Permeation of Ca²⁺ through K⁺ channels in the plasma membrane of Vicia faba guard cells

The whole-cell patch-clamp method has been used to measure Ca²⁺ influx through otherwise K⁺-selective channels in the plasma membrane surrounding protoplasts from guard cells of Vicia faba. These channels are activated by membrane hyperpolarization. The resulting K⁺ influx contributes to the increase in guard cell turgor which causes stomatal opening during the regulation of leaf-air gas exchange. We find that after opening the K⁺ channels by hyperpolarization, depolarization of the membrane results in tail current at voltages where there is no electrochemical force to drive K⁺ inward through the channels. Tail current remains when the reversal potential for permeant ions other than Ca²⁺ is more negative than or equal to the K⁺ equilibrium potential (-47 mV), indicating that the current is due to Ca²⁺ influx through the K⁺ channels prior to their closure. Decreasing internal [Ca²⁺] (Ca_i) from 200 to 2 n m or increasing the external [Ca²⁺] (Ca_o) from 1 to 10 m m increases the amplitude of tail current and shifts the observed reversal potential to more positive values. Such increases in the electrochemical force driving Ca²⁺ influx also decrease the amplitude of time-activated current, indicating that Ca²⁺ permeation is slower than K⁺ permeation, and so causes a partial block. Increasing Ca_o also (i) causes a positive shift in the voltage dependence of current, presumably by decreasing the membrane surface potential, and (ii) results in a U-shaped current-voltage relationship with peak inward current ca. -160 mV, indicating that the Ca^2- block is voltage dependent and suggesting that the cation binding site is within the electric field of the membrane. K⁺ channels in Zea mays guard cells also appear to have a Ca_i-, and Ca_o-dependent ability to mediate Ca²⁺ influx. We suggest that the inwardly rectiying K⁺ channels are part of a regulatory mechanism for Ca_i. Changes in Ca_oand (associated) changes in Ca_i regulate a variety of intracellular processes and ion fluxes, including the K⁺ and anion fluxes associated with stomatal aperture change.