Distinct modes of channel gating underlie inactivation of somatic K+ current in rat hippocampal pyramidal cells in vitro

1. We have used the cell-attached configuration of the patch-clamp recording method to characterize the biophysical properties of the voltage-gated K+ channel underlying a 4-aminopyridine (4-AP)- and tetraethylammonium (TEA)-sensitive K+ current (I-K(AT)) in pyramidal cells of hippocampal slice cultures. 2. The unitary conductance of channels carrying I-(K(AT)) current (K-AT channels) was 19 . 1+/-5 . 1 pS with a physiological K+ gradient (2 . 7 mM external K+) and 39 . 0+/-3 . 6 pS with high external K+ (140 mM). The reversal potential changed with the external K+ concentration as expected for a channel with a dominant K+ selectivity. Channel activity was blocked under both conditions by either external application of 4-AP at 100 mu M or by including 20 mM TEA in the pipette solution. 3. An analysis of kinetic behaviour showed that open times were distributed as a single exponential. The mean open time (+/- S.D.) was 4 . 4+/-1 . 4 ms at a voltage 30 mV positive to resting potential and increased with further depolarization to reach a value of 16 . 2+/-7 . 4 ms at 70 mV positive to the resting potential. At this depolarized potential, we observed bursts of channel openings with a mean burst duration around 100 ms. 4. With repeated depolarizing pulses, response failures of the K-AT channel occurred in a nonrandom manner and were grouped (referred to as mode 0). This mode was associated with a voltage-dependent inactivation process of the channel and was favoured when the opening probability of the channel was reduced by increasing steady-state inactivation or by bath application of 4-AP. This is consistent with the localization of the binding site for 4-AP at or near the inactivation gate of the channel. 5. When K-AT channel openings were elicited by 500 ms depolarizing steps, activity was either transient or it persisted throughout the duration of the pulse. These two modes of activity alternated in a random manner or occurred in groups giving rise to transient (time constant, 20-100 ms) or sustained ensemble currents. In the presence of low concentrations of 4-AP (20-40 mu M), the transient pattern of activity was more frequently observed. 6. In addition to mode 0, we propose the existence of at least two further gating modes for K-AT channels: mode T (transient current) and mode S (sustained current) that underlie the three decaying components of the I-K(AT) ensemble current. These gating modes are probably under the control of intracellular factors that remain to be identified.