Pathways for the permeation of Na+ and Cl- into protoplasts derived from the cortex of wheat roots

Sodium permeation into cortex cells of wheat roots was examined under conditions of high external NaCl and low Ca2+. Two types of K+ inward rectifier were observed in some cells. The time-dependent K+ inward rectifier was Ca2+-sensitive, increasing in magnitude as external Ca2+ was decreased from 10 mM to 0.1 mM, but did not show significant permeability to Na+. However, the spiky inward rectifier showed significant Na+ permeation at Ca2+ concentrations of 1 and 10 mM. In cells that initially did not show K+ inward rectifier channels, fast and sometimes slowly activating whole-cell inward currents were induced at membrane potentials negative of zero with high external Na+ and low Ca2+ concentrations. With 1 mM Ca2+ in the external solution, large inward currents were carried by Rb+, Cs+, K+, Li+, and Na+. The permeability sequence shows that K+, Rb+ and Cs+ are all more permeant than Na+, which is about equally as permeant as Li+. When some K+ was present with high concentrations of Na+ the inward currents were larger than with K+ or Na+ alone. About 60% of the inward current was reversibly blocked when the external Ca2+ activity was increased from 0.03 mM to 2.7 mM (half inhibition at 0.31 mM Ca2+ activity). Changes in the characteristics of the current noise indicated that increased Ca2+ reduced the apparent single channel amplitude. In outside-out patches inward currents were observed at membrane potentials more positive than the equilibrium potentials for K+ and Cl- when the external Na+ concentration was high. These channels were difficult to analyse but three analysis methods yielded similar conductances of about 30 pS.