Potassium currents across the plasma membrane of protoplasts derived from rye roots: A patch-clamp study

Epidermal-cell protoplasts from rye (Secale cereale L.) roots were voltage-clamped in both the whole-cell and outside-out membrane-patch modes. Time-dependent inwardly-rectified (IR) and outwardly-rectified (OR) K⁺-currents were recorded, as well as a ubiquitous, timeindependent (instantaneous) K⁺-current.The IR current activated at voltages more negative than -100mV with two exponentially rising components. The time-constant of the shorter component was voltage-independent, whereas the time-constant of the longer component was voltage-dependent, increasing as the activating voltage became more negative. The IR current showed no inactivation. The IR current deactivated with a single exponential timecourse. The steady-state IR current could be fitted to a Boltzmann function with -135 mV as the voltage at which the current was half-maximal and a minimal gating charge of 1.93. These parameters were insensitive to changes in E_K. One component of the IR current was K ⁺ , but other ions were also permeable. The IR current was inhibited by extracellular Ca²⁺ , Ba²⁺ , Cs⁺, and TEA⁺, but was insensitive to quinine. Single channels with unitary conductances of 56 pS and 110 pS (in c.100 mM K⁺) were recorded at negative voltages.Two OR currents were observed. One had sigmoidal activation kinetics and activated at low positive voltages. The other activated more rapidly, with apparently exponential kinetics, at voltages 50-100 mV more positive than the first. Neither current showed inactivation and deactivation of OR currents followed a double exponential time-course. Unitary-conductances of the channels mediating these OR currents were 24 pS and 57 pS (in c.100 mM K⁺), respectively. Only the first type of OR current was studied in detail. This current activated with a sigmoidal time-course, which could be described using a Hodgkin-Huxley function with the activation variable raised to the second power. Its voltage-dependence was modulated in response to changes in E^K and analysis of single-channel recordings indicated that the channel was K⁺-selective. The current was inhibited by Ba²⁺ and TEA+, but not Ca²⁺, Cs⁺ or quinine.The instantaneous current was selective for monovalent cations and K⁺ , Na⁺ and Cs⁺ were all permeant. It was inhibited by extracellular quinine and the instantaneous inward K⁺-current was reduced by extracellular Ca²⁺, Ba²⁺ and TEA⁺, as well as by competing permeant monovalent cations.The kinetics and pharmacology of these currents are compared with K⁺-currents across the plasma membrane of protoplasts from other root-derived cells and with K⁺ channels in the plasma membrane of rye roots studied following incorporation into artificial, planar lipid bilayers.