Single-channel analysis of a delayed rectifier K+ channel in Xenopus myelinated nerve

Single-channel properties of a delayed rectifier voltage-gated KC channel (I-type) were investigated in peripheral myelinated axons from Xenopus laevis. Channels activated between -60 and -40 mV with a potential of half-maximal activation, E(50), at -47.5 mV. Averaged single-channel currents activated with a time delay at all membrane potentials tested. Time to half-maximal activation decreased from 80 to 1.6 msec between -60 and +40 mV. The channel inactivated monoexponentially with a time constant of 10.9 sec at -40 mV. The time constant of deactivation was 126 msec at -80 mV and 16.9 msec at -110 mV. In symmetrical 105 mM K+, the single-channel conductance (gamma) was 22 and 13 pS at negative and positive membrane potentials, respectively, at 13-15 degrees C. In Na+-rich solution with 2.5 mM extracellular K+ gamma was 7 pS and the reversal potential was negative to -80 mV, indicating a high selectivity for K+ over Na+. gamma depended on extracellular K+ concentration (K-D = 19.6 mM) and temperature (Q(10) = 1.45). External tetraethylammonium (TEA) reduced the apparent single-channel current amplitude at all potentials tested with a half-maximal inhibiting concentration (IC50) of 0.6 mM. Open probability of the channel, but not single-channel current amplitude was decreased by extracellular dendrotoxin (DTX, IC50 = 6.8 nM) and mast cell degranulating peptide (MCDP, IC50 = 41.9 nM). In Ringer solution the membrane potential of macroscopic I-channel patches was about -65 mV and depolarized under TEA and DTX. It is concluded that besides their activation during action potentials, I-channels may also stabilize the resting membrane potential.