Calciprotein particle inhibition explains magnesium-mediated protection against vascular calcification

BACKGROUND: Phosphate (Pi) toxicity is a strong determinant of vascular calcification development in chronic kidney disease (CKD). Magnesium (Mg(2+)) may improve cardiovascular risk via vascular calcification. The mechanism by which Mg(2+) counteracts vascular calcification remains incompletely described. Here we investigated the effects of Mg(2+) on Pi and secondary crystalline calciprotein particles (CPP2)-induced calcification and crystal maturation. METHODS: Vascular smooth muscle cells (VSMCs) were treated with high Pi or CPP2 and supplemented with Mg(2+) to study cellular calcification. The effect of Mg(2+) on CPP maturation, morphology and composition was studied by medium absorbance, electron microscopy and energy dispersive spectroscopy. To translate our findings to CKD patients, the effects of Mg(2+) on calcification propensity (T(50)) were measured in sera from CKD patients and healthy controls. RESULTS: Mg(2+) supplementation prevented Pi-induced calcification in VSMCs. Mg(2+) dose-dependently delayed the maturation of primary CPP1 to CPP2 in vitro. Mg(2+) did not prevent calcification and associated gene and protein expression when added to already formed CPP2. Confirmatory experiments in human serum demonstrated that the addition of 0.2 mmol/L Mg(2+) increased T(50) from healthy controls by 51 ± 15 min (P < 0.05) and CKD patients by 44 ± 13 min (P < 0.05). Each further 0.2 mmol/L addition of Mg(2+) led to further increases in both groups. CONCLUSIONS: Our results demonstrate that crystalline CPP2 mediates Pi-induced calcification in VSMCs. In vitro, Mg(2+) delays crystalline CPP2 formation and thereby prevents Pi-induced calcification.