External protons enhance the activity of the hyperpolarization-activated K channels in guard cell protoplasts of Vicia faba

Hyperpolarization-activated K channels (K(H) channels) in the plasmalemma of guard cells operate at apoplastic pH range of 5 to over 7. Using patch clamp in a whole-cell mode, we characterized the effect of varying the external pH between 4.4-8.1 on the activity of the K(H) channels in isolated guard cell protoplasts from Vicia faba leaves. Acidification from pH 5.5 to 4.4 increased the macroscopic conductance of the K(H) channels by 30-150% while alkalinization from pH 5.5 to 8.1 decreased it only by roughly 15%. The voltage-independent maximum cell conductance, increased by ~60% between pH 8.1 and 4.4 with an apparent pK(a) of 5.3, most likely owing to the increased availability of channels. Voltage-dependent gating was affected only between pH 5.5 and 4.4. Acidification in this range shifted the voltage-dependent open probability by over 10 mV. We interpret this shift as an increase of the electrical field sensed by the gating subunits caused by the protonation of external negative surface charges. Within the framework of a surface charge model the mean spacing of these charges was ~30 Å and their apparent dissociation constant was 10 ^-4.6. The overall voltage sensitivity of gating was not altered by pH changes. In a subgroup of protoplasts analyzed within the framework of a Closed-Closed-Open model, the effect of protons on gating was limited to shifting of the voltage-dependence of all four transition rate constants.