The mechanisms by which PTH and thrombin mobilize intracellular Ca2+ (Cai 2+) were examined in UMR 106-H5 rat osteosarcoma cells. Bovine PTH-(1-34) (24 pM to 240 nM) produced a dose-dependent increase in Cai 2+ (EC50, 3 nM), which returned to baseline within 75 sec. Human α-thrombin produced an increase in Cai 2+ (ECmax, 10 U/ml) which was similar to that of PTH with respect to both magnitude and time course. Chelation of extracellular calcium with 5.0 mM EGTA did not alter the Cai 2+ response to either PTH or thrombin. When added together at maximally effective concentrations, PTH and thrombin produced additive effects on Cai 2+ in the presence and absence of EGTA. The additive effects of PTH and thrombin on Cai 2+. were confirmed at the single cell level, using laser-based image analysis. Bradykinin (1 μM) produced a significant increase in Cai 2+ in UMR 106-H5 cells which was of lesser magnitude than the peak 2- to 3-fold increase elicited by PTH or thrombin. Preexposure of cells to 10 U/ml thrombin for 2 min abolished the Cai 2+ response to bredykinin, whereas preexposure to 240 nM PTH had no effect on the Cai 2+ response to bradykinin. Thrombin elicited a rapid increase in the accumulation of 3H-labeled inositol phosphates (IP2 and IP3) in UMR 106-H5 cells, with increases in [3H]1,4,5-IP3 detectable as early as 15 sec after the addition of thrombin. Bradykinin increased [3H]IP production to a lesser extent than thrombin, whereas PTH neither increased [3H]IP accumulation nor potentiated the [3H]IP response to thrombin. The results suggest that thrombin and bradykinin mobilize Cai 2+ from a shared IP3-responsive calcium pool, whereas PTH may use signals in addition to 1,4,5-IP3 to mobilize calcium from a distinct cellular calcium pool. Alternatively, specific calcium compartmentalization exists, and there is differential coupling of these agonists to the 1,4,5-IPa/Cai 2+. pathway.
|Original language||English (US)|
|Number of pages||8|
|State||Published - Sep 1991|