New transgenic evidence for a system of sympathetic axons able to express tissue plasminogen activator (t-PA) within arterial/arteriolar walls

Sympathetic axons embedded in a few arterioles and vasa vasora were recently shown to store tissue plasminogen activator (t-PA) in vesicles. But the extension of such t-PA axons to arteries and arterioles throughout the organism has not been verified. Confirmation of this anatomy would identify a second significant source of vessel wall t-PA. To visualize fine embedded axons independent of endothelium, we created a transgenic mouse whose expressions of the t-PA promoter and enhanced green fluorescent protein are confined to sympathetic neurons and other neural crest derivatives. Confocal images reveal the extension of t-PA axons to arterioles serving heart, brain, kidney, lung, mesentery, and skin; plus aortic, carotid, and mesenteric artery walls. Ganglion neurons and adrenal chromaffin cells also show strong expressions. These new sightings confirm the existence of a system of t-PA axons that is prominent in arterioles, and compatible with the release of neural t-PA into their walls.

Cardiac fibrinolytic capacity is markedly increased after brief periods of local myocardial ischemia, but declines following successive periods in anesthetized pigs

BACKGROUND

Fibrinolysis in blood is mainly reflected by the activities of tissue plasminogen activator (tPA) and of plasminogen activator inhibitor-1 (PAI-1). The effect of myocardial ischemia on their activities in the coronary circulation is, however, not established.

OBJECTIVES

With an improved experimental model, we therefore examined the effect of a brief period of myocardial ischemia on their activities. Furthermore, the consequences of repeated periods of ischemia, mimicking the situations in patients with unstable angina, were investigated.

METHODS

In six anesthetized pigs, we occluded the distal left anterior descending coronary artery (LAD) four times for 10 min with 40 min intervals and determined the activities of tPA and PAI-1 in arterial and coronary venous blood. By simultaneously recording LAD flow, we could estimate cardiac release of these factors at baseline conditions and during reperfusion.

RESULTS

Neither net cardiac release of PAI-1 nor alterations in plasma PAI-1 levels were demonstrated during the experiment. However, a significant net release of tPA activity of 10.4 +/- 3.2 IU mL(-1) (P < 0.005) was recorded during baseline conditions. During reperfusion following the first period of ischemia, the cardiac release of tPA activity increased to a peak of 103 +/- 30-fold baseline release, but declined progressively after repeated periods of ischemia. After the fourth period, tPA release did not exceed an estimated baseline accumulation during ischemia and early reperfusion.

CONCLUSIONS

In this porcine model, a substantial local increase in fibrinolytic capacity was observed after brief periods of ischemia, but declined subsequently by repeated periods of ischemia.

Distribution of sympathetic tissue plasminogen activator (tPA) to a distant microvasculature

Tissue plasminogen activator (tPA) is the predominant plasminogen activator present in the vascular and nervous systems. Prior studies of the two have emphasized different tPA sources; respectively, endothelium and neurons. A closer relationship is now suggested by evidence that the peripheral sympathetic nervous system synthesizes and infuses enzymatically active tPA into small artery walls and the microcirculation. TPA may thus be the only known neural product able to effect degradation of the artery wall extracellular matrix. This brief review considers historical and current indications for the existence of such an autonomically controlled system and some physiologic implications. Immunohistochemical tPA expression in small arteries and arterioles is more prominent in the outer wall sympathetic axon plexus than in endothelium. Its presence in nerve filaments beneath the seldom-studied adventitia was obscured in earlier localizations. The systemic impact of a neural distribution is suggested by a 60% reduction of blood tPA activity after chemical sympathectomy. TPA-bearing axons extend outward from ganglion neuron cell bodies to reach even thin-walled vasa vasora and uveal microvessels. Ganglion cell bodies synthesize and package tPA in vesicles for the long axoplasmic transport. Densely innervated intact vessels release much greater amounts of tPA in vitro than do larger vessels, indicating a high neuron tPA production capacity and a large storage reservoir available within axon networks. The influence of an autonomically controlled plasmin production within small artery walls on regulation of blood pressure and capillary perfusion awaits further investigation. Its possible role in the pathogenesis of vessel wall matrix degradations in aging, hypertension, and diabetes may also merit further consideration.

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.

Additional evidence that the sympathetic nervous system regulates the vessel wall release of tissue plasminogen activator

It is established that sympathetic neurons can synthesize, transport and store tissue plasminogen activator (t-PA) within axon terminals in the smooth muscle of vessel walls. Moreover, sympathetic excitations (e.g. physical and mental stress) are known to induce an acute release of t-PA into the circulation. However, relatively little is known about the nature and extent of sympathetic nervous system involvement in the release process. We inquired whether a chemical sympathectomy will alter the release of t-PA into the blood, and the intrinsic release of stored t-PA from isolated whole vessel explants. A long-term sympathectomy was induced in adult Sprague-Dawley rats by injection of guanethidine during a 5-week course. The destruction of ganglion neurons and vessel wall axons was verified immunohistochemically. t-PA release was assayed as the free activity in hind limb plasma and explant culture medium. Following sympathectomy: (i) the basal t-PA activity in plasma was 70% less than controls (2.92 +/- 1.96 versus 9.33 +/- 1.72 IU/ml; </= 0.001); (ii) the acute release from isolated vessels induced by bradykinin or phenylephrine was comparably reduced; and (iii) the greatest reductions occurred in densely innervated small vessel explants. The results provide new support for an autonomic regulation of neural t-PA release into the vessel wall matrix and blood of densely innervated thin-walled microvessels.