Endothelin stimulated Ca2+ signaling and endothelin receptor expression ave decreased by parathyroid hormone treatment in UMR-106 Osteoblastic osteosarcoma cells

Modulation of endothelin (ET-1)-induced [Ca2+](i) transients and receptor e xpression by parathyroid hormone (PTH) was studied in UMR-106 osteoblastic osteosarcoma cells. Ca2+ signaling was assessed with Fura-2, and ET recepto r mRNA expression was determined using ETA- and ETB-specific primers and RT -PCR amplification. ET-1 binding in UMR-106 cell membranes was also measure d. PTH pretreatment for 8 h decreased the [Ca2+](i) transients elicited by ET-1 and by the ETB-selective agonist sarafotoxin 6c (S6c), When ETB recept ors were desensitized by pretreatment with S6c or blocked with the ETB-sele ctive antagonist BQ-788, the remaining ETA component of the signal was also decreased by PTH pretreatment. In contrast, [Ca2+](i) transients elicited by PGF(2 alpha) and ionomycin were increased following PTH pretreatment, in dicating that the effect of PTH to decrease ET-1-stimulated transients was selective. PTH pretreatment also decreased [I-125]ET-1 binding and ETA and ETB mRNA, with maximal effects at approximately 8 h. ET-1 was not detectabl e in medium from either control or PTH treated UMR-106 cultures, suggesting that the decreased expression of ET receptors was not due to enhanced ET p roduction and subsequent homologous desensitization. The downregulation of ET receptors in osteoblasts by PTH pretreatment may serve as a homeostatic mechanism in bone. (C) 2000 Harcourt Publishers Ltd.