Role of Sphingosine-1-Phosphate in β-adrenoceptor Desensitization via Ca2+ Sensitization in Airway Smooth Muscle

Background: The correlation between inflammatory cells and airway smooth muscle plays fundamental roles in the pathophysiology of asthma. This study was designed to determine whether pre-exposure of airway smooth muscle to sphingosine-1-phosphate (S1P), which is released from mast cells by allergic reactions, causes a deterioration of β-adrenoceptor function. Methods: Isometric tension and the ratio of fluorescence intensities at 340 and 380 nm (F340/F380), an indicator of intracellular Ca2+ levels, were simultaneously measured using fura-2 loaded guinea-pig tracheal tissues. Intracellular cAMP levels were also measured. Results: Pre-exposure to S1P caused a reduction in the inhibitory effects of 0.3 μM isoprenaline, a β-adrenoceptor agonist, and 10 μM forskolin, a direct activator of adenylyl cyclase, against 1 μM methacholine-induced contraction in concentration- and time- dependent manners. In contrast, the values of F340/F380 were not augmented under this experimental condition. After incubation with S1P in the presence of 0.001-1 μM Y-27632, a Rho-kinase inhibitor, the reduced responsiveness to forskolin induced by S1P was reversed in a concentration-dependent manner. Moreover, pre-treatment with pertussis toxin (PTX), an inhibitor of Gi, suppressed the loss of forskolin-induced relaxation induced by S1P. Pre-exposure to S1P markedly inhibited the augmentation of cAMP accumulation induced by forskolin. However, addition of Y-27632 and pre-exposure to PTX returned forsokin-induced cAMP accumulation to the control level. Conclusions: Pre-exposure to S1P causes heterologus desensitization of β-adrenoceptors by increasing the sensitivity of airway smooth muscle to intracellular Ca2+. Ca2+ sensitization regulated by Gi and Rho-kinase is involved in this phenomenon