Background: Asthma is characterized by airway remodeling, altered mucus production and airway smooth muscle cell (ASMC) contraction causing extensive airway narrowing. In particular, alterations of ASMC contractility seem to be of crucial importance. The elevation of the cytoplasmic Ca²⁺ concentration is a key event leading to ASMC contraction and changes in the agonist-induced Ca²⁺ increase in ASMC have been reported in asthma. Objective: The aim of this study was to investigate mechanisms underlying these changes. Methods: Murine tracheal smooth muscle cells (MTSMC) from T-bet KO mice and human bronchial smooth muscle cells (HBSMC) incubated with IL-13 and IL-4 served as asthma models. Acetylcholine-induced changes in the cytoplasmic Ca²⁺ concentration were recorded using fluorescence microscopy and the expression of Ca²⁺ homeostasis regulating proteins was investigated with Western blot analysis. Results: Acetylcholine-induced Ca²⁺ transients were elevated in both asthma models. This correlated with an increased Ca²⁺ content of the sarcoplasmic reticulum (SR). In MTSMC from T-bet KO mice, the expression of the SR Ca²⁺ buffers calreticulin and calsequestrin was higher compared to wild-type mice. In HBSMC incubated with IL-13 or IL-4, the expression of ryanodine receptors, inositol-3-phosphate receptors and sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPases 2 was increased compared to HBSMC without incubation with interleukins. The enlarged acetylcholine-induced Ca²⁺ transients could be reversed by blocking inositol-3-phosphate receptors. Conclusions: We conclude that in the murine asthma model the SR Ca²⁺ buffer capacity is increased, while in the human asthma model the expression of SR Ca²⁺ channels is altered. The investigation of the Ca²⁺ homeostasis of ASMC has the potential to provide new therapeutical options in asthma.

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