Role of endothelium in ischemic coronary syndromes

Role of Endothelium in lschemic Coronary Syndromes Ian T. Meredith,

Todd J. Anderson, MD, Akimi Uehata, MD, Alan C. Yeung, Andrew P. Selwyn, MD, and Peter Ganz, MD

MBBS, PhD,

lhe recent dkcovq that the endothellum syntheslzes !several powdul vasodllators, inclucllng prodacyclin and endoUwllum4erlvedrelaxing factor, has substanUally changed our view of the importance of the vascular endothellum and the pwspecthfefromwhkihwe conskkrthemechanisms of vascular control. It is now known that thevascularelhthdh mplaysakeyroleincirculatoly homeo&a& through its abiltty to sense the local vascular milieu and respond to it by the synthe&andreleaseofavarletyofbio~lly active-The-influence!5 not only vascular tone, but also vawxdar remodenngviathe productkn of growth-promoting and growth-inhlblttngsubstances; henmhds and thrombosh through antlplatele+t, antkwgulant, and fHMnolyUc effectq and inflammation through theexpredonof S and adhesion molecules on the cell membrane. In diseases such as athemderosiq however, these functions of the endothellum are lmpalred and may even become counterproductive and disease-promoting. The “acthrated”’ or injureul endothelal cells paradoxicauybecomeprothro4nbotk,growth-promote and leukocyteadheshre. The endothalium also loses Its vasodilating abillty, rewMng the underlybqgvascularsmoothmusdeswwptHetoa

MD,

arecurrentlybeingappt&dtothestudyofcoronary artery disease in humans. (Am J Cardiol l99~72:27C-32C)

A

ngiographic assessment of the burden of coronary atherosclerosis is insufficient to predict the clinical course of patients with coronary artery disease. Recent observations clearly demonstrate that the complications of atherosclerosis, including myocardial infarction, often develop in the setting of mild-to-moderate angiographic stenosis. 1-3 Coronary angiography also provides little information regarding the cellular and extracellular compositions of atherosclerotic plaques-factors that may closely relate to a lesion’s biologic activity, vascular reactivity, and propensity toward plaque rupture and thrombosis.4-s Therefore, it is important to learn more about the biology of the arterial wall and to determine how best to apply this knowledge to the clinical management of patients with coronary disease. This review focuses on perhaps the most important regulator of arterial wall function, the vascular endothelium. In health, the endothelium conducts and orchestrates a variety of beneficial functions that protect and maintain the status quo. In diseases such as preponderalMBOfV-liCtiVeforces.ltiS atherosclerosis, however, these functions are disnowreu~@fzedthatthesedisturbancesinendoturbed and may even become counterproductive thelial functkm are prlndpal players in the b5chand disease-promoting. Healthy endothelial cells emkman-of coronaryarterydisease. provide an antithrombotic surface that discourages Endeavors to modtfy or reverse endothellal dysthe migration of inflammatory cells (leukocytes) iimctkmmaytherefore beofslgniflcanttberainto the vessel wall and inhibits vascular growth peudc benefit in the treatment of myocardlal and smooth muscle proliferation (Figure 1). Healthy bhchemia. This review outlines several Important endothelium also produces a powerful array of insights krto the biolagy of the arterial wall that vasodilator and vasoconstrictor substances that are pivotal to the normal regulation of basal and stimulated blood flow. When “activated” in response to injury, endothelial cells paradoxically From the Cardiovascular Division, Department of Medicine, Brigham and Women’s Hospital and Harvard Medical School, become prothrombotic, growth-promoting, and leuBoston, Massachusetts. kocyte-adhesive. The endothelium also loses its Address for reprints: Peter Ganz, MD, Brigham and Women’s vasodilating ability, resulting in a preponderance Hospital, Cardiac Catheterization Laboratory, L2-196, 75 Francis Street, Boston, Massachusetts 02115. of vasoconstrictive forces. A SYMPOSIUM: MANAGING MYOCARDIAL ISCHEMIA

27C

ADHESION PROPERTIES OF ENDOTHEUUM

Monocytes and their tissue form, macrophages, play an important role in all stagesof atherosclerosis. In both animal models9and human atherosclerosis5J0the earliest proliferative lesions are comprised almost entirely of monocytes and macrophages. Macrophages are also prominent in advanced stages of atherosclerosis at the growing edges,or shoulders, of atheromatous plaqueslo and are particularly pronounced in unstable lesions prone to fissuring and rupture.” The migration and accumulation of monocyte/ macrophages in the vesselwall are initiated by the expression of leukocyte receptors (leukocyte adhesion molecules) on the surface of endothelial cells.t2 These adhesion molecules are most likely expressed in response to injury. In cell culture, endothelial cells stimulated by inflammatory mediators (cytokines) expressleukocyte adhesion molecules on their cell membranes.13,14Three major classesof leukocyte adhesion molecules have been characterized. These are: endothelial-leukocyte adhesion molecule-l (ELAM-l), which specifically interacts with polymorphonuclear leukocytes; vascular cell adhesion molecule-l (VCAM-l), which is specific for monocytes and lymphocytes; and intercellular adhesion molecules (ICAM-l), which interact with any leukocytes. Cybulski and Gimbrone13 have shown that the elevation of serum cholesterol levels by dietary fat supplementation in rabbits results in the expression of leukocyte adhesion molecules that are specific for monocytes/macrophages. These adhesion molecules havebeen termed “athero-ELAMs,” NORMAL ENDOTHELIAL CELL

Vasodllatory

Growth-inhibitory

“ACTIVATED” ENDOTHELIAL CELL increased leukocyte adhesion

Vasoconstrlctor

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Growth-promoting

THE AMERICAN JOURNAL OF CARDIOLOGY VOLUME 72

or atherosclerosis-associatedendothelial leukocyte adhesion molecules,and are analogousto VCAM-1. Within several days of increasing serum cholesterol levels, monocytes and macrophages begin to penetrate the intima in areas that express atheroELAMs. In areas where adhesion molecules are absent, the intima remains unaffected, and foam cells do not develop. Although these observations establish a link between cholesterol, adhesion molecules, and the development of early atherosclerotic lesions in experimental animals, human “athero-ELAMs” have not yet been identified. GROWTH-PROMOTING PROPERTIES OF ENDOTliEUUM

Normal, healthy endothelium manufactures very few growth factors and secretes mainly growthinhibiting substancessuch as nitric oxide, prostacyclin (prostaglandin I& and heparin. “Activated” endothelial cells, however, can produce a variety of growth factors. If, for example, endothelial cells in culture are stimulated by the addition of thrombin, their production of platelet-derived growth factor dramatically increases.This observation has particular relevance since pathologists have suggested that atherosclerotic lesions may undergo a “growth spurt” when a thrombus is incorporated into the arterial wa11.7*8 COAGUIATION PROPERTIES OF ENDOTHEUUM

Another characteristic change in “activated” or dysfunctional endothelium is the development of a procoagulant surface rather than an anticoagulant one. Although this process is undoubtedly multifactorial, 2 obvious factors contribute. The first of these factors is the impairment of the normal endothelial defenses against platelet deposition, and the second is the increased endothelial production of factors that stimulate the extrinsic pathway of coagulation. Under normal conditions, the endothelium produces several vasodilating substances, including endothelium-derived relaxing factor (EDRF) and prostacyclin (discussed below), which provide a powerful defense against platelet deposition. Both EDRF and prostacyclin inhibit platelet aggregation,15-19and EDRF may inhibit platelet adhesionm The impaired production of these substances by dysfunctional endothelium therefore favors the forces promoting platelet deposition. Healthy endothelial cells also normally produce only very small quantities of tissue factor (thromboplastin). However, the converse is true of activated SEPTEMBER 9, 1393

endothelial cells. If interleukin-l-which is one of Many endogenous substances, such as acetylchoa class of chemicals (cytokines) believed to be line, serotonin, bradykinin, histamine, adenosine relevant to atherosclerosis-is added to endothe- triphosphate (ATP), adenosine diphosphate lial cells in culture, the endothelial cells produce an (ADP), thrombin, and substance P, as well as abundant amount of tissue factor.14 physical factors, including blood flow and pulse pressure, stimulate the release of EDRF by activating endothelial surface receptors and/or ion chanVASODIIATOR PROPERTIES OF ENDOTREUUM nels.30-36Interestingly, substancesthat act as vasoAbnormal vasoconstriction of epicardial coro- constrictors, such as catecholamines, may also nary arteries is important in all forms of angina, stimulate the release of EDRF.37>38 These simultaincluding stable and unstable angina. Studies per- neous vasodilator signals, mediated through the formed in the cardiac catheterization laboratory endothelium, most likely subserve an important have shown that normal coronary arteries dilate in role in governing the direct vasoconstrictor reresponse to stimuli commonly encountered in daily sponsesto a given agonist. Only a few vasodilators life, such as exercise, mental stress, and cold (e.g., nitroglycerin) bypass the endothelium to pressor stimulus, whereas irregular or stenotic exert their action directly on smooth muscle. arteries paradoxically constrict to these stimuli.21-24 The ability of the endothelium to release EDRF These disturbances in vasomotion were initially in human coronary arteries in vivo is often tested considered to be due to increases in the vasocon- with the endothelium-dependent agonist acetylchostrictor influences. However, coronary vasomotor line (Figure 2) .39-42In patients with angiographitone represents a balance of opposing forces, both tally normal coronary arteries and few coronary vasoconstrictor and vasodilator. Augmented constriction could therefore equally result from the removal of a powerful vasodilating force “tipping Ach y ENDOTHELIAL CELL the balance” in favor of vasoconstriction. Indeed, \ current evidence suggests that impaired vasodila* ,.,.:::::::.: ...... ~~~ _... tion is the predominant mechanism underlying the :.:.:.:.:.:.:.:&,& ,.,.:.~:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.: .::~~f~ijiiijiiiiiiiiiiii:i:i:,: :.:.,., inappropriate constriction in atherosclerosis. The vascular endothelial cell (now known to be a powerful source of vasodilators) and its disturcGMP-b bance in atherosclerosis has therefore become an soconstriction important focal point in efforts to understand SMOOTH MUSCLE abnormal coronary vasomotion and, in turn, the CELL pathophysiology of myocardial ischemia. EDRF is perhaps the most important vasodilatAch DYSFUNCTIONAL ing substance produced by the endothelium. Its ENDOTHELIAL CELL 7 discovery by Furchgott and Zawadski25 resulted from the simple but important finding that intact endothelium was mandatory for acetylcholineinduced vasodilation. In the presence of intact endothelium, acetylcholine produced dose-depencGMP+ Vasodilationl dent relaxation of isolated arterial segments but I soconstriction produced contraction in segmentswhere the endoJ thelium had been removed. These findings were EKIFTH MUSCLE explained on the basis that acetylcholine had both a direct constrictor effect on vascular smooth FIGURE 2. Schema& ~oftheeuectaofacemuscle and an indirect vasodilator effect mediated lmsomow~toIlcaylchdlnebnet through the release of a substance (EDRF) from redomlMntInRuonoeoftherethe endothelium. The net effect on an artery v-duetothep ceptcw~releaseot-ra&xlng therefore resulted from the sum of these two factor (EDRF), or nRrk oxide (No), fmn the mWtha&m influences. Current knowledge indicates that EDRF is a nitric oxide (NO) radica126T27 or a complex contain- sk,t~dlmhWmdendotheMmleaseorincmamdInacof EDRF reaulb In a pmdondnant vammnWkta# ing NO (e.g., nitrosothiol),B most likely derived thmtkn lllfbnw,whkhkmaEatedbythacEreot-ofacefrom the terminal guanidino moiety of L-arginine.29 Qkhdl~olMtSCUlarSmOOth~ A SYMPOSIUM: MANAGING MYOCARDIAL ISCHEMIA

=

risk factors, acetylcholine commonly produces vaso- lowering, may allow healing of the endothelial cell dilation, which is consistent with the normal, recep- and restoration of normal endothelial function. tor-mediated release of EDRF from the endothelium.3941However, in patients with atherosclerosis and multiple risk factors, acetylcholine frequently REFERENCES i Ambrose .I& Tannenbaum MA, AIexopoulos D, Hjemdahl-Monsen CE, produces a vasoconstrictor response that is consis- HeavyJ, Weiss M, Borrico S, Gorlin R, Fuster V. Angiographic progressionof tent with the impaired release or possibly the coronary artery disease and the development of myocardial infarction. I Am Cardid 1988;12:5662. inactivation of EDRF and a direct action on Cdl 2. Hacket D, Vetwilghen J, Davies G, Maseri A. Coronary stenosesbefore and vascular muscle.23,39~40~42~43 Although acetylcholine after myocardial infarction.Am J Car&i 1989;63:1517-1518. Little WC, CimstaninescuM, Applegate RI, Kutcher MA, Burrows MT, servesas a useful indicator of endothelial function 3). KahI FR, Santamore WP. Can coronary angiography predict the site of a in health or disease, it is unlikely to be an impor- subsequentmyocardial infarction in patients with mild-&moderate coronary artery disease?Circularion 1988;78:1157-1166. tant physiologic stimulus to EDRF release. 4. Davies MJ, Thomas AC. Plaque fissuring-the cause of acute myocardii In patients with stable angina, the stimuli that infarction, sudden ischaemicdeath, and crescendoangina Br Heerr I 198533: are relevant to the control of epicardial coronary 363-373. Mitchinson UT, Bali RY. Macmphagesand athemgenesis.Lancet 1987;ii%6vasomotion during normal daily activities include S. 148. catecholamines and endothelial cell shear stress 6. Richardson PD, Davies MJ, Born GVR. Inthtence of plaque configuration stressdistribution on fissuring of coronary atherosclerotic plaques.Lancer induced by increases in blood flow. A recent study and 1989;8:941-944. from our laboratory has shown that human coro- 7. Fuster V, Stein B, Ambrose JA, Badimon L Badimon JJ, Chesebro JH. nary arteries with normal endothelial function are Atherosclerotic plaque rupture and thrombosis: evolving concepts. Cirnclafion 1990;82(supp111):47-59. quite resistant to the constrictor actions of catechol- LFuster V, Badirnon L, Badimon JJ, Chesebro JH. The pathogenesisof amines such as phenylephrine.M In fact, constric- comnaty artery diseaseand the acute coronary syndmmcs(part 1). N .EngfJ 19!?2;326:242-250. tion was observed only with relatively high doses of Med 9. Weidinger FF, McLenachan JM, Cybulski MI, FaIlon JT, HoIIenberg N, phenylephrine. When diseased arteries, i.e., those Cooke JP, Ganz P. Hypercholestemlemia enhances macmphagerecruitment and dysfunction of regenerated endothelium after balloon injury of the rabbit with endothelial vasodilator dysfunction, were chal- iliac artery. Circularion 1991;1991:755-767. lenged with the same concentrations of phenyleph- 10. Ross R. The pathogenesisof atherosclerosis.N End J Med 1986;314:48& rine, they were easily constricted by the catechol- 500. iL Lendon CL, Davies MJ, Born GVR, Richardson PD. Atherosclerosis amine. In addition, diseased arteries with plaque caps are locally weakened when macmphage density is increased. endothelial vasodilator dysfunction have been Atherosclerosis 1991;87:87-91. 12. Butcher EC. Leucocyte-endothel celI recognition: three (or more) steps shown to lack the vasodilator response that is to specificity and diversity. Cell 1991;67:103~1036. normally triggered by increasing blood flow and l3. CybuIski MI, Giibmne MA Jr. Endothelial expressionof a mononuclear adhesionmolecule during athemgenesis.Science1991;251:78&791. shear stress.45*46 Thus, in patients with stable an- Ieukccyte l4. Bevilaqua MP, Pobcr JS, Wheeler ME, Cotran RS, Gimbrone MA Jr. gina, the paradoxic vasoconstriction of epicardial Interleukin-1 activation of vascular endothelium. Am I Parhol 1985;121:39% arteries during stressful activities (such as exercise, 403. l5. Gerlach JP, Abbott WM. Modulation of endothelial ceUhemostaticpmpercold pressor stimulation, and mental stress) is ties: an active role in the host response.Annu RevMed 1990;41:1$24. probably explained by the combination of impaired u). BassengeE. Antiplatelet effects of endotheliumderived relaxing factor and nitric oxide donors.EuHean J 1991;12(supplE):12-15. vasodilation in response to increased blood flow 17. Moncada S, Gtyglewski R, Bunting S, Vane JR. An enzyme-isolatedfrom and by augmented vasoconstriction in response to arteries transforms pmstaglandinendoperoxidaseto an unstable substancethat inhibits platelet aggregation.Nati 197~263:663&65. catecholamines. lRBusse R, Liickoff A, BassengeE. Endotheliumderived relaxant factor ,4&t Pharmacd 1987336556 In unstable angina, the abnormal constriction of inhibits platelet activation.Naunyn-SCepicardial arteries is likely related to the presence 571. lS. Hogan JC, Lewis UT, Henderson AH. In viva EDRF activity intluences of platelet aggregation and thrombosis.30v47 Studies platelet function. Br J Phannacd 1988;94:1020-1022. in patients have shown that a release product of 20. Radomski MW, Palmer RM, Moncada S. Endogenousnitric oxide inhibits human platelet adhesionto vasctdarendothelium.Luncet 1987;1:1057-1058. platelet aggregation, serotonin, is a vasodilator of 21 Nabel EG, Ganr P, Gordon JB, Alexander RW, Selwyn AP. Dilation of healthy coronary arteries but a vasoconstrictor of normal and constriction of atherosclerotic coronary arteries causedby the cold test. Circulation 1988;n:43-52. atherosclerotic arteries4s50The paradoxic constric- pressor 22. Yeung AC, Vekshtein VI, Krantx DS, Vita JA Jr, Ryan TJ Jr, Ganx P, tion of atherosclerotic arteries was associatedwith Selwyn AP. The effect of atherosclerosison the vasomotor responseof coronary arteries to mental stress.N Engl J Med 1991;325:1551-1556. evidence of impaired endothelium-dependent vaso- 21). Zeiher AM, Drexler H, Wollscblager H, Saurbier BHJ. Coronary vasomo dilation. tion in response to sympathetic stimulation in humans: importance of the The challenge for the future, therefore, is to functional integrity of the endothelium.JAm Coil Canfio~1989;14:1181-1190. 24. Gordon JB, Ganz P, Nabel EG, Zebede J, Mudge GH, Alexander RW, find treatments that can convert the activated Selwyn AP. Atherosclerosis and endothelial function influence the cornnary endothelial cell to its normal functional state. responseto exercise.J C/in Invesr 1989;83:1!+46-1952. Furchgott RF, Zawadski JV. Tbe obligatory role of endotbelial ceils in the Experimental evidence suggeststhat the control of 25. relaxation of arterial smooth muscleby acetylcholine.Nafwe 1980;28&373-376. cardiovascular risk factors, such as by cholesterol 26. Ignarm LJ, Byms RE, Buga GM, Wood KS. Endotheliumderived relaxing 30c

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factor from puhnonary artery and vein possessespharmacologic and chemical properties identical to those of nitric oxide radical. Circ Res 1987;61:86&879. 27.Pahner RMJ, Fen&e AG, Moncada S. Nitric oxide accounts for the biological activity of endotheliumderived relaxing factor. Natme 1987;327:524 526.

26. Myers PR, Guerra R, Bates JN, Harrison DG. The vasorelaxantproperties of the endothelium-derived relaxing factor more closely resemblesS-nitrosocysteine than nitric oxide. Natwe 1990,345:161-163. 29. Palmer RMJ, Ashton D, Moncada S. Vascular endothelial cells synthesize nitric oxide from t-arginine. Nahrre 1988;333:664666. 30. Vanhoutte PM, ShimokawaH. Endothelium-derived relaxing and contracting factors. Ciznlariott 1989;80:1-9. 31 Pohl II, Busse R, Juan E, BassengeE. Pulsatile perfusion stimulates the releaseof endothelial autocoids.JqPpl Can&~ 1986;1:215-235. 32. Pohl IJ, Holtz J, Busse R, BassengeE. Crucial role of endothelium in the vasodilator responseto increasedflow in viva. Hyper%nsron1986;8:37+. 33. Rubanyi GM, Romero JC, Vanhoutte PM. Flow-induced release of endo thelium-derived relaxing factor. Am J Phytii 1986;250:H1145-H1149. 34. Inoue T, Tomoike H, Hisano K, Nakamura M. Endothelium determines flow-dependent dilation of the epicardial coronary in dogs.J Am CON Cardiol 1988:11:187-191. 35. Furchgott RF, Vanhoutte PM. Endothelium-derived relaxing and contracting factors. FASEB J 1989,3:2(X%2018. 36. Ignarro IJ. Biological actions and properties of endothelium-derived nitric oxide formed and released from attery and vein. Circ Res 1989;65:1-21. 37. Angus JA Cocks TM, Satoh K. Alpha*-adrenoreceptorsand endotheliumdependent relaxation in canine large arteries. Er J Phamcol1986;8-%767-777. 38. Cocks TM, Angus JA. Endothelium-dependent relaxation in canine large arteries. Nahm 1983;305:627-630.

39. Ludmer PL, Selwyn AP, Shook TL, Wayne RR, Mudge GH, Alexander RW, Gam P. Paradoxical vasoconstriction induced by acetylcholme in atherosclerotic coronary arteries. N En& JMed 1986;315:10461051. 40. Vita JA, Treasure CB, Nabel EG, McLenachan JM, Fish RD, Yeung AC, Vekshtein VI, Selwyn AP, Ganz P. Coronary vasomotor responseto acetylcholine relates to risk factors for coronary artery disease.Circularion 1990,81:491497.

4F Vrints CJM, Bult H, Hitter E, Herman AG, SnoeckJP. Impaired endothelium-dependentcholine@ coronary vasodilatation in patients with angina and normal coronary arteriograms.IAm CoUCur&~ 1992$9:21-31. 42. Werns SW, Walton JA, Hsia HH, Nabel EG, Sam ML, Pitt B. Evidence of endothelial dysfunction in angiographically normal coronary arteries of patients with coronary artery disease.Cinulatin 1989;79:287-291. 43. Zieher AM, Drexler H, Wollschlager J, Just H. Modulation of coronary vasomotor tone in humans: progressive endothelial dysfunction with different early stagesof coronary atherosclerosis.Circulotim 1991;83:391401. 44. Vita JA, Treasure CB, Yeung AC, Vekshtein VI, Fantasia GM, Fish RD, Ganz P, Selwyn AP. Patientswith evidenceof coronary endothelial d@imction as assessedby acetylcholine infusion demonstratemarked increasein sensitivity to constrictor effects of catecholamines.Circulation 1992;85:13!%-1397. 48. Cox DA, Vita JA, Treasure CB, Fish RD, Alexander RW, Ganz P, Selwyn AP. Impairment of flow-mediated coronary dilation by atherosclerosisin man. Circulation 1989;80:458-465. 46. Nabel EG, Selwyn AP, Ganz P. Large coronary arteries in humans are responsiveto changingblood flow: an endothelium-dependentmechanismthat fails in patients with atherosclerosis.JAm Co11Car&~ 1990;16:34%356. 47. Willerson JT, Golino P, Eidt J, Campbell WB, Buja LM. Specific platelet mediators and unstable coronary lesions: experimental evidence and potential implications. Circulation 1989;80:19%205. 4S.Vrints C, Bosmans J, Bult H, Claeys M, Herman A, Snoeck J. Close parallelism between the coronary vasomotor responsesto acetylcholine and to serotonin. (Abstr.)JAm CoUCardi 1992;19:323A. 49. McFadden EP, Clarke JG, Davies GJ, K&i JC, Haider AW, Maseri A. Effect of intracoronary serotonin on coronary vessels in patients with stable angina and patients with variant angina.N EIlgl I Med 1991;324:64&654. 50. Golino P, Piscione F, Willetson JT, Cappelli-Bigazzi M, FocaccioA, Viiari B, Indolfi C, Russolillo E, Condorelli M, Chiariello M. Divergent effects of serotonin on coronary artery dimensions and blood flow in patients with coronary atherosclerosisand control patients.N Engl J Med 1991;324:641648.

DISCUSSION Dr. HO-WI@ FUN (Bufbl~, New York): Dr. Ganz, you mentioned that when endothelial dysfunction occurred, there seemed to be up-regula-

tion of nitric oxide synthesis. And yet you say that supplementation of L-arginine may be a treatment approach. How would you reconcile those two observations? Dr. G~IK: Yes, the L-arginine story has been puzzling. Experimental studies in the rabbit have provided clear-cut evidence that L-arginine will normalize endothelium-dependent relaxation both in conduit arteries and resistance vessels.ly2Moreover, data from Germany have shown that the resistance of coronary arteries in patients can also be at least partially normalized by L-arginine.3 That finding is puzzling because L-arginine is not thought to be rate-limiting for nitric oxide synthase. The dissociation constant (Kn) for the enzyme is thought to be in the micromolar range, whereas the concentration of L-arginine within the endothelial cells is probably at least 1,000 times higher. Thus, it is puzzling why the additional administration of a substrate that is not thought to be rate-limiting would be of benefit. It is a question that still needs to be resolved. Dr. Gary Heller (Pawtucket, Rhode Island): A template in patients in terms of inappropriate vasoconstriction is the ischemic threshold model. Recent data have suggested that the onset of ischemia is variable rather than fixed. Much of this may be related to vasoconstriction, which I would consider inappropriate. Dr. Ganz, could you comment on suggestions that nitrates and perhaps calcium antagonists may play a key role clinically in managing this inappropriate vasoconstriction? Dr. G~Iu: I have indicated that atherosclerotic arteries will inappropriately constrict during the activities that cause ischemia in patients. The question is: Can that be normalized by the administration of nitrates? A study done in Switzerland has shown that arteries constrict during exercise if they are atherosclerotic.4 When the patients were pretreated with intracoronary nitroglycerin and then the exercise test was repeated, the arteries no longer exhibited abnormal constriction. Thus, we know that at least intracoronary nitroglycerin can replace the normal function of endotheliumderived relaxing factor to prevent vasoconstriction. Dr. John D. Parker (Toronto, Canada): In many coronary disease states, such as congestive heart failure, we see an initial insult, which is then followed by a variety of physiologic responses. At one point, these responses are helpful, and then, at another point, they are believed to become counterproductive. This is also true in atherosclerosis, where the assumption is that atheromatous disease leads to endothelial dysfunction and to problems A SYMPOSIUM:

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ISCHEMIA

3%

with the metabolism surrounding nitric oxide. Dr. Ganz, do you foresee a role for exogenousmanipulation of the nitric oxide pathway in the management of chronic atherosclerosis? Dr. Ganz: I spoke about nitric oxide mainly in terms of its vasodilatory capacity. It is clear, however, that nitric oxide also has important antiatherogenie properties. First, it is a reasonably potent antiplatelet agent. It is also one of the few drugs that can inhibit monocyte adhesion. Returning to the L-arginine story, a recent article in the Journal ofCZinicaZInvestigation showed that atherosclerosis can be reduced in the hypercholesterolemic rabbit model by feeding the animals L-arginine.s In fact, nitric oxide, even in tissue culture, will inhibit the proliferation of vascular smooth muscle. Therefore, we are talking about smooth muscle effects of nitric oxide, monocyte effects, and platelet effects. The ingredients are there for nitric oxide to be antiatherogenic, which it seems to be in animal models. Whether that is the case in the clinical setting has yet to be determined, however. Dr. Richard A. Cohen (Boston,

M~u-

are treated with angiotensin II, thrombin, and growth factors, the earliest events include the induction of other growth factors and of white cell chemoattractants. The unfortunate implication is that if you injure the endothelium slightly so that it remains intact, you essentially induce a program that results in a clot and an inflammatory response. If you injure the endothelium a little more, causing it to slough, you get the same response in the smooth muscle. Therefore, the propensity of the vesselwall to clot and to inflame appears to extend through several layers and may limit what we can do, even if we were to work only on the endothelium. Further, we have always thought of the endothelium as being something that protects smooth muscle cells from being activated, but it now appears that both are activated in the same way. Thus, we may be in a no-win situation. In regard to the types of cell that may be present in culture, I think that probably all of these cellsboth smooth muscle cells and endothelial cellsmay be in the activated state in vitro. In fact, tissue factor has not been induced in vivo in the endothelium, even though it clearly can be in vitro. I wonder whether we are always going to be forced to go back to in vivo models, such as the vasoconstriction model, and ask whether the induction of tisue factor or cytokines really occurs or whether the 2 types of cells actually protect each other? Dr. Galu: I agree with everything you have said. The beauty of studying the vasodilator of endothelium is that it is something that can be studied in real patients. That is why we have taken that direction.

setts): Dr. Ganz, have there been any clinical studies based on altering endothelial function to support the idea that microvascular reserve can be enhanced? Dr. Ganz: That work has yet to be done. You bring up an important issue, however. We can show an impairment in the resistance vesselswhen we apply certain pharmacologic test agents, but what about results in a more realistic clinical setting? We have reason to believe that disturbances occur in the microvasculature in atherosclerosis. I would refer you to an article that Drs. Nabel, Selwyn, and I6 published in which we measured coronary blood flow in response to rapid pacing. In that study, I REFERENCES was surprised to find that if rapid atria1 pacing is LCmke JP, Andon NA, Girerd XJ, Hirsch AT, Creager MA Arginine choline@ relaxation of hypercholestemlemic rabbit thoracic aorta. applied to arteries that do not have significant restores Ci~ularion 1991;83:1057-1062. obstructions, patients actually have a less-than- 2. Girerd XJ, Hirsch AT, Cooke JP, Dzau V, Creager MA L-arginine augexpected increase in coronary blood flow in re- ments endotheliumdependent vawdilation in cholesterol-fed rabbits. Cirr Res sponse to this metabolic stimulus. We are now 3.1990;67:1301-1308. Drexler H, Zeiher AM, Meinzer K, Just H. Correction of endothelial looking to see whether that is due to endothelial dysfunction in coronary micrwirculation of hypercholesterolemic patients by L-arginine.Lancer 1991;338:154~1550. dysfunction. 4. Gage JE, Hess OM, Murakami T, Ritter M, Grimm J, Krayenbuehl HP. Dr. Mark B. Taubman (New York, New York):

Dr. Ganz, in your discussion of activated endothelial cells, which can make growth factors, attract leukocytes, and induce procoagulants, the parallel with smooth muscle cells is striking. During the past few years, in our laboratory we as well as others have found that when smooth muscle cells

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Vaswonstriction of stenotic coronary arteries during dynamic exercise in patients with classic angina pectorisz reversibility by nitroglycerin. CLfulafion 1986;73:865-876. 5. Cooke JP, SingerAH, Tsao P. Zera P, Rowan RA, BiUinghamME. Antiatherogenic effects of L-arginine in the hypercholestemlemicrabbit. J Ch Invest

1992;90:1168-1172. 8.Nabel EG, Selwyn AP, Ganz P. Paradcxical narrowing of atherosclerotic coronary arteries induced by increasesin heart rate. Cirnrlation 1990;81:850859.

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