Isolated bone cells are often described according to the presence of PTH- and calcitonin (CT)-sensitive adenylate cyclase activities. Osteoblasts are thought to be cells with PTHsensitive adenylate cyclase but without CT response, and osteoclasts are thought to be CT-sensitive cells. We have studied the adenylate cyclase of a cloned bone cell line (UMR-106) derived from a rat osteosarcoma and used as a model of osteoblastic cells. Cells maintained in continuous culture for over 2 yr contain adenylate cyclase responsive to CT as well as PTH. The stimulatory effects of both hormones are dependent on hormone concentration, time, and the guanine nucleotide GTP. PTH and CT may activate the same adenylate cyclase in UMR-106 cells, since the stimulatory effects of the two hormones are not additive when combined at concentrations giving maximal activity. The β-adrenergic agonist isoproterenol also stimulates adenylate cyclase in these cells. Unlike late passages of UMR-106 cells, cells of earlier passages (< 50) showed only slight CT-sensitive adenylate cyclase activity. Our results suggest that studies of hormone effects attributed to the osteoblast phenotype should consider possible alteration of hormone responsiveness in cloned tumor cells during long term culture.
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