Characterization of potassium-dependent currents in protoplasts of corn suspension cells

Protoplasts obtained from corn (Zea mays) suspension cells were studied using the whole cell patch-clamp technique. One time-independent current, as well as two time-dependent currents were identified. All three currents were reduced by tetraethylammonium (9 millimolar), a K⁺ channel blocker. The time-independent current had a nearly linear current-voltage relationship and its reversal potential, defined as the voltage at which there is zero current, was highly dependent on the extracellular potassium concentration. One of the two time-dependent currents was activated, with rapid kinetics, by membrane hyperpolarization to potentials more negative than −100 millivolts. The second time-dependent current was activated with a sigmoidal time course by membrane depolarization to potentials more positive than −60 millivolts. It exhibited no inactivation and was carried primarily by potassium ions. These characteristics suggest that this latter current is caused by the voltage-dependent opening of delayed-rectifier K⁺ channels. These three currents, which are not generated by the plasmalemma H⁺-ATPase, are likely to assist in the regulation of the cellular K⁺ fluxes and membrane potential.