Proton/chloride cotransport in Chara: mechanism of enhanced influx after rapid external acidification

Abstract

Rapid lowering of the external pH (“pH jump”) enhances Cl⁻ influx in Chara. Experiments were conducted to distinguish between two factors which have previously been proposed to mediate in the response: raised cytoplasmic pH and lowered cytoplasmic Cl⁻ concentration. It is concluded that the latter alternative is more likely because: i) Cl⁻ influx is reduced at high external pH; ii) influx following the pH jump is never greater than that following pretreatment in Cl⁻-free solution, which reduces cytoplasmic Cl⁻ concentration (“Cl⁻ starvation”); iii) the joint application of pH jump and Cl⁻ starvation does not result in a greater Cl⁻ influx than does Cl⁻ starvation alone; and iv) addition of NH ⁺₄ , which increases cytoplasmic pH, does generate an additional stimulation of Cl⁻ influx following a pH jump. It is suggested that the increased cytoplasmic pH at the end of pretreatment at high external pH decays rapidly during the pH jump, and thus any effect on Cl⁻ influx is so transient as to be undetectable by the methods used. The results are discussed in terms of a reaction kinetic model for 2H⁺/Cl⁻ cotransport (Sanders, D. and Hansen, U.-P, 1981, J. Member. Biol. 58, 139–153) which describes quantitatively; i) the effects of NH ⁺₄ on Cl⁻ influx in terms involving only a change in cytoplasmic pH; and ii) the combined effects of Cl⁻ starvation and NH ⁺₄ in terms involving only changes in Cl⁻ concentration and cytoplasmic pH. Conversely, the combined effects of Cl⁻ starvation and pH jump cannot be described by the model if the effect of the pH jump is the consequence of increased cytoplasmic pH. The simple interpretation of experiments on whole cells involving manipulation of $\Delta \bar \mu _{{\text{H}}^ + }$ (the electrochemical potential difference for protons across the plasma membrane) is questioned in the light of these and previous findings that secondary factors can determine the response of Cl⁻ transport in Chara.