['Hyaluronidase, an endogenous enzyme that hydrolyzes mucopolysaccharides, has been shown to enhance myocardial protection when added to preservation solutions. In addition, hyaluronidase infusion reduces injury to ischemic myocardium. Endothelium-derived nitric oxide is an endogenous vasodilator that prevents leukocyte adhesion to the intima and inhibits platelet adhesion and aggregation in the coronary artery. Experiments were undertaken to determine whether the protective action of hyaluronidase could be mediated by the endogenous release of nitric oxide.', 'Segments of coronary artery, with and without endothelium, were placed in organ chambers (25 mL) to measure isometric force. Blood vessel segments were contracted with prostaglandin F(2)(alpha) (2 x 10(-6) mol/L) and exposed to hyaluronidase (3-15 units).', 'Hyaluronidase induced vasodilation of arteries with intact endothelium but not of arteries without endothelium (n = 6, P<.05). Endothelium-dependent vasodilation to hyaluronidase was blocked by N(G)-monomethyl-L -arginine (10(-5) mol/L), an inhibitor of nitric oxide synthesis from L -arginine (n = 6, P<.05). Inhibition of vasodilation by N(G)-monomethyl-L -arginine was reversed by L -arginine (10(-4) mol/L) but not D -arginine (10(-4) mol/L; n = 6, each group). Vasodilation to hyaluronidase also was inhibited by hemoglobin (2 x 10(-6) mol/L), a scavenger of the nitric oxide radical (n = 6, P<.05).'] Does exogenous hyaluronidase induce release of nitric oxide from the coronary endothelium?