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 μm in the plasmalemma. The current-voltage relationship ( 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 curve of the open 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. [ABSTRACT FROM AUTHOR]