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,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,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 μM) also increased the magnitude of I(K,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 ~ 20%) the activation time constant (τ(n)) of I(K,out). The small decrease in τ(n), in both cases, was found to be independent of the membrane voltage. The results indicate that ABA mimics the effect of increasing cytoplasmic K⁺, and suggest that ABA may increase I(K,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,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.