The contribution of inositol 1,4,5-trisphosphate and ryanodine receptors to agonist-induced Ca signaling of airway smooth muscle cells

Bai Y, Edelmann M, Sanderson MJ. The contribution of inositol 1,4,5-trisphosphate and ryanodine receptors to agonist-induced Ca signaling of airway smooth muscle cells. Am J Physiol Lung Cell Mol Physiol 297: L347–L361, 2009. First published May 22, 2009; doi:10.1152/ajplung.90559.2008.—The relative contribution of inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) and ryanodine receptors (RyRs) to agonist-induced Ca signaling in mouse airway smooth muscle cells (SMCs) was investigated in lung slices with phasecontrast or laser scanning microscopy. At room temperature (RT), methacholine (MCh) or 5-hydroxytryptamine (5-HT) induced Ca oscillations and an associated contraction in small airway SMCs. The subsequent exposure to an IP3R antagonist, 2-aminoethoxydiphenyl borate (2-APB), inhibited the Ca oscillations and induced airway relaxation in a concentration-dependent manner. 2-APB also inhibited Ca waves generated by the photolytic release of IP3. However, the RyR antagonist ryanodine had no significant effect, at any concentration, on airway contraction or agonistor IP3-induced Ca oscillations or Ca wave propagation. By contrast, a second RyR antagonist, tetracaine, relaxed agonist-contracted airways and inhibited agonist-induced Ca oscillations in a concentration-dependent manner. However, tetracaine did not affect IP3-induced Ca release or wave propagation nor the Ca content of SMC Ca stores as evaluated by Ca -release induced by caffeine. Conversely, both ryanodine and tetracaine completely blocked agonist-independent slow Ca oscillations induced by KCl. The inhibitory effects of 2-APB and absence of an effect of ryanodine on MCh-induced airway contraction or Ca oscillations of SMCs were also observed at 37°C. In Ca -permeable SMCs, tetracaine inhibited agonist-induced contraction without affecting intracellular Ca levels indicating that relaxation also resulted from a reduction in Ca sensitivity. These results indicate that agonist-induced Ca oscillations in mouse small airway SMCs are primary mediated via IP3Rs and that tetracaine induces relaxation by both decreasing Ca sensitivity and inhibiting agonist-induced Ca oscillations via an IP3-dependent mechanism.