THE CYTOSOLIC INACTIVATION DOMAINS OF BKI CHANNELS IN RAT CHROMAFFIN CELLS DO NOT BEHAVE LIKE SIMPLE, OPEN-CHANNEL BLOCKERS

Most BK-type voltage- and Ca2+-dependent K+ channels in rat chromaffin cells exhibit rapid inactivation. This inactivation is abolished by b rief trypsin application to the cytosolic face of membrane patches. He re we examine the effects of cytosolic channel blockade and pore occup ancy on this inactivation process, using inside-out patches and whole- cell recordings. Occupancy of a superficial pore-blocking site by cyto solic quaternary blockers does not slow inactivation. Occupancy of a d eeper pore-blocking site by cytosolic application of Cs+ is also witho ut effect on the onset of inactivation. Although the rate of inactivat ion is relatively unaffected by changes in extracellular K+, the rate of recovery from inactivation (at -80 and -140 mV with 10 mu M Ca2+) i s faster with increases in extracellular K+ but is unaffected by the i mpermeant ion, Na+. When tail currents are compared after repolarizati on, either while channels are open or after inactivation, no channel r eopening is detectable during recovery from inactivation. BK inactivat ion appears to be mechanistically distinct from that of other inactiva ting voltage-dependent channels. Although involving a trypsin-sensitiv e cytosolic structure, the block to permeation does not appear to occu r directly at the cytosolic mouth or inner half of the ion permeation pathway.