Effects of tetraethylammonium and 4‐aminopyridine on outward currents and excitability in canine tracheal smooth muscle cells

1. The effects of tetraethylammonium (TEA) and 4-aminopyridine (4-AP) on membrane currents and on single channel K currents in smooth muscle cells isolated from canine trachea were examined by use of tight seal whole cell- and patch-clamp techniques. 2. Depolarizing current applied through a recording pipette did not elicit an action potential under current clamp. A strong outward rectification was observed. 3. In most cells under voltage-clamp, only an outward current was observed upon depolarization from -60 mV when a pipette solution contained mainly KCl. The outward current consisted of three components; a large initial transient, a following sustained component and an additional component of irregular small transients on the sustained one. The two transient components were almost abolished when extracellular and pipette solutions contained 2.2 mM Cd2+ (0 mM Ca2+) and 10 mM EGTA, respectively. The sustained component was well maintained under these conditions. 4. TEA at low concentrations (less than 1 mM) effectively decreased the transient components and made the outward current smooth; it also suppressed the sustained component at higher concentrations. In outside-out patches, external 1 mM TEA reduced the single channel conductance of Ca-activated K channels by about 87% whereas 3 mM 4-AP did not. 4-AP at low concentrations (less than 3 mM) selectively reduced the sustained component of the outward current. 5. A Ca current recorded after the suppression of outward current by internal Cs+ had a peak of approximately 200 pA at +10 mV (holding potential: -60 mV). The half inactivation voltage in the steady-state was approximately -30 mV. 6. Simultaneous application of 1 mM TEA and 4-AP reduced the outward current and unmasked a Ca current. Under these conditions, an action potential with overshoot was easily elicited under current clamp. 7. It is concluded that the low excitability of canine tracheal smooth muscle cell upon depolarization is due to a large outward K current which consists of Ca-dependent and Ca-independent components. The peak amplitude of the Ca current is similar to that in highly excitable smooth muscle cells such as those of the ureter.