Abstract
Ion transport across the plasma membrane of suspension-culture cells derived from immature barley embryos has been studied in low (15 mM KCl) and high (additional 150 mM NaCl) salt conditions to understand how plants discriminate between K(+) and Na(+) during ion uptake. In both media about 50% of the cells exhibited resting potentials more negative than any of the passive diffusion potentials. In whole-cell patch clamp experiments membrane hyperpolarization activated large inward currents. Whilst the instantaneous current components did not discriminate between K(+) and Na(+), the time-dependent current, I(in), was selective for K(+) over Na(+). Further analysis of I(in) revealed the following properties: double exponential current activation (time-constants 0.03 s and 0.3 s, half activation potential - 171 mV); no inactivation; complete block by Ba(2+) (30 mM in 100 mM KCl) and part block by TEA(+) (maximum 50% with 20 mM); dependence on millimolar concentrations of cytoplasmic ATP; no block by external or cytoplasmic Na(+). The selectivity sequences K(+) ≫ Rb(+) > NH(+)(4) > Na(+) ≫ Cl(-) and K(+) ≫ NH(+)(4) > Na(+) > Rb(+) were determined from measurements of reversal potentials and relative steady-state currents respectively. P(Na):P(K) was 0.07 ± 0.02 (from reversal potentials) and I(Na):I(K) was 0.17 + 0.05 (from relative currents). A high variance among the observed permeability ratios suggested that several channels with different ion-selectivities contributed to the time-dependent whole-cell currents. In single channel experiments, several inward channels with distinct properties were found. The major channels were (i) a voltage-gated, K(+)-selective channel (12 pS), (ii) an ATP-activated non-selective cation channel (7 pS) and (iii) an inward-rectifying anion-channel (150 pS, all unitary conductances given for 100 mM KCI). No significant differences were found in whole-cell currents or single-channel characteristics between cells that had been adapted to a high-salt growth-medium (150 mM NaCl) and non-adapted cells. The idea that differential regulation of plasma membrane ion channels gives rise to a physiological flexibility, allowing the cells to control Na(+) uptake under varying external conditions, is discussed.
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Claims 20
Claim · model-written summary
About half the barley cells in both media had resting potentials below passive diffusion potentials.
- Taxon
- barley
.jpg?width=60)
Hordeum vulgaret0 - Outcome
- resting potentials more negative than any of the passive diffusion potentialse0
- Value
- about 50%e0
- Qualifier
- both mediae0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
.jpg?width=500)
Verbatim
e0“In both media about 50% of the cells exhibited resting potentials more negative than any of the passive diffusion potentials”
t0“barley”
Claim · model-written summary
Membrane hyperpolarization activated large inward currents.
- Taxon
- barleyHordeum vulgaret0
- Treatment
- membrane hyperpolarizatione0
- Outcome
- large inward currentse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“membrane hyperpolarization activated large inward currents”
t0“barley”
Claim · model-written summary
Instantaneous current components did not discriminate K(+) from Na(+).
- Taxon
- barleyHordeum vulgaret0
- Outcome
- instantaneous current components did not discriminate between K(+) and Na(+)e0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the instantaneous current components did not discriminate between K(+) and Na(+)”
t0“barley”
Claim · model-written summary
Time-dependent inward current was selective for potassium over sodium.
- Taxon
- barleyHordeum vulgaret0
- Outcome
- I(in), was selective for K(+) over Na(+)e0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the time-dependent current, I(in), was selective for K(+) over Na(+)”
t0“barley”
Claim · model-written summary
I(in) exhibited double exponential activation with 0.03 s and 0.3 s time constants.
- Taxon
- barleyHordeum vulgaret0
- Outcome
- double exponential current activatione0
- Value
- 0.03 s and 0.3 se0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“double exponential current activation (time-constants 0.03 s and 0.3 s, half activation potential - 171 mV)”
t0“barley”
Claim · model-written summary
I(in) half activation potential was - 171 mV.
- Taxon
- barleyHordeum vulgaret0
- Outcome
- half activation potentiale0
- Value
- - 171 mVe0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“half activation potential - 171 mV”
t0“barley”
Claim · model-written summary
I(in) showed no inactivation.
- Taxon
- barleyHordeum vulgaret0
- Outcome
- no inactivatione0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“no inactivation”
t0“barley”
Claim · model-written summary
Ba(2+) completely blocked I(in).
- Taxon
- barleyHordeum vulgaret0
- Treatment
- Ba(2+) (30 mM in 100 mM KCl)e0
- Outcome
- complete blocke0
- Direction
- blocke0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“complete block by Ba(2+) (30 mM in 100 mM KCl)”
t0“barley”
Claim · model-written summary
TEA(+) partly blocked I(in), up to 50%.
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“part block by TEA(+) (maximum 50% with 20 mM)”
t0“barley”
Claim · model-written summary
I(in) depended on millimolar cytoplasmic ATP.
- Taxon
- barleyHordeum vulgaret0
- Compound
- cytoplasmic ATPe0
- Outcome
- I(in)e1
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“dependence on millimolar concentrations of cytoplasmic ATP”
t0“barley”
e1“Further analysis of I(in)”
Claim · model-written summary
External or cytoplasmic sodium did not block I(in).
- Taxon
- barleyHordeum vulgaret0
- Outcome
- no block by external or cytoplasmic Na(+)e0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“no block by external or cytoplasmic Na(+)”
t0“barley”
Claim · model-written summary
Reversal-potential measurements ranked ion selectivity K(+) ≫ Rb(+) > NH(+)(4) > Na(+) ≫ Cl(-).
- Taxon
- barleyHordeum vulgaret0
- Outcome
- K(+) ≫ Rb(+) > NH(+)(4) > Na(+) ≫ Cl(-)e0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“K(+) ≫ Rb(+) > NH(+)(4) > Na(+) ≫ Cl(-)”
t0“barley”
Claim · model-written summary
Steady-state current measurements ranked ion selectivity K(+) ≫ NH(+)(4) > Na(+) > Rb(+).
- Taxon
- barleyHordeum vulgaret0
- Outcome
- K(+) ≫ NH(+)(4) > Na(+) > Rb(+)e0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“K(+) ≫ NH(+)(4) > Na(+) > Rb(+)”
t0“barley”
Claim · model-written summary
Sodium to potassium permeability ratio was 0.07 ± 0.02.
- Taxon
- barleyHordeum vulgaret0
- Outcome
- P(Na):P(K)e0
- Value
- 0.07 ± 0.02e0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“P(Na):P(K) was 0.07 ± 0.02 (from reversal potentials)”
t0“barley”
Claim · model-written summary
Sodium to potassium current ratio was 0.17 + 0.05.
- Taxon
- barleyHordeum vulgaret0
- Outcome
- I(Na):I(K)e0
- Value
- 0.17 + 0.05e0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“I(Na):I(K) was 0.17 + 0.05 (from relative currents)”
t0“barley”
Claim · model-written summary
Variable permeability ratios suggested contributions from multiple channels.
- Taxon
- barleyHordeum vulgaret0
- Outcome
- time-dependent whole-cell currentse0
- Qualifier
- several channels with different ion-selectivitiese0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“A high variance among the observed permeability ratios suggested that several channels with different ion-selectivities contributed to the time-dependent whole-cell currents”
t0“barley”
Claim · model-written summary
Single channel experiments identified a 12 pS voltage-gated potassium-selective channel.
- Taxon
- barleyHordeum vulgaret0
- Outcome
- voltage-gated, K(+)-selective channele0
- Value
- 12 pSe0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“a voltage-gated, K(+)-selective channel (12 pS)”
t0“barley”
Claim · model-written summary
Single channel experiments identified a 7 pS ATP-activated nonselective cation channel.
- Taxon
- barleyHordeum vulgaret0
- Outcome
- ATP-activated non-selective cation channele0
- Value
- 7 pSe0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“an ATP-activated non-selective cation channel (7 pS)”
t0“barley”
Claim · model-written summary
Single channel experiments identified a 150 pS inward rectifying anion channel.
- Taxon
- barleyHordeum vulgaret0
- Outcome
- inward-rectifying anion-channele0
- Value
- 150 pSe0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“an inward-rectifying anion-channel (150 pS, all unitary conductances given for 100 mM KCI)”
t0“barley”
Claim · model-written summary
High salt adaptation caused no significant difference in whole-cell currents or single-channel characteristics.
- Taxon
- barleyHordeum vulgaret0
- Treatment
- high-salt growth-medium (150 mM NaCl)e0
- Outcome
- whole-cell currents or single-channel characteristicse0
- Direction
- No significant differencese0
- Comparator
- non-adapted cellse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“No significant differences were found in whole-cell currents or single-channel characteristics between cells that had been adapted to a high-salt growth-medium (150 mM NaCl) and non-adapted cells”
t0“barley”