Electrophysiology of the marine diatom Coscinodiscus wailesii II. Potassium currents

Abstract

The dominating mechanism of K+ uniport through the plasmalemma of Coscinodiscus wailesii has been studied in some detail as part of a general study of ionic relations in marine diatoms. Electrical measurements with double-barrelled glass-microelectrodes have been made in intact cells (diameter ≈250 mm, height ≈100 mm) bathed in artificial sea-water in which [K+] has been changed from 3 mM to 100 mM. Using a modified Goldman equation, these results provide an estimate of [K+]i of about 400 mM and a selectivity for K+ over Na+ and Cl−, which could spontaneously vary by orders of magnitude and reach values of about 1000. Voltage-clamp experiments have been carried out in these states of high K+ selectivity using bipolar staircase command voltages over a range from −180 to +60 mV. The resulting steady-state current-voltage relationships have inward rectifying sigmoid characteristics with a negative saturation current around −300 nA, a positive saturation current of about 30 nA, and a slope conductance of the order of 1 µS at free running voltages < −60 mV. Temporal responses of the clamp currents upon rectangular voltage steps were basically rectangular, i.e. they did not show the familiar relaxation kinetics of voltage-induced activation/inactivation. The sigmoid steady-state current- voltage relationships could not be described by a usual model of constant-field currents through a voltage-gated pore, where the positive current of an inward rectifier would not saturate but vanish. Alternatively, the observed steady-state inward rectifying current-voltage relationship and its changes upon changes in [K+]o, are well described by a three-state reaction cycle for catalysis of K+ translocation with a steady activity.