Background: Vascular calcification (VC) is closely related to cardiovascular events in chronic kidney disease (CKD). Apelin has emerged as a potent regulator of cardiovascular function, but its role in VC during CKD remains unknown. We determined whether apelin plays a role in phosphate-induced mineralization of human aortic smooth muscle cells (HASMCs) and in adenine-induced CKD rats with aortic calcification. Methods and results: In vitro, apelin-13 was found to inhibit calcium deposition in HASMCs (Pi(+) Apelin(+) group vs Pi(+) Apelin(-) group: 50.1 +/- 6.21 ug/mg vs 146.67 +/- 10.02 ug/mg protein, p = 0.012) and to suppress the induction of the osteoblastic transformation genes BMP-2, osteoprotegerin (OPG) and Cbfa1. This effect was mediated by interference of the sodium-dependent phosphate cotransporter (Pit-1) expression and phosphate uptake. In vivo, decreased plasma apelin levels (adenine(+) apelin(-) vs vehicle: 0.37 +/- 0.09 ng/ml vs 0.68 +/- 0.16 ng/ml, p = 0.003) and downregulation of APJ in the aorta were found in adenine-induced CKD rats with hyperphosphatemia (adenine(+) apelin(-) vs vehicle: 6.91 +/- 0.23 mmoL/L vs 2.3 +/- 0.07 mmoL/L, p = 0.001) and aortic calcification. Exogenous supplementation of apelin- 13 normalized the level of the apelin/APJ system and significantly ameliorated aortic calcification, as well as the suppression of Runx2, OPG and Pit-1 expression. Conclusions: Apelin ameliorates VC by suppressing osteoblastic differentiation of VSMCs through downregulation of Pit-1. These results suggest apelin may have potential therapeutic value for treatment of VC in CKD. (C) 2015 Elsevier Ireland Ltd. All rights reserved.