THE DESENSITIZATION OF OXYTOCIN RECEPTORS IN HUMAN MYOMETRIAL CELLS IS ACCOMPANIED BY DOWN-REGULATION OF OXYTOCIN RECEPTOR MESSENGER-RNA

We have recently provided evidence for the desensitization of oxytocin receptors in human myometrial cells. In the present study, we have in vestigated the possible mechanisms by which oxytocin (OT) might regula te OT receptor density. The steady state level of OT binding in cultur ed myometrial cells was 220 x 10(3) binding sites/cell, but this was t ime-dependently reduced to 27 x 10(3) sites/cell by exposure to OT for up to 20 h. Similarly, OT exposure decreased the binding of OT to cel l membranes. In contrast, Western blotting data showed that the total amount of OT receptor protein was not affected by OT treatment for up to 48 h. Flow cytometry experiments demonstrated that OT receptors are not internalized during prolonged exposure of the cells to OT. Howeve r, RNase protection assays and Northern analysis showed that OT recept or mRNA was reduced by OT treatment to reach a new low steady state le vel with a time course similar to that of the disappearance oi cell su rface OT binding sites. Possible mechanisms involved in mRNA down-regu lation include transcriptional suppression and destabilization of mRNA by RNA binding proteins.