CONTROL OF RESTING MEMBRANE-POTENTIAL BY DELAYED RECTIFIER POTASSIUM CURRENTS IN FERRET AIRWAY SMOOTH-MUSCLE CELLS

1. In order to determine the physiological role of specific potassium currents in airway smooth muscle, potassium currents were measured in freshly dissociated ferret trachealis cells using the nystatin-permeab ilized, whole-cell method, at 35-degrees-C. 2. The magnitude of the ou tward currents was markedly increased as bath temperature was increase d from 22 to 35-degrees-C. This increase was primarily due to the incr ease in maximum potassium conductance (g(K,max)), although there was a lso a small leftward shift in the relationship between g(K) and voltag e at higher temperatures. The maximum conductance and the kinetics of current activation and inactivation were also temperature dependent. A t 35-degrees-C, gating of the current was steeply voltage dependent be tween -40 and 0 mV. Current activation was well fitted by fourth-order kinetics; the mean time constants of activation (30 mV clamp step) we re 1.09+/-0.17 and 1.96+/-0.27 ms at 35 and 22-degrees-C, respectively . 3. Outward currents using the nystatin method were qualitatively sim ilar to delayed rectifier currents recorded in dialysed cells with hig h calcium buffering capacity solutions. 4-Aminopyridine (4-AP; 2 mM), a specific blocker of delayed rectifier potassium channels in this tis sue, inhibited over 80% of the outward current evoked by voltage-clamp steps to between -10 and +20 mV (n = 6). Less than 5% of the outward current was blocked over the same voltage range by charybdotoxin (100 nM; n = 15), a specific antagonist of large-conductance, calcium-activ ated potassium channels in this tissue. 4. The degree to which delayed rectifier and calcium-activated potassium conductances control restin g membrane potential was examined in current-clamp experiments. The re sting membrane potential of current clamped cells was -33.6+/-1.0 mV ( n = 62). Application of 4-AP (2 mM) resulted in a 14.4+/-1.0 mV depola rization (n = 8) and an increase in input resistance. Charybdotoxin (1 00 nM) had no effect on resting membrane potential (n = 6). 5. Force m easurements were made in isolated strips of trachealis muscle to deter mine the effect of pharmacological blockade of individual potassium co nductances on resting tone. In the presence of tetrodotoxin (1 muM) an d atropine (1 muM), 4-AP increased baseline tension in a dose-dependen t manner, with an EC50 of 1.8 mM (n = 13); application of 5 mM 4-AP in creased tone to 86.8+/-8.1% of that produced by 1 muM methacholine, an d this tone was almost completely inhibited by nifedipine (1 muM). Con versely, antagonism of calcium-activated potassium (K(Ca)+) channels w ith charybdotoxin (100 nM) resulted in only a slight increase in resti ng tone (4.9+/-3.0% of methacholine response). 6. We conclude that del ayed rectifier potassium channels play a critical role in maintaining resting membrane potential in ferret trachealis smooth muscle. Calcium -activated potassium channels appear to comprise a smaller component o f the resting potassium conductance, and accordingly are less importan t in defining the passive behaviour of this muscle.