HETEROGENEITY OF INTRACELLULAR CALCIUM RESPONSES TO PARATHYROID-HORMONE AND THROMBIN IN PRIMARY OSTEOBLAST-LIKE CELLS AND UMR106-01 CELLS -CORRELATIONS WITH CULTURE CONDITIONS, INTRACELLULAR CALCIUM-CONCENTRATION AND DIFFERENTIATION STATE

The present study evaluates the effect of parathyroid hormone (PTH) on intracellular calcium. Intracellular calcium ion concentrations ([Ca2 +]i) in fetal rat osteoblasts in primary culture (ROB) and in UMR106-0 1 osteogenic sarcoma cells were monitored as changes in the ratio (R) of Fura-2 fluorescence intensities in single cells as well as populati ons of cells. In both single ROB and UMR106-01 cells, addition of 10(- 7) M rat PTH1-34 and 3 NIH units/ml human thrombin resulted in heterog eneous responses in R values and therefore [Ca2+]i. PTH-induced calciu m responsiveness of ROB was dependent on culture conditions, such that response frequencies were positively correlated with the percentage o f fetal calf serum in the culture medium. PTH responsive ROB and UMR10 6-01 cells had significantly higher resting [Ca2+]i than unresponsive cells. PTH- or thrombin-mediated calcium signalling appeared not to be correlated to alkaline phosphatase activity in single ROB. Low percen tages of cells responded to PTH in comparison to thrombin suggesting t hat an increase in [Ca2+]i is not a common PTH signalling pathway in o steoblasts in primary culture. Our data suggest that activation of thi s signalling pathway by PTH is culture condition dependent, possibly v ia a cell-cycle related increase in sensitivity of the pathway.