Effects of internal K+ and ABA on the voltage- and time-dependence of the outward K+-rectifier in Vicia guard cells

One of the main effects of abscisic acid (ABA) is to induce net loss of potassium salts from guard cells enabling the stomata to close. K+ is released from the vacuole into the cytosol and then to the extracellular space. The effects of increasing cytosolic K+ on the voltage- and time-dependence of the outwardly rectifying K+-current (I-K,I-out) in guard cell protoplasts (GCP) was examined in the whole-cell configuration of the patch-clamp technique. The same quantitative analysis was performed in the presence of ABA at different internal K+ concentrations ([K+](i)). Varying [K+](i) in the patch pipette from 100 to 270 mM increased the magnitude of I-K,I-out in a nonlinear manner and caused a negative shift in the midpoint (V-0.5) of its steady-state activation curve. External addition of ABA (10-20 mu M) also increased the magnitude of I-K,I-out at all [K+](i), but caused a shift in V-0.5 of the steady-state activation curve only in those GCP loaded with 150 mM internal K+ or less. Indeed, V-0.5 did not shift upon addition of ABA when the [K+](i) was above 150 mM and up to 270 mM, i.e., the shift in V-0.5 caused by ABA depended on the [K+](i). Both increase in [K+](i) and external addition of ABA, decreased (by approximate to 20%) the activation time constant (tau(n)) of I-K,I-out. The small decrease in tau(n), in both cases, was found to be independent of the membrane voltage. The results indicate that PLEA mimics the effect of increasing cytoplasmic K+, and suggest that ABA may increase I-K,I-out and alter V-0.5 of its steady-state activation curve via an enhancement in cytosolic K+. This report describes for the first time the effects of [K+](i) on the voltage- and time-dependence of I-K,I-out in guard cells. It also provides an explanation for the quantitative (total membrane current) and qualitative (current kinetics) differences found between intact guard cells and their protoplasts.