Store-operated calcium entry is required for sustained contraction and Ca2+ oscillations of airway smooth muscle

Key points: Airway hyper-responsiveness in asthma is driven by excessive contraction of airway smooth muscle cells (ASMCs). Agonist-induced CA²⁺ oscillations underlie this contraction of ASMCs and the magnitude of this contraction is proportional to the CA²⁺ oscillation frequency. Sustained contraction and CA²⁺ oscillations require an influx of extracellular CA²⁺ , although the mechanisms and pathways mediating this CA²⁺ influx during agonist-induced ASMC contraction are not well defined. By inhibiting store-operated calcium entry (SOCE) or voltage-gated CA²⁺ channels (VGCCs), we show that SOCE, rather than CA²⁺ influx via VGCCs, provides the major CA²⁺ entry pathway into ASMCs to sustain ASMCs contraction and CA²⁺ oscillations. SOCE may therefore serve as a potential target for new bronchodilators to reduce airway hyper-responsiveness in asthma.

Abstract: Asthma is characterized by airway hyper-responsiveness: the excessive contraction of airway smooth muscle. The extent of this airway contraction is proportional to the frequency of CA²⁺ oscillations within airway smooth muscle cells (ASMCs). Sustained CA²⁺ oscillations require a CA²⁺ influx to replenish CA²⁺ losses across the plasma membrane. Our previous studies implied store-operated calcium entry (SOCE) as the major pathway for this CA²⁺ influx. In the present study, we explore this hypothesis, by examining the effects of SOCE inhibitors (GSK7975A and GSK5498A) as well as L-type voltage-gated CA²⁺ channel inhibitors (nifedipine and nimodipine) on airway contraction and CA²⁺ oscillations and SOCE-mediated CA²⁺ influx in ASMCs within mouse precision-cut lung slices. We found that both GSK7975A and GSK5498A were able to fully relax methacholine-induced airway contraction by abolishing the CA²⁺ oscillations, in a manner similar to that observed in zero extracellular CA²⁺ ([CA²⁺ ]_e ). In addition, GSK7975A and GSK5498A inhibited increases in intracellular CA²⁺ ([CA²⁺ ]_i ) in ASMCs with depleted CA²⁺ -stores in response to increased [CA²⁺ ]_e , demonstrating a response consistent with the inhibition of SOCE. However, GSK7975A and GSK5498A did not reduce CA²⁺ release via IP₃ receptors stimulated with IP₃ released from caged-IP₃ . By contrast, nifedipine and nimodipine only partially reduced airway contraction, CA²⁺ oscillation frequency and SOCE-mediated CA²⁺ influx. These data suggest that SOCE is the major CA²⁺ influx pathway for ASMCs with respect to sustaining agonist-induced airway contraction and the underlying CA²⁺ oscillations. The mechanisms of SOCE may therefore form novel targets for new bronchodilators.