The NO content of medium supernatant normalized to the Con (n = 1). Stimulation of immortalized chondrocytes (a) and articular cartilage explants (b) with PMA (μg per 1 ml of medium) or SIN-1 (μM). The level of NO is increased after stimulation with reactive oxygen species donors. Results are shown as mean ± standard error of the mean for five separate experiments. * p < 0.05 versus Con. Con, control; NO, nitric oxide; PMA, phorbol myristate acetate; SIN-1, 3-morpholino-sydnonimine hydrochloride.