Mechanisms of Interleukin-4 Effects on Calcium Signaling in Airway Smooth Muscle Cells

In airway smooth muscle cells, interleukin (IL)-4 inhibited both carbachol- and caffeine-induced calcium mobilization from the sarcoplasmic reticulum (SR). Because of the known signaling pathways for IL-4 and importance of calcium uptake in maintaining SR calcium stores shared by agonists and caffeine, it was hypothesized that this rapid inhibitory effect might depend on phosphatidylinositol 3-kinase (PI3K) and on inhibition of calcium uptake by the SR. Enzyme-dispersed bovine trachealis cells were loaded with Fura-2/acetoxymethyl ester, and changes in cytosolic calcium were imaged in single cells. Cells were pretreated with inhibitors of PI3K, either wortmannin (100 nM), LY294002 \[2-(4-morpholinyl)-8-phenyl-4 H -1-benzopyran-4-one\] (50 μM), or deguelin (100 nM). Calcium transients in response to carbachol (10 μM) were significantly decreased to 0.34 ± 0.10 of control after 20-min treatment with IL-4 but were 1.10 ± 0.26 and 1.08 ± 0.23 when wortmannin or deguelin, respectively, was added along with IL-4. LY294002 alone had nonspecific effects on transients. In other experiments, cyclopiazonic acid (CPA) (5 μM), an inhibitor of SR calcium uptake, decreased carbachol-stimulated transients within 4 min to 0.83 ± 0.08 of control ( n = 6). However, for cells treated with IL-4 (50 ng/ml) plus CPA, transients decreased significantly more, to only 0.51 ± 0.05 ( n = 6; p < 0.05). Longer exposures to IL-4 and a higher concentration of CPA (30 μM) gave similar results. It was concluded that IL-4 did not inhibit transients in the presence of PI3K antagonists but that it did in the presence of CPA. This suggested that IL-4 inhibited calcium transients by mechanisms dependent upon a wortmannin-sensitive PI3K but not by inhibition of calcium uptake into the SR.