Prevention of Vascular Calcification by Polyphosphates and Nucleotides - Role of ATP

Background: In recent decades, the prevention of vascular calcification (VC) by pyrophosphate (PPi), bisphosphonates, and polyphosphates has been extensively reported. However, the possibility of direct inhibition of calcium phosphate deposition (CPD) by nucleoside-associated polyphosphates has not been addressed. We analyzed the role of ATP as an inhibitor of calcification in 2 ways: by characterizing the extracellular hydrolysis of ATP as source of PPi in the aorta, and by demonstrating the ability of ATP to prevent CPD by acting as a polyphosphate. Methods and Results: In our study, both PPi and ATP hydrolysis in the rat aorta was kinetically characterized, thereby resulting in apparent Michaelis-Menten constants of 179 and 435 mu mol/L, respectively, with the corresponding maximal velocities of 55.1 and 6,177 nmol.g(-1).min(-1). According to these kinetic parameters, the theoretical PPi concentration in the aortic wall was 0.4-3.5 mu mol/L (for an ATP concentration range of 0.1-1.0 mu mol/L). In addition, we showed that nonhydrolyzable molecules are more efficient as CPD inhibitors than endogenous compounds, in accordance with the IC50 values: 1.2-2.4 mu mol/L for bisphosphonates vs. 8.8 mu mol/L for PPi, and 0.5-1.5 mu mol/L for nonhydrolyzable ATP analogs vs. 3.2 mu mol/L for ATP. Conclusions: Extracellular ATP can play an important role in the prevention of VC, not only as the source of PPi but also as a direct inhibitor of CPD.