Induction of a myogenic response in tonic airway smooth muscle by tetraethylammonium

Thromboxane prostanoid receptor activation amplifies airway stretch-activated contractions assessed in perfused intact bovine bronchial segments

A deep inspiration (DI) produces bronchodilation in healthy individuals. Conversely, in asthmatics, DIs are less effective in producing bronchodilation and can cause more rapid airway renarrowing and even bronchoconstriction in moderate to severe asthmatics. It is noteworthy that the manner by which a DI is able to cause bronchoconstriction via a stretch-activated contraction (R(stretch)) is thought to correlate positively with airway inflammation. Asthmatic airway inflammation is associated with increased production of thromboxane A(2) (TxA(2)) and subsequent thromboxane prostanoid (TP) receptor activation, causing the heightened contractility of airway smooth muscle. In this study, we sought to investigate the effect of TxA(2) on airway R(stretch) by using bovine bronchial segments. In brief, these intact bronchial segments (2 mm in diameter) were dissected, side branches were ligated, and the tissues were mounted horizontally in an organ bath. R(stretch) was elicited by varying the transmural pressure under isovolumic conditions. Using a pharmacological approach, we showed a reduced R(stretch) response in tissues pretreated with indomethacin, a cyclooxygenase inhibitor, a result mimicked by pretreatment with the TP-selective receptor antagonist 4-(Z)-6-(2-o-chlorophenyl-4-o-hydroxyphenyl-1,3-dioxan-cis-5-yl)hexenoic acid (ICI 192605) and the selective p42/p44 mitogen-activated protein kinase inhibitor 2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one (PD 95089) and by airway epithelial denudation. 9,11-Dideoxy-9α,11α-methanoepoxy-prosta-5Z,13E-dien-1-oic acid (U46619), a TP receptor agonist, elicited enhanced R(stretch) responses in a dose-dependent manner. Pretreatment with 6-isopropoxy-9-oxoxanthene-2-carboxylic acid (AH 6809), a prostaglandin E (EP) receptor 1/prostaglandin D2 (DP)-selective receptor antagonist, and 9α,15R-dihydroxy-11.β-fluoro-15-(2,3-dihydro-1H-inden-2-yl)-16,17,18,19,20-pentanor-prosta-5Z,13E-dien-1-oic acid (AL 8810), a prostaglandin F (FP)-selective receptor antagonist, had no effect, suggesting EP, DP, and FP receptor activation is not involved in amplifying airway smooth muscle R(stretch). These data suggest a role for TP receptor activation and epithelial release of TxA(2) in amplifying airway R(stretch), thus providing novel insights into mechanisms regulating the DI-induced bronchoconstriction seen in asthmatics.

Involvement of the neurokinin-2 receptor in airway smooth muscle stretch-activated contractions assessed in perfused intact bovine bronchial segments

The airway response to deep inspirations (DIs) in asthmatics has been shown to be ineffective in producing bronchodilation and can even cause bronchoconstriction. However, the manner by which a DI is able to cause bronchoconstriction remains ambiguous. We sought to investigate the pathway involved in this stretch-activated contraction and whether this contraction is intrinsic to airway smooth muscle (ASM). In brief, intact bovine bronchial segments were dissected, and side branches were ligated and then mounted horizontally in an organ bath. Intraluminal pressure was measured under isovolumic conditions. Instantaneously opening and then closing the tap on a column of fluid 5 to 30 cm high evoked a sudden increase in intraluminal pressure (equivalent to the height of the column of fluid) followed by a stress relaxation response of the ASM. When tissues were stimulated with carbachol (10(-8) M) or serotonin (10(-7) M) for 10 min, and the consequent agonist-evoked pressure response was dissipated manually, the response to the same transmural stretch was accompanied by a slowly developing and prolonged increase in intraluminal pressure. This stretch-activated response was significantly diminished by the stretch-activated cation channel blocker gadolinium (10(-3) M), the L-type Ca2+ channel blockers nifedipine (2 x 10(-6) M), diltiazem (10(-5) M), and verapamil (10(-5) M), the sensory neurotoxin capsaicin (10(-5) M), and the neurokinin (NK)(2) receptor antagonists MEN 10376 ([Tyr(I),d-Trp(6,8,9),Lys(10)]-NKA(4-10)) (10(-5) M) and SR48968 (N-[(2S)-4-(4-acetamido-4-phenylpiperidin-1-yl)-2-(3,4-dichlorophenyl)butyl]-N-methylbenzamide) (3 x 10(-6) M). These results show the ability of isolated airways to exhibit stretch-activated contractions and suggest a role for stretch-activated cation channels, sensory afferent neurons, the neurotransmitter NKA, and L-type Ca(2+) channels in these isolated airway responses.

Deep inspiration and airway smooth muscle adaptation to length change

In normal subjects a deep inspiration (DI) taken during bronchoconstriction substantially reduces airway narrowing (bronchodilation) and a DI taken prior to bronchoconstriction attenuates subsequent airway narrowing (bronchoprotection). Although the exact mechanism(s) for these phenomena are unclear the time course of these effects supports the hypothesis that they are mediated through actions of airway smooth muscle (ASM). There is convincing evidence that both the bronchodilation and bronchoprotection actions of DI are deficient or absent in asthmatic subjects. Various theories have been proposed such as a failure of transmission of stress and strain to the ASM in asthma, stretch-induced contraction of smooth muscle in asthmatics, a failure to release bronchodilating substances and differential effects on cross-bridge dynamics or contractile element rearrangement. In this brief review we focus on the mechanical consequences of DI on the ASM. We suggest that a failure of plastic rearrangement of the contractile apparatus following DI is at the basis of the abnormal response to DI in asthma.

Small airway morphology and lung function in the transition from normality to chronic airway obstruction

This study investigated the relationships between pathological changes in small airways (<6 mm perimeter) and lung function in 22 nonasthmatic subjects (20 smokers) undergoing lung resection for peripheral lesions. Preoperative pulmonary function tests revealed airway obstruction [ratio of forced expiratory volume in 1 s to forced vital capacity (FEV1/FVC) < 70%] in 12 subjects and normal lung function in 10. When all subjects were considered together, total airway wall thickness was significantly correlated with FEV1/FVC (r2 = 0.25), reactivity to methacholine (r2 = 0.26), and slope of linear regression of FVC against FEV1 values recorded during the methacholine challenge (r2 = 0.56). Loss of peribronchiolar alveolar attachments was significantly associated (r2 = 0.25) with a bronchoconstrictor effect of deep inhalation, as assessed from a maximal-to-partial expiratory flow ratio <1, but not with airway responses to methacholine. No significant correlation was found between airway smooth muscle thickness and lung function measurements. In conclusion, this study suggests that thickening of the airway wall is a major mechanism for airway closure, whereas loss of airway-to-lung interdependence may contribute to the bronchoconstrictor effect of deep inhalation in the transition from normal lung function to airway obstruction in nonasthmatic smokers.

Deep breaths, methacholine, and airway narrowing in healthy and mild asthmatic subjects

Deep breaths taken before inhalation of methacholine attenuate the decrease in forced expiratory volume in 1 s and forced vital capacity in healthy but not in asthmatic subjects. We investigated whether this difference also exists by using measurements not preceded by full inflation, i.e., airway conductance, functional residual capacity, as well as flow and residual volume from partial forced expiration. We found that five deep breaths preceding a single dose of methacholine 1) transiently attenuated the decrements in forced expiratory volume in 1 s and forced vital capacity in healthy (n = 8) but not in mild asthmatic (n = 10) subjects and 2) increased the areas under the curve of changes in parameters not preceded by a full inflation over 40 min, during which further deep breaths were prohibited, without significant difference between healthy (n = 6) and mild asthmatic (n = 16) subjects. In conclusion, a series of deep breaths preceding methacholine inhalation significantly enhances bronchoconstrictor response similarly in mild asthmatic and healthy subjects but facilitates bronchodilatation on further full inflation in the latter.

The Schultz-Dale response of sensitized canine bronchial smooth muscle

We wished to determine why in vitro agonist dose-response curves show reduced leftward shift (1/2-1 log dose units) in sensitized canine airway smooth muscle compared to curves elicited in vivo (2-3 log dose unit). The Schultz-Dale response was studied in sensitized dog tracheal (TSM) and bronchial (BSM) smooth muscle. Sensitized TSM challenged with specific antigen showed greater mechanical response, but only on exposure to 300 micrograms/ml; BSM responded to concentrations of ragweed as low as 0.001 microgram/ml. This result resolved the problem cited at the outset. Control TSM and BSM showed no response. The response in BSM is mediated through histamine release, and to a smaller extent by acetylcholine. With challenge release of histamine and acetylcholine increased significantly in sensitized airway smooth muscle. Integrated contractile responses obtained with high and low concentrations of antigen showed a dose-response relationship. Increased sensitivity of BSM to antigen compared to TSM indicates the former is the preparation of choice for study of allergic bronchoconstriction.

Airway smooth muscle relaxation

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Stretch-induced myogenic responses of airways after histamine and carbachol

The purpose of the study described here was to determine the possible role of a myogenic response of bronchial smooth muscle in deep inspiration (DI)-induced bronchoconstriction. Model experiments were performed on sheep tracheal strips. The effect of sudden stepwise elongation on isometric tension of tracheal muscle was studied in the absence and presence of the bronchoconstrictors carbachol (10(-8) M) and histamine (10(-4) M). In control strips tension increased rapidly with stretch and was followed by stress relaxation which corresponds to creep or bronchial dilatation. In histamine- and carbachol-treated strips a reactive contraction with a rhythmic pattern interrupted the process of stress relaxation. These responses appeared after only 20% elongation and were characteristic of a myogenic contraction which in the in vivo situation would correspond to a bronchoconstriction. These findings are interpreted as a functional transformation of multiple- to single-unit smooth muscle due to the influence of carbachol and histamine. This suggests that stretching (DI) of bronchial smooth muscle in the presence of carbachol and histamine induces a protracted myogenic contraction, which may explain bronchoconstriction after DI in severe asthma.

Assessment of airway tone in asthma. Comparison between double lung transplant patients and healthy subjects

We investigated the hypothesis that asthmatic patients have an increased cholinergic tone by measuring tracheobronchial cross-sectional areas during transient voluntary apnea. This allowed us to assess bronchomotor tone without the influence of changes in lung recoil or lung volume. Three groups of subjects with potentially different levels of tracheobronchial tone were studied: 14 healthy volunteers (N), 18 stable asthmatic patients (A), and 10 double lung transplant recipients (T). Using the acoustic reflection technique, we measured changes in tracheobronchial cross-sectional areas during short periods (5 to 10 s) of voluntary apnea. In a subset of subjects, studies were repeated before and after the inhalation of the muscarinic antagonist ipratropium. During breath-holding, glottis and extrathoracic trachea remained unchanged but intrathoracic tracheal area decreased by 30 +/- 8% (mean +/- standard error of the mean) in N, by 27 +/- 3% in A, and by 9 +/- 4% in the T group. Bronchial areas decreased by 24 +/- 8% in N, by 45 +/- 3% in A, and by 10 +/- 4% in T. These differences among groups were statistically significant at the tracheal and bronchial levels (p < 0.05), and ipratropium significantly inhibited this airway constriction (p < 0.05) only in the asthmatic group. Assuming that changes in cross-sectional airway areas voluntary apnea reflect airway tone, these results support the view that in humans this tone is mainly vagally controlled and that it is significantly increased in asthmatic compared with nonasthmatic subjects.

Resistance to stretch, [Ca2+]i, and activation of swine arterial smooth muscle

Rapid stretching of smooth muscle induces a large increase in stress (resistance to stretch), and stress gradually decreases to intermediate values (stress relaxation). This study elucidated whether [Ca2+]-dependent crossbridge activation or passive mechanical structures were responsible for resistance to stretch and stress relaxation in swine carotid media, a tissue that does not exhibit a myogenic response. Tissues were equilibrated at the optimal length for stress development (L0) and loaded with aequorin to estimate myoplasmic [Ca2+]. In tissues activated with contractile agonists, both resistance to stretch and the rate of stress relaxation appeared to correlate best with the stress present before stretch. Stretch-induced [Ca2+] transients had no major role in determining resistance to stretch or stress relaxation. In the unstimulated swine carotid, resistance to stretch was only slightly reduced and stress relaxation not affected by removal of extracellular Ca2+, suggesting that resistance to stretch and the rate of stress relaxation in unstimulated tissues was predominantly dependent on the passive components of smooth muscle rather than attached, Ca(2+)-dependent crossbridges. Incubation of tissues with tetraethylammonium ion had no measurable effect on stretch-induced [Ca2+] transients but increased resistance to stretch, suggesting that stretch-induced myogenic contractions may be mediated by an increase in the sensitivity of the contractile apparatus to [Ca2+]. Despite the ability of stretch to produce substantial increases in myoplasmic [Ca2+], the contractile apparatus of swine carotid is quite insensitive to the stretch-induced [Ca2+] elevations.

Effects of tetraethylammonium and 4-aminopyridine on outward currents and excitability in canine tracheal smooth muscle cells

1. The effects of tetraethylammonium (TEA) and 4-aminopyridine (4-AP) on membrane currents and on single channel K currents in smooth muscle cells isolated from canine trachea were examined by use of tight seal whole cell- and patch-clamp techniques. 2. Depolarizing current applied through a recording pipette did not elicit an action potential under current clamp. A strong outward rectification was observed. 3. In most cells under voltage-clamp, only an outward current was observed upon depolarization from -60 mV when a pipette solution contained mainly KCl. The outward current consisted of three components; a large initial transient, a following sustained component and an additional component of irregular small transients on the sustained one. The two transient components were almost abolished when extracellular and pipette solutions contained 2.2 mM Cd2+ (0 mM Ca2+) and 10 mM EGTA, respectively. The sustained component was well maintained under these conditions. 4. TEA at low concentrations (less than 1 mM) effectively decreased the transient components and made the outward current smooth; it also suppressed the sustained component at higher concentrations. In outside-out patches, external 1 mM TEA reduced the single channel conductance of Ca-activated K channels by about 87% whereas 3 mM 4-AP did not. 4-AP at low concentrations (less than 3 mM) selectively reduced the sustained component of the outward current. 5. A Ca current recorded after the suppression of outward current by internal Cs+ had a peak of approximately 200 pA at +10 mV (holding potential: -60 mV). The half inactivation voltage in the steady-state was approximately -30 mV. 6. Simultaneous application of 1 mM TEA and 4-AP reduced the outward current and unmasked a Ca current. Under these conditions, an action potential with overshoot was easily elicited under current clamp. 7. It is concluded that the low excitability of canine tracheal smooth muscle cell upon depolarization is due to a large outward K current which consists of Ca-dependent and Ca-independent components. The peak amplitude of the Ca current is similar to that in highly excitable smooth muscle cells such as those of the ureter.

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

1. Tissue taken at operation was used to study the electrical and mechanical properties of human bronchial smooth muscle with intracellular microelectrodes and isometric recording of tension changes. 2. Over 90% of the muscle strips exhibited spontaneous tone and 70% produced spontaneous phasic contractions. The resting membrane potential of the smooth muscle cells ranged between -40 to -50 mV with a mean value of -44.9 +/- 5.2 mV (n = 92 +/- s.d.). Spontaneous oscillations of the membrane potential (slow waves) were observed in 90% of the cells examined. 3. The electrical slow waves, phasic contractions and spontaneous tone were greatly reduced by FPL 55712 (10(-6)-10(-5) M). Indomethacin (1-5 x 10(-5) M), atropine (10(-6) M) or a 5-lipoxygenase inhibitor, AA 861 (5 x 10(-6) M) each reduced spontaneous mechanical tone. Indomethacin and atropine each caused minor reduction in the amplitude of electrical slow waves. 4. Leukotriene C4 (10(-8) M), physostigmine (10(-6) M) and K+-rich physiological salt-solution (containing atropine 10(-6) M) each caused tone development in tissue treated with AA 861 (5 x 10(-6) M). In the case of leukotriene C4 and physostigmine, phasic contractions were superimposed on the developed tone. 5. Electrical field stimulation evoked an excitatory junction potential (e.j.p.) followed by a small group of slow waves. Repetitive field stimulation (2-20 stimuli at 20 Hz) markedly enhanced the amplitude of oscillatory slow waves. FPL 55712 (1.9 x 10(-6) M) abolished the oscillatory slow waves following the e.j.p., and physostigmine (10(-6) M) enhanced the amplitude of the e.j.p. and slow waves. 6. These results indicate that, under in vitro conditions, the electrical activity of human bronchial smooth muscle comprises slow wave discharge which may be related to the spontaneous contractions and generation of basal tone.

The effect of ouabain on tension in isolated respiratory tract smooth muscle of humans and other species

1. The Na+,K+-pump has been implicated in animal models of airway hyperreactivity. We examined the effects of inhibiting the Na+,K+-pump and Na+,Ca2+-exchange on isometric tone of isolated trachealis from humans and other species. 2. In preparations from 5 out of 9 humans, strong spontaneous contractions (36-48 h-1; up to 1.8 g) developed within 25 min. 3. Ouabain (10(-7)-10(-5) M) caused an immediate and sustained contraction. This response was not blocked by atropine, diphenhydramine, or cimetidine. 4. Contractions were also elicited when the normal physiological solution was changed to a K+-free solution, a procedure which inhibits the Na+,K+-pump, and in reduced (15 mM) Na+ solution, which inhibits Na+,Ca2+ exchange. 5. In preparations of dog and guinea-pig isolated trachea, ouabain (10(-5) M) caused a multiphasic response; in the rabbit, ouabain was without effect. K+-free solution was without effect in the dog preparations and produced relaxation of the guinea-pig trachea. Guinea-pig tracheae responded to a low Na+ solution with a strong contraction. 6. Our findings indicate that: (a) human airway smooth muscle may be a spontaneously contracting muscle, at least in vitro, (b) a prolonged contraction to ouabain is unique for the human airway smooth muscle among the animals tested, as is the contraction in a K+-free medium, and (c) the contractile response does not involve acetylcholine or histamine release, but may involve a Na+,Ca2+-exchange mechanism. These results suggest that the level of Na+,K+-pump activity could play a role in determining the degree of bronchomotor tone in humans.

Myosin light chain phosphorylation during phasic contractions of tracheal smooth muscle

Rapid, coordinated contractions of tracheal smooth muscle were elicited by either direct electrical depolarization of muscle cells or treatment with tetraethylammonium which produced spontaneous phasic contractile activity. Both types of contraction were blocked by the calcium channel antagonist verapamil, indicating that these contractions are supported primarily by calcium of extracellular origin. With direct electrical stimulation, force was biphasic and phosphate content of the phosphorylatable light chain (P-light chain) of myosin increased rapidly (approximately 2.5 s) from 0.1 to 0.4 mol phosphate/mol P-light chain, then decreased to levels above resting values. Phosphorylation increased more rapidly than force. Under conditions of spontaneous activity, phasic contractions occurred above a level of basal tone significantly greater than resting force, and minimum values of phosphorylation measured at the base of contraction were significantly greater than those observed in the resting muscle. Phosphorylation oscillated with force (from 0.2 to 0.4 mol phosphate/mol P-light chain) and peak values occurred during the rising phase of contraction. Time courses of phosphorylation and force showed evidence of a prolonged state of activation of myosin following dephosphorylation. These results suggest that phosphorylation and dephosphorylation of myosin P-light chain are sufficiently rapid to participate in regulation of contractility during phasic mechanical activity.

Some properties of calcium-induced contraction in the isolated human and guinea-pig tracheal smooth muscle

We studied the effects of the cyclooxygenase inhibitors, indomethacin and aspirin, the leukotriene antagonist, FPL 55712 and the Ca antagonist, verapamil on basal tone and Ca-induced contractions in the human and guinea-pig tracheal muscle. Indomethacin and aspirin usually increased spontaneous tone or Ca-induced contractions in human strips, while consistently decreased tension development in the guinea-pig muscle. FPL 55712 strongly reduced contractions in the human, whereas it had a very weak effect on the guinea-pig muscle. Verapamil had a small inhibitory effect in the human trachea, either at 5.9 or 40 mM K, but markedly suppressed Ca-induced contractions of guinea-pig trachea in 40 mM K and had little effect on contractions in 5.9 mM K in this tissue. It is concluded that active tone in the two different tracheal muscles were controlled by different processes; that different arachidonate by-products may exert an effect on basal tone in the two different muscles; and that different plasma membrane Ca channels may be operative under basal conditions in the different muscles.

Effect of histamine on the length-tension properties of guinea pig trachealis muscle

The isotonic length-tension properties of excised live guinea pig trachealis muscle have been determined as a function of histamine concentration. Over the range of histamine concentrations studied, a family of curves corresponding to the total and active tensions were obtained. Reciprocal plots of the tension-concentration data revealed a length dependence of the affinity constant of histamine for its receptor The tensions and lengths below resting were considered to be of special importance because of the role of muscle contraction in airway closure. A mechanochemical model for shortening and reelongation of the trachealis muscle during anaphylaxis has been proposed on the basis of this data. By comparing the force-length properties of the muscle at the lowest histamine concentration, it was suggested that the formation of a histamine-Ca++ receptor complex causes an increase in the rigidity of the muscle membrane. The complete range of lengths, tensions, and concentrations must be employed in order to propose a mechanism of action from mechanical data alone.

Mechanical activities of trachea as measured in vitro and in vivo

The phenomenon of spontaneous contractions as measured in vivo and in vitro (isolated guinea pig trachea) is described. In vitro contractility is defined in terms of active tension (AT), the maximum rate of tension development (dT/dt) and time to peak tension (TTP). The length-tension relationship was established for spontaneous contractions of isolated guinea pig trachea. The effect of the temperature on spontaneous activities was also studied. An increase in temperature from 37.5 to 41.5C produced an increase in the maximum rate of tension development (dT/dt( and a decrease in the time to peak tension (TTP). A comparable decrease in temperature, from 37.5 to 33.5C produced slight but not statistically significa-nt increases in active tension, dT/dt and TTP. Frequency of the spontaneous contractions varied directly with changes in temperature. Employing a micro-strain-gauge transducer a rhytmic change of tracheal diameter has been observed in vivo in both guinea pigs and rabbits. Administration of atropine and isoproterenol in those experiments transiently abolished the appearance of these rhythmic changes of tracheal diameter.