Controlled deposition of calcium phosphate (CaP) in bone matrix is essential for stability and strength of bones and teeth, but such deposits are also found within other tissues in association with several diseases including atherosclerosis. CaP can interact with host proteins in vivo, including albumin and fetuin-A, which may modulate crystal interactions with cells (1). We recently demonstrated that CaP nanoparticles induced rapid rises in intracellular calcium ((Ca2+)i) in vascular smooth muscle cells (VSMCs) and subsequent cell death (2). Incubation of CaP crystals with serum prior to addition to VSMCs abolished elevation of (Ca2+)i suggesting that serum may offset the detrimental effects of CaP crystals. Compared to synthetic CaP, crystals isolated from calcified atherosclerotic tissue had little effect on (Ca2+)i and were less potent in inducing cell death, possibly due to their association with endogenous serum proteins such as fetuin-A, which has been previously shown to inhibit VSMC calcification and apoptosis (3). Due to the important role of VSMC in stabilising atherosclerotic plaques, we determined whether fetuin-A and albumin can prevent CaP-induced VSMC death. In parallel, we also investigated changes in (Ca2+)i in response to CaP crystal stimulation in the presence and absence of a range of physiological and sub-physiological concentrations of fetuin-A and albumin. Cytotoxicity assessment (MTT) revealed that the crystals reduced cell viability in a dose-dependent manner. Pre-incubation with albumin or fetuin-A (10, 3 and 1 μM but not 0.1 μM) in serum-free medium blocked the cytotoxic effects of the CaP crystals (25 µg/ml) (P < 0.0001). Furthermore, functionalised particles (fetuin-A or albumin attached to CaP) were less toxic than naked CaP. CaP-fetuin particles were less potent at evoking cell death compared to CaP-albumin. The initial (Ca2+)i oscillations appeared after 14.6 ± 1.3 min (n=16) and cells that died exhibited a large, unrecoverable (Ca2+)i peak just prior to cell death which occurred at 31.3 ± 1.76 min (n=17) after CaP addition. Only those cells where (Ca2+)i levels increased to a particular threshold died (peak amplitude (PA) 0.93 ± 0.03; n=17). In surviving cells, the (Ca2+)i oscillations were maintained or dampened over time, and were of much smaller amplitudes (PA 0.58 ± 0.04 (n=8)) compared to cells that died (P < 0.0001). Pre-incubation with 1 μM but not 0.1 μM fetuin-A or albumin silenced (Ca2+)i activity resulting in protection from cell death. The results suggest that CaP-induced cytotoxicity in VSMCs depends on (Ca2+)i, and that presence of fetuin-A or albumin can provide protection against CaP-induced cell death. The cellular mechanisms underlying this protection are currently under investigation.