Purinergic transmitters inhibit bone formation by cultured osteoblasts

Adenosine triphosphate (ATP) and other purinoceptor agonists cause a transient rise in [Ca2+]i in cultured osteoblast-like cells and have a mitogenic effect, as does parathyroid hormone (PTH), and there is evidence that ATP and PTH can act synergistically on osteoblasts. The likelihood that nucleotides, acting through purinoceptors, are important local factors in bone remodeling is therefore considerable. However, their effect on bone formation is unknown. We recently developed a culture system in which appositional bone formation occurs only in narrow grooves cut in a substratum. We have used this as an assay to measure the effects of ATP (50 and 500 μmol/L), ATPγS (20 μmol/L), 2-MeSATP (2 and 20 μmol/L), uridine triphosphate (UTP) (0.2, 2, and 20 μmol/L), adenosine (20 μmol/L), bovine PTH (0.25 and 0.5 IU/mL), rat PTH1–34 (10−8 and 10−7 mol/L), and rat PTHrP1–40 (10−9 and 10−8 mol/L) on bone formation by rat calvarial osteoblasts. The culture medium was renewed 3 times/week (every 2 or 3 days), and the number of bone loci and length and area of Alizarin red-stained mineralized bone formed in the grooves of each specimen in 16–29 days were measured. Compared with controls, ATPγS, 2-MeSATP, and ATP reduced the amount of bone formed in a 2–3 week culture period. Adenosine had no effect, and UTP either had no effect or at 2 μmol/L stimulated bone formation. PTH and PTHrP completely abolished bone formation in 4 week cultures. Our findings are consistent with evidence for more than one P2 purinoceptor subtype in bone, and show for the first time that the effect of ATP on appositional bone formation by osteoblasts in vitro is, like PTH and PTHrP, inhibitory.