolarizes due to inhibition of the proton pump by salinity, the H~+/OH~- channels remain open to dominate the I/V characteristics, which can also be modeled by GHK equation. The addition of 1 mM ZnCl2, the main known blocker of animal H~+ channels, abolishes both the PD noise and the H~+/OH~- I/V characteristics in high pH state and in salt stressed cells. Confocal microscopy and fluorescein isothiocyanate (FITC)-dextran illuminate the pH changes around cells and reveal transient bright spots early in saline exposure, becoming more numerous and fixed in space with time in saline APW.