Calcium-Activated, Voltage-Dependent, Non-Selective Cation Currents in Endosperm Plasma Membrane from Higher Plants

Single-channel and whole-cell patch-clamp techniques were used to characterize the electrophysiological behaviour of plasma membranes from freshly isolated, non-enzyme-treated endosperm protoplasts. A non-selective monovalent cation channel with a single-channel conductance of 22 pS in solutions with physiological potassium concentrations was observed in inside-out patches. The channel passes outward current at depolarized potentials and is highly selective for cations over anions, but discriminates poorly between lithium, sodium, potassium, rubidium and caesium ions. Specific potassium channel blockers were ineffective. The channel kinetics were apparently complex, with burst-like openings and rapid closures within a single burst. Single-channel openings were more frequent both for depolarizing pulses and maintained positive potentials. Channel activity was also increased by elevated cytoplasmic concentrations of either calcium or barium. Subsequent exposure of patches to low calcium, EGTA-buffered solutions resulted in large decreases in activity. Under whole-cell current clamp, small negative resting potentials were observed. A slowly developing outward current evoked by depolarizing pulses was seen in whole-cell recordings.