Repeated allergen inhalation induces cytoskeletal remodeling in smooth muscle from rat bronchioles

Airway hyperresponsiveness (AHR) is associated with airway wall structural remodeling and alterations in airway smooth muscle (ASM) function. Previously, in bronchioles from Brown Norway rats challenged by repeated ovalbumin (OVA) inhalation, we have reported increased force generation and depletion of smooth muscle contractile proteins. Here, we investigated if cytoskeletal changes in smooth muscle could account for this paradox. Sensitized rats (n = 5/group) were repeatedly challenged with OVA or saline, and the lungs were removed 24 h after the last challenge. Levels of globular (G) and filamentous (F) actin in bronchioles were determined by DNase I inhibition and contraction assessed in intact small bronchioles using a myograph. DNase I inhibition assays showed that G-actin monomers were more abundant ( approximately 1F:2G) in extracts from resting small bronchioles from OVA- or saline-challenged animals. However, while contractile protein levels in bronchioles were reduced by OVA (P < 0.05), the proportion of F:G actin was 1.8-fold greater compared with saline challenge (P < 0.05). Consistent with induction of F-actin after OVA challenge, increases in maximum tension development to carbachol or KCl in small bronchioles from OVA-challenged animals were abrogated (P < 0.01) by actin cytoskeleton disruption with 0.5 microM latrunculin A. Cytoskeletal stabilization of F-actin with 0.1 microM jasplakinolide potentiated maximum contractions to carbachol or KCl (P < 0.05) in bronchioles from OVA- but not saline-treated rats. We conclude that alterations in the composition and/or arrangement of the contractile apparatus after OVA exposure confer enhanced contractile responses, possibly as a result of increased F-actin content. Such a mechanism may have relevance for AHR found in allergic asthma.