Abstract
Airway diseases such as asthma are triggered by inflammation and mediated by proinflammatory cytokines such as tumor necrosis factor alpha (TNFα). Our goal was to systematically examine the potential mechanisms underlying the effect of TNFα on airway smooth muscle (ASM) contractility. Porcine ASM strips were incubated for 24 h with and without TNFα. Exposure to TNFα increased maximum ASM force in response to acetylcholine (Ach), with an increase in ACh sensitivity (hyperreactivity), as reflected by a leftward shift in the dose–response curve (EC50). At the EC50, the [Ca2+]cyt response to ACh was similar between TNFα and control ASM, while force increased; thus, Ca2+ sensitivity appeared to increase. Exposure to TNFα increased the basal level of regulatory myosin light chain (rMLC) phosphorylation in ASM; however, the ACh‐dependent increase in rMLC phosphorylation was blunted by TNFα with no difference in the extent of rMLC phosphorylation at the EC50 ACh concentration. In TNFα‐treated ASM, total actin and myosin heavy chain concentrations increased. TNFα exposure also enhanced the ACh‐dependent polymerization of G‐ to F‐actin. The results of this study confirm TNFα‐induced hyperreactivity to ACh in porcine ASM. We conclude that the TNFα‐induced increase in ASM force, cannot be attributed to an enhanced [Ca2+]cyt response or to an increase in rMLC phosphorylation. Instead, TNFα increases Ca2+ sensitivity of ASM force generation due to increased contractile protein content (greater number of contractile units) and enhanced cytoskeletal remodeling (actin polymerization) resulting in increased tethering of contractile elements to the cortical cytoskeleton and force translation to the extracellular matrix.
Highlights
A methacholine (MCh) challenge test is usually used to help diagnose asthma, which is associated with an increase in airway smooth muscle (ASM) sensitivity to muscarinic stimulation and/or with increased ASM force response to muscarinic stimulation
Force was normalized to Fmax to determine the EC50 for ACh, which was ~twofold lower in tumor necrosis factor alpha (TNFa)-treated ASM strips compared controls (P = 0.001, n = 6; Figs. 2B, 3B)
The results of the present study showed that exposing intact porcine ASM to TNFa for 24 h induces both increased sensitivity to ACh stimulation and an increased force response to ACh
Summary
A methacholine (MCh) challenge test is usually used to help diagnose asthma, which is associated with an increase in airway smooth muscle (ASM) sensitivity to muscarinic stimulation (hypersensitivity, shift in the EC50) and/or with increased ASM force response to muscarinic stimulation (hypercontractility). ASM hyperreactivity is used to characterize asthmatic ASM but is not well defined It is not always clear whether ASM hyperreactivity refers to hypersensitivity and/or hypercontractility to muscarinic stimulation. Airway inflammation is a common feature of chronic lung diseases such as asthma and is associated with increased ASM contractility (Martin et al, 2000; Prakash, 2013; West et al, 2013; Wright et al, 2013). Physiological Reports published by Wiley Periodicals, Inc. on behalf of The Physiological Society and the American Physiological Society
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