Inhibitory effects of large Ca2+-activated K+ channel blockers on beta-adrenergic- and NO-donor-mediated relaxations of human and guinea-pig airway smooth muscles.

In human bronchi, relaxations to salbutamol and sodium nitroprusside were performed in the presence or absence of blockers of the large Ca2+-activated K+-channels (BKCa): charybdotoxin (Chtx), iberiotoxin (Ibtx) or tetraethylammonium (TEA). In bronchi under basal tone in presence of indomethacin (1 microM) or precontracted with acetylcholine (in presence or absence of indomethacin), the relaxations to salbutamol or sodium nitroprusside were unaffected or weakly inhibited by pretreatment with the BKca blockers (Chtx (100 nM), Ibtx (100 and 300 nM) and TEA (1 mM)). Significant inhibitions were mainly observed with TEA (1 mM) and iberiotoxin at high concentration (300 nM). These results contrasts with the potent inhibitory effects exerted by Chtx (100 nM) or Ibtx (100 nM) in guinea-pig trachea precontracted with acetylcholine in absence or presence of indomethacin indicating that human airways are less susceptible to BKCa blockade than guinea-pig airways. In addition, the BKCa blockers induced slowly developing contractions of human bronchi at basal tone. The contraction induced by TEA (1 mM) was abolished by verapamil (10 microM) suggesting that BKca blockade promotes an increase in membrane Ca2+-conductance through activation of voltage-gated Ca2+-channels. Verapamil also reversed the effects of TEA on salbutamol-induced relaxations in human bronchi as well as the effects of Ibtx on salbutamol- or sodium nitroprusside-induced relaxations in guinea-pig trachea. These data suggest that BKCa blockers induce activation of voltage-gated Ca2+-channels and therefore influx of Ca2+ which in turn cause a functional antagonism of beta2-adrenoceptor-agonist- and NO-donor-induced relaxations. Moreover, the BKCa opener, NS-1619, induced weak relaxations in human bronchi and guinea-pig trachea which were not blocked by TEA or Ibtx suggesting that BKCa opening is of minor significance for the relaxation of human airway smooth muscles. In conclusion, although a wealth of studies have demonstrated that beta-adrenoceptor agonists or NO-donors activate BKCa, the present study provides evidence that in human bronchi, as recently suggested in guinea-pig trachea, opening of BKCa does not appear to functionally participate in the relaxation to these relaxant agents.