In healthy subjects Deep Inspiration (DI) transiently dilates the airways, while many asthmatics show bronchoconstriction by a mechanism which is incompletely understood.I investigated how the method of assessment affects the response. The response as measured by specific airway conductance (SGaw) appeared to contradict that measured by forced expiration. This led to the formulation of a novel hypothesis to explain the asthmatic bronchoconstrictor response: That the negative intra-thoracic pressure associated with DI may temporarily increase airway oedema and thus reduce lumenal diameter. This was tested by comparing the effects of non-forced with forced inspiration (through resistance). In the asthmatic group, forced inspiration produced significantly more bronchoconstriction. Airway hyperresponsivenessin asthma has been attributed to impaired ability of DI to stretch airway smooth muscle. The seminal study `confirming' this, I argue, is flawed. I have re-tested the hypothesis. The asthmatic response was significantly greater than the control response even when DI was prohibited. Asthmatic hyperresponsivenessis therefore not attributable entirely to an abnormal asthmatic response to DI. Many asthmatics display an apparent capacity for unlimited airway narrowing in response to bronchial challenge; most healthy subjects demonstrate a maximal (limited) response. The maximal response measured by a DI independent index represented a greater % change from baseline than the maximal established by a DI dependent index. This suggested some bronchoprotection resulting from DI but also the existence of a distinct mechanism which ultimately limited narrowing. I reasoned that the capacity for unlimited airway narrowing is most likely a function of smaller airways. I investigated indices of small airway function and found they predicted the ultimate response much earlier in challenge than FEV 1, suggesting a possible practical test of the capacity for unlimited narrowing. I postulate that the clearly established but limited relationship between the responses to DI and bronchial challenge may reflect the dependence of the response to DI on the degree of inflammation within the airway wall whereas the response to challenge may be determined by its overall thickness.

Morbidity and mortality attributable to asthma arise mainly from contraction of airway smooth muscle (ASM) and resulting bronchospasm. Bronchospasm that is induced in the laboratory is easily reversed by a spontaneous deep inspiration (DI) whereas bronchospasm that occurs spontaneously in asthma is not. In response to a spontaneous DI, contracted ASM fluidizes rapidly and then resolidifies slowly, but molecular mechanisms accounting for these salutary bronchodilatory responses -and their dramatic breakdown in asthma- are unknown. Using a multi-scale approach, I show here that both the baseline contractile force and the fluidization response of ASM are independent of the cytoskeletal protein zyxin, whereas the resolidification response is zyxin-dependent. At the levels of the stress fiber, the isolated cell, and the integrated airway, zyxin acts to stabilize the contractile apparatus and promote the resolidification response. More than just the motor of contraction, ASM is thus viewed in the broader context of a self-healing active material wherein resolidification and its molecular determinants contribute to the biology of bronchospasm.

Airway hyperresponsiveness (AHR) is a hallmark of asthma where constriction of airway smooth muscle (ASM) causes excessive airway narrowing. Asthmatics, unlike healthy subjects, cannot prevent or reverse this narrowing by stretching their airways with a deep inspiration (DI). Since stretching of isolated ASM causes dramatic reductions in force generation and asthmatics tend to have stiffer airways, researchers hypothesize that reduced ASM stretching during breathing and DIs results in hyperreactive airways. However, counterintuitively, excised measurement on intact airways show narrowing is minimally reversed by pressure oscillations simulating breathing and DIs. We hypothesized that AHR does not result from reduced capacity to stretch the airways; furthermore, each constituent of the airway wall experiences different strain magnitude during breathing and DIs. To test this, we used an intact airway system which controls transmural pressure (Ptm) to simulate breathing while measuring luminal diameter in response to ASM agonists. An ultrasound system and automated segmentation algorithm were implemented to quantify and compare the ability of Ptm fluctuations to reverse and prevent narrowing in larger (diameter=5.72±0.52mm) relative to smaller airways (diameter=2.92±0.29mm). We found the ability of Ptm oscillations to reverse airway narrowing was proportional to strain imposed on the airway wall. Further, tidal-like breathing Ptm oscillations (5-15cmH2O) after constriction imposed 196% more strain in smaller compared to larger airways (14.6% vs. 5.58%), resulting in 76% greater reversal of narrowing (41.2% vs. 23.4%). However, Ptm oscillations applied before and during constriction resulted in the same steady-state diameter as when Ptm oscillations were applied only after constriction. To better understand these results, we optimized an ultrasound elastography technique utilizing finite element-based image registration to estimate spatial distributions of displacements, strains, and material properties throughout an airway wall during breathing and bronchoconstriction. This required we formulate and solve an inverse elasticity problem to reconstruct the distribution of nonlinear material properties. Strains and material properties were radially and longitudinally heterogeneous, and patterns and magnitudes changed significantly after induced narrowing. Taken together, these data show AHR likely does not emerge due to reduced straining of airways prior to challenge, but remodeling that stiffens airway walls might serve to sustain constriction during an asthmatic-like attack.

No description available.

<p>Asthma is a disease characterized by transient airway smooth muscle contraction leading to episodes of reversible airway narrowing. It affects over 300 million people worldwide and is implicated in over 250 000 deaths annually. The primary clinical features of asthma include airway inflammation, hyperresponsiveness, and remodeling. Generally, asthmatic patients experience exacerbations between periods of diminished symptoms. Interestingly, in addition to these above mentioned hallmarks, asthmatics have also been shown to react differently to ventilatory mechanical strain. This is most evident when assessing the effect of a deep inspiration (DI), clinically measured as a breath taken from functional residual capacity to total lung capacity, in healthy individuals <em>versus</em> asthmatics. These deep inspiratory efforts have been shown to produce a bronchodilatory response in healthy individuals, whereas in asthmatics, DIs are less effective in producing bronchodilation, can cause more rapid airway re-narrowing, and even bronchoconstriction in moderate to severe asthmatics. The mechanism by which a DI is able to cause bronchoconstriction remains ambiguous. Previous theories suggest that this phenomenon is intrinsic to airway smooth muscle (ASM) itself. However, the airway inflammation present in asthmatic airways may also add to the increased ASM contractility following stretch, by the release of mediators that can prime the contractile apparatus to react excessively in the presence of stretch.</p> <p>Thus, collectively, the studies contained in this thesis are linked to the general theme of greater characterization of the signalling mechanisms that regulate airway stretch-activated contractions using a pharmacological approach in intact bovine bronchial segments, with the hope of providing novel insights into the mechanisms that regulate the DI-induced bronchoconstriction seen in asthmatics.</p> / Doctor of Philosophy (Medical Science)