Presence of tissue plasminogen activator (t-PA) in the adventitial sympathetic nerves that innervate small arteries: morphologic evidence for a neural fibrinolysis

Abstract Objective : Previous functional studies have indicated that: (i) sympathetic neurons synthesize, store and release tissue plasminogen activator (t-PA); (ii) small, densely innervated arteries release more t-PA than large sparsely innervated ones; and (iii) sympathectomy greatly reduces arterial t-PA release. This study was done to provide morphologic evidence of the existence of a non-endothelial fibrinolytic system within the adventitial nerve plexus of small arteries and arterioles. Methods : t-PA and neuropeptide Y (NPY) localizations in the adventitial nerve fibers of small and large arteries were examined. Whole mount preparations were used to emphasize immunostaining of the adventitial plexus and the iris. Using confocal and electron microscopy, t-PA was also immunolocalized in isolated sympathetic neurons and the axons of small arteries. The previous comparative t-PA release from cultured small and large artery explants was re-examined. Results : Small arteries – surface views of the mesenteric adventitia showed confinement of t-PA to a dense plexus of NPY-positive sympathetic nerve fibers. Cryosections confirmed the presence within a multilayered plexus at the smooth muscle interface, and in the endothelial monolayer. Comparatively, sections of arterioles showed a heavier layer of NPY- and t-PA-positive material that occupied most of the wall thickness. Electron microscopic views confirmed the discrete confinement of non-endothelial t-PA within the mesenteric artery adventitial plexus. In control plexus immunostainings of the iris, t-PA was largely confined to the densely innervated dilator muscle. Large arteries – surface views of the carotid and aortic adventitia showed a sparse presence of thin NPY and t-PA positive lines that followed the contours of vasa vasora walls, within which endothelial and nerve fiber t-PA could not be separately identified. Cryosections of the carotid and aorta adventitia also showed a sparse NPY and t-PA staining limited to isolated arterioles and axons. Endothelial t-PA immunostaining in carotid and aorta whole mount sections was scant. Neurons – t-PA immunostaining of most sympathetic ganglion neurons was positive. Confocal images of isolated sympathetic neurons revealed the storage of immunoreactive t-PA in closely packed secretory granules within the cell body and axons. Conclusions : • The adventitial sympathetic nerve plexus contains the major portion of the immunoreactive t-PA that is stored in small artery walls. • Like norepinephrine t-PA is transported and stored in secretory granules within the sympathetic axons that preferentially innervate small arteries. • The content and release of t-PA from artery walls appears inversely proportional to vessel diameter, and directly proportional to innervation density. These findings are consistent with the hypothesis that t-PA is supplied to resistance arteries and arterioles by the sympathetic nervous system.