The electrical properties of dissociated canine tracheal smooth muscle cells were examined using the whole cell patch-clamp technique. In current clamp mode, current clamp steps did not initiate action potentials but showed clear outward rectification, which was abolished when cells were loaded with Cs+ ions and when tetraethylammonium (TEA+) ions replaced Na+ in the bath solution. In voltage-clamp experiments, depolarizations positive to -45 mV evoked brief voltage-dependent inward Ca2+ currents [Am. J. Physiol. 254 (Cell Physiol. 23): C793-C801, 1988], followed by sustained outward currents, which did not completely inactivate. Outward currents were identified as K+ currents on the basis of the reversal potential of the current and by ion-substitution experiments. The currents were further defined as Ca2(+)-insensitive delayed rectifier currents, since they were unaltered under conditions in which 1) the Ca2+ current was completely blocked by Mn2+ or nifedipine (10 microM); 2) Ba2+ ions were substituted for Ca2+ as the inward current charge carrier; or 3) charybdotoxin (40 nM) or TEA+ (up to 10 mM) were added to the bath. A Ca2(+)-activated potassium [K(Ca)] current was activated by application of methacholine (100 microM), or A23187 (1 microM), under conditions of low Ca2+ buffering capacity in the internal solution [0.3 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA)]. The K(Ca) current was blocked by 10 mM TEA+ and was not observed under conditions of high intracellular Ca2+ buffering (11 mM EGTA). These data indicate that canine airway smooth muscle cells contain voltage-dependent delayed rectifier channels that underlie membrane rectification and K(Ca) channels that are activated by agents which release intracellular Ca2+ stores.