Endothelin-1-evoked calcium transients in UMR-106 osteoblastic osteosarcoma cells are mediated through endothelin-A and endothelin-B receptors

Endothelin (ET) receptor subtypes involved in the modulation of intracellular calcium were studied in UMR-106 osteoblastic osteosarcoma cells. Calcium signaling in UMR-106 cells in suspension was determined with fluo-3-acetoxymethylester fluorescent dye. ET-1 and the ET(B)-selective agonist sarafotoxin 6c (S6c) elicited rapid calcium transients. Maximally effective concentrations of the ET(A)-selective antagonist BQ-123 [Cyclo(-D- Trp-D-Asp-Pro-D-Val-Leu)] attenuated ET-1-evoked calcium transients by only 50%. BQ-123 had no effect on S6c-stimulated transients. ET-1 and S6c showed homologous desensitization on repeated administration. Pretreatment with ET- 1 completely eliminated S6c-evoked calcium transients, whereas S6c pretreatment only partially (50%) attenuated the calcium transient evoked by ET-1. Joint pretreatment with S6c and BQ-123 or pretreatment with the ET(A)/ET(B) nonselective antagonist PD-142893 (Ac-D-diphenylalanine-Leu-Asp- Ile-Ile-Trp) eliminated the ET-1-stimulated calcium transient. These cells display both ET(A) and ET(B) receptors (60:40), as demonstrated by saturation binding experiments with [¹²⁵I] ET-1 and the ET(B) specific agonist, [¹²⁵I] IRL-1620 (Suc [Glu⁹, Ala^11,15] endothelin-1 [8-21]). This was further confirmed by competition binding experiments using [¹²⁵I] ET-1 and subtype-selective ligands S6c and BQ-123. These data indicate that ET-1 interacts with both ET(A) and ET(B) receptors to elicit calcium transients in UMR-106 osteoblastic osteosarcoma cells.