Cathepsin L is a cysteine protease that can generate endogenous endostatin in vascular and epithelial basement membranes and importantly participates in a variety of pathophysiological processes. The present study was designed to determine whether this cathepsin L-derived endogenous endostatin alters endothelium-dependent vasodilator responses in coronary arteries via NAD(P)H oxidase activation. In isolated and perfused small bovine coronary arteries, administration of cathepsin L (200 ng/ml) markedly attenuated endothelium-dependent vasodilator responses to bradykinin or A23187 by 56.16+/-9.58% and 68.95+/-10.32%, respectively. This inhibitory effect of cathepsin L on endothelium-dependent vasodilator responses could be significantly reversed by pre-incubation of the arteries with O(2)(-) scavenger, Tiron, or neutralizing anti-endostatin antibody. By fluorescent ELISA assay, cathepsin L dose-dependently increased endostatin production in coronary arteries. In situ high-speed dual wavelength switching fluorescent microscopic imaging showed that cathepsin L decreased bradykinin- and A23187-induced NO levels in the intact endothelium, but it had no effect on Ca(2+) response to these vasodilators. This cathepsin L-induced reduction of NO was restored by the pretreatment of an anti-endostatin antibody. Electron spin resonance (ESR) analysis demonstrated that cathepsin L increased O(2)(-) production which could be markedly attenuated by the NAD(P)H oxidase inhibitors, apocynin or anti-endostatin antibody. It is concluded that endostatin could be endogenously produced in coronary arteries when cathepsin L is increased and that this cathepsin L-derived endostatin, if excessive, may result in endothelial dysfunction through enhanced production of O(2)(-) due to NAD(P)H oxidase activation.