Inhibitory effects of large Ca2+-activated K+-channel blockers on β-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 (BK_Ca): charybdotoxin (Chtx), iberiotoxin (Ibtx) or tetraethylammonium (TEA). In bronchi under basal tone in presence of indomethacin (1 μM) 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 BK_Ca 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 BK_Ca blockade than guinea-pig airways. In addition, the BK_Ca blockers induced slowly developing contractions of human bronchi at basal tone. The contraction induced by TEA (1 mM) was abolished by verapamil (10 μM) suggesting that BK_Ca 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 BK_Ca blockers induce activation of voltage-gated Ca²⁺-channels and therefore influx of Ca²⁺ which in turn cause a functional antagonism of β₂-adrenoceptor-agonist- and NO-donor-induced relaxations. Moreover, the BK_Ca opener, NS-1619, induced weak relaxations in human bronchi and guinea-pig trachea which were not blocked by TEA or Ibtx suggesting that BK_Ca opening is of minor significance for the relaxation of human airway smooth muscles. In conclusion, although a wealth of studies have demonstrated that β-adrenoceptor agonists or NO-donors activate BK_Ca, the present study provides evidence that in human bronchi, as recently suggested in guinea-pig trachea, opening of BK_Ca does not appear to functionally participate in the relaxation to these relaxant agents. Received: 25 April 1997 / Accepted: 15 September 1997