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

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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 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+ 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.

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Abbreviations

CKC1in :

Coleoptile K + Channel inward rectifier

U:

membrane voltage

Iss :

steady-state currents

Itail :

tail currents

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Authors and Affiliations

  1. Institut für Biophysik, Universität Hannover, Herrenhäuser Strasse 2, D-30419, Hannover, Germany

    Rainer Hedrich & Ingo Dreyer

  2. Istituto di Cibernetica e Biofisica, CNR, Via de Marini 6, I-16149, Genova, Italy

    Monica Bregante & Franco Gambale

Authors
  1. Rainer Hedrich
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  2. Monica Bregante
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  3. Ingo Dreyer
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  4. Franco Gambale
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Additional information

Experiments were conducted in the laboratory of F.G. during the stay of RHas a guest professor sponsored by Special Project RAISA, subproject N2.1, paper N2155.

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Hedrich, R., Bregante, M., Dreyer, I. et al. The voltage-dependent potassium-uptake channel of corn coleoptiles has permeation properties different from other K+ channels. Planta 197, 193–199 (1995). https://doi.org/10.1007/BF00239956

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  • DOI: https://doi.org/10.1007/BF00239956

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