Localization of nitric oxide synthase type III in the internal thoracic and radial arteries and the great saphenous vein: a comparative immunohistochemical study.

Background: Endothelial nitric oxide synthase type III is the key enzyme of the nitric oxide production in the vessel wall. In this study the localization of endothelial nitric oxide synthase type III within the wall of the human internal thoracic and radial arteries and the great saphenous vein was investigated.

Methods: Specimens were harvested from 23 patients undergoing surgical myocardial revascularization and submitted to light and electron microscope analysis using histochemical stainings and immunohistochemistry with specific antibodies anti-endothelial nitric oxide synthase type III, Factor VIII, and alpha-smooth muscle actin.

Results: Endothelial nitric oxide synthase type III was evident in the intima of all conduits and, unexpectedly, in the muscle cells of the media of muscular internal thoracic arteries and radial arteries. No endothelial nitric oxide synthase type III expression was found in the media of great saphenous veins. Semiquantitative analysis revealed a higher endothelial nitric oxide synthase type III expression in the wall of internal thoracic artery, particularly at the level of the media.

Conclusion: Endothelial nitric oxide synthase type III is expressed in the intima of the internal thoracic and radial artery and the great saphenous vein and in the muscle cells of the media of the internal thoracic and radial arteries. However, the internal thoracic artery shows a higher intensity of endothelial nitric oxide synthase type III expression, particularly within the media. The present study provides the first demonstration of the endothelial nitric oxide synthase type III expression at the level of the smooth muscle cells of the tunica media of systemic human arteries and can provide an histologic explanation for the better results of the internal thoracic artery when used for coronary artery bypass grafting.