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Abstract

We proposed a role for Na-Ca exchange in hormonally mediated bone resorption and recently characterized Na-dependent Ca transport in an osteoblast-like rat osteosarcoma cell line (UMR-106). To test whether calcemic agents alter Na+-Ca2+ exchange in osteoblasts, UMR cells were treated acutely or cultured in the absence or presence of calcemic agent for 24h. Cells were then loaded with the Ca-sensitive dye fura-2 in the presence of 140 mM NaCl, no Ca, and the absence or presence of 0.3 mM ouabain. Cells were resuspended at 22°C, and the fluorescence ratio at excitation wavelength of 340 and 380 nm was measured. An outward Na gradient was generated by removing extracellular Na and maintaining isotonicity with choline chloride. Na+-Ca2+ exchange was demonstrated by enhanced Ca uptake in ouabain-treated (Na-loaded) cells after the addition of 1.5 mM Ca. Acute addition of 10−7 M PTH or 10−6 M PGE2 had no effect on Na-dependent Ca uptake. However, 24 h treatment of cells with PTH, PGE2, or 1,25(OH)2D3 caused a dose-dependent inhibition of Na+-Ca2+ exchange. Using the Na-sensitive dye, SBFI, we also demonstrated that the effect was bidirectional; PTH inhibited Ca-dependent Na uptake comparably to its inhibition of Na-dependent Ca uptake. The effects of the calcemic agents were mimicked by 24 h treatment of the cells with 1 μM forskolin or 2 μM PMA. These results suggest that regulation of Na+-Ca2+ exchange by calcemic agents occurs downstream of signal transduction messengers and that alterations in Na+-Ca2+ exchange may play an integral role in their long-term regulation of osteoblast activity.