Caveolin-1 regulates intracellular Ca2+ in human airway smooth muscle.

Intracellular calcium ([Ca2+]i) in airway smooth muscle (ASM) is key to contractility. In vascular smooth muscle, specialized plasma membrane microdomains (caveolae) expressing the scaffolding protein caveolin are thought to facilitate signal transduction. We hypothesized that caveolae in ASM are important in mediating agonist induced [Ca2+]i responses. In human ASM cells, fluorescence confocal microscopy as well as Western blot analyses were used to determine expression and co-localization of agonist receptors (M3 muscarinic, bradykinin, histamine) as well as TRPC proteins with caveolin-1. The muscle-specific caveolin-3 was barely detectable in these cells, while caveolin-2 (usually associated with caveolin-1) was present. Transfection of ASM cell with 50nM caveolin-1 siRNA significantly knocked down caveolin-1 expression (∼75%), but caveolin-2 expression did not decrease. In ASM cells loaded with 5µM fura2, [Ca2+]i responses to 10nM bradykinin and 10µM histamine, but not 1µM ACh, were attenuated following disruption of caveolae by the cholesterol chelating drug, methyl-β-cyclodextrin (10mM). Transfection of caveolin-1 siRNA significantly blunted [Ca2+]i responses to bradykinin and histamine, but not ACh. Store-operated Ca2+ entry was also significantly blunted by cyclodextrin as well as caveolin-1 knockdown. These results indicate that caveolae are present in ASM and contribute to regulation of agonist induced [Ca2+]i responses, most likely via caveolin-1. [ABSTRACT FROM AUTHOR]