HORMONAL-REGULATION OF NA-CA2+ EXCHANGE IN OSTEOBLAST-LIKE CELLS()

We proposed a role for Na-Ca exchange in hormonally mediated bone reso rption and recently characterized Na-dependent Ca transport in an oste oblast-like rat osteosarcoma cell line (UMR-106). To test whether calc emic agents alter Na+-Ca2+ exchange in osteoblasts, UMR cells were tre ated acutely or cultured in the absence or presence of calcemic agent for 24 h. Cells were then loaded with the Ca-sensitive dye fura-2 in t he presence of 140 mM NaCl, no Ca, and the absence or presence of 0.3 mM ouabain. Cells were resuspended at 22 degrees C, and the fluorescen ce ratio at excitation wavelength of 340 and 380 nm was measured. An o utward Na gradient was generated by removing extracellular Na and main taining isotonicity with choline chloride. Na+-Ca2+ exchange was demon strated by enhanced Ca uptake in ouabain-treated (Na-loaded) cells aft er the addition of 1.5 mM Ca. Acute addition of 10(-7) M PTH or 10(-6) M PGE(2) had no effect on Na-dependent Ca uptake. However, 24 h treat ment of cells with PTH, PGE(2), or 1,25(OH)(2)D-3 caused a dose-depend ent 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 C a uptake. The effects of the calcemic agents were mimicked by 24 h tre atment of the cells with 1 mu M forskolin or 2 mu M PMA. These results suggest that regulation of Na+-Ca2+ exchange by calcemic agents occur s downstream of signal transduction messengers and that alterations in Na+-Ca2+ exchange may play an integral role in their long-term regula tion of osteoblast activity.