Effect of transforming growth factor-beta-1 (TGF-β1) on extracellular inorganic pyrophosphate (ePPi) and nucleotide pyrophosphatase phosphodiesterase (NPPase) activity. (a) Kinetics of ePPi levels in culture supernatant of rat chondrocytes stimulated with TGF-β1 (10 ng/mL). ePPi was assayed radiometrically and normalized to the amount of total cell proteins (n = 6). Data are expressed as mean (± standard deviation [SD]) in picomoles per microgram of protein. (b) NPPase activity in cultured rat chondrocytes stimulated with 10 ng/mL of TGF-β1. Enzyme activity was normalized to the amount of total cell proteins (n = 3). Results are expressed as mean (± SD) in micromoles of paranitrophenol per minute per milligram of protein. Statistically significant differences from the control are indicated as *p < 0.05.