CURRENT-VOLTAGE RELATIONSHIPS OF POTASSIUM CHANNELS IN THE PLASMALEMMA OF ACETABULARIA

The outer membrane of mechanically prepared protoplasmic droplets from Acetabularia mediterranea has been investigated by patch-clamp techniques. These membranes are shown to consist of physiologically intact plasmalemma. With the Cl- pump inhibited, microscopic currents through K+-selective channels were studied. These currents compare well with macroscopic K+ currents as previously determined by standard microelectrode techniques and tracer flux measurements. There is about one K+ channel per .mu.m2 in the plasmalemma. The current-voltage relatioship (I-V curve) of the main open channel (channel A) is sigmoid over a voltage range between about -100 and +100 mV with saturation currents of about .+-. 10 pA. A second species (or different state of channel A) of K+-selective channels (channel B) differs from channel A by smaller saturation currents (about .+-. 7 pA) and a much smaller open probability. The open probability of channel A increases from almost zero at large negative voltages to about 1/2 at large positive voltages. Taking the closed times into account, the mean steady-state I-V curve of channel A displays outward rectification about the equilibrium voltage for K+ and negative slope conductance at larger negative voltages. The open channel I-V curves of the channels A and B, the changes of the I-V curve of the open channel A upon variation of the external K+ concentration, as well as the mean steady-state I-V curves of channel A are described by simple reaction kinetic models, the parameters of which are determined to fit the experimental data. The results are discussed with respect to data from other K+ channels in plants and with respect to regulation of the cytoplasmic K+ concentration in Acetabularia.