XDH and cystic fibrosis: To determine the sources of O2− production involved in the induction of apoptosis in CF cells, both pancreatic and tracheal CF cells were incubated in the presence of inhibitors of xanthine oxidase (allopurinol), NADPH oxidase (apocynin) or mitochondrial complex I (rotenone), and Act D-induced apoptosis was evaluated.