Endothelial cell products may affect bone cell function, since trabecular and cortical bone are in close proximity to vascular endothelial cells. Incubation of cultured human osteoblastic cells with the endothelial cell polypeptide endothelin-1 (ET-1) resulted in a time- and dose-dependent stimulation of cell proliferation. Furthermore, markers of differentiated osteoblastic function, i.e., alkaline phosphatase and type-I collagen, were dose-dependently increased in response to ET-1. The effects of ET-1 on cell growth and function reached a maximum at higher ET-1 concentrations, and osteoblastic cells bound ET-1 specifically with a KD of 35 pM, corresponding to the biologic effects of ET-1 on bone cells. Under baseline conditions osteoblastic cells expressed 16,800 binding sites per cell. The effect of ET-1 was dependent on its binding to the endothelin-1 receptor A (ETRA), since an inhibitor of ET-1 binding blocked the biologic effects of ET-1. Northern blot analyses revealed that cultured human osteoblastic cells possess the transcript for the ETRA. Expression of ETRA mRNA was under control of 1,25-dihydroxyvitamin D3 [1,25 (OH)2D3]. Incubation of osteoblastic cells with 1,25(OH)2D3 increased ETRA mRNA levels, corresponding to an increased effect of ET-1 on osteoblastic proliferation and function. Thus, a concerted action of the endothelial cell polypeptide ET-1 and 1,25(OH)2D3 may mediate an osteoanabolic effect of the vascular and endocrine vitamin D system.