K+ outward rectifying channels as targets of phosphatase inhibitor deltamethrin in Vicia faba guard cells

The dominant outward rectifier K+ currents were examined in protoplasts from Vicia faba guard cells. In whole-cell patch-clamp recordings, we generally observed that the conductance of the K+ inward and the outward rectifier gradually decreases with a half time in the order of 2.3 +/- 0.7min. As a consequence of this rundown, a new steady state was achieved which was 90 +/- 5% lower than that obtained at the beginning of the recording. The rundown of the outward rectifier could be greatly reduced by pre-treating protoplasts either with the membrane permeable drug deltamethrin or by perfusing protoplasts with a pipette solution containing 5 mumol/L cyclosporine A. Furthermore, after the rundown, the conductance of the outward rectifier could be partially restored upon addition of 5 mumol/L deltamethrin to the bath medium. Since deltamethrin and cyclosporine A are established inhibitors of the calcium sensitive phosphatase calcineurin, the data argue for a participation of this type of phosphatase in the control of the activity of K+ outward rectifier channels in guard cells.