Opposing effects of extracellular nucleotides on TNAP activity in VSMCs and osteoblasts. (a) ATP, (b) UTP, and (c) PPi (≥1 μM) increased VSMC TNAP activity by up to 88%. (d) ATP and (e) UTP reduced osteoblast TNAP activity by up to 50%. (f) No effect of PPi on osteoblast TNAP activity. Note that the basal levels of TNAP activity are approximately 100-fold higher in mineralizing osteoblasts than calcifying VSMCs. Values are mean ± SEM (n = 6), *p < 0.05, **p <0.01, *** p< 0.001