CA2-DEPENDENT INACTIVATION OF LARGE-CONDUCTANCE CA2+-ACTIVATED K+ (BK) CHANNELS IN RAT HIPPOCAMPAL-NEURONS PRODUCED BY PORE BLOCK FROM AN ASSOCIATED PARTICLE()

1. Recordings of the activity of the large conductance Ca2+-activated K+ (BK) channel from over 90% of inside-out patches excised from acute ly dissociated hippocampal CA 1. neurones revealed an inactivation pro cess dependent upon the presence of at least 1 mu M intracellular Ca2. Inactivation was characterized by a sudden snitch from sustained hig h open probability (P-o) long open time behaviour to extremely low P-o , short open time channel activity. The low P-o state (mean P-o, 0.001 ) consisted of very short openings (time constant (tau), similar to 0. 14 ms) and rare longer duration openings (tau, similar to 3.0 ms). 2. Channel inactivation occurred with a highly variable time course being observed either prior to or immediately upon patch excision, or after up to 2 min of inside-out recording. Inactivation persisted whilst re cording conditions were constant. 3. Inactivation was reversed by memb rane hyperpolarization, the rate of recovery increasing with further h yperpolarization and higher extracellular K+. Inactivation was also re versed when the intracellular Ca2+ concentration was lowered to 100 nM and was permanently removed by application of trypsin to the inner pa tch surface. In addition, inactivation was perturbed by application of either tetraethylammonium ions or the Shaker (Sh)B peptide to the inn er membrane face. 4. During inactivation, channel P-o was greater at h yperpolarized rather than depolarized potentials, which was partly the result of a greater number of longer duration openings. Depolarizing voltage steps (-40 to +40 mV) applied during longer duration openings produced only short duration events at the depolarized potential, yiel ding a transient ensemble average current with a rapid decay (tau, sim ilar to 3.8 ms). 5. These data suggest that hippocampal BK channels ex hibit a Ca2+-dependent inactivation that is proposed to result from bl ock of the channel by an associated particle. The findings that inacti vation was removed by trypsin and prolonged by decreasing extracellula r potassium suggest that the blocking particle may act at the intracel lular side of the channel.