Pump and K+ inward rectifiers in the plasmalemma of wheat root protoplasts

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 m m, the pump can maintain the membrane potential difference (PD) down to –180 mV, 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 –140 mV. 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.We thank Don Mackenzie and Kay Morris for their valuable technical assistance, particularly in the preparation of protoplasts. The project is funded by the Australian Research Council.