Research in fibrinolysis and proteolysis is focusing on extracellular matrix degradation and vascular biology

Stimulated release of tissue plasminogen activator from artery wall sympathetic nerves: implications for stress-associated wall damage

Recurrent stress is clinically associated with early onset hypertension and coronary artery disease. A mechanism linking emotion to pathogenic remodeling of the artery wall has not been identified. Stress stimulates acute regulated release of tissue plasminogen activator (t-PA) into the circulation, which is presently attributed to the vascular endothelium. Sympathetic neurons also synthesize t-PA and axonally transport it to the arterial smooth muscle. Unlike release by the endothelium, a stress-stimulated sympathetic discharge would potentially accelerate degradation of the wall matrix by plasmin. To assess whether sympathetic axons are the principal source of acute stress-induced arterial release of t-PA, we compared the output from small densely innervated and large sparsely innervated isolated artery segments before and after sympathetic stimulation, and after ablations. Following phenylephrine infusion densely-innervated microvessels in uveal eyecups were released over 60-fold greater amounts of active t-PA per milligram than the sparsely innervated aorta; and ten-fold more than carotid artery segments. Mesenteric artery release was 4.8-fold greater than release by the carotid artery. In vivo, uveal release of t-PA increased more than three-fold within one minute following superior cervical sympathetic ganglion electrical stimulation, and after phenylephrine, or nicotine infusions of the anterior chamber. Circulating levels of t-PA fell 70% following chemical sympathectomy. We propose that sympathetic nerves are the primary source of stress-induced release of t-PA into and from the densely innervated resistance arteries and arterioles, where dysregulated plasmin-induced proteolysis could damage the wall matrix.

Enhanced tissue plasminogen activator synthesis by the sympathetic neurons that innervate aging vessels

We investigated the source of the increased release of tissue plasminogen activator (t-PA) into the circulation that occurs during natural aging. Both the basal release and the acute stress-associated release induced by sympathetic stimulations are greater in older subjects. It is widely assumed that the source of these increases is vascular endothelium. However, the sympathetic neurons that densely innervate resistance vessel walls were recently shown to synthesize and transport active t-PA to axon terminals in vascular smooth muscle, suggesting an alternative source. These fine t-PA-bearing axons lie in the seldom-studied deep adventitia of vessel walls, where they are less visible than endothelium in tissue sections. Using Northern blot analysis, we observed that t-PAmRNA synthesis is increased 54% in the ganglion parent neuron cell bodies that innervate aged vessels. The t-PA release from isolated, aged ganglia in cultures was twofold greater than that from younger controls. In addition, aged whole-artery explants showed a 20% greater basal and a 50% greater acute release of stored t-PA in vitro. In vivo levels of active t-PA were 33% greater in the blood and 40% greater in the aqueous humor. These results are consistent with an increased infusion of the active t-PA protease from sympathetic axon terminals into the vessel wall extracellular matrix and the blood during natural aging, in addition to the basal endothelial release. We suggest that the cumulative impact of an accelerated plasmin production and matrix degradation within vessel walls, especially during repetitive stress, may play an unrecognized role in the pathogenesis of vascular aging. The possibility that increased sympathetic nervous system plasminogenesis influences the aging process in nonvascular tissues also deserves further investigation.

Storage and release of tissue plasminogen activator by sympathetic axons in resistance vessel walls

We studied the immunolocalization of tissue plasminogen activator (t-PA) in rat precapillary arteries, arterioles, and terminal arterioles. Lack of information about the precise location of t-PA within small vessel walls has contributed to uncertainty about its cellular source. The presumed origin has been an endothelial phenotype largely restricted to certain small vessels. However, vessel wall sympathetic axons were recently also shown to store significant amounts of a neuron-generated t-PA in secretory vesicles. Using immunolocalizations we determined the extension of t-PA-bearing axons into the resistance vasculature. Light and confocal images revealed the persistence of t-PA-bearing sympathetic nerve filaments down to the level of 15-microm-diameter terminal arterioles in vasa vasora and the choroidal microvasculature. Immunoelectron localizations confirmed the confinement of t-PA within individual nerve filaments in the deep adventitia. A complete plasminogen activator system (t-PA, plasminogen, and plasmin) was localized in the arteriolar wall matrix. Isolated iris-choroid and mesenteric artery explants from sympathectomized animals released 65 and 43% less t-PA, respectively, than controls. These data support the hypothesis that resistance vessel sympathetic axons release neural t-PA into the wall matrix and the microvascular plasma.