The coumarin osthol attenuates the binding of thyrotropin-releasing hormone in rat pituitary GH(4)C(1) tells

The influence of two plant coumarins, osthol and xanthotoxin, on intracellu lar Ca2+ ([Ca2+](i)) transients evoked by TRH were studied in clonal rat pi tuitary CH4C1 cells. Osthol, but not xanthotoxin, decreased the TRH-induced transient increase in [Ca2+](i) in Fluo-3 loaded cells incubated in Ca2+-f ree buffer. Binding experiments with [H-3]TRH showed that osthol decreased the binding of TRH to its receptor, whereas the affinity of the receptor fo r TRH increased. This resulted in a decreased TRH-evoked production of IP3 in cells treated with osthol, and a decreased mobilization of sequestered c alcium. Osthol did not inhibit the release of calcium evoked by exogenous I P3 in permeabilized cells. Furthermore, osthol decreased the uptake of Ca-4 5(2+) in response to high K+. Xanthotoxin had no effects in these experimen ts. The results show that osthol modulates TRH-evoked responses by interact ing with the TRH receptor.