Heterogeneous intracellular free calcium responses to parathyroid hormone correlate with morphology and receptor distribution in osteogenic sarcoma cells

The osteogenic sarcoma cell line UMR 106-01 exhibits heterogeneous morphology and hormone response in subconfluent monolayer cultures. In these studies we have explored the correlation between morphological profiles and patterns of cytosolic calcium [Ca²⁺]_i response to PTH and other agonists in single UMR 106-01 cells loaded with the Ca²⁺-sensitive fluorescent indicator fura-2. Realtime recording of [Ca²⁺]_i revealed that PTH (10^-7 M) produced a transient [Ca²⁺]_i rise in 19% of the cells studied. [Ca²⁺]_i transients were also induced by prostaglandins E₂ and F_2α, and fetal bovine serum, but with different response frequencies (20%, 12%, and 58%, respectively). Spatial resolution of changes in [Ca²⁺]_i by video image analysis revealed that the response to PTH was more frequent in large polygonal cells with long cytoplasmic processes and less common in smaller cells growing in clusters, whereas there was no clear subtype specificity for the effects of epidermal growth factor and fetal bovine serum on [Ca²⁺]_i. Autoradiographic analysis of cell monolayers demonstrated a higher density of PTH-binding sites on cells with cytoplasmic extensions, whereas epidermal growth factor-binding sites were largely on colony-forming cells. Thus, the [Ca²⁺]_i response to hormonal stimulation is heterogeneous within UMR 106-01 cell populations and within single cells, and it correlates with receptor density. This suggests that osteoblastic cells respond to PTH by activation of changes in [Ca²⁺]_i only at certain specific steps during osteoblast development or stages of the cell cycle.