Airway responsiveness depends on the diffusion rate of methacholine across the airway wall

Abstract

Bates JH, Stevenson CA, Aliyeva M, Lundblad LK. Airway responsiveness depends on the diffusion rate of methacholine across the airway wall. J Appl Physiol 112: 1670 -1677, 2012. First published March 1, 2012 doi:10.1152/japplphysiol.00703.2011.-During methacholine challenge tests of airway responsiveness, it is invariably assumed that the administered dose of agonist is accurately reflected in the dose that eventually reaches the airway smooth muscle (ASM). However, agonist must traverse a variety of tissue obstacles to reach the ASM, during which the agonist is subjected to both enzymatic breakdown and removal by the bronchial and pulmonary circulations. This raises the possibility that a significant fraction of the deposited agonist may never actually make it to the ASM. To understand the nature of this effect, we measured the time course of changes in airway resistance elicited by various durations of methacholine aerosol in mice. We fit to these data a computational model of a dynamically contracting airway responding to agonist that diffuses through an airway compartment, thereby obtaining rate constants that reflect the diffusive barrier to methacholine. We found that these barriers can contribute significantly to the time course of airway narrowing, raising the important possibility that alterations in the diffusive barrier presented by the airway wall may play a role in pathologically altered airway responsiveness. airway resistance; hyperresponsiveness; computational model; mouse model A VARIETY OF DISPARATE MECHANISMS have been invoked to explain measurements of airway hyperresponsiveness (AHR) (2,7,11). Virtually all, however, make the assumption that the administered dose of agonist is accurately reflected in the dose that eventually reaches the airway smooth muscle (ASM). In fact, this is not the case. It is well appreciated, for example, that the fraction of inhaled agonist that is deposited on the airway walls and the sites of deposition are markedly influenced by the pattern of respiratory flow, droplet size, and the geometry of the airway tree (20,22). These can both change substantially as bronchoconstriction proceeds. Indeed, it has been suggested that the response elicited by aerosol challenge is self-limited due to the fact that, when airways become severely narrowed, they prevent further accumulation of agonist (10).Reaching the airway wall, however, is not the only challenging leg of an agonist's journey to the ASM. Once impaction has occurred, the agonist must traverse layers of fluid and mucus, epithelium, basement membrane, connective tissue, and interstitium to reach the ASM and elicit a response. During this diffusive transport phase, the agonist is subjected to both enzymatic breakdown and removal by the bronchial and pulmonary circulations (15,16,26,27). This raises the possibility that a significant fraction of the deposited agonist may never actually make it to the ASM, which, in turn, suggests that changes in the diffusive barrier of the airway wall will affect airway responsiven...

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“…The level of ASM activation in vivo depends on the quantity of spasmogen reaching ASM and acting on it simultaneously. This, in turn, depends on the localized spasmogen de position, the permeability of the epithelium and the lamina propria, as well as the rate of clearance from both catabolism and blood circulation [19]. The total level of ASM activation in vivo is also influence by the presence and the concentration of all the endogenous constrictors and dilators acting on the muscle.…”

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