GLUCOCORTICOIDS INCREASE VASOPRESSIN V1B RECEPTOR COUPLING TO PHOSPHOLIPASE-C

Vasopressin (VP) stimulates pituitary ACTH secretion after binding to V1b VP receptors (V1b-R) coupled to phospholipase C (PLC). This effect of VP on ACTH secretion, unlike that of CRH, is resistant to glucocor ticoid feedback. To determine whether changes in V1b-R expression or s ignaling mediate the refractoriness to glucocorticoids, the effects of glucocorticoids on pituitary VP binding, V1b-R messenger RNA (mRNA) a nd VP-stimulated inositol phosphate (IP) formation were studied in viv o and in vitro in the rat. Dexamethasone injection for 7 days decrease d VP binding but increased V1b-R mRNA, indicating that mRNA levels do not reflect receptor number. In spite of the binding loss, VP-stimulat ed TP formation was enhanced in dexamethasone-treated rats, suggesting that glucocorticoids increase the coupling efficiency of the V1b rece ptor to phospholipase C. Pretreatment of pituitary cells in vitro with dexamethasone or corticosterone, also potentiated IP formation by low and high doses of VP, indicating that glucocorticoids act directly in the pituitary and not through changes in hypothalamic factors. The ef fect is mediated by glucocorticoid receptors because it was blocked by glucocorticoid but not mineralocorticoid antagonists. Dexamethasone p otentiated the stimulation of IP by other PLC-dependent ligands (GnRH, TRH) but not that by the calcium ionophore, ionomycin, suggesting a s ite of action between the receptor and PLC. After treatment with dexam ethasone, in vivo or in vitro, Western blot analysis revealed marked i ncreases in the GTP binding protein, Ga,, which may account for the po tentiating effect of glucocorticoid on ligand-stimulated IF. The data demonstrate that glucocorticoids increase coupling of the V1b-R with P LC thereby providing a mechanism by which VP facilitates corticotroph responsiveness in spite of elevated levels of plasma glucocorticoids d uring stress.