GLUCOCORTICOIDS REDUCE TACHYKININ NK2 RECEPTOR EXPRESSION IN BOVINE TRACHEAL SMOOTH-MUSCLE

Neurokinin A is not only a potent bronchoconstrictor, but also has imm uno-modulatory effects in animals and man, mediated via tachykinin NK2 receptors. We have examined the effect of the glucocorticoid, dexamet hasone, on tachykinin NK2 receptor mRNA and the number of tachykinin N K2 receptors in bovine tracheal smooth muscle in vitro by Northern blo t analysis using a human tachykinin NK2 receptor cDNA probe and recept or binding assay using [H-3]SR48968 tylamino-4-phenylpiperidino-2-(3,4 -dichlorophenyl) butyl]benzamide). Tachykinin NK2 receptor mRNA showed a time-dependent suppression (62% reduction after 6 h at 10(-7) M of dexamethasone), as well as a concentration-dependent suppression after the incubation with dexamethasone (IC50 = 1.3 x 10(-8) M). This suppr ession was abolished by the glucocorticoid receptor antagonist, mifepr istone (RU38486), indicating that dexamethasone acts via the glucocort icoid receptor. It was also abolished by the protein synthesis inhibit or, cycloheximide (10 mu g/ml), indicating that new protein synthesis is required on this suppression. Using the RNA polymerase inhibitor ac tinomycin D (5 mu g/ml), we showed that the stability of tachykinin NK 2 receptor mRNA was not affected by dexamethasone (t(1/2) = 5 h). Nucl ear run-on assays revealed a 51% reduction in the rate of tachykinin N K2 receptor gene transcription after treatment with dexamethasone for 6 h. Radioligand binding assay using an selective tachykinin NK2 recep tor antagonist, [H-3]SR48968 showed a significant decrease in the numb er of receptor binding sites after 16 h (B-max = 262 +/- 23 versus 213 +/- 13 fmol/mg protein for vehicle and dexamethasone treatment respec tively, P < 0.05), with no significant change at the earlier time poin ts. These results suggest that glucocorticoids act on glucocorticoid r eceptors to decrease tachykinin NK2 receptor expression by decreasing the rate of tachykinin NK2, receptor gene transcription. (C) 1998 Else vier Science B.V.