Elementary K+ currents through cardiac outwardly rectifying K+ channels were recorded in inside-out patches excised from cultured neonatal rat cardiocytes at 19 degrees C and at 9 degrees C. By studying the inhibitory effects of tetraethylammonium (TEA), quinidine and verapamil, the properties of this novel type of K+ channel were further characterized. 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 kinetics too fast to be resolvable at 1 kHz. This TEA blockade was e-fold voltage-dependent, with a decrease of the apparent KD(TEA) from 102 mmol/l at -37 mV to 65 mmol/l at +33 mV and, furthermore, became accentuated on lowering the internal K+ concentration. Thus, TEA competes with the permeant K+ for a site located in some distance from the cytoplasmic margin, within the K+ pore. Quinidine (100 mumol/l), like verapamil (40 mumol/l) reversibly depressed i unit to about 80% of the control value (at -7 mV), but drug-induced fast flicker blockade proved voltage-insensitive between -27 mV and +23 mV. These drugs gain access to a portion of the pore distinct from the TEA binding site whose occupancy by drugs likewise blocks K+ permeation. Both drugs showed a greater potency to depress Po which, with quinidine, decreased reversibly 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 (C2). This alteration of the gating process also includes a sometimes dramatic shortening of the open state. Most probably, cardiac K+(outw.-rect.) channels possess a second drug-sensitive site whose occupancy by quinidine or verapamil may directly or allosterically stabilize their non-conducting configuration.