The voltage-dependent potassium-uptake channel of corn coleoptiles has permeation properties different from other K+-channels

The initial response of coleoptile cells to growth hormones and light is a rapid change in plasma-membrane polarization. We have isolated protoplasts from the cortex of maize (Zea mays L.) coleoptiles to study the electrical properties of their plasma membrane by the patch-clamp technique. Using the whole-cell configuration and cell-free membrane patches we could identify an H+-ATPase, hyperpolarizing the membrane potential often more negative than 150 mV, and a voltage-dependent, inward-rectifying K+ channel (unit conductance approximately 5-7 pS) as the major membrane conductances. Potassium currents through this channel named CKC1in (for Coleoptile K+ Channel inward rectifier) were elicited upon voltage steps negative to -80 mV, characterized by a half-activation potential of -112 mV. The kinetics of activation, well described by a double-exponential process, were strongly dependent on the degree of hyperpolarization and the cytoplasmic Ca2+ level. Whereas at nanomolar Ca2+ concentrations K+ currents increased with a t1/2=16 ms (at -180 mV), higher calcium levels slowed the activation process about four- to fivefold. Upon changes in the extracellular K+ concentration the reversal potential of the K+ channel followed the Nernst potential for potassium with a 56-mV shift for a tenfold increase. The absence of a measurable conductance for Na+, Rb+, Cs+ and a permeability ratio PNH+4/PK+ around 0.25 underlines the high selectivity of CKC1in for K+. In contrast to Cs+, which at submillimolar concentration blocks the channel in a voltage-dependent manner, Rb+, often used as a tracer for K+, does not permeate this type of K+ channel. The lack of Rb+ permeability is unique with respect to other K+ transporters. Therefore, future molecular analysis of CKC1in, considered as a unique variation of plant inward rectifiers, might help to understand the permeation properties of K+ channels in general.