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. Received: 1 December 1995/Revised: 8 May 1996