Planta

, Volume 197, Issue 1, pp 193–199 | Cite as

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

  • Rainer Hedrich
  • Monica Bregante
  • Ingo Dreyer
  • Franco Gambale
Article

Abstract

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 techniqueUsing 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 ≈5–7 pS) as the major membrane conductan-ces 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 fourto fivefoldUpon 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 increaseThe absence of a measurable conductance for Na+, Rb+, Cs+ and a permeability ratio PNH4+/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+ channelThe 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.

Key words

Coleoptile (K+ uptake) K+ channel selectivity, voltage-dependence Zea 

Abbreviations

CKC1in

Coleoptile K+ Channel inward rectifier

U

membrane voltage

Iss

steady-state currents

Itail

tail currents

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Copyright information

© Springer-Verlag 1995

Authors and Affiliations

  • Rainer Hedrich
    • 1
  • Monica Bregante
    • 2
  • Ingo Dreyer
    • 1
  • Franco Gambale
    • 2
  1. 1.Institut für Biophysik, Universität HannoverHannoverGermany
  2. 2.Istituto di Cibernetica e Biofisica, CNRGenovaItaly

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