Protein kinase C-activated calcium channel in the osteoblast-like clonal osteosarcoma cell line UMR-106

The effects of protein kinase C stimulation on free cytosolic Ca2+ ( Ca2+]i) were studied in Fura 2-loaded UMR-106 cells. Stimulation of the protein kinase C with the tumor-promoting phorbol esters 12-O-tetradecanoylphorbol 13-acetate (TPA) and phorbol 12,13-diacetate or 1-oleoyl-2-acetylglycerol was followed by an increase in Ca2+]i. The protein kinase C-induced increase in Ca2+]i has a lag period, the duration of which was dependent on the stimulant and medium Ca2+ concentrations. With 2 microM TPA, the rise in Ca2+]i peaked within 1.5 min, after which Ca2+]i returned partially toward base line. The increase in Ca2+]i was absolutely dependent on the presence of medium Ca2+ and was inhibited by the Ca2+ channel blockers nicardipine and verapamil. Cell stimulation also results in Ca2+ release from intracellular pool(s) which appears to be mediated by a Ca2+-dependent Ca2+ release mechanism. The reduction in Ca2+]i was due to channel inactivation. Pretreatment of the cells with 1 nM TPA, 2 units/ml parathyroid hormone (PTH), or 15 microM forskolin blocked the effect of 2 microM TPA on Ca2+]i. TPA and PTH were more potent inhibitors than was forskolin. The properties of this channel are compared to the cAMP-independent PTH-stimulated Ca2+ channel present in these cells.