Influence of cGMP modulators. Shown is the influence of cGMP modulators on the hyaluronan (open bars), proteoglycan (solid bars), and collagen (cross-hatched bars) production of bovine cartilage. Cartilage explants were cultured in the presence of 2 ng/ml IL-1 and the guanylate cyclase inhibitor ODQ (1H-[1,2,4]-oxadiazolo [4,3a]quinoxaline-1-one; 25 μmol/l) or dibutyryl-cGMP (25 μmol/l). The incubation periods were for 3 days for hyaluronan, 5 days for proteoglycans and 28 days for collagens. For stimulation of collagen degradation, the cartilage explants were supplemented with 25 ng/ml IL-17, and 2 μmol/l retinoic acid. The concentrations of hyaluronan in the supernatant, proteoglycans and collagen (as hydroxyproline) were determined as described under Materials and methods. The error bars represent the standard deviation of three determinations.