Arachidonic acid metabolites and glucocorticoid regulatory mechanism in cultured porcine tracheal smooth muscle cells

To elucidate the signal transduction system in the production of prostaglandin E2 (PGE2) by porcine tracheal smooth muscle cells in culture (PTSMC), we examined the pattern of arachidonic acid metabolites released from PTSMC and the relationship between bradykinin-stimulated rises in intracellular calcium concentration ([Ca2+]i) and PGE2 production by PTSMC. We next examined the effect of dexamethasone on these parameters. Bradykinin induced a dose-dependent increase in both the rise in [Ca2+]i and PGE2 production by PTSMC. The increase in [Ca2+]i paralleled an increase in PGE2 production. High-performance liquid chromatography (HPLC) revealed that dexamethasone-treated PTSMC were suppressed to release arachidonic acid metabolites such as PGE2 and prostaglandin F2α (PGF2α). Incubation of PTSMC with 10−6M dexamethasone for 24 h significantly suppressed both the rise in [Ca2+]i and PGE2 production by PTSMC in response to bradykinin, and also significantly suppressed bradykinin-stimulated release of radioactivity from PTSMC prelabeled with 3H-labeled arachidonic acid (3H-AA). When PTSMC pretreated with dexamethasone were incubated with 170 nM prostaglandin H2 (PGH2) or 20 μM arachidonic acid; PTSMC synthesized less PGE2 than control PTSMC. Results suggest that bradykinin stimulates PTSMC to produce PGE2 via the signal transduction system including Ca2+, and dexamethasone appeared to suppress PGE2 production by reducing the activity of cytosolic phospholipase A2 (cPLA2) and PGE2 synthase. However, we failed to demonstrate the suppression of the activity of cyclooxygenase in PTSMC by dexamethasone. Since the elevation of [Ca2+]i is necessary for the contraction of airway smooth muscles, dexamethasone seems to reduce the contraction of airway.