A depolarization-activated outwardly-rectifying channel (OR), most likely involved in the passive release of K + from the root symplasm into the stelar apoplast (for subsequent transport to the shoot via the xylem vessels), has been characterized in the plasma membrane of maize root stelar cells (Roberts and Tester, 1995). In the present study, the selectivity of this channel was further characterized using single channel current-voltage curves generated using a voltage ramp protocol. This protocol permitted the accurate and unambiguous measurement of the reversal potentials of currents resulting from single channel openings. Using the voltage ramp protocol, it was shown that the OR allows both K + efflux and Ca 2+ influx at potentials positive of EK and negative of E^,. The OR had a PC./PK of 1.72-0.21 decreasing as extracellular Ca 2+ was increased. The permeability of the OR for monovalent cations other than K + was also investigated. In biionic conditions, a relative permeability sequence of K + ^Rb +>Cs +>Na + =NH4 + was determined (i.e. Eisenman sequence IV). The physiological implications of the selectivity of the OR are discussed
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