Pharmacological properties of potassium currents in swine tracheal smooth muscle

The pharmacological properties of large conductance Ca(++)-activated K+ channels and spontaneous transient outward currents (STOCs) in swine tracheal smooth muscle (TSM) were studied using enzymatically dissociated single cells and patch clamp techniques. Recording from inside-out patches showed that extracellular tetraethylammonium caused a concentration-dependent decrease in single channel current amplitude. Acetylcholine (ACh; 10(-8) M) induced repetitive outward current transients in cell-attached patches due to simultaneous opening of many large conductance Ca(++)-activated K+ channels. Whole-cell recordings revealed the presence of STOCs in TSM membrane that were inhibited by tetraethylammonium at concentrations similar to those which inhibited the large conductance Ca(++)-activated K+ channels. Caffeine (5 mM) stimulated and then inhibited STOCs, suggesting that they are induced by the release of calcium from internal stores. ACh (1 microM) initially increased STOC frequency followed by an inhibition of STOCs. The effects of ACh occurred in control solution (2 mM Ca++) or when calcium influx was eliminated (0 Ca++, 2 mM Mn++). ACh also induced an inward current at negative membrane potentials associated with an increase in conductance. We conclude that inhibition of STOCs (i.e., potassium channel activity) and induction of an inward cation current by ACh may give rise to the depolarization observed in the presence of ACh and be important in the development of ACh-induced contractions in TSM.