DIFFERENCES IN INTRACELLULAR CALCIUM SIGNALING AFTER ACTIVATION OF THE THROMBIN RECEPTOR BY THROMBIN AND AGONIST PEPTIDE IN OSTEOBLAST-LIKECELLS

Thrombin and the thrombin receptor agonist peptide (TRAP) caused a ris e in intracellular calcium concentration ([Ca2+](i)) in the human oste oblast like cell line Saos-2. Striking differences in the [Ca2+](i) si gnals elicited by these agonists were revealed. In cell populations, t hrombin induced a transient increase in [Ca2+](i) while TRAP caused a biphasic [Ca2+](i) response consisting of an initial peak and a sustai ned plateau phase. In individual cells, thrombin mainly caused a singl e [Ca2+](i) transient while TRAP induced repetitive [Ca2+](i) spikes. Neither tyrosine phosphorylation, cAMP-dependent phospho rylation, nor pertussis toxin-sensitive G proteins appeared to be involved in throm bin receptor [Ca2+](i) signaling in this cell line. However, the susta ined [Ca2+](i) response caused by TRAP was converted into a transient, thrombin-like response by pretreatment with serine/threonine phosphat ase inhibitors. Pretreatment with the phorbol ester phorbol 12-myrista te 13-acetate (PMA) abrogated thrombin receptor [Ca2+](i) signaling, a nd TRAP-induced Ca2+ entry was inhibited by the acute treatment with P MA In contrast, Ca2+ entry stimulated by thapsigargin was not sensitiv e to agents affecting serine/threonine phosphorylation. The observatio n that thrombin and TRAP, despite being agonists for a common receptor , induce dissimilar [Ca2+](i) responses indicates that binding of TRAP alone is insufficient to fully regulate the thrombin receptor in Saos -2 cells.