Kinetic analysis of two simultaneously activated K+ currents in root cell protoplasts of Plantago media L.

Two different, simultaneously activated outward rectifying K+ currents were analyzed in the plasmalemma of root cortex protoplasts of Plantago media. Their gating is dependent on the diffusion potential for K+(EK). The threshold potential was more negative than EK allowing small inward currents at potentials below EK thereby keeping cells with little pump activity in the K state (Vogelzang & Prins, 1994). Time and voltage dependence of the outward rectifying K+ currents have been analyzed with Hodgkin-Huxley-like (HH) models. Dynamic responses of whole cell currents to pulse potentials were analyzed with two voltage dependent functions, the Boltzmann distribution for open probability per gate and the transition rate towards the open state (alpha). The transition rate in the opposite direction (beta), was calculated from alpha and the Boltzmann distribution. These functions were used for an integral analysis of activation and deactivation currents measured over a range of pulse potentials. Both whole cell and single channel data were used for the determination of the number of closed and open states. The effects of single channel flickering on time response and amplitude of tail currents were added to the model. The dominant K+ channel present in the plasmalemma of P. media has a characteristic nonlinear single channel I-V curve reducing the amplitude of whole cell currents at positive potentials. To compensate for this nonlinearity, a four state translocator model was added to the whole cell open probability model. The analysis presented here provides a general basis for the study and comparison of K+ channel kinetics in plant protoplasts.