Maturational changes in airway smooth muscle shortening and relaxation. Implications for asthma

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.

Efficiency of physiotherapy with Caycedian Sophrology on children with asthma: A randomized controlled trial

BACKGROUND

Asthma is the most common chronic disease in pediatrics. Along with the usual drug therapy using corticosteroids and bronchodilators, some interest has been shown for adjuvant therapies, such as sophrology. However, the level of evidence for non-pharmaceutical therapies in asthma remains low, especially in children. This study aimed to assess whether in children with asthma, peak expiratory flow (PEF) improved more after a sophrology session alongside standard treatment than after standard treatment alone.

METHODS

We carried out a prospective randomized controlled clinical trial among 74 children aged 6-17 years old, hospitalized for an asthma attack. Group 1: conventional treatment (oxygen, corticosteroids, bronchodilators, physiotherapy) added to one session of sophrology. Group 2: conventional treatment alone. The primary outcome was the PEF variation between the initial and final evaluations (PEF₂ -PEF₁ ).

RESULTS

Demographic and clinical characteristics were similar in both groups at baseline. Measures before and after the sophrology session showed that the PEF increased by mean 30 L/min in the sophrology group versus 20 L/min in the control group (P = 0.02). Oxygen saturation increased by 1% versus 0% (P = 0.02) and the dyspnea score with visual analogue scale improved by two points point (P = 0.01). No differences were observed between the two groups in terms of duration of hospitalization, use and doses of conventional medical treatment (oxygen, corticosteroids, and bronchodilators), and quality of life scores.

CONCLUSIONS

Sophrology appears as a promising adjuvant therapy to current guideline-based treatment for asthma in children.

Incidence and characteristics of early childhood wheezing, Dhaka, Bangladesh, 2004-2010

BACKGROUND

Early childhood wheezing substantially impacts quality of life in high-income countries, but data are sparse on early childhood wheezing in low-income countries. We estimate wheezing incidence, describe wheezing phenotypes, and explore the contribution of respiratory viral illnesses among children aged <5 years in urban Bangladesh.

METHODS

During 2004-2010, respiratory illness surveillance was conducted through weekly home visits. Children with fever or respiratory illness were referred for examination by study physicians including lung auscultation. During 2005-2007, every fifth referred child had nasal washes tested for human metapneumovirus, respiratory syncytial viruses, and influenza and parainfluenza viruses.

RESULTS

During April 2004-July 2010, 23,609 children were enrolled in surveillance. Of these, 11,912 (50%) were male, median age at enrollment was 20 months (IQR 5-38), and 4,711 (20%) had ≥1 wheezing episode accounting for 8,901 episodes (733 [8%] associated with hospitalization); 25% wheezed at <1 year of age. Among children aged <5 years, incidences of wheezing and wheezing hospitalizations were 2,335/10,000 and 192/10,000 child-years. Twenty-eight percent had recurrent wheezing. Recurrent versus non-recurrent wheezing episodes were more likely to be associated with oxygen saturation <93% (OR 6.9, 95%CI 2.8-17.3), increased work of breathing (OR 1.6, 95%CI 1.4-1.8), and hospitalization (OR 2.0, 95%CI 1.6-2.4). Respiratory viruses were detected in 66% (578/873) of episodes with testing.

CONCLUSION

In urban Bangladesh, early childhood wheezing is common and largely associated with respiratory virus infections. Recurrent wheezing is associated with more severe illness and may predict children who would benefit most from closer follow-up and targeted interventions. Pediatr Pulmonol. 2016;51:588-595. © 2015 Wiley Periodicals, Inc.

Smooth muscle in human bronchi is disposed to resist airway distension

Studying airway smooth muscle (ASM) in conditions that emulate the in vivo environment within which the bronchi normally operate may provide important clues regarding its elusive physiological function. The present study examines the effect of lengthening and shortening of ASM on tension development in human bronchial segments. ASM from each bronchial segment was set at a length approximating in situ length (Linsitu). Bronchial tension was then measured during a slow cyclical strain (0.004Hz, from 0.7Linsitu to 1.3Linsitu) in the relaxed state and at graded levels of activation by methacholine. In all cases, tension was greater at longer ASM lengths, and greater during lengthening than shortening. The threshold of methacholine concentration that was required for ASM to account for bronchial tension across the entire range of ASM lengths tested was on average smaller by 2.8 logs during lengthening than during shortening. The length-dependency of ASM tension, together with this lower threshold of methacholine concentration during lengthening versus shortening, suggest that ASM has a greater ability to resist airway dilation during lung inflation than to narrow the airways during lung deflation. More than serving to narrow the airway, as has long been thought, these data suggest that the main function of ASM contraction is to limit airway wall distension during lung inflation.

Vitamin D deficiency and exercise-induced laryngospasm in young competitive rowers

Exercise-induced dyspnea is common among adolescents and young adults and often originates from exercise-induced bronchoconstriction (EIB). Sometimes, dyspnea corresponds to exercise-induced laryngospasm (EILO), which is a paradoxical decrease in supraglottic/glottic area. Vitamin D deficiency, which occurs frequently at northern latitudes, might favor laryngospasm by impairing calcium transport and slowing striate muscle relaxation. The aim of this study was to evaluate whether vitamin D status has an influence on bronchial and laryngeal responses to exercise in young, healthy athletes. EIB and EILO were investigated during winter in 37 healthy competitive rowers (24 males; age range 13-25 years), using the eucapnic voluntary hyperventilation test (EVH). EIB was diagnosed when forced expiratory volume in the first second decreased by 10%, EILO when maximum mid-inspiratory flow (MIF50) decreased by 20%. Most athletes (86.5%) had vitamin D deficiency (below 30 ng/mL), 29 mild-moderate (78.4%) and 3 severe (8.1%). EVH showed EIB in 10 subjects (27%), EILO in 16 (43.2%), and combined EIB and EILO in 6 (16.2%). Athletes with EILO had lower vitamin D (19.1 ng/mL vs. 27.0 ng/mL, p < 0.001) and higher parathyroid hormone (30.5 pg/mL vs. 19.2 pg/mL, p = 0.006) levels. The degree of laryngoconstriction (post-EVH MIF50 as a percentage of pre-EVH MIF50) was related directly with vitamin D levels (r = 0.51; p = 0.001) and inversely with parathyroid hormone levels (r = -0.53; p = 0.001). We conclude that vitamin D deficiency is common during winter in young athletes living above the 40th parallel north and favors laryngospasm during exercise, probably by disturbing calcium homeostasis. This effect may negatively influence athletic performance.

Ovalbumin sensitization of guinea pig at birth prevents the ontogenetic decrease in airway smooth muscle responsiveness

Airway smooth muscle (ASM) displays a hyperresponsive phenotype at young age and becomes less responsive in adulthood. We hypothesized that allergic sensitization, which causes ASM hyperresponsiveness and typically occurs early in life, prevents the ontogenetic loss of the ASM hyperresponsive phenotype. We therefore studied whether neonatal allergic sensitization, not followed by later allergen challenges, alters the ontogenesis of ASM properties. We neonatally sensitized guinea pigs to ovalbumin and studied them at 1 week, 3 weeks, and 3 months (adult). A Schultz‐Dale response in isolated tracheal rings confirmed sensitization. The occurrence of inflammation was evaluated in the blood and in the submucosa of large airways. We assessed ASM function in tracheal strips as ability to produce force and shortening. ASM content of vimentin was also studied. A Schultz‐Dale response was observed in all 3‐week or older sensitized animals. A mild inflammatory process was characterized by eosinophilia in the blood and in the airway submucosa. Early life sensitization had no effect on ASM force generation, but prevented the ontogenetic decline of shortening velocity and the increase in resistance to shortening. Vimentin increased with age in control but not in sensitized animals. Allergic sensitization at birth without subsequent allergen exposures is sufficient to prevent normal ASM ontogenesis, inducing persistence to adulthood of an ASM hyperresponsive phenotype.

Airway smooth muscle (ASM) displays a hyperresponsive phenotype at young age and becomes less responsive in adulthood. In this study, we found that allergic sensitization at birth without subsequent allergen exposures is sufficient to prevent normal ASM ontogenesis, inducing persistence to adulthood of an ASM hyperresponsive phenotype.

Assessment of exercise-induced bronchoconstriction in adolescents and young children

Recent research shows important differences in exercise-induced bronchoconstriction (EIB) between children and adults, suggesting a different pathophysiology of EIB in children. Although exercise can trigger classic symptoms of asthma, in children symptoms can be subtle and nonspecific; parents, children, and clinicians often do not recognize EIB. With an age-adjusted protocol, an exercise challenge test can be performed in children as young as 3 years of age. However, an alternative challenge test is sometimes necessary to assess potential for EIB in children. This review summarizes age-related features of EIB and recommendations for assessing EIB in young children and adolescents.

An imbalance in C/EBPs and increased mitochondrial activity in asthmatic airway smooth muscle cells: novel targets in asthma therapy?

The asthma prevalence was increasing over the past two decades worldwide. Allergic asthma, caused by inhaled allergens of different origin or by food, is mediated by inflammatory mechanisms. The action of non-allergic asthma, induced by cold air, humidity, temperature or exercise, is not well understood. Asthma affects up to 15% of the population and is treated with anti-inflammatory and muscle relaxing drugs which allow symptom control. Asthma was first defined as a malfunction of the airway smooth muscle, later as an imbalanced immune response of the lung. Recent studies placed the airway smooth muscle again into the focus. Here we summarize the molecular biological basis of the deregulated function of the human airway smooth muscle cell as a cause or important contributor to the pathology of asthma. In the asthmatic human airway smooth muscle cells, there is: (i) a deregulation of cell differentiation due to low levels of maturation-regulating transcription factors such as CCAAT/enhancer binding proteins and peroxisome proliferator-activated receptors, thereby reducing the cells threshold to proliferate and to secrete pro-inflammatory cytokines under certain conditions; (ii) a higher basal energy turnover that is due to increased number and activity of mitochondria; and (iii) a modified feedback mechanism between cells and the extracellular matrix they are embedded in. All these cellular pathologies are linked to each other and to the innate immune response of the lung, but the sequence of events is unclear and needs further investigation. However, these findings may present the basis for the development of novel curative asthma drugs.

Reduced spontaneous relaxation in immature guinea pig airway smooth muscle is associated with increased prostanoid release

Airway smooth muscle (ASM) from infant guinea pigs has less spontaneous relaxation during stimulation than ASM from adults. Inhibition of cyclooxygenase (COX), which catalyzes the production of prostanoids, increases this relaxation in infant ASM and abolishes age differences, thus suggesting that prostanoids reduce relaxation in infant ASM. In this study, we investigated whether leukotrienes are also involved in reducing spontaneous relaxation; whether the two COX isoforms, COX-1 and COX-2, differentially regulate spontaneous relaxation; and whether prostanoid release is developmentally regulated in guinea pig ASM. In different age groups, we measured relaxation during and after electrical stimulation in tracheal strips as well as prostanoid release from tracheal segments. Relaxation was studied in the absence and in the presence of a lipoxygenase inhibitor, a cysteinyl leukotriene receptor-1 antagonist, a COX-1 inhibitor, or a COX-2 inhibitor. We found that inhibition of lipoxygenase or cysteinyl leukotriene receptor-1 antagonism did not increase spontaneous relaxation at any age, thus excluding a role for leukotrienes in this phenomenon. Inhibition of COX-2, but not COX-1, promoted spontaneous relaxation. The basal release of prostanoids was more abundant in tissue from infant animals and decreased significantly with age. Thromboxane B2 was the most abundant metabolite released at all ages. Electrical stimulation and epithelium removal did not affect the age difference in prostanoid release. We conclude that increased basal prostanoid release contributes to the reduced spontaneous relaxation in immature guinea pig ASM compared with older animals. By regulating ASM relaxation, prostanoids may play a role in the airway hyperresponsiveness at a young age.

Interleukin-4 and interleukin-13 enhance human bronchial smooth muscle cell proliferation

BACKGROUND

T(H)2 inflammation and bronchial smooth muscle cell (BSMC) hyperplasia are characteristic features of asthma, but whether these phenomena are linked remains unknown. This study aims to define the effect of the T(H)2 cytokines IL-4 and IL-13 on human BSMC proliferation when administered alone or in combination with the fibroblast growth factor 2 (FGF2) growth factor. In addition, the effects of the proinflammatory mediators TNFalpha and IL-1 beta and the involvement of members of the well-known family of platelet-derived growth factor (PDGF) mitogens were tested.

METHODS

BSMC proliferation was measured by crystal violet staining and PDGF and PDGF receptor (PDGFR) expression were determined by RT-PCR, immunocytochemistry, ELISA, flow cytometry and dot plot analysis.

RESULTS

Neither IL-4 nor IL-13 alone induced BSMC proliferation, despite both being potent inducers of PDGF-CC. However, following a pretreatment with FGF2, which increased PDGFR alpha chain expression, both IL-4 and IL-13 increased FGF2-induced BSMC proliferation in a time- and concentration-dependent manner. TNFalpha and IL-1 beta did not affect basal or FGF2-induced BSMC proliferation, but both proinflammatory mediators enhanced the proliferative synergism between FGF2 and the T(H)2 cytokines.

CONCLUSIONS

IL-4 and IL-13 potently induce FGF2-primed BSMC proliferation via an autocrine loop involving PDGFRalpha and PDGF-CC, and this proliferative synergism is amplified by proinflammatory cytokines.

Disruption of NO-cGMP signaling by neonatal hyperoxia impairs relaxation of lung parenchyma

Exposure of immature lungs to hyperoxia for prolonged periods contributes to neonatal lung injury and airway hyperreactivity. We studied the role of disrupted nitric oxide-guanosine 3',5'-cyclic monophosphate (NO-cGMP) signaling in impairing the relaxant responses of lung tissue from hyperoxia-exposed rat pups. Pups were exposed to >/=95% O(2) or room air for 7 days starting from days 1, 5, or 14. The animals were killed, lungs were removed, and 1-mm-thick lung parenchymal strips were prepared. Lung parenchymal strips of room air or hyperoxic pups were preconstricted using bethanechol and then graded electrical field stimulation (EFS) was applied to induce relaxation. EFS-induced relaxation of lung parenchymal strips was greater at 7 and 12 days than at 21 days in room air-exposed rat pups. Hyperoxic exposure significantly reduced relaxation at 7 and 12 days but not 21 days compared with room air exposure. NO synthase blockade with N(omega)-nitro-l-arginine methyl ester diminished relaxant responses in room air but not in hyperoxic pups at 12 days. After incubation with supplemental l-arginine, the relaxation response of hyperoxic strips was restored. cGMP, a key mediator of the NO signaling pathway, also decreased in strips from hyperoxic vs. room air pups and cGMP levels were restored after incubation with supplemental l-arginine. In addition, arginase activity was significantly increased in hyperoxic lung parenchymal strips compared with room air lung parenchymal strips. These data demonstrate disruption of NO-cGMP signaling in neonatal rat pups exposed to hyperoxia and show that bioavailability of the substrate l-arginine is implicated in the predisposition of this model to airway hyperreactivity.

Age differences in cholinergic airway responsiveness in relation with muscarinic receptor subtypes

Children seem more susceptible to increased airway reactivity than adults. Such an age-dependent discrepancy in airway reactivity may involve different airway smooth muscle functions. Therefore, we compared the in vivo and in vitro responsiveness of airway smooth muscles between two age groups of animals. Rats of 6 and 21 weeks old were challenged in vivo with acetylcholine (ACh) infused intravenously and airway resistance (R(aw)) was measured. Tracheal muscle was also isolated and the isometric force developed to ACh or KCl was measured. Furthermore, the level of genes encoding muscarinic receptor subtypes (M(1-3)) and acetylcholinesterase (AChE) expressed in the tracheal muscle was determined by RT-PCR. In results, the basal R(aw) was similar in the two age groups. The R(aw) at each ACh dose was significantly greater in young rats than older rats (p<0.05, n=22-27). Tracheal muscles from young rats were more sensitive to ACh than older rats (p<0.05, n=20-21), while receptor-independent muscle contraction to KCl was greater in older rats (p<0.05, n=10-19). Genes encoding AChE, M(2) and M(3) muscarinic receptors were more highly expressed in the tracheal muscles from young than older rats (p<0.05, n=4-6). In conclusion, airway smooth muscle in young rat is more sensitive to cholinergic stimulation in vivo and in vitro compared to older rats, which may be due to a higher expression of M(2) and M(3) muscarinic receptors in airway smooth muscle.

Soluble guanylate cyclase-dependent relaxation is reduced in the adult rat bronchial smooth muscle

Cyclic nucleotides are relaxants of the airway smooth muscle, yet most of the available data were obtained in adult animals. The expression and activity of cyclases have been reported to be developmentally regulated in the lung, and little is known about the age-related changes in their bronchial muscle relaxation potential. We evaluated and compared the newborn and adult rat bronchial smooth muscle response to cyclic AMP- and GMP-dependent agonists in isometric mounted bronchial rings. In acetylcholine-precontracted bronchial muscle, the relaxant response to the cAMP agonist forskolin was not age dependent, but the relaxant response to the nitric oxide (NO) donor sodium nitroprusside (SNP) was significantly greater (P<0.01) in the newborn. To further evaluate the cGMP pathway, we stimulated the soluble guanylate cyclase (sGC) with the specific agonists BAY 41-2272 and YC-1. In keeping with the SNP dose-response curves, the sGC agonists significantly relaxed the newborn, but not the adult bronchial muscle. Protein expression of the sGC alpha1- and beta1-subunits were significantly lower (P<0.01) in the adult compared with the newborn bronchial tissue. Consistent with these results, the NO-stimulated sGC activity was significantly greater in the newborn compared with the adult (P<0.01). In conclusion, the bronchial smooth muscle cGMP-, but not cAMP-dependent, relaxant response is developmentally regulated and significantly reduced in the adult rat.

Primary prevention of asthma: age and sex influence sensitivity to allergen-induced airway inflammation and contribute to asthma heterogeneity in Guinea pigs

BACKGROUND

Limiting allergen exposure in the sensitization phase has been proposed as a means of primary prevention of asthma, but its effectiveness is debated.

HYPOTHESIS

Primary prevention of asthma is more effective in limiting asthma symptoms in young guinea pigs compared with adults, whether males or females.

METHODS

The following experimental groups were used: young/young, sensitized and challenged before sexual maturity; young/adult, sensitized young and challenged after sexual maturity; adult/adult, sensitized and challenged after sexual maturity. Males and females were sensitized intraperitoneally with varying doses of ovalbumin (OVA) and challenged intratracheally with a constant OVA dose. Cellular infiltration into lung and lavage fluid as well as airway hyperresponsiveness to intravenous methacholine was determined 24 h later.

RESULTS

In unsensitized animals, density of resident inflammatory cells as well as baseline pulmonary function differed with age and sex. Maximum OVA-induced eosinophilia in females occurred at a lower sensitizing dose of OVA than in males, and the slopes of the dose-response relationship differed significantly between sexes. Young females had more pronounced increases in eosinophils compared with some adult treatment groups. The concentrations of OVA-specific antibodies were not directly related to differences in cellular infiltration. Airway hyperresponsiveness to methacholine challenge was observed in all treatment groups.

CONCLUSION

Young animals require major reductions in allergen exposure compared with adults to effectively limit airway inflammation in primary prevention. Heterogeneity of asthma symptoms seen with age and sex suggests that primary prevention by limiting allergen exposure or treatment with anti-inflammatory or bronchodilator drugs may be more effective strategies for specific age and gender populations.

Ontogenesis of myosin light chain phosphorylation in guinea pig tracheal smooth muscle

Increased airway responsiveness occurs in normal young individuals compared to adults. A maturation of airway smooth muscle (ASM) contractility is likely a mechanism of this juvenile airway hyperresponsiveness. Indeed, we showed in guinea pig tracheal smooth muscle (TSM) that maximum shortening velocity decreases dramatically after the first 3 weeks of life. Because the phosphorylation of the 20-kDa myosin light chain (MLC(20)) was shown to be a key event in ASM contractility, in the present work we sought to investigate it during ontogenesis. In three age groups (1-week-old, 3-week-old, and adult guinea pigs), we assessed the amount of MLC(20) phosphorylation achieved either in TSM crude protein homogenates exposed to Mg(2+) . ATP . CaCl(2) or in tracheal strips during electrical field stimulation (EFS). Phosphorylated and unphosphorylated MLC(20) were separated on nondenaturing 10% polyacrylamide gels, and the ratio of phosphorylation was obtained by densitometric analysis of chemiluminescent Western immunoblots. Maximum MLC(20) phosphorylation (% of total MLC(20)) in TSM tissue homogenate was, respectively, 32.6 +/- 5.7, 32.2 +/- 5.7, and 46.8 +/- 5.8 in 1-week, 3-week, and adult guinea pigs. Interestingly, in nonstimulated intact tracheal strips, we found a substantial degree of MLC(20) phosphorylation: respectively, 42.2 +/- 5.8, 36.5 +/- 7.8, and 46.4 +/- 4.7 in 1-week, 3-week, and adult guinea pigs. Maximal EFS-induced MLC(20) phosphorylation (% increase over baseline) in the 3-week age group was attained after 3 sec of EFS, and was 161.2 +/- 17.6, while in 1-week and adult guinea pigs, it was attained at 1.5 sec of EFS and was, respectively, 133.3 +/- 9.3 and 110.2 +/- 3.9 (P < 0.05). We conclude that MLC(20) phosphorylation in guinea pig intact tracheal strips correlates with ontogenetic changes in shortening velocity and changes in myosin light chain kinase content. These results further suggest that the maturation of ASM contractile properties plays a role in the greater airway responsiveness reported in children and young animals.

Smooth muscle development during postnatal growth of distal bronchioles in infant rhesus monkeys

Development of smooth muscle in conducting airways begins early in fetal life. Whereas the pattern and regulation of smooth muscle differentiation are well-defined, the impact of airway growth on the process is not. To evaluate the transformations in organization during postnatal growth, smooth muscle bundle organization (size, abundance, and orientation) was mapped in five generations of distal airways of infant rhesus monkeys (5 days and 1, 2, 3, and 6 mo old). On the basis of direct measurement of the bronchiole proximal to the terminal bronchiole, length increased by 2-fold, diameter by 1.35-fold, and surface area by 2.8-fold between 5 days and 6 mo of age. Smooth muscle bundle size was greater in proximal bronchioles than in respiratory bronchioles and did not change with age. However, relative bundle size decreased in proportion to airway size as the airways grew. Relative bundle abundance was constant regardless of airway generation or age. The distribution of smooth muscle bundle orientation changed with age in each airway generation, and there were significant changes in the terminal and respiratory bronchioles. We conclude that smooth muscle undergoes marked organizational changes as airways grow during postnatal development.

Ontogenesis of myosin light chain kinase mRNA and protein content in guinea pig tracheal smooth muscle

We previously reported in guinea pig tracheal smooth muscle that maximal shortening velocity decreases from 3 weeks of age to adulthood. It is not known whether myosin light chain kinase (MLCK), a key enzyme determining the velocity of smooth muscle contraction, undergoes maturational changes. In the present work, we investigated MLCK protein content and mRNA expression in 1-week-old, 3-week-old, and adult guinea pigs. We extracted either proteins or RNA from isolated tracheal smooth muscle. The content of MLCK was assessed by Western immunoblots. MLCK mRNA was evaluated by Northern analysis and by quantitative real time reverse transcriptase-polymerase chain reaction (RT-PCR). The content of MLCK increased 3-fold at 3 weeks of age and then decreased in adults, being 0.116 +/- 0.042, 0.330 +/- 0.125 (P < 0.05), and 0.153 +/- 0.054 microg/mg of total protein, respectively, in 1-week, 3-week, and adult animals. Quantitative RT-PCR revealed that MLCK mRNA increased with age to 135 +/- 35% and 177 +/- 23% (P < 0.01) in 3-week and adult animals, respectively, compared to 1-week animals. The transient increase of MLCK content in juvenile guinea pig tracheal smooth muscle may contribute to the increased shortening velocity at this age. We suggest that this increased content of MLCK is one of the mechanisms leading to maturation of airway smooth muscle contractility, which in turn contributes to the airway hyperresponsiveness reported in children and young animals.

Cardiotrophin-1 alters airway smooth muscle structure and mechanical properties in airway explants

Induction of hypertrophy and inhibition of apoptosis may be important mechanisms contributing to increased airway smooth muscle (ASM) mass in asthma. Data from our laboratory indicate that cardiotrophin-1 (CT-1) induces hypertrophy and inhibits apoptosis in isolated human ASM cells. To determine whether these novel effects of CT-1 also occur in the airway tissue milieu and to determine whether structural changes are accompanied by functional changes, matched pairs of guinea pig airway explants were treated with or without CT-1 for 7 days, and structural features as well as isometric and isotonic contractile and relaxant mechanical properties were measured. CT-1 (0.2-5 ng/ml) increased both myocyte mass and extracellular matrix in a concentration-dependent fashion. CT-1 (10 ng/ml)-treated tissues exhibited a significant increase in passive tension at all lengths on day 7; at optimal length, passive tension generated by CT-1-treated tissues was 1.72 +/- 0.12 vs. 1.0 +/- 0.1 g for control. Maximal isometric stress was decreased in the CT-1-treated group on day 7 (0.39 +/- 0.10 kg/cm(2)) vs. control (0.77 +/- 0.15 kg/cm(2), P < 0.05). Isoproterenol-induced relaxant potency was reduced in CT-1-treated tissues, log EC(50) being -7.28 +/- 0.34 vs. -8.12 +/- 0.25 M in control, P < 0.05. These data indicate that CT-1 alters ASM structural and mechanical properties in the tissue environment and suggest that structural changes found in the airway wall in asthma are not necessarily associated with increased responsiveness.