DELAYED RECTIFIER POTASSIUM CHANNELS IN CANINE AND PORCINE AIRWAY SMOOTH-MUSCLE CELLS

1. In order to define the ion channels underlying the inactivating, calcium-insensitive current in airway smooth muscle cells, unitary potassium currents were recorded from canine and porcine trachealis cells, and compared with macroscopic currents. On-cell and inside-out single-channel currents were compared with whole-cell recordings made in dialysed cells. 2. Depolarizing voltage steps evoked outward unitary currents. In addition to a large conductance, calcium-activated potassium channel (K(Ca)), a lower conductance potassium channel was identified. This channel has a conductance of 12.7 pS (on-cell; 1 mM-K+ in the pipette). 3. The lower conductance channel (K(dr)) was not sensitive to cytosolic Ca2+ concentration and unitary current openings occurred following a delay after the voltage step. The time course of activation of the current composed of averaged single-channel events was very similar to that of the whole-cell, delayed rectifier potassium current (I(dK)), recorded under conditions of low intracellular calcium (Kotlikoff, 1990). 4. K(dr) channels also inactivated with kinetics similar to those of the macroscopic current. Averaged single-channel records revealed a current that inactivated with kinetics that could be described by two exponentials (tau(1) = 0.14 s, tau(2) = 1.1 s; at 5 mV). These values corresponded well with previously determined values for time-dependent inactivation of I(dK). Inactivation of K(dr) channels was markedly voltage dependent, and was well fitted by a Boltzmann equation with V50 = -53 mV; this was similar to measurements of the macroscopic current, although the V50 value was shifted to more positive potentials in whole-cell measurements. When only the inactivating component of the macroscopic current was considered, the voltage dependence of inactivation of the single-channel current and macroscopic current were quite similar. 5. Single-channel kinetics indicated that K(dr) channels occupy one open and two closed states. The mean open time was 1.7 ms. Inactivation results in a prominent increase in the long closed time, with little effect on the mean open time or short closed time. 6. The K(dr) channel was not blocked by tetraethylammonium (TEA; 1 mM), charybdotoxin (ChTX; 100 nM) or glibenclamide (20-mu-M), but was blocked by 4-aminopyridine (4-AP; 1 mM). Similarly, 4-AP blocked the inactivating component of the macroscopic current, but a non-inactivating current remained. K(Ca) currents were blocked by TEA (0.5-1 mM) and charybdotoxin (40 nM), but were insensitive to 4-AP (1 mM) and glibenclamide (20-mu-M). 7. Since the activation and inactivation kinetics, voltage dependence of inactivation, and the pharmacology of K(dr) channels are the same as those of the inactivating component of the delayed rectifier current, I(dK), it is suggested that these channels mediate the inactivating component of the calcium-insensitive macroscopic current and account for a substantial component of the outward rectification that is characteristic of airway smooth muscle.