Inhibition of hyaluronan export in bovine chondrocytes. (a) Cartilage explants were incubated in the absence and presence of IL-1α and the drugs tadalafil (○), zaprinast (□), or vardenafil (△). The concentration of hyaluronan was determined in the supernatant after 3 days. The error bars indicate the standard deviation of three determinations. (b) Effect of inhibitors on hyaluronan synthase activity. A particulate fraction of chondrocytes was prepared and incubated with the substrates UDP-GlcNac and UDP-[14C]GlcA and increasing concentrations of tadalafil (○), zaprinast (□), or vardenafil (△), and the incorporation into [14C]hyaluronan was determined.