A membrane-delimited effect of internal pH on the K+ outward rectifier of Vicia faba guard cells

ABA stimulation of outward K+ current (I-K,I-out) in Vicia faba guard cells has been correlated with a rise in cytosolic pH (pH(i)). However, the underlying mechanism by which IK,out is affected by pH(i) has remained unknown. Here, we demonstrate that pH(i) regulates outward K+ current in isolated membrane patches from Vicia faba guard cells. The stimulatory effect of alkalinizing pH(i) was voltage insensitive and independent of the two free calcium levels tested, 50 nM and 1 mu M. The single-channel conductance was only slightly affected by pH(i). Based on single-channel measurements, the kinetics of time-activated whole-cell current, and the analysis of current noise in whole-cell recordings, we conclude that alkaline pH(i) enhances the magnitude of I-K,I-out by increasing the number of channels available for activation. The fact that the pH(i) effect is seen in excised patches indicates that signal transduction pathways involved in the regulation of I-K,I-out by pH(i) and by implication, components of hormonal signal transduction pathways that are downstream of pH(i) are membrane-delimited.