Inhibition of protein infiltration into bovine cartilage explants. Cartilage explants were incubated in the absence or presence of IL-1α and (a) tadalafil, (b) zaprinast, or (c) vardenafil for 14 days at 37°C. The explants were then incubated with horseradish peroxidase as indicator protein for 1 hour. After washing, the amount of enzyme that had infiltrated the explants was determined by a colour reaction. The error bars represent the standard deviation of eight determinations.