Cardiovascular injury induced by sympathetic catecholamines

Cardiovascular Injury Induced by Sympatbetie Catecholamines

Jacob I. Haft

The damaging effect of sympathetic catecholamines on phenomenon are presented, and the recent data that sug-

the cardiovascular system is reviewed. The catecholamines gests that intravascular platelet aggregation is of etiologic

in high dose cause diffuse necrotic lesions in the myo- importance is reviewed. The possible relation of ,these

cardium with accompanying inflammation and subsequent effects of catecholaminas to myocardial infarction that

fibrous scarring. This can be readily demonstrated in the occurs during stress is discussed. High doses of catechol-

experimental animal. The clinical counterpart is seen in amines causa medial necrosis, destruction of the elastic

patients dying after prolonged treatment with norepi- lamellae, and atrophic scarring of the wallsof large vessels.

nephrine and among patients dying with pheochromocy- The relationship of this lesion and other effects of

toma. The various theories suggested to explain this catecholamines to atherosclerosis is reviewed.

T HE SYMPATHETIC catecholamines have pro- found physiologic effects on the heart and

the blood vessels. In the heart, inotropy is enhanced with an increase in the amount and rate of tension developed with each beat, with a resulting decrease in the end-diastolic pressure and the end- systolic volume. Cardiac automaticity is increased as manifested by an acceleration of the sinus rate and by the development of ectopic atria1 and ventricular arrhythmias. Oxygen and substrate con- sumption by the heart is increased as is the amount of lipid available in the blood for use as fuel. In the blood vessels, the various catecholamines lead to constriction and/or dilatation, depending on the specific vascular bed and on the specific sympathetic catecholamine. In addition to altering the function of the cardiovascular system, effects that can be either beneficial or deleterious to the organism, catecholamines have been demonstrated to cause anatomic damage to the heart and to the blood vessels. It is the purpose of this report to review the latter effects.

intramyocardial hemorrhage, edema, patchy areas of myofibrillar damage, round cell infiltration,, and proliferation of fibrous tissue in the hearts of rabbits subjected to repeated injections of adrenalin. He found these lesions as early as 6 days after the onset of injections and noted areas of increased connective tissue and “anemic infarcts” after 10 days of adrenalin administration. Pearce’ in 1906 found similar lesions with myofibrillar edema, loss of striations, and increase in fibrous tissue in the heart after seven to 15 injections of adrenalin.

MYOCARDIAL DAMAGE DUE TO CATECHOLAMINES

The injurious effect of catecholamines on cardiac muscle has been known since the beginning of this century. With the availability of relatively purified extracts of the adrenal medulla, numerous investigators studied the pathologic effects of parenter- ally administered catecholamines on the heart.

In 1905, Ziegler’ reported the occurrence of

From the Cardiac Section, Bronx V.A. Hospital, and the Mount Sinai School of Medicine, New York, N. Y.

Reprint requests should be addressed to Dr. Jacob I. Haft, Chief Cardiac Section, V.A. Hospital, I30 West Kingsbridge Road, Bronx, N, Y. 10468.

0 1974 by Grune & Stratton, Inc.

During studies of induced cardiac hypertrophy, Josue in 1907 noted myocardial degeneration and foci of sclerosis (ilots de tissu sclereux) in rabbits who received adrenalin intravenously in conjunc- tion with subcutaneous injections of sterilized typhoid bacteria. Fleisher and Loeb4 in 1909 produced similar lesions with either spartein or caffein in combination with adrenalin. They found that production of the lesions was dependent on the adrenalin, but in the absence of the additional substances higher doses of adrenalin were neces- m-y. In 19Y1, Christian et al.5”‘.fbund cardiac necrosis and fibrosis of the same type while in- vestigating the effects of adrenalin injections on the production of renal and cardiac injury.

These cardiotoxic effects of adrenalin have been confirmed repeatedly. 9-16 With demonstration that norepinephrine is also secreted by the sympathetic nervous system, and the development of isoproterenol, it was shown that these catecholamines are also markedly effective in producing experimental myocardial injury.rr-r5 ,17-31 Characteristic myocardial lesions induced by catecholamines have been produced in many species of laboratory animals, including rabbits, rats, dogs, cats, pigs, hamsters, and turtles.

Catecholamine-induced necrosis and fibrosis of

Progress in Cardiovascuiar Diseases, Vol. XVII. No. 1 (July/August). 1974 73

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the myocardium had also been documented to occur in man. In 1937, Franz,32 and in 1939, Gormsen33 each reported deaths related to an overdose of epinephrine that had been used for treatment of asthma. On postmortem examination, these patients were found to have lesions in the myocardium similar to those described in experimental animals. Szakacs and Cannon’7 in 1958 de&bed two patients who died after prolonged infusion of norepinephrine for the treatment of shock in whom focal myocarditis with degeneration of myofibrils and infiltration of leukocytes were found on histologic study of their hearts. In a further report in 1960, Szakacs and Mehlman” noted the occurrence of similar lesions in three additional patients treated with norepinephrine.

The spontaneous counterpart of administration of excess amounts of exogenous sympathetic catecholamines is seen in patients with pheochromocytomas. In a review of the literature in 1957, Wat- kins34 drew attention to the frequently reported finding of multiple sites of focal fibrosis in the myocardium in patients dying with pheochromo-

JACOB I. HAFT

cytomas. Szakacs and Cannon” found evidence of acute myocarditis in three of 17 patients with pheochromocytoma and myocardial disease in the others. The lesions were identical with those seen in patients dying after prolonged norepinephrine infusion and in animals experimentally infused with norepinephrine. Kline35 reported similar findings in four of seven patients with pheochromocytoma. Van Vliet et al.,22 on review of 26 cases seen at the Mayo Clinic from 1928 to 1964, found active myocarditis in 15 patients and myocardial fibrosis in eight of the others, including two that had distribution of the fibrotic lesions similar to that of the active myocarditic lesions. On comparison of the myocardial lesion found in their patients with the experimental cardiac lesion produced by norepinephrine injection in rats, these investigators found a marked similarity. Those rats killed shortly after injection showed lesions similar to the patients with active myocarditis, and those rats killed weeks after injection showed the diffuse pattern of myocardial scarring seen in the pheochromocytoma patients with myocardial fibrosis.

Fig. 1. Photomicrograph of the heart of a dog sacrificed 7 days after receiving a 4-hr infusion of epinephrine (4 @g/kg/ min). Note the diffuse destruction of myofibers with round cell infiltration and early fibrosis.

INJURY INDUCED BY SYMPATHETIC CATECHOLAMINES

THE MYOCARDIAL LESION

The myocardial lesion produ,ced by catecholamine administration is similar in all species that have been studied (including man) and does not differ with the catecholamine used or the route of administration. The severity and extent varies directly with the dose and rate of administration; the pathologic appearance of the lesion is deter- mined by the time that has elapsed between administration of the drug and sacrifice of the animal.

On gross examination, in those animals studied shortly after infusion (or in those patients dying during norepinephrine infusion or with acute myocarditis due to activity of a pheochromocytoma), subendocardial and intramyocardial hemorrhages are seen.‘1~‘2~18~20~21 These are found predomi- nantly in the left ventricle13y20P21 but occur in all chambers of the heart. Edema and hemorrhages into the valve leaflets and in the subendocardial

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myocardium also are seen occasionally.‘1Y’2 Within hours to a few days, patches of anemic infarction with the appearance of pale, yellow-brown areas are found in the ventricular myocardium.4’13725 The myocardium in these areas is stiffened and loses its pliability? After weeks, areas of gray, scarred fibrosis may be seen.

Immediately after infusion, on microscopic examination, congestion of the vasculature,5T’3 interstitial edema,2P47s313 focal myofiber degeneration with loss of striations of individual muscle fibers, vacuolization, and accumulation of lipid droplets within myofibers is seen.12~13~1s~1gy20~21~ 27T31 Subendocardial vascular congestion and hemorrhages” ,13 especially in the regions of thebesian vessels2’ are a common finding. Within hours 15Y2sy30 interstitial edema and infiltration with ’ polymorphonuclear leukocytes and then mononuclear cells appearsy13 frequently in perivascular areas or surrounding necrotic myocardial

Fig. 2. Electron micrograph of the heart of a rat sacrificed 30 min after the administration of 3 mg of isoproterenol intraperitoneally. Note the presence of wide 2 lines in many of the sarcomeres, with other areas showing sarcomeres that are pale and appear pulled apart. Part of a capillary containing aggregated platelets is present at the bottom of the section.

76

cells. Perivascular Anitschkow myocyte type cells are also seen.13~18920 Fibrinoid degeneration of arterioles has been noted.11718~z2

After 3 days, 2~4~14Y15~20,22 distinct foci of necrosis with disolution of the myolibrils and sarcoplasmic fragmentation are present (Fig. 1). Infiltration with histiocytes, lymphocytes, and occasionally plasma cells” are found. Polymorphonuclear cells are rare at this stage.30 Starting from about this time, fibrotic bands appear in the interstitium. Over the next few weeks, the necrotic debris of the destroyed myocardial cells are removed by phago- cytes, and the acute inflammatory changes abate. 2p4920 Fibrous replacement with collaginous bands, especially in areas surrounding the microvascula- ture, becomes prominent as healing progresses, 2p4~16 and late lesions show only these scars as evidence of the earlier acute necrotic 1esions.4920922t30

On electronmicroscopic examination, the myocardial cells show distinct changes in the myo-

JACOB I. HAFT

fibers, the mitochondria, and the endoplasmic reticulum (Figs. 2-6).‘5*21Y27~30~31~36 In specimens obtained minutes after exposure to catecholamines, areas of myotibrillar damage contain sarcomeres that are hypercontracted with wide 2 lines and “contraction bands.“15~30931y37 Evidence of tearing away of myofilaments from the sarco- lemma has been noted.31 The mitochondria in these areas are enlarged and swollen with increase in the intracristal space. r5s27530 Electron dense de- posits in the mitochondria are also occasionally seen21s3’ and have been thought to be calcium.30 The number of glycogen granules are decreased.15Y27Y30 Between the hypercontracted areas, the sarcomeres appear pale. Dilatation of the sarcoplasmic reticulum and transverse tubules is seen. 1sy27930931 As the time from the onset of catecholamine exposure to sacrifice increases, the number and extent of the contraction bands increases and the structural integrity and fine archi-

Fig. 3. Electron micrograph of the heart of a rat sacrificed 30 min after administration of 3 mg of isoproterenot intraperitoneaily. Note the irregularly contracted sarcomeres with areas of fragmentation, contraction bands, and edema. The mitochondria appear swollen. In the center of the section, there is a platelet aggregate within a capillary, partially occluding it. The granules of the platelets have centralized, although most have not yet extruded their contents.


tecture of the sarcomeres begin to show evidence of destruction. Swelling and dilatation of the endoplasmic reticulum15~30 and development of lipid droplets is seen. 15~27 In specimens obtained be- yond 8 hr after onset of catecholamine exposure, dissolution of myofibrils with loss of striations and Z lines is seen; mitochondria are markedly swollen with destruction of cristae,” and leukocytes make their appearance. Evidence of phagocytosis of myotibrillar debris appears.30 An increase in the number of lysosomes has been noted.r5 In our studies of the hearts of animals sacrificed shortly after administration of catecholamines,36p37 many small myocardial vessels were found to contain aggregated platelets.

THEORIES

There have been numerous theories suggested to explain the damaging effect of catecholamines on the heart. Pearce’ in 1906 considered the lesion to be related to constriction of the coronary arteries

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caused directly by adrenalin. Such constriction was felt to be sufficient to interfere with augmentation of the blood supply that was needed by the more vigorously contracting myocardium. He noted that lesions were similar to those found after ligation of a coronary artery. Pearce considered but rejected a direct toxic effect of adrenalin on heart muscle and did not feel that lesions in the aorta at the ostia of the coronaries were sufficient to explain the findings.

Fleisher and Loeb4F38 felt that a mechanical factor played the significant role, with the violent contraction caused by adrenalin causing the resultant damage. They also suggested that direct toxic effects of adrenalin may be related. Because they considered adrenalin not to cause constriction of coronary vessels they did not feel that the myocardial lesions were related to a decrease in myocardial nutrition.

Vishnevskayar’ studied the relationship of the nervous system to the myocarditis that occurs

Fig. 4. Electron micrograph of the heart of a dog sacrificed after a 4-hr infusion of norepinephrine (4 pg/kg/min), show-

ing detail of two contraction bands, and the presence of swollen mitochondria with damaged cristae.

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after adrenalin administration. He found that the incidence of the myocardial lesion was signifi- cantly decreased in rats that had their spinal cords severed at C7-Tl prior to adrenalin administration and that the severity of the lesion increased in rats that had been decorticated. He interpreted these results to indicate that reflex arcs play a part in the etiology of the lesion.

Szakacsr7 suggested three possible mechanisms to explain the myocarditic damage associated with norepinephrine. The vagal stimulation seen during administration of both epinephrine and norepinephrine was felt to possibly play a role, since it had been previously shown that vagal stimulation alone can lead to subendocardial hemorrhages. The role of catecholamines in high energy phosphate transfer that is necessary for dissociation of actin and myosin during the relaxation phase might be of etiologic importance. An overabundance of epinephrine or norepinephrine may be toxic, increas-

JACOB I. HAFT

ing the dissociation of actin and myosin molecules of the contractile apparatus to an irreversible ex- treme. He also, considered that anoxia might be related but did not feel that direct mechanical effects on contraction played a significant role.

Maling and Highman12 noted fat droplets in myocardial cells after catecholamine administration and suggested that these were similar in etiology to the fat infiltration seen at the periphery of infarcts, suggesting that ischemia plays an important role. This ischemia might be due either to increased oxygen utilization because of increased work of the heart, or to coronary vasoconstriction. They found that the myocardium was protected from these effects by dibenzyline, and interpreted this to suggest that hypertension and coronary vasoconstriction were of etiologic importance. Direct alterations in lipid metabolism might also play a role.

Nahas et al.rr isolated the heart from nervous

Fig. 5. Electron micrograph of the heart of a rat that received intraperitoneally administered isoproterenol 30 min before sacrifice. Note the contraction bands, the pale sarcomeres, and the capillary almost totally occluded by a platelet aggregate.


and hormonal influences by studying the effects of catecholamines on the heart-lung preparation. They found the same cardiac lesions, suggesting that reflex arcs do not play a role. They suggested that the prolonged increased myocardial tension induced by the catecholamines may be responsible for cardiac lesions. Furthermore, they also proposed that catecholamine effects on lipid metabolism may be important, and that the,alteration in lymphatic flow, with dilation and enlargement of lymphatics that they observed, may be related to the interstitial edema that occurred as an early manifestation of damage.

Chappel et al. compared the cardiotoxic effects of isoproterenol with epinephrine, norepinephrine, and ephedrine. Finding that a much lower per- centage of the median lethal dose of isoproterenol (although a much larger amount in milligrams) re- sulted in larger amounts of necrosis, they reasoned that coronary constriction did not play a role. However, they feel that the ischemia due to a disproportion between the enhanced myocardial re- quirement for oxygen from the increased inotropy

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of the heart and the oxygen available was the cause of the lesion. The depressor effect of isoproterenol on the systemic vasculature widens the in- equality between oxygen supply and demand.

By injection of india ink into the coronary arteries of hamsters sacrificed shortly after injection of isoproterenol, Handforth demonstrated that blood flow to the inner areas of the left ventricular wall was markedly reduced, even prior to the development of myocardial edema or necrosis. He felt that the necrosis that developed was due to ischemic infarction and suggested that either local- ized vasoconstriction or shunting of blood to effectively bypass the subendocardial areas had occurred.

Studying the effects of isoproterenol on serum enzymes and adrenal steroids, Wexler and Kittin- ger*’ noted the close resemblance between the myocardial lesion and that due to myocardial infarction in breeder rats with spontaneous atherosclerosis. The effects on serum enzymes and adrenal steroids showed a close similarity to the effects of myocardial infarction in humans.

Fig. 6. Electron micrograph of the heart of a dog that received an infusion of norepinephrine (4 tigglkglmin) for 4 hr. Note the changes in the muscle cells. In the center is a mature platelet clot with fibrin surrounding the platelets. Sections of the wall of the capillary that is occluded by the thrombus are destroyed, probably as a result of the proteolytic enzymes extruded from the platelet granules.

80 JACOB I. HAFT

Changes in serum lipids and serum proteins that occurred after isoproterenol administration were felt to possibly be related to the occurrence of myocardial infarction rather than the infarction to be secondary to the metabolic effects of the catecholamines.

From their electronmicroscopic studies Ferrans et al.15~27 concurred that ischemia is the apparent etiology for the myocardial lesion caused by isoproterenol and reasoned that it was the increased demand for oxygen caused by the inotropic stimu- lus of the catecholamine that led to the ischemia. In comparing the morphological changes after isoproterenol with those seen after myocardial infarction, they found marked similarities, with only the rate of development of the lesion being different (morphological changes occurred more slowly with isoproterenol). They also have suggested that isoproterenol, by its metabolic effects, may enhance the amount of lipid that is found in the cell. They also felt that the fibrosis found after isoproterenol may be related to a direct stimulating effect of the drug on interstitial cell growth and multiplication.

Rosenblum et al.r4 compared the myocardial toxic effect of 15 sympathomimetic drugs. They found lesions to occur in more than 50% of their rats after various doses of dl-isoproterenol, l-epinephrine, 1-norepinephrine, dl-methoxamine, dl- nordefrin, dl-phenylephrine, dl-hydroxyampheta- mine, dl-norephedrine, tranylcypromine, and tryramine. D-amphetamine, dopamine, dl-ephedrine, mephenteramine, and dl-isoetharine did not lead to similar lesions (possibly because high doses led to rapid death before these effects could develop). ,Rosenblum. et al. were unab1e.t.o explain why these latter compounds, many of which have inotropic, metabolic, and vascular effects similar to the drugs of the former group, did not produce cardiac lesions.

Schenk and Moss,~’ studying the effects of norepinephrine, agreed that the lesion was an ischemic one and suggested that the drop in sys-

temic blood pressure shortly after the onset of infusion led to a further decrease in coronary blood flow that was already not sufticient to supply the oxygen needed by the heart because of the aug- mented inotropic effect of the catecholamines.

Ostadal et al.** tested the hypothesis that the lesion produced by isoproterenol was related to either coronary vasoconstriction or to local redis-

tribution of blood within the heart by studying the effect of isoproterenol on the hearts of turtles. The blood supply to the heart of the turtle is different from that of mammals in that the outer part of the heart wall is supplied by coronary arteries, but the inner part is a spongy layer with lacunae in direct communication with the lumen of the ventricle. Necrosis occurred in the spongy myocardium just below the compact layers. These findings suggested that the vascular effects per se play little role in isoproterenol-induced cardiac necrosis.

Hoak et al.*l proposed that the stimulating effect of catecholamine on lipid metabolism plays an important part in the production of cardiac lesions. With the mobilization of lipid from adipose tissue induced by catecholamines, the plasma level of free fatty acids increases and may lead to their accumulation within the myocardial cell. Such accumulation of long chain fatty acids within the cell may lead to uncoupling of oxidative phos- phorylation and also to direct tissue damage.

Csapo et aL3r studying the early ultramicro- scopic anatomic changes after isoproterenol administration found that the first changes were related to hyperconstriction of the myofilaments with contraction bands and disorganized and frag- mented myofibrils, some torn and separated from each other. They suggested that the etiology of the myocardial lesion is multifactorial with the toxic mechanical direct effect of the catecholamines playing an important role.

THROMBOGENIC THEORY OF THE

ETIOLOGY OF CATECHOLAMINE-INDUCED MYOCARDIAL lN.FARCT.LfJN

As noted above, the striking similarity between the microscopic lesions induced by catecholamines and those seen after myocardial infarction has been recognized by many investigators. The mito- chondrial changes, the sarcoplasmic edema, the appearance of fat droplets, and the myofibrillar changes can all be induced by myocardial ischemia, and thus, ischemia has been favored as the etiologic mechanism by many authors. Through their marked inotropic stimulation, catecholamines induce ischemia by disproportion between oxygen need and oxygen availability. In addition, the blood pressure depression caused by prolonged pressor amine or isoproterenol infusion further in-

INJURY INDUCED BY SYMPATHETIC CATECHOLAMINES 81

hibits sufficient coronary blood flow from supply- ing the inotropically stimulated myocardium.

Recently, it has been proposed that occlusion of small cardiac vessels by platelet aggregates may be part of the mechanism that leads to myocardial ischemia.‘6Y36p37

In 1914, Cannon demonstrated that sympathetic catecholamines enhance coagulation of the blood. This work has been repeatedly confirmed.” In the 1960s it was shown that platelets would aggregate in vitro in the presence of either epinephrine or norepinephrine. 41-43 The sensitivity of platelets to varying doses of catecholamines in vitro is used as a measure of the propensity for platelets to aggregate in vivo. It has also been suggested that physiologic concentrations of epinephrine will cause the arterial wall to release thromboplastic substances that induce platelets and leukocytes to stick to each other.68

The ability of intravascular platelet aggregates within the coronary vessels to cause myocardial necrosis similar to that seen after catecholamine i?#usion WXY demonstmted by “Wt%tard and’ co- workers.44 They infused adenosine diphosphate, a potent platelet aggregating agent, directly into the coronary arteries of pigs and showed in histologic specimens that platelet aggregates plugging the vessels occurred and caused myocardial infarctions and sudden death. In those animals that were sacrificed immediately after infusion, intravascular platelet aggregates were numerous. In those that were sacrificed some time after completion of the infusion, platelet aggregates were less commonly found, but patchy necrosis was present even where the platelet aggregates were not found, suggesting that platelet plugs had occurred in these areas, had occluded the vessels for a sufficient time to cause cardiac necrosis, but had then broken up and dis- appeared so that they were no longer seen at time of sacrifice. This interpretation fits with what is known about platelet aggregation. Under ADP or catecholamine stimulation, platelet aggregation occurs in two phases. 45 The first immediate phase is reversible and consists of the platelets sticking to each other and the vessel wall. The second phase, somewhat later, occurs as the platelets become spherical or ovoid, losing their pseudopods and centralizing their granules. The granules, which contain catecholamines, ADP, and lysosomal enzymes, then extrude their contents. These extruded substances cause further aggregation of

platelets, loss of the fine architecture of the platelets (viscous metamorphosis), and irreversible aggregation of the platelets4’ Thromboplastic: substances also are released at this stage and the cas- cade system that results in the formation of Ifibrin and a macroscopic clot is initiated. If the process stops prior to the significant progression of the second stage, the platelet aggregate may break up and be carried away by the bloodstream.

To investigate whether the effect of sympat.hetic catecholamines on myocardial necrosis might be related to the intravascular aggregation of platelets that act as plugs in the small vessels of the heart, a series of experiments were embarked upon in 197 1. l6 It was reasoned that if platelet aggregation played a significant part in the etiology o-f the catecholamine-induced myocarditic lesion, pre- treatment of animals with drugs that would inhibit platelet aggregation might prevent the lesion in animals subsequently infused with catecholamines. A number of pharmacologic agents have been demonstrated to exert an antiaggregating effect on

-’ platelets.@?4spirin4*“and dipyridamole49 .‘have been studied extensively and have been shown to have a significant effect on platelet function both in vitro and in viva. Ten dogs were pretreated with aspirin 10 grains b.i.d. for 4 days, and ten dogs with dipyridamole 50 mg b.i.d. for 4 days and then infused with epinephrine 4 pg/kg/min for 4 hr. The animals were sacrificed 7 days later and the histologic findings in their hearts were compared with the findings in ten dogs that had not been pretreated but had been subject to similar infusions. Myocardial necrosis was graded 0 to 3+ with 3t signifying diffuse confluent myocardial necrosis, 2t, diffuse necrosis without confluence, and l+, a few small patches of isolated necrosis, Whereas all of the ten control animals were found to have necrosis, (eight with 2 or 3t) only three of each of the pretreated groups had any necrosis. None of the pretreated animals were found to have 3+ necrosis, three had It among the ani.mals pretreated with aspirin, and one each had trace (a few areas less than would be graded 1 t), It, and 2+ necrosis among the dipyridamole pretreated animals. Since aspirin and dipyridamole have little in common other than their platelet-inhibiting effects, the protection afforded by both of these drugs suggests that intravascular platelet aggregation plays a significant role in the etiology of catecholamine-induced cardiac necrosis.r6

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To further support the concept that platelet aggregates in the small vessels of the heart played a role in the myocardial damage induced by catecholamines, it was necessary to demonstrate the presence of such platelet plugs. Platelets are diffi- cult to identify using the light microscope because they frequently appear as amorphous debris, but can be readily recognized using the electron microscope. The hearts of six dogs sacrificed immediately after 4-hr infusions of norepinephrine 4 pg/kg/min were studied with the electronmicro- scope. All the animals were found to have platelet aggregates within the small vessels of their hearts.36 Intravascular platelets in all stages of aggregation were found. These ranged from a few platelets adherent to. each other, to platelet plugs completely occluding vessel lumina with platelets that had already undergone viscous metamorphosis. In most of the vessels the endothelial surfaces appeared intact, suggesting that the platelet aggregation was primary, rather than secondary to vessel wall damage. In some of the vessels containing occlusive mature platelet plugs there were areas of vessel wall breakdown that suggested damage caused by the extruded lysosomal enzymes” from the platelets that had already undergone viscous metamorphosis.

Electronmicroscopic studies of the hearts of 14 rats given 3 mg isoproterenol i.p. 1 hr prior to sacrifice revealed similar platelet aggregates within the small vessels of the myocardium.37 Platelets in all degrees of aggregation ranging from two and three platelets adherent to each other and to the vessel wall to platelet plugs completely occluding small vessels were found.

An intriguing aspect of the study of catecholamine-induced cardiac necrosis has been the relationship of endogenous catecholamine secretion during stress and cardiac damage. The cardiac lesions induced by various forms of stress in the adrenal corticoid-pretreated animal reported by Selyes 1 are similar to those caused by excess catecholamines. In studies by Raab et al.,52 adrenergic- blocking agents were found effective in preventing these stress lesions in corticoid-pretreated animals, suggesting that these stress lesions require an intact sympathetic adrenergic system for their production. If the thrombogenic effect of catecholamines secreted during stressful situations plays a role in the etiology of these lesions, this finding might be of etiologic importance in the relation of acute myocardial infarction to stress. To determine if

JACOB I. HAFT

Table 1. Stress Studies

Cold stress 16 rats 100% with intravascular

platelet aggregation

Hot water stress 8 rats 87.5% with intravascular


Shocking stress 15 rats 87.8% with intravascular


Controls 14 rats 7.3% with intravascular


endogenous catecholamines were able to induce intravascular platelet aggregates, studies on rats exposed to immersion in ice cold water,53 exposed to immersion in hot water,54 and who had received repeated painful shocks to their pawss4 were carried out. In all of these stress studies, platelet aggregates similar to those seen after exogenous catecholamine administration were found in most of the rats (Table 1).

There is a large literature correlating stress with acute myocardial infarction or sudden death.ssYs6 It has also been demonstrated that catecholamine secretion is enhanced during stressful situations.52 It is possible that the link between stress and these myocardial events may be via a platelet aggregate induced to form intravascularly by endogenously secreted catecholamines, and traveling to an area previously narrowed by atherosclerosis. Such an aggregate might totally occlude the vessel lumen and lead to either a myocardial infarction, or to the induction of a fatal ventricular arrhythmia. If this is the case, chronic treatment with drugs that inhibit platelet aggregation might be effective in preventing myocardial infarction and sudden death. There are currently a number of large scale studies underway to determine if aspirin, an active platelet aggregation-inhibiting drug, will have an effect on the prognosis in patients at risk for myocardial infarction.

DIRECT VASCULAR INJURY INDUCED BY CATECHOLAM INES

Very early in this century, attention was drawn to adrenalin as a possible etiologic agent for the development of arteriosclerosis. Josues in 1904 demonstrated that injections of adrenalin in rabbits would cause degenerative changes and scarring in the wall of the aorta. A large number of investigators throughout the world confirmed these findings over the subsequent years9.59160 (this work was exhaustively reviewed by Heupe? in 1948). Although most investigators have agreed that


adrenalin can affect the vessel wall and damage it, the relationship of these lesions to atherosclerosis remains in question.

Grossly,58-60 the early vascular lesion induced by adrenalin is a punctate or linear, depressed, grayish area in the ascending or thoracic aorta. With time, confluent, irregular, brittle pearly gray areas develop that at first are depressed but later begin to bulge slightly. Later the aortic wall involved in the lesion becomes thin and hard, similar to parchment, and aneurysmal dilatation of these lesions may develop. The aorta may eventually be completely involved with numerous areas of nod- ularity and sacular dilatation. The late plaques are hard, shiny, with brownish discoloration and are arranged in groups or in longitudinal lines. Dissect- ing aneurysms that originate in these plaques are seen not infrequently. Occasionally hemorrhagic necrosis in the vessel wall is also seen. Involvement of middle-sized vessels including the coronary arteries also occasionally occurs in animals with the late lesion in the aorta.’

Histologically, 58-60 the initial lesion is a focus of necrosis of the muscularis with atrophic degenerative changes and eventual necrosis occurring in the muscle cells. Calcium salts enter these necrotic areas early in their development and the medial layer of the vessel wall becomes calcified. The elastic lamellae become stretched out and com- pacted and eventually they degenerate with swelling, fragmentation, and clumping of the elastic fibers. Reactive inflammation with Iymphocytes and polymorphonuclears accumulation is seen. Giant cells are an inconstant finding, and occasionally granulation tissue develops. The intimal layer of the wall is usually not primarily involved but occasionally shows endothelial thickening and proliferation9 over the area of the medial lesion. In areas of sacculation of the wall, hyperpiasia, hyalinization, and fibrosis of the intimal layer may occur. The adventitia remains normal. In the smaller arteries, thickening with degenerating muscle cells are seen and occasionally perivascular cuff- ing with inflammatory cells occurs. The vasa vasorum of the larger vessels occasionally are found to have obliterative endothelial proliferation. It has been suggested that the lesions in the medial layer of large- and medium-sized vessels may be due to ischemia secondary to compromise of blood flow through the vasa vasorum.

These lesions caused by large doses of sympathetic catecholamines are quite different, how-

ever, from atherosclerosis as seen in humans,58a59 especially with respect to location, the early development of calcification, and the absence Iof fat. Whereas the location of atherosclerosis is primarily at the sites of branching of vessels and involves the smaller vessels very soon after the aorta is involved, adrenalin-induced lesions are diffuse, occur almost exclusively in the larger vessels, and only much later are smaller vessels involved. The intima is the layer most prominently affected in atherosclerosis with the media not involved in the early lesions, whereas adrenalin first and most prominently af- fects the muscularis. Lipid infiltration and proliferation of the intima is an early event in atherosclerosis; necrosis of the media is not seen, and calcification, rather than occurring early im the course, is a late manifestation, while calcification is early and prominent in adrenalin-induced lesions, and lipid infiltration is rarely seen. The hallmark of human atherosclerosis is the presence of yellow, fatty, cholesterol-containing plaques, whereas this is not seen in adrenalin-induced vascular sclerosis. The lesions produced by large dose adrenalin are more similar to those seen with Monckeberg’s sclerosis.

Although the lesions produced after ,large Idoses of adrenalin are quite different, lesions simil.ar to human atherosclerosis have been produced by repeated administration of small amounts of adrenalin Braun61 produced marked fibrous intimal thickening in the aorta, renal, pulmonary, and tibial arteries of rabbits by this technique, lesions that were quite reminiscent of early arteriosclerosis. Similar lesions,58 but also containing cholesterol, have been found in patients dying with pheochromocytoma.

An interesting aspect of the re1ationshi.p of catecholamines and atherosclerosis has been noted by Anitschkow . 59 He found that the combination of adrenalin injection and cholesterol feeding enhanced the production of atherosclerotic plaques in the experimental animal, although the location of the lesion was more typical of adrenalin sclerosis. He considered these changes to be secondary atherosclerotic lesions; similar to the cholesterol infiltration occasionally seen in syphil- litic aortas and not related to primary atherosclerosis as usually seen in humans. It is likely that degenerative changes in the vessel wall induced by catecholamines predispose to the infiltration of the wall by lipids.

The etiology of the vascular lesion induced by

84 JACOB I. HAFT

catecholamines remains unclear. It has been proposed that constriction of the vasa vasorum with resultant medial ischemia,s9 a direct toxic effect of adrenalins or the mechanical trauma to the vessel wall by the induced rise in blood pres- sure9ps9 may each be of etiologic importance.

Catecholamines may play an indirect role in the development of atherosclerosis. Epidemiologically, hypertension is found to be one of the major risk factors in the development of atherosclerotic cardiovascular disease. 62 Although, the relationship between catecholamines and hypertension, other than when a pheochromocytoma is present, is still not clear, there has been recent evidence presented that norepinephrine blood levels are elevated among patients with essential hypertension.63 Such a finding would help to explain the effectiveness of antihypertensive drugs that work by interfering with catecholamine action (e.g., alpha methyldopa, reserpine, and guanethidine). The relationship of hypertension to atherosclerosis is felt to be via enhancement of the ultrafiltration of plasma into vessel walls and by the increase in turbulence around areas of vessel branching.

Another action of catecholamines on the vessel wall that might indirectly lead to vessel wall damage is the relation of catecholamines to intravascular clotting. In the middle 18OOs, Rokitan- Sky64 suggested that arteriosclerotic plaques were the final result of degeneration of mural thrombi. This theory lay dormant until the 1940s when Duguid6sS66 demonstrated that mural thrombi become covered with endothelium and then degenerate, liquify, and are replaced by fatty debris and cholesterol crystals. He found lesions that suggested various phases of this progression.

In the late 1950s Woolf and &stair@’ studied atherosclerotic plaques by staining them with fluorescein-tagged antibodies to platelets. Deep within mature atherosclerotic lesions, these investigators found material that reacted antigeni-

tally like platelets. Their work suggests that plaques may grow by the repeated formation of mural thrombi, endothelialization of the clot, and degeneration of the thrombi, with the process repeat- ing itself over and over again, until a large atherosclerotic plaque is formed.

As noted previously, the relationship of clotting to catecholamines has been recognized since the work of Cannon in the early I~OOS.~’ A direct effect of adrenalin and noradrenalin on platelet aggregation has been well documented in vitro,41-43 and catecholamine-induced intravascular platelet aggregation in vivo has been demonstrated.36337 If the thrombogenic thesis for the production of atherosclerosis is valid, catecholamines may affect the induction of atherosclerotic plaques by causing the formation of mural thrombi.

In addition, catecholamine-induced platelet aggregates that adhere to vessel walls may damage the endothelium during the release action that occurs as platelets undergo viscous metamorphosis. During this second, irreversible stage of platelet aggregation, the substances present within the platelet granules are released.46 In addition to the secretion of ADP, serotonin and sympathetic catecholamines that tend to perpetuate and prolif- erate the platelet aggregate, lysosomal enzymes are released. So These enzymes may damage the endothelial surface to make it more permeable to the ultrafiltration of lipids, and in such a manner the development of an atherosclerotic plaque may be initiated.69 If catecholamines are related to atherosclerosis by way of these thrombogenic effects, the recent finding that emotional and physical stress, probably by way of endogenous catecholamines, can lead to intravascular platelet aggregates in the experimental anima137S53*54 may help to explain the epidemologic correlation that individuals who are driving, aggressive, and frequently exposed to stress are those who are prone to develop atherosclerosis.”

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Cardiovascular injury induced by sympathetic catecholamines