Outward K+ channels in Brassica chinensis pollen protoplasts are regulated by external and internal pH

Patch-clamp whole-cell and single-channel recording techniques were used to investigate the regulation of outward K+ channels by external and internal protons in Brassica chinensis pollen protoplasts. Outward K+ currents and conductance were insensitive to external pH (pH(o)) except at pH 4.5. Maximal conductance (G(max)) for the outward K+ currents was inhibited at acidic external pH. Half-activation voltage (E-1/2) for the outward K+ currents shifted to more positive voltages along with the decrease in pH. E-1/2 can be described by a modified Henderson-Hasselbalch equation expected from a single titratable binding site. The activation kinetics of the outward K+ channels was largely insensitive to pH(o). An internal pH (pH(i)) of 4.5 significantly increased outward K+ currents and conductance. G(max) for the outward K+ currents decreased with elevations in pHi. In contrast to the effect of pH(o), E-1/2 was shifted to more positive voltages with elevations in pHi. The outward K+ currents, G(max) and E-1/2 can be described by the modified Henderson-Hasselbalch equation. Furthermore, acidifying pHi accelerated the activation of the outward K+ currents significantly. The differences in electro-physiological properties among previously reported and currently described plant outward K+ channels may reflect differences in the structure of these channels.