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 intrac ellular calcium were studied in UMR-106 osteoblastic osteosarcoma cell s. Calcium signaling in UMR-106 cells in suspension was determined wit h fluo-3-acetoxymethylester fluorescent dye. ET-1 and the ET(B)-select ive agonist sarafotoxin 6c (S6c) elicited rapid calcium transients, Ma ximally 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 transi ents. ET-1 and S6c showed homologous desensitization on repeated admin istration. Pretreatment with ET-1 completely eliminated S6c-evoked cal cium transients, whereas S6c pretreatment only partially (50%) attenua ted the calcium transient evoked by ET-1. Joint pretreatment with S6c and BQ-123 or pretreatment with the ET(A)/ET(B) nonselective antagonis t 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 experim ents with [I-125] ET-1 and the ET(B) specific agonist, [I-125] IRL-162 0 (SUc [Glu(9), Ala(11,15)] endothelin-1 [8-21]), This was further con firmed by competition binding experiments using [I-125] ET-1 and subty pe-selective ligands S6c and BQ-123. These data indicate that ET-1 int eracts with both ET(A) and ET(B) receptors to elicit calcium transient s in UMR-IOB osteoblastic osteosarcoma cells.