ENDOTHELIN-1 MODULATES CALCIUM SIGNALING BY EPIDERMAL GROWTH-FACTOR, ALPHA-THROMBIN, AND PROSTAGLANDIN-E(1) IN UMR-106 OSTEOBLASTIC CELLS

Local factors play an important role in the regulation of bone metabol ism. The homologous and heterologous desensitization of responses to t hese factors may be crucial in the modulation of bone cell signaling. In this study, the effects and interactions of endothelin-1 (25 nM), a lpha-thrombin (0.9 muM), epidermal growth factor (40 nM), prostaglandi n E1 (5 muM), and prostaglandin F1alpha (5 muM) were examined on calci um signaling in UMR-106 rat osteoblastic osteosamoma cells. Intracellu lar calcium was measured using fluo-3 fluorescent dye. All agents elic ited calcium transients at these concentrations and showed homologous desensitization to their repeated administration. Preincubation for 60 minutes with 500 muM monodansylcadaverine and 30 minutes or 24 h prei ncubation with 0.5 muM indomethacin did not affect homologous desensit ization, suggesting that neither the internalization of receptors nor prostaglandins are involved in this event. Pretreatment for 3 minutes with 2 muM 4beta-phorbol-12beta, 13alpha-dibutyrate significantly redu ced the calcium elevations elicited by the first application of these compounds, whereas an inactive phorbol, 12,13-didecanoate, had no effe ct. Pretreatment for 4 minutes with 0.5 muM forskolin decreased the ca lcium signal response to PGE, only. Pretreatment with endothelin-1 for 3 minutes significantly decreased the calcium signals elicited by epi dermal growth factor and alpha-thrombin. Prior administration of endot helin-1 significantly increased prostaglandin E1-stimulated calcium tr ansients, whereas prostaglandin F1alpha responses were not affected. P reincubation with indomethacin did not alter any of the interactions. Responses to endothelin-1 were not significantly altered by 2-3 minute s pretreatment with the other factors, nor was there cross-desensitiza tion among the other factors. The results could indicate that endothel in-1 has a unique and specific role in the modulation of bone cell sig naling.