- Email: [email protected]
Platelet-leukocyte-endothelial interactions in coronary artery disease
Platelet4eukocyt~Endothelial Interactions Coronary Artery Disease
in
Jawahar L. Mehta, MD, PhD, Francesca A. Nicolini, MD, William H. Donnelly, MD, and Wilmer W. Nichols, PhD
P
athologic examination of coronary arteries of patients with coronary artery disease generally shows extensive luminal narrowing with atherosclerosis as the dominant feature. Atherosclerosis appears in layers, with frequent organized thrombi in the arterial wall, suggesting that each layer of atherosclerosis may have evolved following a thrombotic event. In patients dying of acute myocardial ischemia (unstable angina or acute myocardial infarction), the epicardial large coronary arteries supplying the involved region usually show a recent thrombus that almost totally occludes the arterial lumen.‘92 Carefully conducted pathologic studies have revealed that the thrombus begins with disruption of the endothelial lining from a fissure or hemorrhage in the atherosclerotic region, resulting in exposure of subendothelial layers to circulating blood. Platelets adhere to subendothelial collagen, undergo activation, and release a variety of vasoactive substances that may cause dynamic vasoconstriction of the downstream coronary tree. Platelet activation also initiates the coagulation pathway involving formation of fibrin. Other cellular elements, such as leukocytes and erythrocytes, may also become incorporated into the plateletfibrin thrombus. Occlusion of the atherosclerotic coronary artery, if not opposed by naturally occurring defense mechanisms, may lead to severe reduction in blood flow to the myocardium. If the thrombus is not dissolved, myocardial injury may ensue, followed by myocyte death. The reestablishment of coronary blood flow by pharmacologic (e.g., streptokinase or tissue plasminogen activator) or physical (e.g., angioplasty and bypass surgery) means has become a therapy of choice in acute myocardial ischemia. However, reperfusion From the Division of Cardiology, Department of Medicine, University of Florida College of Medicine, and the Veterans Administraof the previously ischemic myocardium is often tion Medical Center, Gainesville, Florida. This study was supfollowed by morphologic and functional changes in ported by a VA Merit Review and Clinical Investigator Awards the affected coronary arteries and myocardial tis(JLM), American Heart Association Research Fellowship (FAN), and grants-in-aid from the American Heart Association, Florida sues, which may by themselves be considered Affiliate, St. Petersburg, Florida. Address for reprints: Jawahar L. Mehta, MD, PhD, Box J-277, detrimental. Hence, reperfusion therapy has been considered a “double-edged sword.” JHMHC, University of Florida, Gainesville, Florida 32610.
lttsgenerallyrecognizedthatformationofa platdet-#Jrin-rkh tluombus In an atherosclerotkwronaryarterylsthebasisofunstaMean= ginaandacute+myocafdialinfaMfon.Platelet hyperactfvltyhasbeenldenwIedincwonaryrlsk factorssuchashyperlipklemlaanddlabetesmellmls.Pwsktmtactivatknoftheseceusresldts in~ofgrowthfactorsthatmayconkibute to the prorpeaskn ofatherosderosia several recentstudlesshowthatendotheRum,bygen8rat~orm&&HMngahostof~substancerr, plays a crMcal rok in the modulation of vasculartone.lmportantamongthesesubstances are prostacyclln (PGQ and endothelkmderived rekud~i’actor (EDRF). The endatheliumdepe+ndeMmodulatkmofcoronaryarteryton8 -wCththeseverttyofandthenumberof coronaryrlskfactors.duressuchasangk@a&yandcoronarybypass surgery in/we the endothelium.Thekwsofendothelialsmoothmusderelaxantfunctionmaycontribute to the vrktion and thrombo&s of&enobserwdsoonafterthese#procedures. llwombolysis(andwbsquwtrqwfu&mofthe comnary arteq) is also associateul wtth severe endoWelialdysfunctkm,witharesultingvastrktoritiuenceonthecoronaryvasoular bed. AcWation of leukocytes and their presence in the bmpdmed myacardium contribute to progression of myocardtal INmy by release of oxygen free radicals and pMeolytk enzymes. Thus, tt-thataperturbatkninthlsde&ateequC Hum in celhdar itieractkns relates to gene& and progrw&m ofmyocardlal isch8mia. (Am J Cardk~I -69:8Sl3B)
88
THE AMERICAN JOURNAL OF CARDIOLOGY VOLUME 69
MARCH 6, 1992
In this report, we shall review our current understanding of the role of three major cell lines-platelets, leukocytes, and endotheliumthat may have a bearing on the genesis and progression of myocardial ischemia. PlAlELeeFUNCWNINCDRDNARYARTERY DISEASE
Nature has provided us with platelets as a first-line defense against bleeding when the integrity of the blood vessel is disrupted. Besides their dynamic role as a physiologic plug, platelets also provide nutrients for the maintenance of endothelial integrity. When activated, these tiny cells produce a variety of vasoactive substances and growth factors. Their hyperactive status in coronary risk factors, such as hyperlipidemia and diabetes mellitUS,3,4is well known and could result in vascular smooth muscle proliferation leading to vascular occlusion. Dbring acute myocardial ischemia, platelet activity in response to a variety of agonists such as catecholamines and thromboxane is markedly enhanced and in response to antagonists such as prostacyclin (PGI,) is diminished.s7 At the same time, platelet microaggregates are often seen in the peripheral circulation.8 Some vasoactive substances released from activated platelets (e.g., thromboxane A, and serotonin) are potent vasocon-
strictors, whereas others (e.g., adenosine diphosphate [ADP]) are vasorelaxants. The important role of platelets in the genesis of acute myocardial ischemia became evident from experiments in narrowed coronary arteries of dogs, which show spontaneous cyclic reductions (gradual decrease followed by abrupt increase) in coronary blood flow. These cyclic changes in blood flow are due to formation of occlusive platelet aggregates followed by their spontaneous dissolution.’ These cyclic changes can be abolished by antiplatelet agents, thromboxane inhibitors, and serotonin receptor antagonists.” Similar cyclic changes in blood flow have also been shown in human arteries. Since these changes in blood flow may be followed by total occlusion of the coronary artery, it may be hypothesized that platelet aggregation occurring in atherosclerotic coronary arteries may contribute to acute myocardial ischemia. Cyclic flow reductions are also observed in the therapeutically reperfused coronary arteries prior to their reocclusion (Figure 1). The morphology of these arteries often reveals a large number of platelets and fibrin strands on the intimal surface of the artery. The endothelial lining almost always shows extensive disruption (Figure 2). Based on these considerations, use of antiplatelet drugs, particularly aspirin, is advocated in a
180 120 60
Control
DC Current
2mcIA
Coronary Occlusion
t-PA 20 rs/kg/min
large number of cardiac conditions, e.g., in patients with unstable angina, myocardial infarction, and coronary artery bypass surgery, and in others receiving thrombolytic therapy.” ENDOTHEIJAI. FUNCTION IN WRONARY ARTERYDISEASE Endothelium lines all blood vessels in the human body, weighs in total about 12 lb, comprises approximately 1 x lo’* cells, and traditionally has been thought to be a protective lining. However, it also secretes or metabolizes a large number of substances with vasodilator, vasoconstrictor, anticoagulant, and procoagulant activities. As such, it may be considered the largest paracrine organ in the body.‘* Among the important vasodilator species generated by the endothelial cells are PGI, and endothelium-derived relaxing factor (EDRF). PGI, is derived from the cyclooxygenase pathway of arachidonic acid metabolism; and it is rapidly released locally in response to thrombin, bradykinin, high density lipoproteins, platelet-derived ADP
ififE!!&IBB
-WE-
and growth factor, tissue hypoxia, and hemodynamic stress. Aspirin, other nonsteroidal antiinflammatory drugs, and nicotine inhibit its release. Although short-acting (half-life is approximately 30 s), PGI, is a powerful vasorelaxant and platelet inhibitor, and these effects of PGI, are mediated by an increase in intracellular cyclic adenosine monophosphate (CAMP). EDRF is also a potent vasorelaxant and platelet inhibitor, but its effects are mediated by an increase in intracellular cyclic guanosine monophosphate (cGMP). It has been characterized as nitric oxide or a related substance, derived from the amino acid L-arginine, and is released locally in response to acetylcholine and many of the same stimuli as is PGI,. EDRF, being a non-prostaglandin-related substance, is not inhibited by aspirin or nonsteroidal antiinflammatory drugs. As such, vasodilation is still possible when the cyclooxygenase pathway is blocked. ii Both PGI, and EDRF serve to regu! ate vascular tone in response to physiologic and pathologic
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THE AMERICAN JOURNAL OF CARDIOLOGY VOLUME 69
MARCH 6, 1992
stimuli. The vasoconstrictor effects of smooth mus- mal models of atherosclerosis. Fortunately, the cle agonists is enhanced, and the effect of several endothelial function reverts to normal as serum smooth muscle relaxants is decreased, in the ab- cholesterol is lowered and atherosclerosis resence of endothelium. Given an intact endothe- gresses.15Unlike EDRF, local PGI, synthesis in lium, aggregating platelets, by release of ADP, atherosclerosis may not be diminished, since the elicit EDRF and relax the blood vessels. In con- release of arachidonic acid is enhanced in these trast, given a dysfunctional endothelium, aggregat- vascular segments.16 Following coronary artery occlusion and repering platelets constrict blood vessels via release of thromboxane A2 and serotonin.‘* These consider- fusion, oxygen free radicals are released; these ations may be particularly relevant after coronary superoxide radicals injure the endothelium (Figure angioplasty and thrombolysis, when the endothe- 3). The superoxide radical-mediated endothelial lial lining is disrupted and platelet activation in situ injury results in increased smooth muscle contracmay be responsible for dynamic coronary constric- tion and decreased endothelium-dependent contion. A recent study13shows that endothelial func- traction.*’ The oxygen free radical-mediated vascution remains abnormal for several weeks even if the lar injury may relate to decreased coronary blood endothelial morphologic integrity is restored. This flow reserve” and coronary reactivity” after reperwould imply the need for the continued use of fusion. Recent evidence indicates that the endothelium spasmolytic and platelet inhibitory agents for sevalso generates a potent vasoconstrictor named eral weeks after such procedures. In patients with stable coronary artery disease, endothelin, which is a 21-amino acid-containing release of EDRF in response to acetylcholine is peptide. The levels of endothelin are increased in markedly diminished.14 Furthermore, the endothe- acute myocardial ischemia. Besides inducing polial dysfunction correlates with the extent of coro- tent vasoconstriction and enhancing the effect of nary artery disease as well as with the number of circulating catecholamines, endothelin also stimucoronary risk factors. In atherosclerotic coronary lates release of both PGI, and EDRF.20 arteries, the release of EDRF is markedly diminIn addition to the several pathophysiologic states ished, and this decrease becomes evident even described above, endothelium also determines the before development of luminal narrowing in ani- vascular smooth muscle response to anoxia and
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A SYMPOSIUM: MYOCARDIAL ISCHEMIA
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reoxygenation.” Anoxia induces endotheliumAnother mechanism by which leukocytes could dependent vasoconstriction and reoxygenation- participate in ischemic injury of myocardium is by mediated initial vasorelaxation. In the absence of release of leukotrienes. We have shown that peptifunctional intact endothelium, the primary re- doleukotrienes (LTC,, LTD,, and LTE,) potentisponse of vascular smooth muscle to anoxia is ate platelet aggregation and vasoconstriction and, vasorelaxation and to reoxygenation is intense in synergy with platelet-released thromboxane A,, vasoconstriction. reduce coronary blood flow in the intact animal.“,24 Recently, neutrophils and monocytes have been shown to produce a nitric oxide-like material that LEUKOCYTE FUNCTlON IN CORONARY ARTERY decreases platelet aggregation by a cGMP-depenDlSEASE dent mechanism.*’ This material also decreases Leukocytes are a first-line defense against inva- vascular smooth muscle tone, and this effect of sion of the body by pathogens. These cells protect neutrophils is more pronounced in the absence of the tissues against the noxious effect of pathogens endothelium.26 The leukocyte-derived nitric oxide by their migration to the injury site,and activation may have a physiologic role in preventing platelet and release of proteolytic enzymes, oxygen free deposition on the vessel wall and maintaining radicals, and arachidonic acid metabolites. Informa- vascular tone. It may also have a pathologic role in tion developed in the past decade indicates that the development of shock-like state, particularly these cells, like platelets, have the potential to when leukocyte number is elevated and the endoenhance myocardial ischemic injury. Monocytes thelial integrity is disrupted. appear early at the potential site of atherosclerosis and subsequently transform into macrophages. The number of leukocytes in peripheral blood ROLE OF SLOW CHANNEL CALCIUM correlates with future cardiac events in both asymp- ANTAGONISTS IN CELLULAR ImRACTlONS Calcium mobilization is a key step in platelet tomatic and symptomatic patients. Recent electron microscopic studies in patients with acute myocar- activation.27Release of vasoactive substancessuch dial ischemia show intense activation of neutro- as thromboxane A,, serotonin, and adenine nuclephils, involving an increase in their chemotactic otides is modulated by voltage-dependent calcium ability, release of leukotriene B,, and release of an antagonists. For example, verapamil and diltiazem decrease platelet aggregation, adenosine triphosimportant proteolytic enzyme, elastase.** In animals subjected to coronary artery occlu- phate (ATP) release, and thromboxane A, generasion followed by reperfusion, leukocytes appear in tion. Although early studies showed that the inhiblarge numbers in the myocardial interstitium and itory effect of these calcium antagonists is observed partially plug the microvasculature.” Leukocyte only in very high concentrations, other studies deposition in the reperfused regions could be in show that these agents, in therapeutically achieved response to expression of complement fragments concentrations, do significantly inhibit platelet actiand adhesion molecules as well as to generation of vation induced by a variety of stimuli.27 Calcium mobilization is also involved in leukoa superoxide radical-stimulated chemotactic factor cyte migration.** Release of oxygen free radicals in plasma. The leukocytes then become activated from activated cells parallels the increase in extraand release oxygen free radicals and proteolytic enzymes, which in turn could be responsible for cellular calcium and its subsequent influx. At progression of endothelial and myocardial injury. therapeutic concentrations the voltage-dependent Physical obstruction of the microvascular bed by calcium antagonists verapamil and diltiazem deactivated leukocytes probably contributes to de- crease leukocyte chemotaxis and superoxide radicreased coronary flow reserve and myocardial stun- cal formation*’ (Figure 4). The vascular smooth muscle contraction in rening. Early studies in which superoxide radical scavengers were given prior to reperfusion have sponse to a variety of agonists is decreased by shown much promise in terms of modification of calcium antagonists. This has been the basis for the coronary flow reserve and myocardial dysfunction use of these agents in coronary artery disease.The seen after coronary artery occlusion and reperfu- agonist-induced release of PGI, and EDRF is also sion.‘8*‘9The release of elastase” along with super- a calcium-dependent phenomenon. Diltiazem has oxide radicals may also be an important pathogenic been shown to enhance PGI, synthesis by isolated factor in “reperfusion injury.” Recent studies in human blood vessels,*’whereas some studies show which elastase inhibitors were used suggest that a decrease in EDRF synthesis following calcium antagonists. These agents markedly decrease reoxthese agents may have a beneficial role. l2B
THE AMERICAN JOURNAL OF CARDIOLOGY VOLUME 69
MARCH 6, 1992
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ygenation-induced (reperfusion) increase in vascular smooth muscle tone,*l which most likely represents calcium influx across the sarcolemma, and this may be the basis of the beneficial effect of diltiazem in animal models of myocardial injury induced by coronary artery occlusion and reperfusion.*’
REFERENCES l. Falk E. Thrombosis in unstable angina: Pathologic aspects.In: Mehta JL, ed. Thrombosis and Platelets in Myocardiil Ischemia. Cardiovasc Clin Series, ~0118.Philadelphia: FA Davis, 1987137-149. 2. Davies MJ, Thomas A Thrombosis and acute coronary-artq lesions in sudden cardiac ischemicdeath. NEnglJMed 1984;310:1137-1140. 3. ShattiI SJ, Anaya-GaImdo R, Ben&t J, Colman RW, Cooper RA. Platelet hypersensitivity induced by cholesterol incorporation. J Clin Invest 197$55:63& 643. 4. Mustard JF, Packham MA. Platelets and diabetes meIIitus. N Et& .I Med 1977$97:1345-1347. 5. Mehta JL, Mehta P, Ostrowski N. Increase in human platelet alph+adrenergic receptor affinity for agonist in unstable angina. J Lab Chin Med 1985;106%61666. 6. Mehta JL, Mehta P, Conti CR. Platelet function studies in coronary heart disease,IX. Increased platelet prostaglandin generation and abnormal platelet sensitivity to prostacyclin and endoperoxide analog in angina pectoris. Am .I cardid 1980@943-947. 7. Neri Serneri GG, Mcdesti PA, Fortini A, Abbate R, Lombardi A, Gemini GP. Platelet receptors in active spontaneousangina.Loncet 1984;1383%841. 8. Mehta P, Mehta JL. Platelet function studies in coronary artery disease,V. Evidence for enhanced platelet aggregateformation activity in acute myocardii infarction. Am J Caniiol1979;43:757-760. 9. Felts JD, Gallagher K, Rowe GG. Blood flow reductions in stenosedcanine coronary arteries: vasospasmor platelet aggregation?Cimdation 1982;65:24& 255. 10. Golmo P, Buja LM, Ashton JH, KuIkarni P, Taylor A, WilIerson JT. Effect of thromboxane and serotonin receptor antagonists on intracoronary platelet deposition in dogs with experimentally stenosed coronary arteries. C&x&&n 1988;78:701-711. ll. Mehta JL, Conti CR. Aspirin in myccardial ischemia:why, when and how much? C/in Cardid 1989,12:179-184. l2. Dinerman JL, Mehta JL. Endothelial, platelet and leukocyte interactions in ischemic heart disease:insights into potential mechanismand their clinical relevance.JAm Call Caniiol1990;16;2O-222. 15 Shimokawa H, Flavahan NA, Shepherd JT, Vanhoutte PM. Endothelium-
Diltiazem @g/ml)
dependent inhibition of ergonovine-inducedcontraction is impaired in porcine coronary arteries with regenerated endothelinm. Clrculntion 1989;80:643-650. l4. Ludmer PI Selwyn AP, Shook TL, Wayne RR, Mudge GH, Alexander RW, Ganz P. Paradoxical vasoconstriction induced by acetylcholine in athemsclerotic human coronary arteries. N Et& J Med 1986;315:1046-1051. lS. Harrison DG, Armstrong ML, Freiman PC, Heistad DD. Restoration of endothelium-dependent relaxation by dietary treatment of atherosclerosis.J Clin Inw? 1987;80:18@3-1811. ui. Mehta JL, Lawson DL, Mehta P, SaIdeen TGP. Increased prostacyclin and thromboxane A2 biosynthesis in atherosclerosis.&c Nad Acad Sci USA 1988;85:4511~515. 17. Lawson DI., Mehta JL Nichols WW, Mehta P, Donnelly WH. Superoxide radii-mediated endothelial injury and vasoconstriction of rat thoracic aortic rings.J&b C&n Med 1990;115:541-548. 10. Mehta JL, Nichols WW, Donnelly WH, Lawson DL, Thompson LV, ter Riet M, Saldeen TGP. Protection by superoxide dismutase from myocardial dysfunction and attenuation of vasodilator reserve following coronary occlusion and reperfusion in dog. Cix Res 1989;65%283-1295. 19. Mehta JL,, Lawson DL, Nichols WW. Attenuated of oxonary relaxation after reperfusion: effects of superoxidediiutase and TXA, inhibitor U63J57A. Am J Physiol1989;257:H1240-H1246. 20. Lawson DL, Mehta P, Mehta JL. Vascconstrictor and vasorelaxant effects of endothelin-one on rat aorta. Clin Res 1991;39:262A. 2L Haught H, Lawson DL, Mehta JL. Slow channel calcium bkxkers attenuate reoxygenation-mediatedvascular contraction, but augment severe hypoxiamediated vascular contraction.J Cardiovasc Pharmacd 1992(in press). 22. Mehta JL, Dinerman J, Mehta P, Saldeen TGP, Lawson DL, Donnelly WH, Wallin R. Neutrophil function in ischemic heart disease. CirnJalion 1989;79:54%556. 23. Mehta P, Mehta JL, Lawson DL Krop I, Letts GL. Leukotrienes potentiate the effects of epinephrine and thromvm on human platelet aggregation. Z7wmb Res 1986;41:731-738. 24. Nichols WW, Mehta JL Thompson LV, Donnelly WH. Synergisticeffects of LTC, and TXA, on coronary flow and myocardial function. Am J Phpiol 1988,255:H15>H159. 2S. Nimlini FA, Wilson AC, Mehta P, Mehta JL. Comparative platelet inhiiitory effects of human neutrophils and lymphocytes.JLab C&zMed 1990,116:1471.52. 28. Mehta JL, Lawson DI+ Nichols WW, Mehta P. Modulation of vascular tone by neutrophils: dependenceon endotheiial integrity.Am J Physiol1989;257: H131>H1320. 27. Mehta JL. Influence of calcium-channelblockers on platelet function and arachidonic acid metabolism.Am J Cardiol1985;55:158B-168B. 28. Mehta P, Nimlini FA, Mehta JL Influence of extracellular calcium on superoxide radical generation in cholesterol-poor and cholesterol-rich neutrophiIs: effect of slowchannel calcium blockers. C&z Res 1991;39:459A. 29. Latour JG, RousseauG, ST-JeanG. Diltiazem reducesneutrophil a-ulation and infarct size in reperfused myocardium. Circularion 1989?3O(suppl 2):114cr2.
A SYMPOSIUM: MYOCARDIAL ISCHEMIA
18B
in
Jawahar L. Mehta, MD, PhD, Francesca A. Nicolini, MD, William H. Donnelly, MD, and Wilmer W. Nichols, PhD
P
athologic examination of coronary arteries of patients with coronary artery disease generally shows extensive luminal narrowing with atherosclerosis as the dominant feature. Atherosclerosis appears in layers, with frequent organized thrombi in the arterial wall, suggesting that each layer of atherosclerosis may have evolved following a thrombotic event. In patients dying of acute myocardial ischemia (unstable angina or acute myocardial infarction), the epicardial large coronary arteries supplying the involved region usually show a recent thrombus that almost totally occludes the arterial lumen.‘92 Carefully conducted pathologic studies have revealed that the thrombus begins with disruption of the endothelial lining from a fissure or hemorrhage in the atherosclerotic region, resulting in exposure of subendothelial layers to circulating blood. Platelets adhere to subendothelial collagen, undergo activation, and release a variety of vasoactive substances that may cause dynamic vasoconstriction of the downstream coronary tree. Platelet activation also initiates the coagulation pathway involving formation of fibrin. Other cellular elements, such as leukocytes and erythrocytes, may also become incorporated into the plateletfibrin thrombus. Occlusion of the atherosclerotic coronary artery, if not opposed by naturally occurring defense mechanisms, may lead to severe reduction in blood flow to the myocardium. If the thrombus is not dissolved, myocardial injury may ensue, followed by myocyte death. The reestablishment of coronary blood flow by pharmacologic (e.g., streptokinase or tissue plasminogen activator) or physical (e.g., angioplasty and bypass surgery) means has become a therapy of choice in acute myocardial ischemia. However, reperfusion From the Division of Cardiology, Department of Medicine, University of Florida College of Medicine, and the Veterans Administraof the previously ischemic myocardium is often tion Medical Center, Gainesville, Florida. This study was supfollowed by morphologic and functional changes in ported by a VA Merit Review and Clinical Investigator Awards the affected coronary arteries and myocardial tis(JLM), American Heart Association Research Fellowship (FAN), and grants-in-aid from the American Heart Association, Florida sues, which may by themselves be considered Affiliate, St. Petersburg, Florida. Address for reprints: Jawahar L. Mehta, MD, PhD, Box J-277, detrimental. Hence, reperfusion therapy has been considered a “double-edged sword.” JHMHC, University of Florida, Gainesville, Florida 32610.
lttsgenerallyrecognizedthatformationofa platdet-#Jrin-rkh tluombus In an atherosclerotkwronaryarterylsthebasisofunstaMean= ginaandacute+myocafdialinfaMfon.Platelet hyperactfvltyhasbeenldenwIedincwonaryrlsk factorssuchashyperlipklemlaanddlabetesmellmls.Pwsktmtactivatknoftheseceusresldts in~ofgrowthfactorsthatmayconkibute to the prorpeaskn ofatherosderosia several recentstudlesshowthatendotheRum,bygen8rat~orm&&HMngahostof~substancerr, plays a crMcal rok in the modulation of vasculartone.lmportantamongthesesubstances are prostacyclln (PGQ and endothelkmderived rekud~i’actor (EDRF). The endatheliumdepe+ndeMmodulatkmofcoronaryarteryton8 -wCththeseverttyofandthenumberof coronaryrlskfactors.duressuchasangk@a&yandcoronarybypass surgery in/we the endothelium.Thekwsofendothelialsmoothmusderelaxantfunctionmaycontribute to the vrktion and thrombo&s of&enobserwdsoonafterthese#procedures. llwombolysis(andwbsquwtrqwfu&mofthe comnary arteq) is also associateul wtth severe endoWelialdysfunctkm,witharesultingvastrktoritiuenceonthecoronaryvasoular bed. AcWation of leukocytes and their presence in the bmpdmed myacardium contribute to progression of myocardtal INmy by release of oxygen free radicals and pMeolytk enzymes. Thus, tt-thataperturbatkninthlsde&ateequC Hum in celhdar itieractkns relates to gene& and progrw&m ofmyocardlal isch8mia. (Am J Cardk~I -69:8Sl3B)
88
THE AMERICAN JOURNAL OF CARDIOLOGY VOLUME 69
MARCH 6, 1992
In this report, we shall review our current understanding of the role of three major cell lines-platelets, leukocytes, and endotheliumthat may have a bearing on the genesis and progression of myocardial ischemia. PlAlELeeFUNCWNINCDRDNARYARTERY DISEASE
Nature has provided us with platelets as a first-line defense against bleeding when the integrity of the blood vessel is disrupted. Besides their dynamic role as a physiologic plug, platelets also provide nutrients for the maintenance of endothelial integrity. When activated, these tiny cells produce a variety of vasoactive substances and growth factors. Their hyperactive status in coronary risk factors, such as hyperlipidemia and diabetes mellitUS,3,4is well known and could result in vascular smooth muscle proliferation leading to vascular occlusion. Dbring acute myocardial ischemia, platelet activity in response to a variety of agonists such as catecholamines and thromboxane is markedly enhanced and in response to antagonists such as prostacyclin (PGI,) is diminished.s7 At the same time, platelet microaggregates are often seen in the peripheral circulation.8 Some vasoactive substances released from activated platelets (e.g., thromboxane A, and serotonin) are potent vasocon-
strictors, whereas others (e.g., adenosine diphosphate [ADP]) are vasorelaxants. The important role of platelets in the genesis of acute myocardial ischemia became evident from experiments in narrowed coronary arteries of dogs, which show spontaneous cyclic reductions (gradual decrease followed by abrupt increase) in coronary blood flow. These cyclic changes in blood flow are due to formation of occlusive platelet aggregates followed by their spontaneous dissolution.’ These cyclic changes can be abolished by antiplatelet agents, thromboxane inhibitors, and serotonin receptor antagonists.” Similar cyclic changes in blood flow have also been shown in human arteries. Since these changes in blood flow may be followed by total occlusion of the coronary artery, it may be hypothesized that platelet aggregation occurring in atherosclerotic coronary arteries may contribute to acute myocardial ischemia. Cyclic flow reductions are also observed in the therapeutically reperfused coronary arteries prior to their reocclusion (Figure 1). The morphology of these arteries often reveals a large number of platelets and fibrin strands on the intimal surface of the artery. The endothelial lining almost always shows extensive disruption (Figure 2). Based on these considerations, use of antiplatelet drugs, particularly aspirin, is advocated in a
180 120 60
Control
DC Current
2mcIA
Coronary Occlusion
t-PA 20 rs/kg/min
large number of cardiac conditions, e.g., in patients with unstable angina, myocardial infarction, and coronary artery bypass surgery, and in others receiving thrombolytic therapy.” ENDOTHEIJAI. FUNCTION IN WRONARY ARTERYDISEASE Endothelium lines all blood vessels in the human body, weighs in total about 12 lb, comprises approximately 1 x lo’* cells, and traditionally has been thought to be a protective lining. However, it also secretes or metabolizes a large number of substances with vasodilator, vasoconstrictor, anticoagulant, and procoagulant activities. As such, it may be considered the largest paracrine organ in the body.‘* Among the important vasodilator species generated by the endothelial cells are PGI, and endothelium-derived relaxing factor (EDRF). PGI, is derived from the cyclooxygenase pathway of arachidonic acid metabolism; and it is rapidly released locally in response to thrombin, bradykinin, high density lipoproteins, platelet-derived ADP
ififE!!&IBB
-WE-
and growth factor, tissue hypoxia, and hemodynamic stress. Aspirin, other nonsteroidal antiinflammatory drugs, and nicotine inhibit its release. Although short-acting (half-life is approximately 30 s), PGI, is a powerful vasorelaxant and platelet inhibitor, and these effects of PGI, are mediated by an increase in intracellular cyclic adenosine monophosphate (CAMP). EDRF is also a potent vasorelaxant and platelet inhibitor, but its effects are mediated by an increase in intracellular cyclic guanosine monophosphate (cGMP). It has been characterized as nitric oxide or a related substance, derived from the amino acid L-arginine, and is released locally in response to acetylcholine and many of the same stimuli as is PGI,. EDRF, being a non-prostaglandin-related substance, is not inhibited by aspirin or nonsteroidal antiinflammatory drugs. As such, vasodilation is still possible when the cyclooxygenase pathway is blocked. ii Both PGI, and EDRF serve to regu! ate vascular tone in response to physiologic and pathologic
~~-pl-ckposl-
dathmnb-d~a~fdkwlns*~ WWkW---
THE AMERICAN JOURNAL OF CARDIOLOGY VOLUME 69
MARCH 6, 1992
stimuli. The vasoconstrictor effects of smooth mus- mal models of atherosclerosis. Fortunately, the cle agonists is enhanced, and the effect of several endothelial function reverts to normal as serum smooth muscle relaxants is decreased, in the ab- cholesterol is lowered and atherosclerosis resence of endothelium. Given an intact endothe- gresses.15Unlike EDRF, local PGI, synthesis in lium, aggregating platelets, by release of ADP, atherosclerosis may not be diminished, since the elicit EDRF and relax the blood vessels. In con- release of arachidonic acid is enhanced in these trast, given a dysfunctional endothelium, aggregat- vascular segments.16 Following coronary artery occlusion and repering platelets constrict blood vessels via release of thromboxane A2 and serotonin.‘* These consider- fusion, oxygen free radicals are released; these ations may be particularly relevant after coronary superoxide radicals injure the endothelium (Figure angioplasty and thrombolysis, when the endothe- 3). The superoxide radical-mediated endothelial lial lining is disrupted and platelet activation in situ injury results in increased smooth muscle contracmay be responsible for dynamic coronary constric- tion and decreased endothelium-dependent contion. A recent study13shows that endothelial func- traction.*’ The oxygen free radical-mediated vascution remains abnormal for several weeks even if the lar injury may relate to decreased coronary blood endothelial morphologic integrity is restored. This flow reserve” and coronary reactivity” after reperwould imply the need for the continued use of fusion. Recent evidence indicates that the endothelium spasmolytic and platelet inhibitory agents for sevalso generates a potent vasoconstrictor named eral weeks after such procedures. In patients with stable coronary artery disease, endothelin, which is a 21-amino acid-containing release of EDRF in response to acetylcholine is peptide. The levels of endothelin are increased in markedly diminished.14 Furthermore, the endothe- acute myocardial ischemia. Besides inducing polial dysfunction correlates with the extent of coro- tent vasoconstriction and enhancing the effect of nary artery disease as well as with the number of circulating catecholamines, endothelin also stimucoronary risk factors. In atherosclerotic coronary lates release of both PGI, and EDRF.20 arteries, the release of EDRF is markedly diminIn addition to the several pathophysiologic states ished, and this decrease becomes evident even described above, endothelium also determines the before development of luminal narrowing in ani- vascular smooth muscle response to anoxia and
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reoxygenation.” Anoxia induces endotheliumAnother mechanism by which leukocytes could dependent vasoconstriction and reoxygenation- participate in ischemic injury of myocardium is by mediated initial vasorelaxation. In the absence of release of leukotrienes. We have shown that peptifunctional intact endothelium, the primary re- doleukotrienes (LTC,, LTD,, and LTE,) potentisponse of vascular smooth muscle to anoxia is ate platelet aggregation and vasoconstriction and, vasorelaxation and to reoxygenation is intense in synergy with platelet-released thromboxane A,, vasoconstriction. reduce coronary blood flow in the intact animal.“,24 Recently, neutrophils and monocytes have been shown to produce a nitric oxide-like material that LEUKOCYTE FUNCTlON IN CORONARY ARTERY decreases platelet aggregation by a cGMP-depenDlSEASE dent mechanism.*’ This material also decreases Leukocytes are a first-line defense against inva- vascular smooth muscle tone, and this effect of sion of the body by pathogens. These cells protect neutrophils is more pronounced in the absence of the tissues against the noxious effect of pathogens endothelium.26 The leukocyte-derived nitric oxide by their migration to the injury site,and activation may have a physiologic role in preventing platelet and release of proteolytic enzymes, oxygen free deposition on the vessel wall and maintaining radicals, and arachidonic acid metabolites. Informa- vascular tone. It may also have a pathologic role in tion developed in the past decade indicates that the development of shock-like state, particularly these cells, like platelets, have the potential to when leukocyte number is elevated and the endoenhance myocardial ischemic injury. Monocytes thelial integrity is disrupted. appear early at the potential site of atherosclerosis and subsequently transform into macrophages. The number of leukocytes in peripheral blood ROLE OF SLOW CHANNEL CALCIUM correlates with future cardiac events in both asymp- ANTAGONISTS IN CELLULAR ImRACTlONS Calcium mobilization is a key step in platelet tomatic and symptomatic patients. Recent electron microscopic studies in patients with acute myocar- activation.27Release of vasoactive substancessuch dial ischemia show intense activation of neutro- as thromboxane A,, serotonin, and adenine nuclephils, involving an increase in their chemotactic otides is modulated by voltage-dependent calcium ability, release of leukotriene B,, and release of an antagonists. For example, verapamil and diltiazem decrease platelet aggregation, adenosine triphosimportant proteolytic enzyme, elastase.** In animals subjected to coronary artery occlu- phate (ATP) release, and thromboxane A, generasion followed by reperfusion, leukocytes appear in tion. Although early studies showed that the inhiblarge numbers in the myocardial interstitium and itory effect of these calcium antagonists is observed partially plug the microvasculature.” Leukocyte only in very high concentrations, other studies deposition in the reperfused regions could be in show that these agents, in therapeutically achieved response to expression of complement fragments concentrations, do significantly inhibit platelet actiand adhesion molecules as well as to generation of vation induced by a variety of stimuli.27 Calcium mobilization is also involved in leukoa superoxide radical-stimulated chemotactic factor cyte migration.** Release of oxygen free radicals in plasma. The leukocytes then become activated from activated cells parallels the increase in extraand release oxygen free radicals and proteolytic enzymes, which in turn could be responsible for cellular calcium and its subsequent influx. At progression of endothelial and myocardial injury. therapeutic concentrations the voltage-dependent Physical obstruction of the microvascular bed by calcium antagonists verapamil and diltiazem deactivated leukocytes probably contributes to de- crease leukocyte chemotaxis and superoxide radicreased coronary flow reserve and myocardial stun- cal formation*’ (Figure 4). The vascular smooth muscle contraction in rening. Early studies in which superoxide radical scavengers were given prior to reperfusion have sponse to a variety of agonists is decreased by shown much promise in terms of modification of calcium antagonists. This has been the basis for the coronary flow reserve and myocardial dysfunction use of these agents in coronary artery disease.The seen after coronary artery occlusion and reperfu- agonist-induced release of PGI, and EDRF is also sion.‘8*‘9The release of elastase” along with super- a calcium-dependent phenomenon. Diltiazem has oxide radicals may also be an important pathogenic been shown to enhance PGI, synthesis by isolated factor in “reperfusion injury.” Recent studies in human blood vessels,*’whereas some studies show which elastase inhibitors were used suggest that a decrease in EDRF synthesis following calcium antagonists. These agents markedly decrease reoxthese agents may have a beneficial role. l2B
THE AMERICAN JOURNAL OF CARDIOLOGY VOLUME 69
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REFERENCES l. Falk E. Thrombosis in unstable angina: Pathologic aspects.In: Mehta JL, ed. Thrombosis and Platelets in Myocardiil Ischemia. Cardiovasc Clin Series, ~0118.Philadelphia: FA Davis, 1987137-149. 2. Davies MJ, Thomas A Thrombosis and acute coronary-artq lesions in sudden cardiac ischemicdeath. NEnglJMed 1984;310:1137-1140. 3. ShattiI SJ, Anaya-GaImdo R, Ben&t J, Colman RW, Cooper RA. Platelet hypersensitivity induced by cholesterol incorporation. J Clin Invest 197$55:63& 643. 4. Mustard JF, Packham MA. Platelets and diabetes meIIitus. N Et& .I Med 1977$97:1345-1347. 5. Mehta JL, Mehta P, Ostrowski N. Increase in human platelet alph+adrenergic receptor affinity for agonist in unstable angina. J Lab Chin Med 1985;106%61666. 6. Mehta JL, Mehta P, Conti CR. Platelet function studies in coronary heart disease,IX. Increased platelet prostaglandin generation and abnormal platelet sensitivity to prostacyclin and endoperoxide analog in angina pectoris. Am .I cardid 1980@943-947. 7. Neri Serneri GG, Mcdesti PA, Fortini A, Abbate R, Lombardi A, Gemini GP. Platelet receptors in active spontaneousangina.Loncet 1984;1383%841. 8. Mehta P, Mehta JL. Platelet function studies in coronary artery disease,V. Evidence for enhanced platelet aggregateformation activity in acute myocardii infarction. Am J Caniiol1979;43:757-760. 9. Felts JD, Gallagher K, Rowe GG. Blood flow reductions in stenosedcanine coronary arteries: vasospasmor platelet aggregation?Cimdation 1982;65:24& 255. 10. Golmo P, Buja LM, Ashton JH, KuIkarni P, Taylor A, WilIerson JT. Effect of thromboxane and serotonin receptor antagonists on intracoronary platelet deposition in dogs with experimentally stenosed coronary arteries. C&x&&n 1988;78:701-711. ll. Mehta JL, Conti CR. Aspirin in myccardial ischemia:why, when and how much? C/in Cardid 1989,12:179-184. l2. Dinerman JL, Mehta JL. Endothelial, platelet and leukocyte interactions in ischemic heart disease:insights into potential mechanismand their clinical relevance.JAm Call Caniiol1990;16;2O-222. 15 Shimokawa H, Flavahan NA, Shepherd JT, Vanhoutte PM. Endothelium-
Diltiazem @g/ml)
dependent inhibition of ergonovine-inducedcontraction is impaired in porcine coronary arteries with regenerated endothelinm. Clrculntion 1989;80:643-650. l4. Ludmer PI Selwyn AP, Shook TL, Wayne RR, Mudge GH, Alexander RW, Ganz P. Paradoxical vasoconstriction induced by acetylcholine in athemsclerotic human coronary arteries. N Et& J Med 1986;315:1046-1051. lS. Harrison DG, Armstrong ML, Freiman PC, Heistad DD. Restoration of endothelium-dependent relaxation by dietary treatment of atherosclerosis.J Clin Inw? 1987;80:18@3-1811. ui. Mehta JL, Lawson DL, Mehta P, SaIdeen TGP. Increased prostacyclin and thromboxane A2 biosynthesis in atherosclerosis.&c Nad Acad Sci USA 1988;85:4511~515. 17. Lawson DI., Mehta JL Nichols WW, Mehta P, Donnelly WH. Superoxide radii-mediated endothelial injury and vasoconstriction of rat thoracic aortic rings.J&b C&n Med 1990;115:541-548. 10. Mehta JL, Nichols WW, Donnelly WH, Lawson DL, Thompson LV, ter Riet M, Saldeen TGP. Protection by superoxide dismutase from myocardial dysfunction and attenuation of vasodilator reserve following coronary occlusion and reperfusion in dog. Cix Res 1989;65%283-1295. 19. Mehta JL,, Lawson DL, Nichols WW. Attenuated of oxonary relaxation after reperfusion: effects of superoxidediiutase and TXA, inhibitor U63J57A. Am J Physiol1989;257:H1240-H1246. 20. Lawson DL, Mehta P, Mehta JL. Vascconstrictor and vasorelaxant effects of endothelin-one on rat aorta. Clin Res 1991;39:262A. 2L Haught H, Lawson DL, Mehta JL. Slow channel calcium bkxkers attenuate reoxygenation-mediatedvascular contraction, but augment severe hypoxiamediated vascular contraction.J Cardiovasc Pharmacd 1992(in press). 22. Mehta JL, Dinerman J, Mehta P, Saldeen TGP, Lawson DL, Donnelly WH, Wallin R. Neutrophil function in ischemic heart disease. CirnJalion 1989;79:54%556. 23. Mehta P, Mehta JL, Lawson DL Krop I, Letts GL. Leukotrienes potentiate the effects of epinephrine and thromvm on human platelet aggregation. Z7wmb Res 1986;41:731-738. 24. Nichols WW, Mehta JL Thompson LV, Donnelly WH. Synergisticeffects of LTC, and TXA, on coronary flow and myocardial function. Am J Phpiol 1988,255:H15>H159. 2S. Nimlini FA, Wilson AC, Mehta P, Mehta JL. Comparative platelet inhiiitory effects of human neutrophils and lymphocytes.JLab C&zMed 1990,116:1471.52. 28. Mehta JL, Lawson DI+ Nichols WW, Mehta P. Modulation of vascular tone by neutrophils: dependenceon endotheiial integrity.Am J Physiol1989;257: H131>H1320. 27. Mehta JL. Influence of calcium-channelblockers on platelet function and arachidonic acid metabolism.Am J Cardiol1985;55:158B-168B. 28. Mehta P, Nimlini FA, Mehta JL Influence of extracellular calcium on superoxide radical generation in cholesterol-poor and cholesterol-rich neutrophiIs: effect of slowchannel calcium blockers. C&z Res 1991;39:459A. 29. Latour JG, RousseauG, ST-JeanG. Diltiazem reducesneutrophil a-ulation and infarct size in reperfused myocardium. Circularion 1989?3O(suppl 2):114cr2.
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