I. Benz et M. Kohlhardt, BLOCKADE OF CARDIAC OUTWARDLY RECTIFYING K-III ANTIARRHYTHMICS - EVIDENCE AGAINST A SINGLE DRUG-SENSITIVE CHANNEL SITE( CHANNELS BY TEA ANDCLASS), European biophysics journal, 22(6), 1994, pp. 437-446
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.