Effects of some K+-channel inhibitors on the electrical behaviour of guinea-pig isolated trachealis and on its responses to spasmogenic drugs

1 A study has been made of the effects of inhibitors selective among plasmalemmal K+-channels on the sensitivity and responsiveness of guinea-pig trachealis muscle to carbachol, histamine and KCl. The effects of the K+-channel inhibitors on the resting membrane potential and spontaneous electrical activity of the trachealis cells have also been examined. 2 In indomethacin (2.8 mu M)-treated trachealis muscle, dofetilide (1 mu M) and glibenclamide (10 mu M) were each devoid of spasmogenic activity. In contrast 4-aminopyridine (4-AP, 62.5 mu M-8 mM), charybdotoxin (ChTX, 100 nM) and iberiotoxin (IbTX, 100 nM) were each spasmogenic. Spasm evoked by 4-AP, IbTX or ChTX was reduced, though not abolished, by atropine (1 mu M). Spasm evoked by 4-AP (1 mM), ChTX (100 nM) or IbTX (100 nM) was unaffected by tetrodotoxin (TTX; 3.1 mu M) or by tissue pretreatment with capsaicin (1 mu M for 30 min). Spasm evoked by IbTX dr ChTX was abolished by nifedipine (1 mu M). 3 Dofetilide (1 mu M) and glibenclamide (10 mu M) were each without effect on the tracheal sensitivity or responsiveness to carbachol, histamine or KCl. 4-AP (1 mM) antagonized carbachol, potentiated histamine but did not affect tissue sensitivity to KCl. When the effects of 4-AP were examined in the presence of atropine (1 mu M), it potentiated all the spasmogens including carbachol. IbTX and ChTX (each 100 nM) potentiated all three spasmogens. Potentiation of histamine induced by 4-AP (1 mM) or IbTX (100 nM) was also observed in tissues treated with a combination of atropine (1 mu M) and TTX (3.1 mu M). 4 Dofetilide (1 and 10 mu M) was without effect on the resting membrane potential or spontaneous electrical activity of the trachealis cells. 4-AP (1 mM) evoked depolarization and caused a small increase in the frequency of slow wave discharge. The depolarization evoked by 4-AP was abolished by atropine (1 mu M). IbTX (100 nM) and ChTX (100 nM) each evoked little or no change in resting membrane potential but converted the spontaneous slow waves into spike-like, regenerative action potentials. These electrophysiological effects of IbTX and ChTX were unaffected by atropine (1 mu M). 5 It is concluded that the dofetilide-sensitive, cardiac, delayed rectifier K+-channel is either not expressed in trachealis muscle or is of no functional importance in that tissue. The ATP-sensitive K+-channel (K-ATP) does not moderate tracheal sensitivity to spasmogens such as carbachol, histamine and KCl. The 4-AP-sensitive delayed rectifier K+-channel (K-dr) and the large Ca2+-dependent K+-channel (BKCa) each moderate trachealis muscle sensitivity to spasmogens. Neither K-dr nor BKCa plays an important role in determining the resting membrane potential of guinea-pig trachealis cells. However, the BKCa channel is responsible for limiting the effects of the increase in membrane Ca2+ conductance associated with the depolarizing phase of slow waves. It is BKCa channel opening that prevents the development of a slow wave into a spike-like regenerative action potential.