BLOCKADE OF CARDIAC OUTWARDLY RECTIFYING K-III ANTIARRHYTHMICS - EVIDENCE AGAINST A SINGLE DRUG-SENSITIVE CHANNEL SITE( CHANNELS BY TEA ANDCLASS)

Elementary K+ currents through cardiac outwardly rectifying K+ channel s were recorded in inside-out patches excised from cultured neonatal r at cardiocytes at 19 degrees C and at 9 degrees C. By studying the inh ibitory effects of tetraethylammonium (TEA), quinidine and verapamil, the properties of this novel type of K+ channel were further character ized. Internal TEA (50 mmol/l) evoked a reversible decline of i(unit) to 62.7 +/- 2.7% of control (at -7 mV), without significant changes of open state kinetics, indicating a blockade of the open K+ pore with k inetics too fast to be resolvable at 1 kHz. This TEA blockade was e-fo ld voltage-dependent, with a decrease of the apparent K-D(TEA) from 10 2 mmol/l at -37 mV to 65 mmol/l at + 33 mV and, furthermore, became ac centuated on lowering the internal K+ concentration. Thus, TEA compete s with the permeant K+ for a site located in some distance from the cy toplasmic margin, within the K+ pore. Quinidine (100 mu mol/l), like v erapamil (40 mu mol/l) reversibly depressed i(unit) to about 80% of th e control value (at -7 mV), but drug-induced fast flicker blockade pro ved voltage-insensitive between -27 mV and + 23 mV. These drugs gain a ccess to a portion of the pore distinct from the TEA binding site whos e occupancy by drugs likewise blocks K+ permeation. Both drugs showed a greater potency to depress P-o which, with quinidine, decreased reve rsibly to 38.6 +/- 11.1% (at -7 mV) and, with verapamil to 24.9 +/- 9. 1%(at -7 mV), mainly by an increase of the prolonged closed state (C-2 ). This alteration of the gating process also includes a sometimes dra matic shortening of the open state. Most probably, cardiac K-(outw.-re ct)(+) second drug-sensitive site whose occupancy by quinidine or vera pamil may directly or allosterically stabilize their non-conducting co nfiguration.