OSTEOGENIC PROTEIN-1 DOWN-REGULATES ENDOTHELIN-A RECEPTORS IN PRIMARYRAT OSTEOBLASTS

Osteogenesis is a complex process whereby growth factors and mediators from both local and systemic sources modulate the bone-forming activi ties of osteoblasts. In the present study we utilized primary cultures of fetal rat calvarial cells to characterize osteoblast responsivenes s to the vascular mediator endothelin-l (ET-1) and to investigate whet her ET-I responses are regulated by osteogenic protein-1 (OP-1). We fo und that a 1- to 2-day exposure to OP-1 diminished ET-1 receptor ligan d binding and signal transduction by downregulating ET-1 receptor mRNA expression. ET-l-mediated calcium signaling and ligand binding were c ompletely abolished by the ETA receptor antagonist BQ-123, suggesting that ET-1 effects are mediated by this receptor. Northern analysis of total RNA revealed that ETA mRNA expression was inhibited similar to 5 0% by OP-1 treatment, whereas ETB receptor mRNA was not detected by th is method of analysis. In OP-1-treated cultures, the magnitude and dur ation of ET-1 calcium signals varied among individual cells. This find ing may be related to a heterogeneous OP-1 response, indicated by alka line phosphatase induction in only a subpopulation of cells. These res ults suggest that modulation of osteoblast function by ET-1 occurs dur ing distinct periods of phenotypic development and imply that downregu lation of ET-1 responsiveness may be necessary for optimal bone format ion in vivo.