Quantitative analysis of outward rectifying K+ channel currents in guard cell protoplasts from Vicia faba

A quantitative analysis of the time and voltage dependence of outward-rectifying K+ currents (IK+, out) in guard cells from Vicia faba is described using the whole-cell patch-clamp technique. After step depolarizations from -75 mV to potentials positive to -40 mV, time-dependent outward currents were produced, which have recently been identified as K+ channel currents. This K+ current was characterized according to its time dependence and its steady-state activation. IK+, out could be described in terms of a Hodgkin-Huxley type conductance. Activation of the current in time was sigmoid and was well fitted by raising the activation variable to the second power. Deactivating tail currents were single exponentials, which suggests that only one conductance underlies this slow outward K+ current. Rates of channel closing were strongly dependent on the membrane potential, while rates of channel opening showed only limited voltage dependence leading to a highly asymmetric voltage dependence for channel closing and opening. The presented analysis provides a quantitative basis for the understanding of IK+, out channel gating and IK+, out channel functions in plant cells.