Sodium permeation into cortex cells of wheat roots was examined under conditions of high external NaCI and low Ca(2+). Two types of K(+) inward rectifier were observed in some cells. The time-dependent K(+) inward rectifier was Ca(2+)-sensitive, increasing in magnitude as external Ca(2+) 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 Ca(2+) 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 Ca(2+) concentrations. With 1 mM Ca(2+) 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 Ca(2+) activity was increased from 0.03 mM to 2.7 mM (half inhibition at 0.31 mM Ca(2+) activity). Changes in the characteristics of the current noise indicated that increased Ca(2+) 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.

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An electrogenic pump, a slowly activating K+ inward rectifier and an intermittent, "spiky," K+ inward rectifier, have been identified in the plasmalemma of whole protoplasts from root cortical cells of wheat (Triticum) by the use of patch clamping techniques. Even with high external concentrations of K+ of 100 mM, the pump can maintain the membrane potential difference (PD) down to -180mV, more negative than the electrochemical equilibrium potentials of the various ions in the system. The slowly activating K+ inward rectifier, apparent in about 23% of protoplasts, allows inward current flow when the membrane PD becomes more negative than the electrochemical equilibrium potential for K+ by about 50 mV. The current usually consists of two exponentially rising components, the time constant of one about 10 times greater than the other. The longer time constant is voltage dependent, while the smaller time constant shows little voltage dependence. The rectifier deactivates, on return of the PD to less negative levels, with a single exponential time course, whose time constant is strongly voltage dependent. The spiky K+ inward rectifier, present in about 68% of protoplasts, allows intermittent current, of considerable magnitude, through the plasmalemma at PDs usually more negative than about -140mV. Patch clamp experiments on detached outside-out patches show that a possibly multi-state K+ channel, with maximum conductance greater than 400 pS, may constitute this rectifier. The paper also considers the role of the pump and the K+ inward rectifiers in physiological processes in the cell.