Quantification of pro-peptides of collagen type II. (a) Quantification of collagen type II synthesis from all treatments over the entire experimental period. Collagen type II synthesis in cartilage explants was measured by the concentration of N-terminal pro-peptides of type II collagen in the conditioned medium using the PIINP enzyme-linked immunosorbent assay (ELISA). The curves represent the release found at the specific day, where the conditioned medium was fully replaced, and the values were accumulated over the entire period. Vehicle control, metabolically inactive (MI), O + T, oncostatin M plus tumour necrosis factor. (b) Quantification of collagen type II formation 2 weeks after the catabolic induction. The collagen type II synthesis in the identical 14-day periods with or without IGF stimulation following the three different periods of catabolic stimulation was measured by the PIINP ELISA. The conditioned medium was fully replaced three times a week. The results show the accumulated release of collagen type II pro-peptide during the two weeks with anabolic stimulation (insulin growth factor, IGF) and without stimulation (vehicle). IGF-I significantly induced collagen type II formation at low and intermediate catabolic insult, but not at maximal insult. *P < 0.05. PIINP, N-terminal pro-peptide of pro-collagen type II.