Role of K+ channel opening and Na(+)-K+ ATPase activity in airway relaxation induced by salbutamol

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

This study investigated the mechanisms underlying epithelium-derived hyperpolarizing factor (EpDHF)-type relaxation in rat bronchioles. Immunohistochemistry was performed, and rat bronchioles and pulmonary arteries were mounted in microvascular myographs for functional studies. An opener of small (SKCa) and intermediate (IKCa)-conductance calcium-activated potassium channels, NS309 (6,7-dichloro-1H-indole-2,3-dione 3-oxime) was used to induce EpDHF-type relaxation. IKCa and SKCa3 positive immunoreactions were observed mainly in the epithelium and endothelium of bronchioles and arteries, respectively. In 5-hydroxytryptamine (1 μM)-contracted bronchioles (828 ± 20 μm, n = 84) and U46619 (0.03 μM)-contracted arteries (720 ± 24 μm, n = 68), NS309 (0.001–10 μM) induced concentration-dependent relaxations that were reduced by epithelium/endothelium removal and by blocking IKCa channels with charybdotoxin and in bronchioles also by blocking SKCa channels with apamin. Inhibition of cyclooxygenase, nitric oxide synthase, and cytochrome 2C isoenzymes, or blockade of large (BKCa)-conductance calcium-activated potassium channels with iberiotoxin, failed to reduce NS309 relaxation. In contrast to the pulmonary arteries, relaxations to a β2-adrenoceptor agonist, salbutamol, were reduced in bronchioles by removing the epithelium or blocking IKCa and/or SKCa channels. Extracellular K+ (2–20 mM) induced relaxation in both bronchioles and arteries. An inhibitor of Na+-K+-ATPase, ouabain, abolished relaxations to NS309, salbutamol, and K+. These results suggest that IKCa and SKCa3 channels are located in the epithelium of bronchioles and endothelium of pulmonary arteries. Analog to the endothelium-derived hyperpolarizing factor (EDHF)-type relaxation in pulmonary arteries, these channels may be involved in EpDHF-type relaxation of bronchioles caused by epithelial K+ efflux followed by activation of Na+-K+-ATPase in the underlying smooth muscle layer.

Ionic mechanisms and Ca(2+) regulation in airway smooth muscle contraction: do the data contradict dogma?

In general, excitation-contraction coupling in muscle is dependent on membrane depolarization and hyperpolarization to regulate the opening of voltage-dependent Ca(2+) channels and, thereby, influence intracellular Ca(2+) concentration ([Ca(2+)](i)). Thus Ca(2+) channel blockers and K(+) channel openers are important tools in the arsenals against hypertension, stroke, and myocardial infarction, etc. Airway smooth muscle (ASM) also exhibits robust Ca(2+), K(+), and Cl(-) currents, and there are elaborate signaling pathways that regulate them. It is easy, then, to presume that these also play a central role in contraction/relaxation of ASM. However, several lines of evidence speak to the contrary. Also, too many researchers in the ASM field view the sarcoplasmic reticulum as being centrally located and displacing its contents uniformly throughout the cell, and they have focused almost exclusively on the initial single [Ca(2+)] spike evoked by excitatory agonists. Several recent studies have revealed complex spatial and temporal heterogeneity in [Ca(2+)](i), the significance of which is only just beginning to be appreciated. In this review, we will compare what is known about ion channels in ASM with what is believed to be their roles in ASM physiology. Also, we will examine some novel ionic mechanisms in the context of Ca(2+) handling and excitation-contraction coupling in ASM.

Hypokalemia and salbutamol therapy in asthma

Hypokalemia is a common side effect in adult asthmatic patients on beta 2 adrenergic therapy. There is limited information in regard to hypokalemia and its relation to the clinical responses following administration of beta 2 agonist therapy in children with asthma. We observed that salbutamol inhalation significantly improved asthmatic symptoms as demonstrated by increases in peak expiratory flow (PEF: 122.37+/-75.38 vs. 152.59+/-80.29; P < 0.001) and venous oxygen tension (Pv,O2: 33.24+/-4.95 vs. 58.16+/-2.31; P < 0.001), and decreases in respiratory rate (RR: 36.39+/-3.78 vs. 28.62+/-3.12; P< 0.01), clinical scores (CS: 3.59+/-1.28 vs. 1.59+/-0.71), and venous PCO2 tensions (Pv,CO2: 40.84+/-2.67 vs. 34.75+/-2.31; P < 0.001). Salbutamol-induced hypokalemia was correlated with a decrease in RR, and an increase of Pv,O2 and PEF. These findings suggest that the same mechanism is involved in eliciting hypokalemia and bronchodilatation.

Corticosteroid modulation of Na(+)-K+ pump-mediated relaxation in maturing airway smooth muscle

1. The ontogeny of the relaxant influence of the airway electrogenic Na(+)-K+ pump and its potential modulation by corticosteroids were examined in airway smooth muscle (ASM) segments isolated from newborn and adult rabbits. 2. Control and methylprednisolone-treated (MP) ASM segments were half-maximally contracted with methacholine in K(+)-free buffer and the ASM relaxant responses to Na(+)-K+ pump activation were subsequently evaluated. Relative to adult ASM, control newborn ASM showed significantly enhanced maximal relaxation (Rmax) to KCl (62.5 +/- 5.2% vs. 47.8 +/- 5.2%), but no difference in sensitivity (pC2 = -log concentration producing 50% Rmax: 2.18 +/- 0.12 vs. 2.29 +/- 0.09-log M). 3. Exposure of ASM segments to 500 microM methylprednisolone for 1 h potentiated the airway Na(+)-K+ pump activity. A more pronounced effect was obtained in newborn ASM, where both the Rmax and pC2 values were significantly enhanced. In mature ASM, only the Rmax response to KCl was increased in the presence of MP. 4. Collectively, these data demonstrate that: (i) the functional activity of the airway electrogenic Na(+)-K+ pump decreases with post-natal maturation in the rabbit: (ii) corticosteroid treatment potentiates Na(+)-K+ pump activity in rabbit ASM; and (iii) the latter effect of corticosteroids is enhanced in immature airways. 5. The above findings provide new evidence that the airway relaxant response to activation of the electrogenic Na(+)-K+ pump varies ontogenetically and that corticosteroids potentiate the Na(+)-K+ pump activity in an age-dependent manner.

Mechanism of protein kinase C potentiation of airway beta-adrenergic relaxation

To evaluate the regulatory action of protein kinase C (PKC) on airway beta-adrenergic function, the relaxant effects of isoproterenol (ISO) and 8 bromo-cyclic AMP (BrcAMP) were examined in tracheal smooth muscle (TSM) segments half-maximally contracted with acetylcholine in the absence (control) and presence of PKC activation with the phorbol ester, 12-deoxyphorbol 13-isobutyrate (DPB). Relative to control tissues, TSM treated with 0.1 microM DPB depicted significantly enhanced maximal relaxation and sensitivity to ISO but not to BrcAMP. The enhancing effect of DPB on ISO responsiveness was completely inhibited in the presence of the PKC antagonist H-7. Inhibition of the Na(+)-K+ pump with either ouabain or K(+)-free buffer diminished the TSM relaxant response to ISO but not to BrcAMP. Inhibition of the Na(+)-K+ pump also ablated the DPB-induced potentiation of beta-adrenoceptor responsiveness. Collectively, these data demonstrate that: 1) PKC activation enhances TSM relaxant responsiveness to beta-adrenoceptor stimulation; 2) inhibition of the airway Na(+)-K+ pump markedly blunts the relaxant response to beta-adrenoceptor stimulation; and 3) inhibition of the Na(+)-K+ pump abolishes the above potentiating effect of DPB on beta-adrenoceptor-mediated relaxation of rabbit TSM. Thus, the above findings provide new evidence that PKC activation enhances the airway relaxant response to beta-adrenoceptor stimulation, and that the latter effect is dependent on potentiated stimulation of the airway electrogenic Na(+)-K+ pump.