Role of capacitative Ca²⁺ entry in bronchial contraction and remodeling

Asthma is characterized by airway inflammation, bronchial hyperresponsiveness, and airway obstruction by bronchospasm and bronchial wall thickening due to smooth muscle hypertrophy. A rise in cytosolic free Ca²⁺ concentration ([Ca²⁺]_cyt) may serve as a shared signal transduction element that causes bronchial constriction and bronchial wall thickening in asthma. In this study, we examined whether capacitative Ca²⁺ entry (CCE) induced by depletion of intracellular Ca²⁺ stores was involved in agonist-mediated bronchial constriction and bronchial smooth muscle cell (BSMC) proliferation. In isolated bronchial rings, acetylcholine (ACh) induced a transient contraction in the absence of extracellular Ca²⁺ because of Ca²⁺ release from intracellular Ca²⁺ stores. Restoration of extracellular Ca²⁺ in the presence of atropine, an M-receptor blocker, induced a further contraction that was apparently caused by a rise in [Ca²⁺]_cyt due to CCE. In single BSMC, amplitudes of the store depletion-activated currents (I_SOC) and CCE were both enhanced when the cells proliferate, whereas chelation of extracellular Ca²⁺ with EGTA significantly inhibited the cell growth in the presence of serum. Furthermore, the mRNA expression of TRPC1, a transient receptor potential channel gene, was much greater in proliferating BSMC than in growth-arrested cells. Blockade of the store-operated Ca²⁺ channels by Ni²⁺ decreased Isoc and CCE and markedly attenuated BSMC proliferation. These results suggest that upregulated TRPC1 expression, increased Isoc, enhanced CCE, and elevated [Ca²⁺]_cyt may play important roles in mediating bronchial constriction and BSMC proliferation.