JOURNAL

OF SURGICAL

RESEARCH

19, 133-148 (1975)

CURRENT Platelet

RESEARCH

REVIEW

Adhesiveness’

JESPER SWEDENBORG, M.D. AND SEYMOUR

I. SCHWARTZ,

M.D.

The University of Rochester School of Medicine and Dentistry, Department of Surgery, Rochester, New York 14642 Submitted for publication December 6, 1974

Platelets were first observed in 1842, [3, 671and in 1851 Wharton Jones reported that injury to a vessel in a frog’s web caused “the aggregation of colourless corpuscles” [93]. The blood platelets were subsequently more closely identified and their role in hemostasis was pointed out by Bizzorero [21]. Shortly afterward, their tendency to adhere to foreign surfaces was demonstrated [53]. Osler introduced the “blood plaque”, and emphasized the importance of platelets in hemostasis in the human [,l38]. Platelet adhesiveness as a concept was introduced by Wright, who developed the first method to quantify it [204]. Hellem’s extensive study on platelet adhesion and his report on factor R [77], later identified as adenosine diphosphate (ADP) [63], has led to the development of the various methods used in attempting to measure platelet function. The term “platelet adhesion” is somewhat imprecise, since in most tests of adhesion, some aggregation occurs. When platelets adhere to surfaces, they become activated. This caused release of several intracellular substances 1811, one of which is ADP, a potent platelet aggregator. Released ADP causes platelets to aggregate to the already ahered ones and the platelet mass grows by accretion, eventually leading to a white thrombus. Platelet factor 3, with clotpromoting activity, also becomes available on the platelet surface [72, 1751,which triggers the plasma coagulation, causing the platelet thrombus to be strengthened by fibrin strands. This chain of events leads to a ‘Supported by NIH Grant No. 5 ROI-HL-13466-05.

thrombus both on a foreign surface and in the normal hemostatic mechanism. In the latter case, exposed collagen serves as the activator of the platelets. Strictly speaking, only the initial event, i.e., the contact between the platelets and the surface should be termed “adhesion,” whereas the platelet-toplatelet contact is termed “aggregation.” In most tests of adhesion, however, it is impossible to distinguish between the two and so the term “platelet retention” has been proposed for the entity measured by different tests. TESTS OF PLATELET ADHESION In the physiologic hemostatic mechanism, platelets adhere to collagen. No standardized test for platelet adhesion to collagen exists, even though collagen is used to initiate aggregation in platelet-rich plasma [83, 2081. Platelet adhesion to collagen stimulates the release reaction and the released ADP causes aggregation. Thus, although platelet adhesion is bound to occur in such tests, they measure aggregation caused by products of the release reaction rather than adhesion. In an attempt to quantify platelet adhesion in normal hemostasis, an in viuo method has been designed in which the platelet count of blood dripping from a standardized wound is compared to the platelet count of circulating blood [22]. The assumption is that when platelets adhere to collagen and build up the hemostatic plug, they will not escape into the blood flowing out of the wound. This method has been correlated to

133 Copyright cc1975 by Academic Press, Inc. All rights of reproduction in any form reserved.

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the bleeding time and its reproducibility is poor, limiting its clinical use. In tests of adhesiveness,the percentage of platelets adhering to a foreign surface after contact with blood is measured. Contrary to platelet aggregation where one method is used almost exclusively [23], several methods have been designed to measure platelet adhesiveness.Most in vitro tests of platelet adhesion use glass as the surface to which platelets adhere. Platelet adhesion to glass differs from adhesion to collagen, in that the former is dependent upon the concentrations of calcium ions [64, 771 and fibrinogen [64, 112, 113,209]. Even though a foreign surface is introduced in these tests, they are probably one or two steps closer to the in vivo situation than tests of aggregation. Most tests of adhesiveness use whole blood which may or may not be anticoagulated, whereas aggregation tests require platelet-rich plasma or suspensions of washed platelets. Several reviews on the different test systems have been published [74,76, 1161. Wright’s method measures the adhesion of platelets to the walls of a rotating glass bulb [204]. This test differs from many others in that it is partially inhibited by heparin, indicating that it is to some extent dependent on thrombin formation. Moolten and Vroman designed a method in which platelets are retained in a glass wool filter, and related the retention to the red cell count [119]. Since the test is rather complicated, it is rarely employed today. Currently, methods utilizing platelet retention in glass bead columns are most commonly used. The percentage of platelets retained after passageof blood through the column is used as the adhesive index. This method was originally introduced by Hellem [77], but has been modified by others several times. Hellem used titrated blood which was forced through the column by a motor driven syringe. He noted that the presence of red blood cells was necessary for the test, and this has later been shown to be due to a leak of ADP from damaged red cells [ 1561.Con-

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sequently, platelet-rich plasma can only be used for the test if ADP has been previously added [63, 1821.This requirement for ADP indicates that a considerable amount of aggregation takes place in the test. Hellem’s original test is reported to correlate well with that of Wright [80]. Since adhesion to glass is calcium dependent, subsequent modifications have avoided the use of chelating agents as anticoagulants. Consequently either heparin [26, 1331 or no anticoagulant at all was used [133, 1621.Transit time through the column also affects the adhesion [25, 1351,and most tests keep this time constant by using a motordriven syringe to drive the blood through the column. In Salzman’s test, which has many other advantages, the blood is drawn through the column directly from the venipuncture needle using a vacutainer tube attached to the end of the column [ 1621. Glass bead column methods measure, to some extent, aggregation of platelets because platelets stick to those platelets which have adhered to the glass, and aggregates that form during the passage of blood through the column become trapped between the glass beads. In order to eliminate the component of aggregation, tests have been designed in which a glass slide is simply dipped into the blood and the platelets adhering to the slide are counted [27, 1121. Some tests have been developed for specific clinical situations. Lindsay et al., concerned with platelet adhesion in association with hemodialysis, constructed an adhesion test cell, with an inner surface consisting of a dialyzer membrane [105]. In order to estimate the adhesion to vascular catheters, a method has been developed in which platelet retention is measured after blood has been enclosed in a rotating catheter loop [88]. PLATELET ADHESION TO BIOLOGICAL TISSUES Platelets do not adhere to normal vascular endothelium [ 1781, a phenomenon partly ascribed to the negative surface

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charge carried by both platelets and endothelial cells [l]. This negative charge on the platelet surface is provided by sialic acid, which can be partly digested by neuraminidase [92, 1101. Platelets thus treated lose their repulsive forces [82], are more easily aggregated by collagen [41], and their survival time in viva is shortened [65]. If the surface charge of the platelets is made less negative, thrombosis may be induced and this has been proposed as an explanation for intravascular platelet adhesion [42]. In many reports, Sawyer and associates have pointed out that an alteration of the negative charge of the vessel wall, leading to the same net effect as altering the platelet charge, causes thrombosis [165-1671. The platelets, however, share the property of a negative surface charge with other blood cells, which do not adhere to the same extent when their negative charge is reduced. The negative surface charge is only one of many reasons why platelets do not normally adhere to endothelium, and similarly reduction of the charge is only one of several factors initiating a thrombus. Consequently the adhesion of neuraminidase-treated platelets to collagen seemslargely unimpaired [177]. Hugues was the first to point out that in the normal hemostatic mechanism platelets adhere to exposed connective tissue [85]. The component of connective tissue to which platelets most readily adhere is collagen [14]. Recently it has been proposed that the adhesion of platelets to collagen is brought about by enzyme-substrate bindings acting as bridges between collagen and platelets [89]. These enzymes transfer glucose [I 1] and galactose [lo] from the platelet membrane to a collagen acceptor. Fibrinogen has been shown to be required for the initial adhesion of platelets to collagen-coated foreign surfaces [34]. In addition, fibrin is of importance in reinforcing the platelet thrombus which would otherwise have a tendency to embolize 120, 1211. On the other hand, adhesion to connective tissue is normal in afibrinogenemic patients [202] and the primary hemostasis in viva which is

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platelet dependent, is unimpaired in animals lacking fibrinogen [20, 1371. Platelets constitute a major part of an arterial thrombus. There is some disagreement about the mechanisms initiating the adhesion of platelets to the vascular wall, but it is generally accepted that platelet adhesion represents a primary event in arterial thrombosis [4, 35, 1231. While there is considerable agreement on the mechanism of hemostasis after transsection of a blood vessel with consequent exposure of collagen, there is great controversy concerning platelet adhesion to an inner vessel wall. Generally it has been felt that the platelets adhere to subendothelial connective tissue in places where endothelial cells have been damaged or lost [24]. Collagen, however, is scarce in the immediate subendothelial layer [16, 1811, but platelet adhesion to the basement membranes of capillaries has been demonstrated in the interstices between the endothelial cells [ 111, 1931. Platelet adhesion has also been demonstrated after denudation of the endothelium of the rabbit aorta [13, 15, 1941.Electronmicroscopic studies have defined the substances of the immediate subendothelial layer to include amorphous material, microfibrils, and elastin [ 14, 1941.Adhesion has been shown to occur to amorphous material and microfibrils while adhesion to elastin is limited. After treatment of the deendothelialized aorta with collagenase, platelet adhesion continues to occur, but not to the amorphous material which has been digested by collagenase [15, 1811. Thus, platelet adhesion has been shown to occur to immediate subendothelial structures other than collagen. In contrast to this however, evidence has been presented that platelets actually adhere to the injured endothelial cells per se [6, 61, 1471,and it has been suggested that aggregates formed in the streaming blood under the influence of local flow disturbance attach to the endothelium, causing damage to the cells laying the ground for further adhesion and subsequent thrombus formation [94]. Whether or not

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the initial adhesion of platelets requires loss of endothelial cells, a mural thrombus seems to be attached to the subendothelial tissue once it is formed [95]. The future fate and growth of the platelet mass is largely dependent upon flow characteristics [loll. The relation of the growth of the platelet cell mass to flow characteristics is most apparent in the accumulation of a thrombus at bifurcations and within an aneurysm [120]. Platelet adhesion to the arterial wall is also one of many contributing factors in the development of an atherosclerotic plaque [61, 96, 122, 1471. In venous thrombosis, stagnant flow is a crucial factor. Baumgartner has shown that if flow rate is varied in the rabbit aorta, denuded of its endothelium, the amount of platelets adhering to the wall increases with flow [12]. The stagnation, therefore, probably has its effect on its favoring of whole blood coagulation and the formation of a red thrombus. If the flow is sufficiently low, the deposits on the vessel wall consist mainly of fibrin, in contrast to the structure of an arterial thrombus, where platelets constitute the main component. PLATELET ADHESION TO FOREIGN SURFACES Platelets adhere to all foreign surfaces. Whether a subsequent release reaction takes place or not is somewhat dependent upon the surface and the protein coating that develops when blood flows over it. Adhesion of platelets to glass, polymeric surfaces and latex particles is dependent upon the presence of divalent cations [64, 771, which makes it difficult to interpret results of platelet adhesion to foreign surfaces if chelating agents have been used as anticoagulants. To overcome this problem, several different methods have been developed using extracorporeal shunts. This idea was originally introduced by Rowntree and Shionoya who observed the growth of a thrombus in a collodion arteriovenous shunt [ 1571. They concluded that the growth of the platelet mass initiated the thrombosis of the shunt, and their findings have subsequently been

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confirmed [123]. Apart from direct microscopic observation, platelet accumulation in extracorporeal shunts has also been evaluated by measuring the activity in the shunt of 51Cr after labeling of the platelets with the isotope [99]. This method allows good quantitation of the platelet accumulation. Another method measures the bleeding time from standardized holes in the shunt [5 11. Microscopic observation of the growing thrombus is best carried out in ex vivo flow chambers, and studies utilizing them have invariably shown that platelet deposition precedes fibrin formation [9, 62, 1431. The initial adhesion of platelets seems to be a response to a different stimulus than fibrin deposition and plasma coagulation [ 1541. The primary event, however, when blood contacts a foreign surface, is the adsorption of a thin protein film [50, 1531and it is the nature of this protein coating that will determine the blood compatibility of the surface. Dutton et al. observed blood in contact with germanium prisms and found electronmicroscopic evidence of a protein layer bridging the gap between the platelets and the surface [50]. Evidence has suggestedthat this protein is fibrinogen [9]. Platelets have been shown to adhere to fibrinogen-coated surfaces [91, 2021 and adsorbed fibrinogen has been claimed to be a prerequisite for adhesion to glass [64]. Adhesion to glass has been found to correlate well with the fibrinogen level of plasma [ 1131 and it is severely impaired in afibrinogenemia [66, 861. Depending on the surface, adsorbed fibrinogen has been reported to increase or decrease platelet adhesion. Lyman et al. reported a decreased adhesion when a polymeric surface was coated with fibrinogen [ 1061, whereas fibrinogen increases adhesion to glass. Adsorbed fibrinogen eventually changes, at least its antigenic properties, and it has been proposed that once it has undergone these changes, it no longer attracts platelets [195]. Platelets do not adhere to fully polymerized fibrin [84], but they have been reported to adhere to polymerizing fibrin and to enhance further

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polymerization [129]. This could be one explanation for the close proximity of platelets and fibrin strands seenin thrombosis [7]. The nature of the protein coating influences the behavior of the platelets. Packham et al. studied the adhesion of platelets to the surface of extracorporeal shunts and found that a fibrinogen-coated surface allowed the platelets to adhere, but no release reaction occurred. If gamma globulin was adsorbed, both adhesion and release occurred, and if albumin was adsorbed, neither occurred [ 1391.The antiadhesive properties of an albumin coating have been confirmed by others [ 1141, and such coating has been proposed as one method to obtain a nonthrombogenic surface [ 1091. Albumin coating is an example of a method to lower the surface energy and make the surface hydrophobic. Hydrophobic surfaces have antithrombogenic properties, part of which may be due to their antiadhesive properties [log]. Perhaps the most familiar method of making a surface hydrophobic is by siliconization, and indeed this procedure prevents platelet adhesion [168]. Other attempts to make nonthrombogenic surfaces have been, paradoxically enough, highly hydrophilic surfaces (the so-called hydrogels), various polymers, negatively charged surfaces, and surfaces with additives such as heparin or surface active agents. The adhesive properties of hydrogels are not known in detail, but some adhesion appears to take place [29]. Different polymers currently used clinically in contact with blood vary considerably in. their adhesive properties [ 1071.Negatively charged surfaces have the theoretical possibility of repelling the negatively charged platelets. Inhibited platelet adhesion and nonthrombogenicity has been achieved by incorporation of Chicago acid (the active component of Evans blue) into the surface, rendering the surface negatively charged [ 1301. Considering the importance of platelet adhesion in thrombosis on foreign surfaces, it is somewhat surprising that the most commonly employed method of obtaining a nonthrombogenic surface is by hepariniza-

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tion, aimed at inhibition of plasma coagulation. Increased [164] as well as decreased [98, 1501adhesion of platelets to heparinized surfaces has been reported. The different results are probably dependent upon different methods of heparinization. With heparinization of the surface followed by cross linking of the heparin with glutaraldehyde, a very stable heparinization is obtained [98]. This surface shows markedly inhibited platelet adhesion and excellent antithrombogenic properties [87, 991. It has been suggested that heparinized surfaces that do not leak heparin, i.e., surfaces that do not have a micromilieu of heparin close to them, actually are antithrombogenic due to charge effects, or their affinity for certain proteins rather than their inhibition of plasma coagulation [159]. Heparin, as will be discussed later, may cause platelet aggregation [36, 541 and the continuous leak of heparin from some surfaces may enhance aggregation to platelets that may have adhered. This could account for the different results of platelet adhesion obtained with heparinized surfaces. On the basis of present data, one has to conclude that platelet adhesion and growth of the thrombus on a foreign surface most likely is dependent upon different characteristics of the surface such as chemistry, texture, and charge and also oil local flow patterns. Turbulence, for instance, may enhance cell destruction, thus making ADP available and a surface that performs well under ideal flow conditions may show considerable platelet accumulation when the flow is turbulent. Total inhibition of adhesion may be extremely difficult to achieve, and it may be that the best surface is one which allows a monolayer of nonreleasing platelets to adhere, thus preventing secondary aggregation. There is some indication that a monolayer of platelets is actually nonthrombogenic [ 121. PLATELET ADHESIVENESS: SURGICAL CONSIDERATION Many reports using different methods have confirmed Wright’s original ob-

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servation [203] of an increased platelet adhesiveness after surgery [18, 19, 55, 701. The major thromboembolic complication occurring postoperatively, however, is deep venous thrombosis. Since stagnant flow, rather than vessel wall changes and platelet stickiness is primarily important in venous thrombosis, plasma coagulation is considered the more important factor. Postoperative increase in platelet adhesivenessis more marked in patients who develop venous thrombosis [17, 1361,but most attempts to predict postoperative venous thrombosis by an increased preoperative platelet adhesivenesshave failed [17, 127, 1361.It may be that some postoperative venous thrombi are primarily platelet dependent but present knowledge is insufficient to distinguish these. In recurrent venous thrombosis, not associated with surgery, platelet adhesivenesshas been found to be increased [79,179] In arterial thrombosis, platelet adhesion is an important primary step and much evidence points to the fact that a mural platelet thrombus represents part of a multifactorially determined lesion subsequently leading to an atherosclerotic plaque [61, 96, 122, 1461.The question is, how well does adhesion to glass predict the in vivo situation? In support of glass adhesiveness tests, patients with different manifestations of atherosclerotic disease have been found to have increased adhesiveness [ 1151. An ulcerated plaque with exposure of deep vascular structures serves as a receptive surface for platelet adhesion and growth of a platelet thrombus. Platelet embolization from such sites in the carotid bifurcation has been obtransient served in patients with ischemic attacks (TIA) [68, 1581. The symptoms of TIA can be better explained by lodgment of emboli in the microcirculation of the brain, and consequent cerebral vasoconstriction. Platelet release products are pharmacologically active and give rise to hemodynamic changes [149, 1891, and the released substances have been shown to be capable of causing cerebral vasoconstriction [44,97,206].

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This sequencemay also be of importance in other forms of atherosclerotic disease. Decreased platelet adhesivenesscan be a cause of hemorrhage in various bleeding disorders and consequently tests of platelet adhesion have been most valuable clinically in the detection of von Willebrand’s disease [75, 142, 162, 183, 2071 and certain other syndromes with platelet dysfunction [142]. Also, decreased platelet adhesiveness has been noted in patients with bleeding stress ulcers and gastric bleeding after aspirin, but no platelet abnormality could be shown in patients bleeding from chronic duodenal ulcers [8]. Platelet adhesion becomes of great importance when blood is brought in contact with foreign surfaces, either during extracorporeal circulation (E.C.C.), or when foreign material is implanted in the blood stream. During E.C.C. there is a loss of platelets due to activation of them in their contact with the foreign surface [78, 1611. Lindsay et al. found that thrombocytopenia after hemodialysis correlated well with platelet trapping in the dialyzer [103]. deLeva1 et al. however, reported that most of the platelets were trapped in the liver after E.C.C. [43]. Platelet adhesion eventually causing embolization and release of pharmacologically active substances might explain some of the hemodynamic alterations in connection with E.C.C. The thrombocytopenia causes serious bleeding The same problems postoperatively. mechanisms to some extent limit the use of intraoperative autotransfusion [49]. Thrombocytopenia and hemodynamic complications limit prolonged E.C.C., initiating trials with nonthrombogenic surfaces. Successful extracorporeal oxygenation of dogs for 24 hr has been performed with material heparinized and cross linked with glutaraldehyde [152]. With this method of heparinization, there was 100% survival rate, and no heparin activity in plasma as opposed to heparin binding to tridodecyl-methyl-ammonium chloride (TDMAC), which gives a less stable heparin binding, where 100%

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face-induced release [58, 141, 2011, but they have also been reported to inhibit adhesion of platelets on fibrinogen-coated surfaces [90, 1401,where release supposedly does not occur. Adhesiveness tests are not inhibited by aspirin [132, 1991.The scattered reports on such inhibition may be due to the difficulty in distinguishing between adhesion and aggregation [184]. Shepard did not find any inhibition of platelet adhesion to the aortic wall after endothelial injury when aspirin was administered [ 1721.Aspirin has, however, been used to inhibit experimental arterial thrombosis [39, 401. Aspirin and other anti-inflammatory drugs have also been used to prevent platelet adhesion to foreign surfaces, such as dialyzer membranes [ 1041. Since anti-inflammatory drugs affect platelet function, they are not expected to influence venous thrombosis and this has been confirmed in the caseof aspirin in carefully controlled studies using 1251-fibrinogen to detect venous thrombosis [131, 1511.An inhibitory effect of aspirin against postoperative venous thrombosis, has, however, been reported but only when clinical criteria to detect thrombosis were used [163]. The present understanding of aspirin, which is the most extensively studied anti-inflammatory drug, is that it affects the platelet release reaction rather than true adhesion and that it is of minor importance in preventing DRUGS AFFECTING PLATELET venous thrombosis. Weiss has concluded ADHESION that aspirin may be of use in preventing Aspirin, sulfinpyrazone, phenylbutazone, arterial thrombosis, but that it is a weak indomethacin, and related anti-inflammaantiaggregant [ 1991. tory drugs recently have received much atSulfinpyrazone has been reported to tention as inhibitors of platelet function. reduce the mortality in patients with a hisLarge studies are being performed to deter- tory of stroke and this has been attributed to mine the efficacy of aspirin to prevent the se- its inhibition of platelet function [173]. quelae of arterial occlusive disease [60]. Sulfinpyrazone also normalizes the Anti-inflammatory drugs supposedly inhibit shortened survival time of platelets caused platelet release, thus inhibiting growth of the by an arteriovenous shunt [124], and platelet thrombus, once the initial adhesion decreases the number of thromboses in arhas occurred, which explains why aspirin teriovenous shunts used for hemodialysis prolongs the bleeding time [118, 148, 1991. [ 1441.Sulfinpyrazone is reported to normalSeveral reports indicate that aspirin and ize the shortened survival time of platelets in other anti-inflammatory drugs inhibit sur- patients with artificial heart valves [196], mortality was recorded [52]. It is to be noted that the former method of surface treatment works primarily by inhibiting platelet adhesion [99]. Activation of platelets on foreign surfaces can lead to shortened survival time in vivo. This has been observed in patients with artificial heart valves [73, 1961, and the shortening correlates well with the surface area of the valve [73]. In the one study which did not find a significantly shortened survival time, the valve used is known to have a low incidence of thromboembolic complications [197]. The increased consumption of platelets may be augmented by the hemolysis that is present in patients with artificial heart valves [126]. Destruction of red cells by the valve leads to liberation of ADP causing aggregation of platelets. Prosthetic vascular grafts also cause a shortened platelet survival time which does not normalize until the graft becomes endothelialized [ 1701.The adhesion of platelets to the synthetic graft may well be a promoting factor for late failure of vascular grafts. In support of this, it has been shown that a high platelet adhesiveness in vitro is associated with an increased late failure rate of femoropopliteal synthetic grafts [56, 711 as well as saphenous vein grafts [57]. Amazingly few studies, however, have been performed, evaluating platelet adhesivenessto different graft materials.

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while this could not be demonstrated with aspirin [73]. Pyrimidopryimidines, such as dipyridamole and its analogus, RA 233, RA 433, and VK 744, represents another group of antiplatelet drugs. Dipyridamole has not been effective in preventing venous thrombosis, although the studies used only clinical criteria for detection of thrombosis [28, 1631. Reports on inhibition of experimentally induced arterial thrombosis by dipyridamole are conflicting [5, 39, 40, 451 and the results seem to depend upon dosage, route of administration, and technique of producing thrombosis. Dipyridamole has not been effective in preventing TIA’s in patients with cerebrovascular atherosclerotic disease [2]. Most reports on the beneficial effect of the pyrimidopyrimidines, both clinically and in the experimental animal, concern situations in which blood comes into contact with a foreign surface. Thus, dipyridamole inhibits the thrombosis rate in an extracorporeal shunt, but the dosage necessary usually produces hemodynamic side effects [46]. On the other hand, the dose sufficient to inhibit adhesion in an extracorporeal shunt does not produce any effect in in vitro platelet aggregation tests [38]. Harker and Schlicter, in agreement with this, found that platelet survival time was normalized by dipyridamole in patients with prosthetic heart valves, while their platelet aggregation in vitro was normal [73]. Platelet adhesiveness tests, have, however been reported to be inhibited after treatment with dipyridamole [ 1861. Further support for the inhibitory effect of dipyridamole on the adhesion of platelets to foreign surfaces in patients was provided by Sullivan et al. They noted a decreasedrate of thromboembolic complications in patients with prosthetic heart valves [185]. A normalized platelet survival time in patients with synthetic vascular grafts after treatment with dipyridamole has been demonstrated [170]. Pyrimidopyrimidines also inhibit thrombocytopenia after extracorporeal circulation [ 1171and hemodialysis [104]. Thus, there is a considerable amount of indirect evidence indicating that the py-

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rimidopyrimidines interfere with the adhesion of platelets to foreign surfaces. Dextran has been used extensively as an antithrombotic agent. Dextran affects the platelets by increasing their negative surface charge [145, 155, 1711.This effect is more marked the higher the molecular weight, although very large molecular weight dextrans actually cause platelet aggregation. The explanation for this apparent paradox may be that large molecules act as bridges between the platelets [ 1711.This serves as a good example that charge effect is just one of several factors determining platelet adhesion and aggregation. The optimal molecular weight of dextran for inhibition of adhesion is probably 70,000 [32, 37, 1691.Dextran inhibits platelet accumulation at Laser induced endothelial damage [5]. Decreased platelet adhesion has been demonstrated in patients receiving Dextran [33, 371 but no definite changes in platelet function can be demonstrated when Dextran is added to blood in vitro [32, 2001. Because of this, it has been hypothesized that Dextran affects platelets indirectly, e.g., by interfering with some plasma protein [32]. Dextran inhibits postoperative venous thrombosis [30, 100, 163, 1691 and this offers support for the hypothesis that platelets are of significant importance for venous thrombosis. Recently, however, it has been shown that fibrin formed in the presence of Dextran is abnormal and more succeptible to lysis [ 125, 1911, a mechanism that could be of importance for the inhibitory effect of Dextran on venous thrombosis. Some controversy exists about the effect of heparin on platelets, which might be due to the fact that distinction has not been made between in vivo and in vitro studies, and between aggregation and adhesion. Heparin causes aggregation in vitro [54] as well as in vivo [36]. Adhesion to foreign surfaces seemsto be inhibited [99, 1601,but this is not apparent at normal clinical dosagesin vivo [99, 102, 1601.However, increased adhesion in vitro to a cellulose membrane by heparin, has been reported [192]. Low-dose therapy with heparin, which has been suc-

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cessful in inhibiting postoperative venous thrombosis, is reported to inhibit postoperative increases in platelet adhesiveness, and this has been correlated to the increase in lipo protein-lipase activity caused by heparin [69, 1281.The effect is of short duration and it is doubtful that the decreased adhesivenessis caused by heparin per se, but effects upon platelet function other than adhesion by low-dosage heparin has been reported [134]. Thus, heparin does not ordinarily inhibit platelet adhesion but will prevent stabilization of the platelet thrombus by fibrin. Several other compounds including membrane-active drugs, phenothiazines, tricyclic antidepressants, and antihistamines have been shown to affect platelet function. These drugs have been studied mainly in vitro using aggregation tests and few, if any, studies indicate any effect in vivo either in patients or in the experimental animal. For a complete review on drugs affecting all aspects of platelet function, the reader is referred to a thorough review by Weiss [ 1981. Among recently discovered substances, pyridinolcarbamate has been shown to inhibit platelet adhesion to foreign surfaces [47, 481, and also to inhibit other aspects of platelet function [205]. This is an interesting observation which could serve as support for the importance of platelet adhesion in the initial stages of atherosclerosis since pyridinolcarbamate also is said to inhibit the development of atherosclerosis [ 1741. Bygdeman and Lund, however, were unable to demonstrate inhibited platelet adhesion with clinical dosagesof pyridinolcarbamate [31]. Another promising substance is polyphloretin phosphate which effectively inhibits platelet adhesion to the surface of extracorporeal shunts [190]. This substance also has a weak heparin-like activity and inhibits thrombin-induced aggregation in viva [187], features which add to its attractiveness as an antithrombotic drug. Polyphloretin phosphate does not cause inhibition of platelet function when added to blood in vitro. Like Dextran [59], it might even enhance ADP-induced aggregation

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[188], once again demonstrating that aggregation tests may not be appropriate in the clinical situation. SUMMARY The concept of platelet adhesion is confused by the wide variety of tests currently in use. Distinction has to be made between adhesion to collagen or collagen-like structures in vivo and adhesion to foreign surfaces. In the former case, conclusions have to be drawn from adhesion to glass and this may not always be an acceptable extrapolation. In the latter case one has to use the specific material in question, used clinically, in order to evaluate platelet adhesiveness. Platelet adhesiveness tests may have more clinical significance than aggregation test. Platelet adhesion is of primary importance in the development of arterial thrombosis and is probably one of the initiating steps in the development of an atherosclerotic plaque. Platelet adhesiveness to foreign surfaces and its sequelaeis one of the limiting factors for prolonged extracorporeal circulation and adds to the morbidity after implantation of foreign material in the circulation. These problems may be solved by making surfaces more “friendly” toward platelets, and by using different drugs. Many drugs are being tried for platelet inhibition and in many cases their mechanism of action is not fully understood. Further search for more effective drugs and better surfaces is warranted. REFERENCES 1. Abramsort. H., Electrophoresis of blood cells. In Sawyer, P. N., (Ed.) Biophysical mechanisms in vascular homeostasis and intravascular thrombosis, p. 3, 1965. Appleton-Century-Crofts, New York. Acheson, J., Danta, G., and Hutchinson, E. C. Controlled trial of dipyridamole in cerebral vascular disease. Br. Med. J. 1:614, 1969. Addison, W. On the colourless corpuscles and on the molecules and cytoblasts in the blood. Lond. Med. Cm. N.S., 2:144, 1842. Ardlie, N. G., and Schwartz, C. J. A comparison of the organization and fate of autologous pulmonary emboli and of artificial plasma thrombi in the anterior chamber of the eye in normocho-

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