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

Abstract

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.

Novel Small Airway Bronchodilator Responses to Rosiglitazone in Mouse Lung Slices

“…In addition, dilator responses to RGZ were maintained in the presence of validated effective concentrations of indomethacin or NOLA (17,31). These findings allowed us to exclude the possible contributions of cAMP or epithelial-derived relaxing factors such as PGE 2 or NO to RGZ-mediated relaxation of small airways in perfused lung slices.…”

Mechanisms underlying epithelium-dependent relaxation in rat bronchioles: analogy to EDHF-type relaxation in rat pulmonary arteries

“…In addition to bronchodilation induced by reduction of calcium oscillations and lowering of calcium through activation of sarcoplasmic Ca 2ϩ -ATPase as well as dephosphorylation of myosin light chain through increased myosin light chain phosphatase activation (26,41), activation of G-coupled ␤-adrenoceptors may lead to activation of smooth muscle BK Ca channels with subsequent relaxation sensitive to ChTX or IBTX in guinea pig trachea and human bronchi (9,43). However, in the present study, BK Ca channel blockade with IBTX did not change NS309 or salbutamol relaxation.…”

The mechanism of gentisic acid-induced relaxation of the guinea pig isolated trachea: the role of potassium channels and vasoactive intestinal peptide receptors

“…In addition, gentisic acid-induced relaxation was markedly antagonized when a higher K + concentration was present in the medium, further confirming the requirement of the integrity of the cell membrane potential for its action. Also relevant are the results showing that the relaxation caused by gentisic acid in the guinea pig trachea was largely antagonized by charybdotoxin, a neurotoxin known to antagonize selectively high-conductance calcium-activated potassium channels (18). However, glibenclamide, a selective blocker of ATP-sensitive K + channels, at concentrations at which it completely antagonized the relaxation caused by the opener of ATPsensitive potassium channels levcromakalim in the guinea pig trachea, failed to affect gentisic acid-mediated relaxation in this preparation, suggesting that ATP-sensitive potassium channels have no major effect on gentisic acid relaxation.…”

“…Among them are the delayed rectifier channels (17), the large-and low-conductance calcium-activated channels (15) and also the ATP-sensitive potassium channels (16). Potassium channels also are an important effector element for several relaxant agents in the guinea pig isolated trachea, like nitric oxide (18) and VIP (19). The results of the present study provide functional evidence indicating that gentisic acid, an active metabolite of the salicylic route of degradation (1), causes concentration-dependent relaxation of the guinea pig trachea in both the presence and absence of epithelium.…”