Mini-Review

Distribution of Sympathetic TissuePlasminogen Activator (tPA) to aDistant Microvasculature

James O’Rourke,* Xi Jiang, Zhifang Hao, Robert E. Cone, and Arthur R. HandUniversity of Connecticut Health Center, Farmington, Connecticut

Tissue plasminogen activator (tPA) is the predominantplasminogen activator present in the vascular and ner-vous systems. Prior studies of the two have emphasizeddifferent tPA sources; respectively, endothelium andneurons. A closer relationship is now suggested by evi-dence that the peripheral sympathetic nervous systemsynthesizes and infuses enzymatically active tPA intosmall artery walls and the microcirculation. TPA may thusbe the only known neural product able to effect degra-dation of the artery wall extracellular matrix. This briefreview considers historical and current indications for theexistence of such an autonomically controlled systemand some physiologic implications. Immunohistochemi-cal tPA expression in small arteries and arterioles is moreprominent in the outer wall sympathetic axon plexus thanin endothelium. Its presence in nerve filaments beneaththe seldom-studied adventitia was obscured in earlierlocalizations. The systemic impact of a neural distributionis suggested by a 60% reduction of blood tPA activityafter chemical sympathectomy. TPA-bearing axons ex-tend outward from ganglion neuron cell bodies to reacheven thin-walled vasa vasora and uveal microvessels.Ganglion cell bodies synthesize and package tPA in ves-icles for the long axoplasmic transport. Densely inner-vated intact vessels release much greater amounts oftPA in vitro than do larger vessels, indicating a highneuron tPA production capacity and a large storage res-ervoir available within axon networks. The influence of anautonomically controlled plasmin production within smallartery walls on regulation of blood pressure and capillaryperfusion awaits further investigation. Its possible role inthe pathogenesis of vessel wall matrix degradations inaging, hypertension, and diabetes may also merit furtherconsideration. © 2005 Wiley-Liss, Inc.

Key words: tissue plasminogen activator; distribution bysympathetic neurons; degradation

The tissue plasminogen activator (tPA)–plasmin sys-tem degrades fibrin and facilitates the degradation of otherextracellular matrix (ECM) proteins. Plasmin in its zymo-

gen form, plasminogen, is distributed ubiquitously in bodytissues. Once activated, it has broad substrate specificity,directly degrades some matrix proteins, and activates latentmetalloproteinases. It is therefore thought to assist cellmigration, chemotaxis, and tissue remodeling duringmany physiologic and pathologic processes (Kim et al.,1998; Li et al., 2003; Syrovets and Simmet, 2004).

Currently, the endothelial lining of multiple unspec-ified vessels is thought to be the principal source of the tPAthat is released into the circulation (Van Hinsbergh et al.,1991; Emeis and van den Eijnden-Schrauwen, 1996; Steinet al., 1998). Not all vessel categories and sizes have beenshown to express endothelial tPA, however, and signifi-cant regional differences have been described (Levin anddel Zoppo, 1994; Levin et al., 1997). Moreover, sympa-thetic neurons that densely innervate the walls of ratprecapillary resistance arteries and arterioles were shownrecently to store and release much greater amounts of tPAthan the adjacent endothelium (Jiang et al., 2000, 2002).The putative significance of a neural tPA–plasmin prote-olysis in these walls is that smooth muscle and ECMvasomotions critically regulate systemic pressure anddownstream capillary perfusion (Luff, 1991; Guyton andHall, 2000). The prominent storage in resistance arteriesmay therefore suggest a need to concentrate a plasmin-producing serine protease within the arterial microvascu-lature. The extent to which this occurs in all resistancevessels and in other species or neuron types remains un-known. Nonetheless, its prominent storage within the thinwalls of such vessels does offer an alternative (non-

Contract grant sponsor: NIH-NEI; Contract grant number: 522 589;Contract grant sponsor: Connecticut Lions Eye Research Foundation;Contract grant sponsor: Lions Clubs International Foundation.

*Correspondence to: James O’Rourke, MD, University of ConnecticutHealth Center, 263 Farmington Avenue, Farmington, CT 06030-3105.E-mail: jorourke@uchc.edu

Received 10 August 2004; Revised 6 October 2004; Accepted 7 October2004

Published online 27 January 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jnr.20366

Journal of Neuroscience Research 79:727–733 (2005)

© 2005 Wiley-Liss, Inc.

endothelial) explanation for gaps in our understanding ofthe mechanisms whereby adrenergic agonists and otherstimulations induce an acute release of vessel wall tPA intothe blood (Jern et al., 1994). Unlike other neural-releasedneurotransmitters and neuropeptides, tPA has the uniquecapacity to degrade the innervated artery wall matrix andthereby assist or retard its compliance. Past and currentevidence for the existence of such a sympathetic tPAdistribution to small artery walls is outlined below.

PAST SYMPATHETIC–BLOODFIBRINOLYSIS ASSOCIATIONS

Morgagni (1769) and Hunter (1794) over two cen-turies ago described prolonged bleeding during dissectionsof the bodies of obviously stressed executed criminals.Efforts to explicate this presumed sympathetic responsehave tended to emphasize either vessel wall sources of afibrinolytic factor (i.e., tPA), or adrenergic influences onits release, as highlighted below.

VESSEL WALL SOURCESIn 1948, during autopsies of victims of violent death,

Mole reconfirmed prolonged bleeding and prescientlynoted that it principally came from small vessels. On thisbasis, he proposed an inverse relationship between vesselsize and the release of “fibrinolysin,” and nominated en-dothelium as its probable source (Mole, 1948). At thistime, Astrup and colleagues had recently identified tissueplasminogen activator (tPA) as the fibrinolytic factorpresent in extracts of vessel walls and other tissues (Astrupand Permin, 1947). A detailed account of the long evo-lution of this research was published 30 years later (Astrup,1988). Beginning in 1959, innovative fibrin slide histo-chemistry by Todd (1959) made it possible to roughlylocalize vessel wall fibrinolytic activity, later refined bytPA immunohistochemistry. This work located tPA to theendothelium of several vessels, but also showed frequentlya high activity in the outer wall adventitia (Todd, 1959;Hegt and Brakman, 1974), for which the endothelium ofvasa vasora became the widely assumed source. (Emeis andvan den Eijnden-Schrauwen, 1996; Levin et al., 1997).Subsequent studies reported the release of tPA from cul-tured endothelial cells and described rapid release whenstimulated by thrombin, presumably from endothelialstorage vesicles (Van Hinsbergh et al., 1991; Emeis andvan den Eijnden-Schrauwen, 1996). These reports indi-cated that endothelium possesses a stimulated, or regu-lated, release pathway for tPA, in addition to a constitutivepathway. In this period, new attention was drawn to smallvessels by noting the prominence of tPA expression byendothelium of certain small vessels in certain locations,suggesting the existence of a unique small vessel pheno-type (Levin and del Zoppo, 1994; Levin et al., 1997). Theculmination of these source studies was wide acceptance ofendothelium as the sole or predominant source of vesselwall tPA, whereas the tPA source was seldom closelyanalyzed in adventitia. Its location in nerve filaments couldnot be visualized in the few studies of the adventitia thenpublished (Heistad et al., 1979; Martin et al., 1991; Phillips

et al., 1995; De Meyer et al., 1997; van der Loo andMartin, 1997; Scotland et al., 1999).

ADRENERGIC EFFECTSBiggs et al. (1947) demonstrated a sharp rise in the

blood fibrinolytic activity of human subjects after adren-aline injections, exercise and stress. The observation wassoon confirmed by others (Fearnley et al., 1952), whichopened up a new adrenergic phase of fibrinolytic studies.Local perivenous injections of adrenaline, acetylcholine,or serotonin were then shown to increase fibrinolyticactivity in the local venous circulation (Kwaan et al.,1957a,b).

It was next confirmed that electric shock and otherstressors in humans stimulated the release of tPA per sefrom vessel walls into the plasma (Sherry et al., 1959; Jernet al., 1994). The response was assumed widely to reflectan endothelial release triggered by elevated levels of cir-culating adrenal catecholamines (Cash, 1978; Chandler etal., 1995). Direct experimental proof for an in vivo endo-thelial response was lacking, however, and catecholamineresponses were not reproduced later in cultured endothe-lium. Recent studies have also demonstrated an acuteregional release of tPA in vivo after intraarterial infusionsof epinephrine and other vasoactive agents (Bjorkman etal., 2003).

Additional non-fibrinolytic studies by neuroscientistsin the 1980s began to describe a tPA presence in centralnervous system (CNS) pathways including the hypothal-amus and hippocampus (Alvarez-Bullya and Valinsky,1985; Pittman, 1985; Pittman and Buettner, 1989; Garcia-Rough et al., 1994; Strickland, 2001). A key observationin that period was evidence that tPA in the developingmouse is expressed preferentially in neural ectoderm-derived tissues (Soreq and Miskin, 1981; Friedman andSeeds, 1994). An important anatomic observation at thistime was the common tendency of the adventitial sympa-thetic nerve plexus beneath the arterial adventitia to in-crease in density and axon numbers as the vessel sizedecreases down to the level of precapillary terminal arte-rioles (Burnstock, 1975; Luff, 1991; Jiang et al., 2000).More recently, cultured sympathetic neuron analogues,including PC12 and adrenal chromaffin cells, were shownto contain and transport tPA in axoplasmic vesicles (Loch-ner et al., 1998; Parmer et al., 1997), early clues that aneural protease may travel to artery walls.

In sum, past interpretations of the Morgagni-Hunterobservation have confirmed a strong adrenergic associa-tion to vessel wall tPA release, but were unable to establishendothelium as its sole or primary source.

MORPHOLOGIC EVIDENCEFOR A SYMPATHETIC

MICROVASCULAR DISTRIBUTIONPrecise immunolocalization, the most visible evi-

dence of a tPA source, is also a key indicator of its localfunction. Due to excess inhibitors, the free enzyme onlyremains active for a short time in the immediate vicinity ofits release. A long-standing gap in tPA vascular studies has

728 O’Rourke et al.

been the lack of precise localizations of the enzyme sourcewithin the thin walls of a vast microvasculature, whoseendothelium is currently considered the primary source ofblood tPA activity.

MICROVASCULAR DIMENSIONSThe microvasculature (vessels of less than 100-�m

diameter) occupies roughly 95% of the total vascular sys-tem (Guyton and Hall, 2000). Precapillary small arteriesand arterioles are of special interest because they generallyreceive the richest sympathetic innervation in most vas-cular systems (Burnstock, 1975; Luff, 1991; Jiang et al.,2000). Based on tissue counts in animals, the adult humancan be estimated to possess roughly 10 million denselyinnervated arterioles of width 15–100 �m and averagelength 1.2 mm (Milnor, 1989; Luff, 1991). Situated at thehead of a network of 10 billion capillaries, their changingdiameters critically regulate systemic pressure and nutrientperfusion (Guyton and Hall, 2000). The release of a neuralserine protease at such locations would theoretically exerta major influence on microvascular wall and blood plasminproduction, but has not been shown by direct experiment.The microvasculature is dispersed widely, largely invisible,and difficult to access for such studies. Much of what hasbeen assumed about the process has therefore been in-ferred from studies of larger vessels that receive a sparsesympathetic innervation.

An additional limitation has been the general lack ofclose studies of adventitial tPA locations. Bright-fieldimages of vessel cross sections have been unable to distin-guish discrete 0.5-�m tPA bearing nerve filaments in theadventitia of thin-walled microvessels from the adjacentendothelium. The pattern and density of tPA-bearingsympathetic arborizations on the outer surface of small

vessels (including 15-�m wide vasa vasora of large vessels)is most visible in whole-mount preparations of isolatedintact vessels (Burnstock, 1975). Penetration of surfaceaxon terminals into the smooth muscle can also be shownin confocal z-sections of the intact whole mounts (Jiang etal., 2002).

MICROVASCULAR SYMPATHETICTPA PRESENCE

The unanticipated presence of tPA in microvascularaxons was observed first during immunoelectron micro-scopic localizations of the enzyme in the rat uvea (Wang etal., 1997). Based on earlier studies, the intent was todetermine the source of the extravascular tPA found inaqueous humor. Prepared as eyecups, the densely inner-vated iris and choroid provide a unique access to rich,adrenergically innervated microvascular beds, and to theiris dilator (Wang et al., 1995). These electron microscopicimmunogold particle tPA localizations revealed a near-exclusive confinement of the enzyme within the axoplasmof the uveal adventitial sympathetic nerve filaments, ratherthan endothelium.

Subsequent bright-field and ultrastructural localiza-tions of tPA in the adventitia of small mesenteric arteriesand arterioles have since confirmed that the dense plexusof sympathetic nerve fibers presents a much larger area oftPA immunostaining than does the endothelium (Fig. 1).Additional images of the adventitial surface have shown amuch greater density of interlacing tPA-bearing nervefibers in small arteries and arterioles than in larger arteries(Jiang et al., 2000). Consistent with past observations ofthe dense arteriolar innervation (Burnstock, 1975; Luff,1991), these neural depots could account for the capacityof adrenergic stimulations to acutely increase blood fibri-

Fig. 1. TPA immunolocalization in rat small mesenteric artery andheart. Left, center: TPA immunostaining (brown) is most prominent indense subadventitial nerve plexus on smooth muscle surface of smallmesenteric artery. Right: Immunofluorescent localization of tPA incoronary artery and cardiac muscle nerve fibers. Scale bars � 25 �m.

Bottom: Immunoelectron tPA localization in terminal mesenteric ar-teriole (black) confined to subadventitial nerve fibers. Scale bar �1 �m. sm, smooth muscle. (Left, center, and bottom from Jiang et al.,2000, reprinted with permission from Elsevier.)

Sympathetic Tissue Plasminogen Distribution 729

nolysis, postmortem small vessel bleeding, and the ten-dency of only certain small vessels to express tPA (Levinand del Zoppo, 1994; Levin et al., 1997). Confocalz-section images have also confirmed small artery wallpenetrations of tPA-bearing axon terminals into small ar-tery walls to depths greater than 50 �m (Jiang et al., 2002;Wang et al., 2002).

TPA-bearing axons are found less frequently beneaththe venous adventitia and are sparse in large conduitarteries, but do appear in heart muscle (Fig. 1). The thinimmunostained walls of 15-�m wide vasa vasora withinthe adventitia of large vessels, e.g., carotid and aorta, alsocontain tPA-bearing, sympathetic axons (Jiang et al.,2000). Moreover, whole mounts of the immunostainediris and choroidal microvascular beds have revealed theextension of tPA-bearing sympathetic axons to the small-est precapillary microvessels (Jiang et al., 2002). Althoughcapillaries per se have been reported occasionally to con-tain nerve fibers, none have yet been shown to expresstPA.

TPA SYNTHESIS BY DISTANTSYMPATHETIC NEURONS

The tPA protein present in microarterial wall axonscan only be synthesized by distant parent neuron cellbodies located mainly in paravertebral sympathetic ganglia.These typically lie at long distances from innervated mi-crovessels and other tissues. Immunohistochemical local-izations have confirmed a prominent tPA expression inmost but not all cell bodies in the superior cervical gan-glion that innervates cephalic and uveal tissues (Jiang et al.,2000). Rich innervation of the mesenteric artery branchesby tPA-bearing fibers indicates a similar tPA synthesis inthe lower thoracic sympathetic ganglia. In situ hybridiza-tion and Northern blot analysis have shown strong tPAmRNA expression by isolated ganglion neurons (Wang etal., 2002; Jiang et al., 2003). Confocal immunolocaliza-tions in isolated cultured ganglion neurons and axons havevisualized tPA packaged in vesicles during axoplasmictransport, similar to that described earlier in cultured PC12and chromaffin cell analogues (Harrison et al., 1996;Parmer et al., 1997; Lochner et al., 1998; Jiang et al.,2000). In addition, �1 receptors have been morphologi-cally immunolocalized to prejunctional axon terminals onthe smooth muscle surface of small resistance arteries (Jianget al., 2002; Razzaq et al., 2003). These morphologicfindings seem more compatible with an acute exocytoticburst of tPA from microarterial axon terminals per se afteradrenergic stimulation than from a non-innervated endo-thelium. The long axonal distribution of the enzyme topresynaptic clefts in small artery smooth muscle is thus anotable exception to the general belief that active tPA isreleased in the immediate vicinity of its cell sources. Themany tPA-filled axons in microvascular walls instead sug-gest a high-production capacity by individual ganglionneurons, and the existence of a sizable reservoir of auto-nomically regulated, releasable tPA within the systemicmicrovascular sympathetic network.

The unique architecture of this peripheral tPA-producing sympathetic system suggests a substantial capac-ity to autonomically reinforce plasmin production withinthe arterial microvasculature, including the tiny vasa va-sora and vasa nervosa that, respectively, nourish largeartery walls and nerve trunks.

FUNCTIONAL CORROBORATIONA notable limitation of early vascular tPA studies was

the inability to assay directly a stimulated tPA release fromindividual small vessels. Much of what is understood cur-rently about the release has thus been inferred from globalassays of the tPA activity remaining in venous bloodsamples, which is mainly determined by the release fromremote small vessels (Kluft, 1992). Recent studies of iso-lated whole vessel release have tended to support morpho-logic evidence that the arterial microvasculature is prefer-entially supplied with added amounts of tPA by its densesympathetic innervation. (This had been difficult to de-termine from blood assays because the protease only exitsvessel walls in its free active form, and unless bound tofibrin or other matrix or cell surfaces is quickly complexedto surplus plasma inhibitors) (Van Hinsbergh et al., 1991;Emeis and van den Eijnden-Schrauwen, 1996). Packagedin vesicles during its axoplasmic transport, the neural pro-tease is presumably shielded from inhibitors including axo-plasmic neuroserpins. Nonetheless, once released into thebloodstream, its level can be altered further by hepaticclearance, local blood flow changes, or unpredictable sym-pathetic discharges (Jern et al., 1994; Chandler et al.,1995). To offset these effects and compare the releasecapacity of different individual vessels, investigators haveutilized in vitro release assays of isolated whole vesselsegments (Wang et al., 1998, 2002; Jiang et al., 2003). Inthese experiments, individual weighed samples are incu-bated in superfused short-term organ cultures in which thetotal release of stored tPA from endothelium or axonterminals can be serially assayed at brief intervals. Differ-ences in ganglion neuron tPA synthesis have also beenstudied in such cultures (Jiang et al., 2003). In this wayconstitutive and acute regulated releases have been com-pared, and the influences of artery size, agonist stimula-tion, adventitial–endothelial ablations, and chemical sym-pathectomy have been investigated.

Basal release of active tPA (per milligram) fromdensely innervated rat microvascular iris-choroid eyecupsis over twofold greater than is that from small mesentericartery segments, and eightfold greater than that from ca-rotid artery samples (Fig. 2). Reductions of tPA basalrelease after chemical sympathectomy are also far greaterfor microvascular eyecups than for larger vessels. More-over, chemical sympathectomy in these animals reducedbasal and acute releases from isolated vessels, and induceda greater than 60% reduction in circulating levels of activetPA (Wang et al., 2002) (Fig. 2). Mechanical ablations ofthe axon-bearing adventitia from isolated small mesentericarteries generally show greater reductions of tPA releasethan does removal of endothelium. The release from mi-crovascular eyecups induced by the �1 agonist, phenyl-

730 O’Rourke et al.

ephrine, was also more than twofold greater than that fromsmall mesenteric artery samples, and sixfold above therelease from carotid samples (Wang et al., 1998, 2002;Jiang et al., 2000). Consistent with these results, infusionsof 1 � 105 M phenylephrine into the anterior chamber offeline eyes or electrical stimulations of the superior cervicalsympathetic ganglion have each shown greater than three-fold increases in the acute release of tPA into the aque-ous humor (unpublished observations). Recalling Mole(1948), these observations seem compatible with the con-cept that tPA release from an individual innervated vessel(A) is approximately proportional to its sympathetic in-nervation density (B), but inversely proportional to itsdiameter and endothelial surface (C); thus A � k B/C,where k is a proportionality constant. There may thereforeexist two classes of tPA-releasing vessels, in which eitherneural or endothelial tPA release predominate (Jiang et al.,2000).

Studies of tPA synthesis in isolated intact superiorcervical ganglia show a delayed but consistent response toinfused adrenergic agonists. After chemical sympathec-tomy, the ganglia show extensive destruction of neuroncell bodies and greatly reduced tPA release (Wang et al.,2002). It may also be relevant that ganglia from aged ratsrelease more than twofold greater amounts in organ cul-tures than do those from young adults, and their basallevels of circulating tPA activity are also elevated (Jiang etal., 2003). This is of interest because the capacity of tPA toinduce neuronal damage has suggested recently a need toalso regulate its peripheral neural production during aging.In other tissues the destruction of sympathetic innervation,and presumably reduced plasmin production, has beenshown to impair tissue remodeling during wound healing

(Kim et al., 1998; Li et al., 2003), as well as cellularimmunoregulation in the uvea, thymus, and spleen (Li etal., 2004). A direct involvement of neural tPA per se inthese processes, however, has not yet been shown bydirect experiment.

IMPLICATIONS FOR FURTHER STUDIESThe presence of tPA in resistance vessel nerve fibers

does not contest its better-known presence in the endo-thelium of many vessels, nor does it conflict with a basalendothelial release into the circulation. More simply, itcalls attention to a previously undescribed concentrationof a sympathetic protease at a key position in the vascularsystem; small precapillary resistance vessels that regulateblood pressure and capillary flow. In this proposedscheme, endothelial and neural tPA that are distributedintermittently throughout the vascular tree would to-gether regulate blood fibrinolytic activity, but the neuralrelease into smooth muscle would also influence plasminproteolysis within the densely innervated resistance vesselwall matrix. Pertinent to recent studies of an endothelialtPA receptor (annexin), it is also possible that neuralreleased tPA binds to the intimal surface (Hajjar et al.,1994; Ling et al., 2004).

Critical to future studies of microvascular tPA effectswill be clear ultrastructural identifications of the tPAsource in small vessel walls of various organs and tissues,especially tissues that receive a rich sympathetic innerva-tion. Not all vessels receive equal amounts of neural tPA,which could explain important pathophysiologic differ-ences; namely, artery versus vein thrombotic tendencies, aswell as choroid versus retinal microvascular flow. Visibleimmunolocalizations of adrenergic and other agonist re-

Fig. 2. Comparative tPA release per mg. *Basal: 10-min accumulation peak. Acute: 1-min peakresponse to infused phenylepinephrine. Microvascular, iris-choroid eyecups; sympathectomy �guanethidine treatment, 5 weeks. Compiled from Wang et al, 1995 (with permission ARVO) and2002 (with permission Lippencott, Williams and Wilkins); Jiang et al 2002(with permission Elsevier)and 2003(with permission Wiley Liss).

Sympathetic Tissue Plasminogen Distribution 731

ceptors on vessel wall axons or endothelium would alsolocate the site of stressful activations of a tPA release thathave often been only functionally defined by pharmaco-logic or binding criteria.

More speculative, but worthy of new attention, arethe putative consequences of a dysregulated autonomicneural tPA release into the wall matrix of innervatedarteries. Does the active tPA infused into wall matricesduring recurrent stress accelerate matrix destruction, vas-cular aging, or stiffening? Studies of the pathogenesis ofthese wall changes during major vascular diseases have thusfar tended to emphasize intimal and smooth muscle re-sponses, mainly in large vessels. Behavior of the sympa-thetic plasmin producing system during the major clinicalmicrovascular disorders (e.g., diabetes and hypertension)may also merit added investigation. Regarding other or-gans and tissues, it is interesting that cell-mediated im-mune responses regulated by the densely innervated spleenand thymus have been aborted by sympathectomy (Li etal., 2004). Such effects have not yet been linked to changesin sympathetic plasminogenesis per se. The presence oftPA in heart muscle sympathetic fibers (Fig. 1) suggests anew source for the cardiac tPA release induced by sym-pathetic stimulations (Bjorkman et al., 2003).

Influences of a microvascular sympathetic plasminproduction on nutrient capillary perfusion of tissues mayalso be considered. Components of a complete fibrinolyticsystem (tPA, plasminogen, and plasmin) have been immu-nolocalized in the full thickness of thin arteriolar walls, anddestruction of the resistance vessel sympathetic innervationby chemical sympathectomy induces major reductions inblood tPA. Neural tPA released from resistance arteriolesmay therefore directly enter the capillary blood in vivo.The slow passage of viscous blood though 8–10-mmcapillary lumina is thought presently to be assisted byanticoagulants (heparin; antithrombin). An infusion ofneural tPA-plasmin during stress-induced sympatheticstimulations may assist further perfusion and prevent mi-crothrombus formations. Conceptually, the local stimula-tion of neural tPA release for therapeutic purposes bypharmacologic or electrical means has not yet been testedby direct experiment. Finally, the adventitia has beenshown responsive (i.e., fibrosis formation) to changes inthe local plasminogen activator system, but the role oflocal sympathetic tPA release in this change has not beenevaluated (Kaikita et al., 2001).

ACKNOWLEDGMENTSWe thank Roshanak Sharafei for technical assistance.

We also thank Ms. A. Fowler and Ms. J. Wegh foreditorial assistance.

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