A. Amtmann et al., MULTIPLE INWARD CHANNELS PROVIDE FLEXIBILITY IN NA+ K+ DISCRIMINATIONAT THE PLASMA-MEMBRANE OF BARLEY SUSPENSION-CULTURE CELLS/, Journal of Experimental Botany, 48, 1997, pp. 481-497
Ion transport across the plasma membrane of suspension-culture cells d
erived 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 medi
a about 50% of the cells exhibited resting potentials more negative th
an 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 bet
ween 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 similar to 171 mV); no inactivation; com
plete block by Ba2+ (30 mM in 100 mM KCl) and part block by TEA(+) (ma
ximum 50% with 20 mM); dependence on millimolar concentrations of cyto
plasmic ATP; no block by external or cytoplasmic Na+. The selectivity
sequences K+ >> Rb+ > NH4+ > Na+ >> Cl- and K+ >> NH4+ > Na+ > Rb+ wer
e determined from measurements of reversal potentials and relative ste
ady-state currents respectively. P-NA:P-K was 0.07+/-0.02 (from revers
al 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 t
ime-dependent whole-cell currents. In single channel experiments, seve
ral inward channels with distinct properties were found. The major cha
nnels were voltage-gated, K+-selective channel (12 pS), (ii) an ATP-ac
tivated non-selective cation channel (7 pS) and (iii) an inward-rectif
ying anion-channel (150 pS, all unitary conductances given for 100 mM
KCl). 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 ide
a that differential regulation of plasma membrane ion channels gives r
ise to a physiological flexibility, allowing the cells to control Nauptake under varying external conditions, is discussed.