Endothelin‐1 modulates calcium signaling by epidermal growth factor, α‐thrombin, and prostaglandin E₁ in UMR‐106 osteoblastic cells

Local factors play an important role in the regulation of bone metabolism. The homologous and heterologous desensitization of responses to these factors may be crucial in the modulation of bone cell signaling. In this study, the effects and interactions of endothelin‐1 (25 nM), α‐thrombin (0.9 μM), epidermal growth factor (40 nM), prostaglandin E₁ (5 μM), and prostaglandin F_1α (5 μM) were examined on calcium signaling in UMR‐106 rat osteoblastic osteosarcoma cells. Intracellular calcium was measured using fluo‐3 fluorescent dye. All agents elicited calcium transients at these concentrations and showed homologous desensitization to their repeated administration. Preincubation for 60 minutes with 500 μM monodansylcadaverine and 30 minutes or 24 h preincubation with 0.5 μM indomethacin did not affect homologous desensitization, suggesting that neither the internalization of receptors nor prostaglandins are involved in this event. Pretreatment for 3 minutes with 2 μM 4β‐phorbol‐12β,13α‐dibutyrate significantly reduced the calcium elevations elicited by the first application of these compounds, whereas an inactive phorbol, 12,13‐didecanoate, had no effect. Pretreatment for 4 minutes with 0.5 μM forskolin decreased the calcium signal response to PGE, only. Pretreatment with endothelin‐1 for 3 minutes significantly decreased the calcium signals elicited by epidermal growth factor and α‐thrombin. Prior administration of endothelin‐1 significantly increased prostaglandin E₁‐stimulated calcium transients, whereas prostaglandin F_1α responses were not affected. Preincubation with indomethacin did not alter any of the interactions. Responses to endothelin‐1 were not significantly altered by 2–3 minutes pretreatment with the other factors, nor was there cross‐desensitization among the other factors. The results could indicate that endothelin‐1 has a unique and specific role in the modulation of bone cell signaling.