Human cerebromicrovascular endothelial cells (HBEC) in culture express high affinity ET/A receptors coupled to phospholipase C (PLC) activation. Pretreatment of HBEC with 1 microM dexamethasone for 24 hr decreased the number of ET-1 binding sites (Bmax) on HBEC (96 fmol-mg protein vs. 57 fmol-mg protein_) without changing the binding affinity (K/D) (101 pm vs. 92 pM) or displacing profile (ET-1= ET-2>ET-3 >S6c). Dexamethasone- pretreated HBEC also exhibited a 40% reduction in the maximal ET-1 stimulated inositol triphosphate (IP3) production, whereas the half- maximal stimulatory concentration (EC50) was not affected. This effect of dexamethasone was concentration-dependent and most pronounced after 24 hr of pretreatment. The inhibitory effect of dexamethasone on the ET- 1-induced IP3 production was abolished by the glucocorticoid receptor antagonist cortexolone. In contrast, vasopressin-mediated IP3 response in HBEC was not changed by dexamethasone. The cyclooxygenase inhibitors, indomethacin and acetylsalicylic acid, did not influence the ET-1-induced IP3 production by HBEC. The down-regulation of ETA receptors in HBEC by dexamethasone may represent one of the mechanisms involving the described effects of glucocorticoids on cerebromicrovascular function (i.e., changes in blood-brain barrier properties, secretion of vasoactive factors, vascular morphogenesis, etc.).
