Activation Kinetics of the K⁺ Outward Rectifying Conductance (KORC) in Xylem Parenchyma Cells from Barley Roots

Abstract.

The activation kinetics of outward currents in protoplasts from barley root xylem parenchyma was investigated using the patch-clamp technique. The K⁺ outward rectifying conductance (KORC), providing the main pathway for K⁺ transport to the xylem, could be described in terms of a Hodgkin-Huxley model with four independent gates. Gating of KORC depended on voltage and the external K⁺ concentration. An increase in the external K⁺ concentration resulted in a shift in the voltage dependence of gating. This could be explained by a K⁺ dependence of the rate constant β for channel closure, indicating binding of K⁺ to a regulatory site exposed to the bath. Occasionally, KORC was observed to inactivate; this inactivation occurred and vanished spontaneously. In some of the whole cell and excised patch recordings, a stepwise increase in outward current was observed upon a depolarizing voltage pulse, indicating that several populations of `sleepy' channels existed in the plasma membrane that activated with a certain lag time. It is discussed whether this observation can be explained by a putative subunit, which retards channel activation, or by a scheme of cooperative gating. A quantitative description of outward rectifying K⁺ channels in xylem parenchyma cells is a major step forward towards a mathematical model of salt transport into the xylem.