The spontaneous electrical and mechanical activity of human bronchial smooth muscle: its modulation by drugs

Asthma: A Loss of Post-natal Homeostatic Control of Airways Smooth Muscle With Regression Toward a Pre-natal State

The defining feature of asthma is loss of normal post-natal homeostatic control of airways smooth muscle (ASM). This is the key feature that distinguishes asthma from all other forms of respiratory disease. Failure to focus on impaired ASM homeostasis largely explains our failure to find a cure and contributes to the widespread excessive morbidity associated with the condition despite the presence of effective therapies. The mechanisms responsible for destabilizing the normal tight control of ASM and hence airways caliber in post-natal life are unknown but it is clear that atopic inflammation is neither necessary nor sufficient. Loss of homeostasis results in excessive ASM contraction which, in those with poor control, is manifest by variations in airflow resistance over short periods of time. During viral exacerbations, the ability to respond to bronchodilators is partially or almost completely lost, resulting in ASM being "locked down" in a contracted state. Corticosteroids appear to restore normal or near normal homeostasis in those with poor control and restore bronchodilator responsiveness during exacerbations. The mechanism of action of corticosteroids is unknown and the assumption that their action is solely due to "anti-inflammatory" effects needs to be challenged. ASM, in evolutionary terms, dates to the earliest land dwelling creatures that required muscle to empty primitive lungs. ASM appears very early in embryonic development and active peristalsis is essential for the formation of the lungs. However, in post-natal life its only role appears to be to maintain airways in a configuration that minimizes resistance to airflow and dead space. In health, significant constriction is actively prevented, presumably through classic negative feedback loops. Disruption of this robust homeostatic control can develop at any age and results in asthma. In order to develop a cure, we need to move from our current focus on immunology and inflammatory pathways to work that will lead to an understanding of the mechanisms that contribute to ASM stability in health and how this is disrupted to cause asthma. This requires a radical change in the focus of most of "asthma research."

Regulation of Airway Smooth Muscle Contraction in Health and Disease

Airway smooth muscle (ASM) extends from the trachea throughout the bronchial tree to the terminal bronchioles. In utero, spontaneous phasic contraction of fetal ASM is critical for normal lung development by regulating intraluminal fluid movement, ASM differentiation, and release of key growth factors. In contrast, phasic contraction appears to be absent in the adult lung, and regulation of tonic contraction and airflow is under neuronal and humoral control. Accumulating evidence suggests that changes in ASM responsiveness contribute to the pathophysiology of lung diseases with lifelong health impacts.Functional assessments of fetal and adult ASM and airways have defined pharmacological responses and signaling pathways that drive airway contraction and relaxation. Studies using precision-cut lung slices, in which contraction of intrapulmonary airways and ASM calcium signaling can be assessed simultaneously in situ, have been particularly informative. These combined approaches have defined the relative importance of calcium entry into ASM and calcium release from intracellular stores as drivers of spontaneous phasic contraction in utero and excitation-contraction coupling.Increased contractility of ASM in asthma contributes to airway hyperresponsiveness. Studies using animal models and human ASM and airways have characterized inflammatory and other mechanisms underlying increased reactivity to contractile agonists and reduced bronchodilator efficacy of β₂-adrenoceptor agonists in severe diseases. Novel bronchodilators and the application of bronchial thermoplasty to ablate increased ASM within asthmatic airways have the potential to overcome limitations of current therapies. These approaches may directly limit excessive airway contraction to improve outcomes for difficult-to-control asthma and other chronic lung diseases.

Transition From Phasic to Tonic Contractility in Airway Smooth Muscle After Birth: An Experimental and Computational Modeling Study

Fetal airway smooth muscle (ASM) exhibits phasic contractile behavior, which transitions to a more sustained "tonic" contraction after birth. The timing and underlying mechanisms of ASM transition from a phasic to a tonic contractile phenotype are yet to be established. We characterized phasic ASM contraction in preterm (128 day gestation), term (~150 day gestation), 1-4 month, 1 yr, and adult sheep (5yr). Spontaneous phasic activity was measured in bronchial segments as amplitude, frequency, and intensity. The mechanism of phasic ASM contraction was investigated further with a computational model of ASM force development and lumen narrowing. The computational model comprised a two-dimensional cylindrical geometry of a network of contractile units and the activation of neighboring cells was dependent on the strength of coupling between cells. As expected, phasic contractions were most prominent in fetal airways and decreased with advancing age, to a level similar to the level in the 1-4 month lambs. Computational predictions demonstrated phasic contraction through the generation of a wave of activation events, the magnitude of which is determined by the number of active cells and the strength of cell-cell interactions. Decreases in phasic contraction with advancing age were simulated by reducing cell-cell coupling. Results show that phasic activity is suppressed rapidly after birth, then sustained at a lower intensity from the preweaning phase until adulthood in an ovine developmental model. Cell-cell coupling is proposed as a key determinant of phasic ASM contraction and if reduced could explain the observed maturational changes.

A synthetic chloride channel relaxes airway smooth muscle of the rat

Synthetic ion channels may have potential therapeutic applications, provided they possess appropriate biological activities. The present study was designed to examine the ability of small molecule-based synthetic Cl^– channels to modulate airway smooth muscle responsiveness. Changes in isometric tension were measured in rat tracheal rings. Relaxations to the synthetic chloride channel SCC-1 were obtained during sustained contractions to KCl. The anion dependency of the effect of SCC-1 was evaluated by ion substitution experiments. The sensitivity to conventional Cl^– transport inhibitors was also tested. SCC-1 caused concentration-dependent relaxations during sustained contractions to potassium chloride. This relaxing effect was dependent on the presence of extracellular Cl^– and HCO₃⁻. It was insensitive to conventional Cl^– channels/transport inhibitors that blocked the cystic fibrosis transmembrane conductance regulator and calcium-activated Cl^– channels. SCC-1 did not inhibit contractions induced by carbachol, endothelin-1, 5-hydroxytryptamine or the calcium ionophore A23187. SCC-1 relaxes airway smooth muscle during contractions evoked by depolarizing solutions. The Cl^– conductance conferred by this synthetic compound is distinct from the endogenous transport systems for chloride anions.

Mechanisms of intrinsic force in small human airways

We quantified the magnitude and investigated mechanisms regulating intrinsic force (IF) in human airway smooth muscle (hASM). IF was identified by reducing extracellular calcium (Ca2+) concentration to nominally zero in freshly isolated isometrically mounted 2mm human bronchi. Our results show: (1) the magnitude of IF is ∼50% of the maximal total force elicited by acetylcholine (10(-5) M) and is epithelial independent, (2) IF can also be revealed by β-adrenergic activation (isoproterenol), non-specific cationic channel blockade (La3+) or L-type voltage gated Ca2+ channel blockade (nifedipine), (3) atropine, indomethacin, AA-861, or pyrilamine did not affect IF, (4) IF was reduced by the intracellular Ca2+ ([Ca2+]i) chelating agent BAPTA-AM, (5) ω-conotoxin had no effect on IF. In studies in cultured hASM cells nominally zero Ca2+ buffer and BAPTA-AM reduced [Ca2+]i but isoproterenol and nifedipine did not. Taken together these results indicate that rapid reduction of [Ca2+]i reveals a permissive relationship between extracellular Ca2+, [Ca2+]i and IF. However IF can be dissipated by mechanisms effecting Ca2+ sensitivity. We speculate that an increase of IF, a fundamental property of ASM, could be related to human airway clinical hyperresponsiveness and must be accounted for in in vitro studies of hASM.

Role of potassium channels in the relaxant effect of levosimendan in guinea pig tracheal preparations

We investigated both the effect of levosimendan and the role of various potassium channels in carbachol-precontracted tracheal preparations samples obtained from guinea pig. The tracheas were cut into 0.5 cm wide rings and suspended in a 20 ml organ bath. Isometric tension was continuously measured with an isometric force transducer connected to a computer-based data acquisition system. Levosimendan or cromakalim produced concentration-dependent relaxation responses in guinea pig tracheal rings precontracted by carbachol. Incubation of guinea pig tracheal rings with the ATP-dependent potassium channel (K(ATP)) blocker glibenclamide for 30 min significantly inhibited the relaxant responses to both levosimendan and cromakalim. The large conductance Ca(2+)-activated potassium channel (BK(Ca)) blocker iberiotoxin also caused a significant inhibition on relaxant responses to levosimendan. However, incubation of the tracheal rings with the voltage-dependent potassium channel blocker 4-aminopyridine for 10 min did not cause significant alterations on relaxant responses to levosimendan. The present findings suggested that the relaxant effect induced by levosimendan might be partially due to K(ATP) and BK Ca in isolated guinea pig tracheal rings.

Bronchial muscle peristaltic activity in the fetal rat

Aside for the potential for tonic contraction, the airway smooth muscle exhibits intermittent phasic rhythmic activity that may contribute to lung growth during fetal life. Therefore, we examined 4th generation rat 18-22 d gestation fetal, 4-6 d of age newborn and adult bronchial ring from Sprague Dawley rats to compare differences in smooth muscle function. We hypothesized that phasic contractions were greatest before birth. Bronchial muscle spontaneous rhythmic contractions were greatest in the fetus and absent in the adult. In response to KCl stimulation, the fetal bronchial smooth muscle only developed tonic force that was 3.5 +/- 0.6 and lower than measured in the newborn 9.0 +/- 0.3 and adult 13.7 +/- 1.4 mN/mm2. The thromboxane A2 analogue U46619 induced tonic and phasic muscle contractions and the amplitude and frequency of the phasic contractions were greater in the fetus as compared with the adult and increased with gestational age. The U46619-induced rhythmic contractions were abrogated by ryanodine, thapsigargin and reduction of extracellular Na+, suggesting intracellular Ca2+ dependence and involvement of the Na+/Ca2+ exchanger. The inward rectifier K+ blocker BaCl2 induced phasic contractions in unstimulated fetal, but not adult bronchial muscle of the same amplitude and frequency as for the spontaneous and U46619-induced ones. We conclude that the airway smooth muscle phasic activity is greatest in the fetus and tends to disappear post-natally with age suggesting an in utero role during lung development.

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.

Thapsigargin, a Ca(2+)-ATPase inhibitor, relaxes guinea pig tracheal smooth muscle by producing epithelium-dependent relaxing factors

A non-phorbol ester-type tumor promoter, thapsigargin has been reported to deplete Ca(2+) stores in endothelial cells by inhibiting Ca(2+)-ATPase, which in turn increases intracellular Ca(2+) by mobilization of extracellular Ca(2+), leading to activation of constitutive nitric oxide synthase (cNOS) and resultant generation of nitric oxide (NO). In the present study, to evaluate the role of Ca(2+) in the release of epithelium-dependent relaxing factor (EpDRF), we determined the effect of thapsigargin (10(-6) M) on the contraction evoked by exogenous Ca(2+) or acetylcholine (10(-5) M) in epithelium-denuded or epithelium-intact smooth muscle from guinea pig trachea. The following results were obtained: (1) In epithelium-denuded smooth muscle, the contraction evoked by exogenous Ca(2+) in Ca(2+)-free solution or by acetylcholine (10(-5) M) in Ca(2+)-containing solution did not change within 20 min after thapsigargin application, but the contraction evoked by exogenous Ca(2+) increased markedly after 120 min, indicating that thapsigargin had no effect on smooth muscle itself within 20 min of application. The following experiments were performed within 20 min of thapsigargin application. (2) In epithelium-intact smooth muscle, thapsigargin significantly suppressed the contraction evoked by acetylcholine, suggesting that thapsigargin stimulate the epithelium to produce EpDRF. N(G)-nitro-L-arginine methylester (L-NAME) partly, but significantly, attenuated this inhibitory effect of thapsigargin. (3) In epithelium-denuded smooth muscle, atropine (10(-6) M) and L-NAME (10(-5) M) did not change the contraction evoked by exogenous Ca(2+) after application of thapsigargin, suggesting that thapsigargin did not stimulate acetylcholine and NO release from nerve terminals. These results suggest that thapsigargin (10(-6) M) may stimulate EpDRF, including NO and other factor(s) by Ca(2+)-dependent mechanisms.

Effects of load and tone on the mechanics of isolated human bronchial smooth muscle

Isotonic and isometric properties of nine human bronchial smooth muscles were studied under various loading and tone conditions. Freshly dissected bronchial strips were electrically stimulated successively at baseline, after precontraction with 10(-7) M methacholine (MCh), and after relaxation with 10(-5) M albuterol (Alb). Resting tension, i.e., preload determining optimal initial length (Lo) at baseline, was held constant. Compared with baseline, MCh decreased muscle length to 93 +/- 1% Lo (P < 0.001) before any electrical stimulation, whereas Alb increased it to 111 +/- 3% Lo (P < 0.01). MCh significantly decreased maximum unloaded shortening velocity (0.045 +/- 0.007 vs. 0.059 +/- 0.007 Lo/s), maximal extent of muscle shortening (8.4 +/- 1.2 vs. 13.9 +/- 2.4% Lo), and peak isometric tension (6.1 +/- 0.8 vs. 7.2 +/- 1.0 mN/mm2). Alb restored all these contractile indexes to baseline values. These findings suggest that MCh reversibly increased the number of active actomyosin cross bridges under resting conditions, limiting further muscle shortening and active tension development. After the electrically induced contraction, muscles showed a transient phase of decrease in tension below preload. This decrease in tension was unaffected by afterload levels but was significantly increased by MCh and reduced by Alb. These findings suggest that the cross bridges activated before, but not during, the electrically elicited contraction may modulate the phase of decrease in tension below preload, reflecting the active part of resting tension.

Functional activity of bronchi from an organ donor with fatal asthma: studies on cryopreserved bronchi

Human bronchi were taken from the lungs of a single asthmatic and 5 nonasthmatic organ donors. The tissues were slowly frozen to -70 degrees C and stored for 1-28 months in liquid nitrogen (-196 degrees C) while suspended in Krebs-Henseleit solution containing 1.8 M dimethyl sulfoxide and 0.1 M sucrose as cryoprotectants. After thawing, bronchial rings were suspended in 10 ml organ baths for isometric tension recording. Spontaneously developed tone (1.13 +/- 0.12, n = 22, vs. 0.56 +/- 0.07 g, n = 33, p < 0.001) and maximal contractile responses to histamine (1.93 +/- 0.12, n = 34, vs. 1.02 +/- 0.14 g, n = 30, p < 0.001) were significantly stronger in asthmatic than in nonasthmatic bronchi. The potency of histamine was 4 times less in asthmatic than in nonasthmatic bronchi (p < 0.001). Comparison of the maximal responses to histamine after storage at -196 degrees C for up to 28 months revealed no significant reduction of the contractile function by time of cryostorage. Salbutamol and the potassium channel opener SDZ PCO 400 were 3-4 times less potent in asthmatic than in nonasthmatic bronchi. For antagonism of histamine by ketotifen in asthmatic bronchi (pD'2 = 8.04 +/- 0.13, n = 5) 4 times higher concentrations were necessary than in nonasthmatic bronchi (pD'2 = 8.63 +/- 0.06, n = 15, p < 0.001). These data support the contention that in spite of a diminished sensitivity to histamine after fatal asthma, isolated bronchi show enhanced spontaneous and agonist-induced contractile responses whereas relaxant responses appear to be impaired.

Potassium channels in human fetal airway smooth muscle cells

Ion channels underlying the resting membrane potential were examined in human fetal airway smooth muscle (ASM). Tissue was obtained from the Medical Research Council Tissue Bank, London, UK. ASM cells were enzymatically dispersed, and ion currents were examined using a patch clamp. Although all cells were of similar size and stained intensely for vimentin, only approximately 50% stained intensely for smooth muscle alpha-actin or myosin heavy chain. Depolarization induced a tetraethylammonium (TEA)- and charybdotoxin (ChTX)-sensitive outward current that varied widely among cells (<50 to >2000 pA at +100 mV), and a smaller nonselective cation current that was similar in all cells (approximately 20 pA at +100 mV). The TEA-sensitive current was associated with three types of large conductance, ChTX-sensitive K+ channel: a 200-pS channel, which was active at negative potentials and low [Ca2+], as described for freshly isolated adult ASM, and two other K+ channels of 100 and 150 pS, previously observed only in adult ASM proliferating in culture. ChTX, but not 4-aminopyridine, caused a substantial depolarization in the current clamp mode, suggesting that, in contrast to ASM from other species or vascular smooth muscle, large conductance K+ channels rather than a delayed rectifier are the major determinant of membrane potential in this tissue. Our results show a distinct similarity between fetal ASM and adult ASM proliferating in culture. We suggest that the heterogeneity in current density and staining reflect different degrees of differentiation, rather than different cell types, and that the 100- and 150-pS K+ channels are specifically associated with a proliferative phenotype in human ASM.

Interaction between prostaglandins and selective phosphodiesterase inhibitors in isolated guinea-pig trachea in vitro

The possible interaction between spontaneously synthesized relaxant prostaglandins and the relaxation produced by three different isoenzyme-selective phosphodiesterase inhibitors was investigated in the isolated guinea-pig trachea in vitro. The relaxant action of siguazodan (phosphodiesterase III inhibitor), rolipram (phosphodiesterase IV inhibitor) and zaprinast (phosphodiesterase V inhibitor) was investigated in preparations with either spontaneously tone alone or in preparations with spontaneous tone and additionally stimulated with histamine (1 microM). In addition, relaxant effects were assessed in preparations without spontaneous tone (inhibited by indomethacin 2 microM) and precontracted with histamine (1 microM) or prostaglandin F2 alpha (10 microM), either alone or in the presence of a non-relaxant concentration (20 nM) of prostaglandin E2. All three phosphodiesterase inhibitors preferentially relaxed preparations with spontaneous tone and showed increased relaxant effects in preparations with spontaneous tone and additionally stimulated with histamine compared to preparations contracted by histamine alone. This enhanced relaxing effect observed in the presence of initial spontaneous tone was mimicked by exogenous application of prostaglandin E2 to indomethacin treated preparations either precontracted by histamine or prostaglandin F2 alpha. Furthermore, the study revealed marked differences in the relaxant profiles of siguazodan, rolipram and zaprinast, differences which most likely are related to the functional importance of the phosphodiesterase isoenzymes inhibited by these drugs. It is concluded that endogenously synthesized relaxant prostaglandins and exogenously applied prostaglandin E2 are capable of enhancing the relaxant action of the phosphodiesterase inhibitors siguazodan, rolipram and zaprinast and that cyclooxygenase inhibition is an important way to avoid this interaction in experimental studies of airway smooth muscle relaxants in isolated guinea-pig trachea in vitro.

The spasmogenic effects of vanadate in human isolated bronchus

1. Inhalation of vanadium compounds, particularly vanadate, is a cause of occupational bronchial asthma. We have now studied the action of vanadate on human isolated bronchus. Vanadate (0.1 microM-3 mM) produced concentration-dependent, well-sustained contraction. Its -logEC50 was 3.74 +/- 0.05 (mean +/- s.e.mean) and its maximal effect was equivalent to 97.5 +/- 4.2% of the response to acetylcholine (ACh, 1 mM). 2. Vanadate (200 microM)-induced contraction of human bronchus was epithelium-independent and was not inhibited by indomethacin (2.8 microM), zileuton (10 microM), a mixture of atropine, mepyramine and phentolamine (each at 1 microM), or by mast cell degranulation with compound 48/80. 3. Vanadate (200 microM)-induced contraction was unaltered by tissue exposure to verapamil or nifedipine (each 1 microM) or to a Ca2+-free, EGTA (0.1 mM)-containing physiological salt solution (PSS). However, tissue incubation with ryanodine (10 microM) in Ca2+-free, EGTA (0.1 mM)-containing PSS reduced vanadate-induced contraction. A series of vanadate challenges was made in tissues exposed to Ca2+-free EGTA (0.1 mM)-containing PSS with the object of depleting intracellular Ca2+ stores. In such tissues cyclopiazonic acid (CPA; 10 microM) prevented Ca2+-induced recovery of vanadate-induced contraction. 4. Tissue incubation in K+-rich (80 mM) PSS, K+-free PSS, or PSS containing ouabain (10 microM) did not alter vanadate (200 microM)-induced contraction. Ouabain (10 microM) abolished the K+-induced relaxation of human bronchus bathed in K+-free PSS. This action was not shared by vanadate (200 microM). The tissue content of Na+ was increased and the tissue content of K+ was decreased by ouabain (10 microM). In contrast, vanadate (200 microM) did not alter the tissue content of these ions. Tissue incubation in a Na+-deficient (25 mM) PSS or in PSS containing amiloride (0.1 mM) markedly inhibited the spasmogenic effect of vanadate (200 microM). 5. Vanadate (200 microM)-induced contractions were markedly reduced by tissue treatment with each of the protein kinase C (PKC) inhibitors H-7 (10 microM), staurosporine (1 microM) and calphostin C (1 microM). Genistein (100 microM), an inhibitor of protein tyrosine kinase, also reduced the response to vanadate. 6 Vanadate (0.1-3 mM) and ACh (1 microM- 3 mM) each increased inositol phosphate accumulation in bronchus. Such responses were unaffected by a Ca2+-free medium either alone or in combination with ryanodine (10 microM). 7. In human cultured tracheal smooth muscle cells, histamine (100 microM) and vanadate (200 microM) each produced a transient increase in intracellular Ca2+ concentration ([Ca2+]i). 8. Intracellular microelectrode recording showed that the contractile effect of vanadate (200 microM) in human bronchus was associated with cellular depolarization. 9. It is concluded that vanadate acts directly on human bronchial smooth muscle, promoting the release of Ca2+ from an intracellular store. The Ca2+ release mechanism involves both the production of inositol phosphate second messengers and inhibition of Ca-ATPase. The activation of PKC plays an important role in mediating vanadate-induced contraction at values of [Ca2+]i that are close to basal.

Inherent tone of human bronchus: role of eicosanoids and the epithelium

1. Airway preparations of different species possess varying degrees of inherent tone which is the result of different metabolites of arachidonic acid in different species. In human bronchial smooth muscle in vitro we have investigated the effects of 5-lipoxygenase inhibition (zileuton, 10 microM), cyclo-oxygenase inhibition (indomethacin, 1 microM) and mechanical epithelium removal on inherent tone. The shunting of arachidonic acid by inhibition of one or other of these enzymes, as a possible explanation for the effects observed, has also been investigated. 2. Zileuton caused a significant fall in tone either alone (-107 +/- 33 mg) or after cyclo-oxygenase inhibition (-203 +/- 48 mg) and this effect was not significantly altered by epithelial removal (-191 +/- 43 mg alone; -333 +/- 88 mg after indomethacin). Indomethacin increased tone when applied alone (160 +/- 94 mg), but this effect only reached statistical significance after 5-lipoxygenase inhibition, (210+/-81 mg; P<0.05). Epithelial removal did not alter the effect of indomethacin when applied alone (213+/-97 mg), but significantly reduced the effect of indomethacin after 5-lipoxygenase inhibition (34 +/- 23 mg; P<0.05). 3. These data suggest that inherent tone in human bronchus is largely the result of contractile 5-lipoxygenase products. However, the involvement of cyclo-oxygenase products cannot entirely be discounted, since in the presence of 5-lipoxygenase inhibition contractile and relaxant eicosanoids originating from the bronchial epithelium appear to influence significantly inherent tone.

SCA 40: studies of the relaxant effects on cryopreserved human airway and vascular smooth muscle

1. 6-Bromo-8-methylaminoimidazol[1,2-a]pyrazine-2carbonitrile (SCA 40) has been claimed to induce relaxation in guinea-pig trachea by opening high conductance, calcium-activated potassium (BKCa) channels. The mechanism of action of SCA 40 has now been further investigated in ring preparations from cryopreserved human airway and vascular smooth muscle preparations in vitro. 2. Human bronchi with spontaneous tone relaxed in response to SCA 40 in a biphasic way. A high affinity component (pD2 8.61 +/- 0.21; mean +/- s.e.mean) accounted for 30% of the response and a low affinity component (pD2 6.53 +/- 0.14) for the remaining 70%. In contrast, in bronchi contracted with carbachol, 1 microM, the concentration-response curve to SCA 40 was monophasic and yielded a pD2 of 6.31 +/- 0.29. 3. SCA 40 relaxed pulmonary and mesenteric arteries and peripheral veins which had been precontracted by 10 nM U46619 nearly completely and in a monophasic way; the pD2 values were 6.37 +/- 0.08, 6.17 +/- 0.15 and 5.45 +/- 0.25, respectively. 4. Lemakalim, an opener of ATP-dependent potassium (KATP) channels, also relaxed human bronchi under spontaneous tone and the vascular tissues. NS 1619, a recognised opener of BKca channels, was inactive up to 10 microM on bronchial and vascular tissues. 5. The SCA 40-induced relaxation of human bronchi was reduced concentration-dependently in the presence of high potassium chloride (20 and 80 mM). However, in the presence of 80 mM KCl and nifedipine, 30 nM, SCA 40 fully relaxed the remaining contractile response with pD2 values of 8.08 +/- 0.13 and 5.27 +/- 0.13 for the high and low affinity component, respectively. 6. Relaxation responses to SCA 40 in human bronchi were resistant to blockade by glibenclamide at concentrations up to 10 microM (which blocked the relaxant response to lemakalim), quinine (30 microM), apamin (100 nM), tetraethylammonium (0.1-1 mM) and charybdotoxin (10-100 nM), thus excluding the involvement of a variety of K+ channels including KATP and KCa channels. 7. In bronchi contracted with carbachol, 1 microM, the nature of the interaction between SCA 40 and the beta 2-adrenoceptor agonist, salbutamol, was synergistic. 8. These experiments establish that SCA 40 is a potent relaxant of human bronchial smooth muscle manifesting spontaneous tone. A low affinity relaxant component has its counterpart in the relaxation seen in both human arterial and venous smooth muscle. The consensus of the evidence suggests that K+ channel opening is not the basis of the relaxant response to SCA 40. Furthermore, BKCa channels appear to be of minor importance in the regulation of human airway smooth muscle tone. Our data suggest that inhibition of an adenosine 3':5'-cyclic monophosphate phosphodiesterase may contribute, at least to the low affinity relaxant component of SCA 40. However, the exact mechanism mediating the SCA 40-induced relaxation of human airways remains to be defined.

VIP antagonists enhance excitatory cholinergic neurotransmission in the human airway

It has been reported that a low concentration of exogenously applied vasoactive intestinal peptide (VIP) suppresses the release of acetylcholine (ACh) from vagus nerve terminals in the ferret and feline trachea. There has been, however, no documentation of the prejunctional action of VIP in the human airway. We observed the effects of VIP and VIP antagonists on cholinergic excitatory neuro-effector transmission in the human bronchus to study the possible role of endogenous VIP on excitatory neurotransmission. In the human bronchus, VIP (10(-10) to 10(-7) M) showed no effect on either the contractions evoked by electrical field stimulation (EPS) or those evoked by ACh. To investigate the possible role of endogenous VIP on the human bronchus, we observed the effects of the VIP antagonists [4-Cl-D-Phe6,Leu17]-VIP and [Ac-Tyr1,D-Phe2]-GRF(1-29)-NH2 on excitatory neuro-effector transmission. Both VIP antagonists (10(-8) M) significantly enhances the contractions evoked by EFS without affecting the ACh sensitivity of smooth muscle cells. These results indicate that VIP antagonists have a prejunctional action that enhances excitatory neurotransmission. This study suggests that endogenous VIP may suppresses ACh release from the vagus nerve terminals in the human airway. It is also suggested that exogenously applied VIP may be inactivated by some mechanism in the human airway.

Studies of human airways in vitro: a review of the methodology

The pathophysiology of human airway narrowing is only partly understood. In order to gain more insight in the mechanisms of human lung diseases and potential beneficial therapeutic agents, adequate models are needed. Animal airway models are of limited value since lung diseases such as asthma and chronic obstructive pulmonary disease (COPD) are unique to humans and because the mechanisms of airway narrowing differ between species. Therefore, it is important to perform studies on human isolated airways. We describe the models that have been developed to study airway function in vitro, emphasizing human airway preparations. The easily prepared airway strip and ring preparations are described first. The potential damage during preparation and the interference with airway structure are important drawbacks in these preparations. Lung parenchymal strips, described next, were designed in order to study responsiveness of small airways. However, parenchymal strips are anatomically complex, and responsiveness is determined by the relative amounts of airway and vascular smooth muscle. The lack of reproducibility between species and even within one animal limits their usefulness. Airway tube preparations, in which luminal and serosal stimulation can be separated, enable us to study the modulatory role of the airways epithelium in vitro. Furthermore, airway compliance can be measured. In the isolated perfused lung preparation, relationships between the airways and the vascular system are preserved and the interaction between these two systems can be studied. Weight gain due to fluid extravasation is a problem in this model which has not been used yet to study human lungs in vitro. Next, methodological aspects such as tissue handling and storage, recording of responses, removal of the epithelium, and electrical field stimulation are discussed in some detail. Although animal airways tissue can be studied immediately after removal, human tissue is often obtained with some delay. However, this seems tenable since electron microscopy of lung tissue obtained at autopsy showed that recovery of the preparation occurs during incubation of carbogenated Krebs-Henseleit (K-H) buffer. Dissected airways can be stored overnight in cooled K-H buffer until up to 55 hr after resection without losing viability. Commonly used physiological salt solutions which bath the tissue contain osmotic molecules, ions important for contractility, glucose as a substrate, and a bicarbonate-carbon dioxide buffer.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of beta-adrenoceptor agonists in human bronchial smooth muscle

1. We have investigated the potency and duration of action of isoprenaline and a range of beta-adrenoceptor agonists as relaxants of inherent tone in human superfused, isolated bronchial smooth muscle, a tissue reported to contain a homogeneous population of beta 2-adrenoceptors. 2. All of the beta-adrenoceptor agonists caused concentration-related inhibition of inherent tone, with isoprenaline having an EC50 of 27 nM. The rank order of agonist potency was: formoterol > or = -salmeterol > or = clenbuterol > fenoterol = isoprenaline > terbutaline > or = salbutamol > quinprenaline. 3. Relaxant responses to salmeterol were fully reversed by the selective beta 2-adrenoceptor blocking drug, ICI 118551, demonstrating the involvement of beta 2-adrenoceptors. 4. Rt50, i.e. the time taken for 50% recovery from the effects of an EC50 concentration of agonist, differed considerably between the different beta 2-adrenoceptor agonists. Most agonists were short-acting, having Rt50 values less than 13 min. Quinprenaline was of moderate duration, with an Rt50 value of > or = 20 min. In contrast, salmeterol was extremely long-acting, with no sign of recovery within 4 h. 5. Estimates of relative potency and duration of action were similar to those previously determined for these agonists in the guinea-pig isolated trachea. These results suggest, therefore, that guinea-pig trachea is a suitable alternative to human bronchus for the evaluation of the actions of beta-adrenoceptor agonists on airways smooth muscle.

Degradation of acetylcholine in human airways: role of butyrylcholinesterase

1. Neostigmine and BW284C51 induced concentration-dependent contractions in human isolated bronchial preparations whereas tetraisopropylpyrophosphoramide (iso-OMPA) was inactive on airway resting tone. 2. Neostigmine (0.1 microM) or iso-OMPA (100 microM) increased acetylcholine sensitivity in human isolated bronchial preparations but did not alter methacholine or carbachol concentration-effect curves. 3. In the presence of iso-OMPA (10 microM) the bronchial rings were more sensitive to neostigmine. The pD2 values were, control: 6.05 +/- 0.15 and treated: 6.91 +/- 0.14. 4. Neostigmine or iso-OMPA retarded the degradation of acetylcholine when this substrate was exogenously added to human isolated airways. A marked reduction of acetylcholine degradation was observed in the presence of both inhibitors. Exogenous butyrylcholine degradation was prevented by iso-OMPA (10 microM) but not by neostigmine (0.1 microM). 5. These results suggest the presence of butyrylcholinesterase activity in human bronchial muscle and this enzyme may co-regulate the degradation of acetylcholine in this tissue.

Effect of a new leukotriene receptor antagonist, ONO-1078, on human bronchial smooth muscle in vitro

Peptide leukotrienes (LT) have been postulated to play a major role in the etiology of bronchial asthma. The present study investigated the effect of a new peptide LT receptor antagonist, ONO-1078, on isolated human bronchial smooth muscle in vitro. Helical strips of bronchi were suspended in the organ baths filled with 37 degrees C Krebs solution and mechanical responses were recorded isometrically. ONO-1078 produced dose-dependent relaxations, which suggested that the spontaneous basal tone was in part mediated by LT. ONO-1078 caused dose-dependent relaxations of the tissues which were precontracted with LTC4 or LTD4 (3 x 10(-8) M). Pretreatment of bronchi with ONO-1078 (10(-8) M, 10(-7) M) significantly inhibited dose-dependent contractions induced by LTC4 and LTD4. ONO-1078 (10(-6) M) also significantly reduced the antigen-induced contractions in bronchi passively sensitized with atopic serum from mite-allergic patients. Moreover the combination of an H1-receptor antagonist, diphenhydramine (10(-5) M), and ONO-1078 (10(-6) M) completely abolished the antigen-induced contractions. The present findings demonstrate that ONO-1078 is a potent antagonist of exogenous and endogenous LT in the human airway. The selective LT antagonist such as ONO-1078 may be valuable in the therapy of allergic asthma.

Excitatory role of axon reflex in bradykinin-induced contraction of guinea pig tracheal smooth muscle

To elucidate the role of the axon reflex in the airways, we studied the effects of atropine (10(-6) M) and tetrodotoxin (10(-7) M) on the bradykinin-induced contraction of guinea pig tracheal smooth muscle, with or without pretreatment of the animals with capsaicin. The concentration-response curves to bradykinin (10(-9) to 10(-5) M) were measured in the presence of both indomethacin and propranolol. In the guinea pigs not given capsaicin pretreatment, baseline tension values did not differ before versus after the application of atropine or tetrodotoxin. Tetrodotoxin reduced the bradykinin-induced contraction significantly, but atropine did not change the contraction induced by bradykinin. These observations indicate that bradykinin-induced contraction is potentiated by a neurally mediated action, but that is not mediated by acetylcholine released from the efferent vagal nerve terminals. The contractile response to bradykinin was significantly reduced in the animals treated with capsaicin as compared with those administered vehicle only. Furthermore, in the animals treated with capsaicin, tetrodotoxin did not affect the response to bradykinin. These observations indicate that bradykinin-induced airway smooth muscle contraction is mediated in part by tachykinins released from C-fiber endings, presumably via an axon reflex.

NIP-121 and cromakalim, potassium channel openers, preferentially suppress prostanoid-induced contraction of the guinea-pig isolated trachea

We have investigated the relaxant effect of the potassium channel openers, NIP-121 and cromakalim, on spontaneous and spasmogen-induced tone in the isolated guinea-pig trachea. NIP-121 and cromakalim fully suppressed the spontaneous tone in a concentration-dependent manner and the maximal response was 89 and 97% of that to 1 mM aminophylline. The suppressant effect of NIP-121 (pD2 7.39) was 5 times stronger than that of cromakalim (pD2 6.69). Spontaneous tone was completely inhibited by the cyclooxygenase inhibitor, indomethacin, and partially inhibited by the thromboxane A2 (TXA2) antagonist, BM13177. In the presence of indomethacin, the contraction induced by prostaglandin (PG) F2 alpha and PGD2 was reversed by BM13177 to the same extent. NIP-121 and cromakalim reversed the contraction induced by PGF2 alpha, PGD2 and the TXA2 mimetic, U46619, and the effects were more potent than those observed on the contraction induced by leukotriene (LT) D4, LTC4, histamine and acetylcholine. The maximal relaxant responses (%) induced by NIP-121 and cromakalim were 97 and 96 for PGF2 alpha, 94 and 87 for PGD2, 94 and 93 for U46619, 69 and 69 for LTD4, 75 and 58 for LTC4, 73 and 61 for histamine and 1 and 16 for acetylcholine, respectively. The relaxant effect of NIP-121 on responses to these spasmogens (pD2 7.35 for PGF2 alpha, 7.40 for PGD2, 7.31 for U46619, 7.28 for LTD4, 7.09 for LTC4, and 7.15 for histamine) was about 10-20 times stronger than the effect of cromakalim (pD2 6.23 for PGF2 alpha, 6.04 for PGD2, 6.20 for U46619, 6.01 for LTD4, 5.82 for LTC4 and 5.88 for histamine).(ABSTRACT TRUNCATED AT 250 WORDS)

Pre-junctional inhibitory action of prostaglandin E2 on excitatory neuro-effector transmission in the human bronchus

The effects of prostaglandin E2 (PGE2) and indomethacin on excitatory neuro-effector transmission in the human bronchus were investigated by tension recording and microelectrode methods. PGE2 (10(-10)-10(-9)M) suppressed the amplitude of twitch contractions and excitatory junction potentials (e.j.ps) evoked by field stimulation at a steady level of basal tension obtained by the combined application of indomethacin (10(-5) M) and FPL55712 (10(-6) M). In doses over 10(-8)M, PGE2 reduced the muscle tone and dose-dependently suppressed the amplitude of twitch contractions. Indomethacin (10(-5) or 5 x 10(-5) M) reduced the muscle tone and enhanced the amplitude of twitch contractions and e.j.ps evoked by field stimulation in the presence of FPL55712. PGE2 (10(-9) M) had no effect on the post-junctional response of smooth muscle cells to exogenously applied acetylcholine (ACh) (4 x 10(-7) M). However, indomethacin (10(-5) M) significantly enhanced the ACh-induced contraction of the human bronchus. These results indicate that PGE2 in low concentrations has a pre-junctional action to inhibit excitatory neuro-effector transmission in addition to a post-junctional action, presumably by suppressing transmitter release from the vagus nerve terminals in the human bronchial tissues.