PUMP AND K+ INWARD RECTIFIERS IN THE PLASMALEMMA OF WHEAT ROOT PROTOPLASTS

An electrogenic pump, a slowly activating K+ inward rectifier and an i ntermittent, ''spiky,'' K+ inward rectifier, have been identified in t he plasmalemma of whole protoplasts from root cortical cells of wheat (Triticum) by the use of patch clamping techniques. Even with high ext ernal concentrations of KC of 100 mM, the pump can maintain the membra ne potential difference (PD) down to -180 mV, more negative than the e lectrochemical equilibrium potentials of the various ions in the syste m. The slowly activating Kf 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 smalle r time constant shows little voltage dependence. The rectifier deactiv ates, on return of the PD to less negative levels, with a single expon ential time course, whose time constant is strongly voltage dependent. The spiky K+ inward rectifier, present in about 68% of protoplasts, a llows intermittent current, of considerable magnitude, through the pla smalemma 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 c onstitute this rectifier. The paper also considers the role of the pum p and the K+ inward rectifiers in physiological processes in the cell.