Tendon calcification has been widely regarded by researchers to result from the osteogenic differentiation of Tendon-Derived Stem Cells (TDSCs) and ectopic mineralization caused by the calcification of cellular matrix. Recent studies have revealed a correlation between the Mg2+-Ca2+ balance and the degeneration or calcification of tendon tissues. Furthermore, the ATP-P2X/P2Y receptor pathway has been shown to play a decisive role in the process of calcification, with calcium exportation from mitochondria and calcium oscillations potentially representing the cohesive signal produced by this pathway. Our previous study demonstrated that matrix calcification is inhibited by magnesium. In this study, we examined the effects of extracellular Mg2+ on the deposition of calcium phosphate matrix and cellular pathways in TDSCs. The suppression of the export of calcium from mitochondria has also been detected. We found that a high concentration of extracellular Mg2+ ([Mg2+](e)) inhibited the mineralization of the extracellular matrix in TDSCs and that 100 mu M ATP reversed this inhibitory effect in vitro. In addition, the spontaneous release of ATP was inhibited by high [Mg2+](e) levels. A high [Mg2+](e) suppressed the expression of P2X4, P2X5 and P2X7 and activated the expression of P2Y1, P2Y2, P2Y4 and P2Y14. The interaction between Mg2+ and Ca2+ is therefore contradictory, Mg2+ inhibits mitochondrial calcium concentrations, meanwhile it reverses the opening of mPTP that is induced by Ca2+. JC-1 staining verified the protective effect of Mg2+ on mitochondrial membrane potential and the decrease induced by Ca2+. Taken together, these results indicate that high [Mg2+]e interferes with the expression of P2 receptors, resulting in decreased extracellular mineralization. The balance between Mg2+ and Ca2+ influences mitochondrial calcium exportation and provides another explanation for the mechanism underlying matrix calcification in TDSCs. (C) 2016 Elsevier Inc. All rights reserved.