Dose dependence of FGFR1c activation by FGF23 and N, O-SH. A, BaF3 cells expressing FGFR1c were treated with increasing doses of FGF23 in the presence of 2 µg/ml N, O-SH. Mitogenic activity of FGF23 showed dose dependence, with a threshold dose of approximately 25 nM and an ED50 of approximately 50 nM (c, P < 0.0001 vs. N, O-SH alone). B, Increasing doses of N, O-SH resulted in increasing activity of FGF23 [a, P < 0.001 vs. control (CTL); b, P < 0.001 vs. FGF23 alone; c, P < 0.001 vs. heparin alone].