711
REVIEW ARTICLE Shear-induced
platelet aggregation
Two platelet mechanisms contribute to haemostasis and thrombosis. (1) Compounds such thrombin activate glycoprotein as IIb/IIIa; fibrinogen is the ligand. The cyclooxygenase pathway is involved and so this process is aspirin sensitive. (2) Shearing forces alone activate a different domain on glycoprotein IIb/IIIa; von Willebrand’s factor is the ligand. This process is probably non-enzymatic and is aspirin insensitive. The prevention of shear-induced platelet activation may prove to be more rewarding therapeutically than inhibition of aspirin sensitive
pathways. Introduction Platelet activation induced by agonists such as thrombin and adrenaline is one mechanism involved in arterial thrombosis. Activation of the cyclooxygenase pathway results in the exposure of a domain on platelet glycoprotein IIb/IIIa (GPIIb/IIla); fibrinogen is then the ligand binding platelet to platelet or platelet to collagen. Thus aspirin is clinically beneficial because it inhibits the cyclooxygenase pathway. Shear-induced platelet activation is another mechanism, which is not affected by aspirin. Atherosclerotic stenosis, which induces high shearing forces, is almost invariably present in patients with arterial thrombi in the heart or head. Therefore this mechanism may be of great importance in thrombogenesis. The effect of shear cannot be investigated fully in man. We review the various models and discuss the mechanisms involved in shear-induced platelet activation.
Sheared
platelets
Early studies When blood is forced through small holes platelets aggregate and block the holes without the addition of any aggregating agent.2 This method is poorly reproducible, and the importance of shearing forces was little appreciated; and any contribution from the walls of the hole is uncontrolled. Forcing blood through a column of glass beads and measuring platelet retentioncan discriminate between patients with von Willebrand’s disease and controls, and is unaffected by aspirin. The blood must be forced rapidly through the column. However, these results from 1967 were similar to those obtained 20 years later,4 when shear forces and von Willebrand’s factor (vWf) were shown to determine the deposition of platelets on pig de-endothelialised aorta. Thus platelet-to-platelet aggregation induced by shear and platelet adhesion to subintima apparently share basic mechanisms.
Viscometers With a rotational viscometer precisely measured shearing forces can be applied. Large but practically inert surfaces are involved. Usually, but not always,5 the blood is sheared for many seconds or minutes and adenosine diphosphate may be released from platelets and/or red cells. Such release may occur before or after platelet aggregation. So is it cause or effect? In a study6 of patients with congenital deficiencies; for normal aggregation to occur, high shearing forces were required together with GPIIb/IIIa, a normal calcium ion concentration, and vWf, particularly the unusually large multimers. GPIbband fibrinogen7 may also be involved in this system. Aspirin does not inhibit viscometer shearinduced platelet aggregation.6 Filter
technique
Blood is forced at a predetermined pressure through a filter made of fine fibres.8 Particles (eg, platelet aggregates) larger than 10 pm are retained. As blood is forced through the tortuous, capillary sized channels the platelets are activated; they aggregate and the aggregates block the filter. The time for blood to pass through the filter is initially 8 ms so this is a very fast reaction, as is that required for haemostasis and thrombogenesis. Filters made of positively and negatively charged hydrophilic glass fibres and lipophilic polypropylene fibres give similar results. These and electron microscopy studies indicate that the filter surface plays little or no part. Normally platelet retention in the filter and the time and number of platelets required to achieve blockage are monitored. Only at high shear (40 mm Hg) does the filter block. Platelet retention is decreased and filter blockage is delayed in samples from patients with von Willebrand’s disease, and when all six antibodies to vWf so far tested are added to normal blood. Thus vWf is required for normal blocking. Retention is also abnormal in specimens from patients with thrombasthaenia, in which GPIIb/IIIa is decreased or abnormal, and on the addition of five antibodies to GPIIb/IIIa to normal blood. A critical concentration of anti-GPIIb/IIIa was less inhibitory at higher pressure (100 mm Hg), which suggests that the degree of shear determines the degree of exposure of GPIIb/IIIa. One patient with Bernard-Soulier syndrome (abnormal GPIb) and one antibody to GPIb had normal results, so GPIb may not be involved. Removal of calcium ions by edetic acid prevents platelet retention in normal blood. All these abnormal results were obtained in blood with normal fibrinogen, thus ligands such as fibrinogen and ADDRESS: Central Laboratory, St Mary’s Hospital, Portsmouth, Hampshire P03 6AG, UK (Dr J. R. O’Brien, FRCP).
712
fibrinectin cannot replace vWf as the adhesive protein in this model. In this and other systems9 the receptor site on GPIIb/IIIa for vWf differs from that required for fibrinogen. Thus in this system high shearing forces, vWf, GPIIb/IIIa, and calcium ions, but not fibrinogen, are essential for normal platelet aggregation and filter blockage. Other molecules may contribute but since enzyme inhibitors (dinitrophenol, sodium azide, and iodoacetic acid) and lowering the temperature to 15°C have little effect, it seems that the shearing forces induce a physical nonenzymatic exposure of GPIIb/IIIa. Aspirin and ticlopidine have no effect on this system. The filter technique can be used to study native blood passing through capillary sized holes under controlled shear stress. It is simple, quick, and cheap, with good
reproducibility.
Lambrecht et al23 reported that biomaterials coated with vWf in arteriovenous shunts promote platelet deposition and thrombus anchoring. The effect of shear in disease of large vessels such as the carotid and coronary arteries has been considered. vWf is also involved in small vessel haemostasis, as indicated by the long bleeding time in von Willebrand’s disease and its correction by cryoprecipitate and desmopressin. Since the bleeding time in von Willebrand’s disease is more prolonged than that after taking aspirin, in vivo the von Willebrand dependent mechanism may be the more important. Unstable angina is almost always associated with coronary stenosis and Davies et a124 found intramyocardial microthrombi in such patients who die suddenly, these microthrombi may also depend on the same sheardependent mechanism.
Conclusion
Perfusion chambers The Baumgartner technique involves mounting an everted, de-endothelialised ring of aorta on a glass rod. This is placed in an annular chamber and native or anticoagulated blood is forced at known shear rates through the chamber.
Platelet adhesion to the sub-endothelium and thrombus formation is monitored. This method is cumbersome, expensive, and has poor reproducibility; however, it accurately mimics in-vivo events. At low shear rates other mechanisms, probably involving fibrinogen and its appropriate domain on GPIIb/IIIa, may be involved" as well as vWf.12 At high shear vWf and GPIIb/IIIa have repeatedly4,1l,13,14 been shown to be important in platelet deposition and thrombus formation. Again GPIb may be involved. 13 In-vivo studies Patients with von Willebrand’s disease who have normal fibrinogen and platelets have a standard skin bleeding time that is long when the tourniquet is inflated to 40 mm Hg. Cryoprecipitate, which raises the vWf plasma level (and fibrinogen), or desmopressin, which raises both vWf15 and fibrinolytic precursors, shorten the bleeding time. Thus vWf must be involved in some aspect of small vessel haemostasis. These in-vivo observations are supported by Fuster et al.16 They passed heparinised blood at high shear over collagen I in a perfusion chamber. Platelet deposition, relative to controls, was decreased by 30 % in the presence of aspirin, by 90% in blood from patients with von Willebrands disease, and by 80% on adding an antibody to vWf. Incidentally, in the absence of collagen or a damaged vessel wall (ie, in the filter test), similar results can be obtained and therapeutic manoeuvres, giving cryoprecipitate or desmopressin, can be monitoredp.18 Coller et all and others2021 have exposed the carotid and the coronary artery in animals and found that if the endothelium is slightly damaged and a 70-90% stenosis is created by external constriction of the vessel, then platelet thrombi form distal to the stenosis. High shearing forces through the sternotic area are essential for the formation of these in-vivo platelet thrombi. Injecting antibodies to GPIIb/IIla19-21 and to vWf22 inhibits thrombus formation. Thus vWf must act as ligand. It also follows that shear forces induced by the stenosis must activate the vWf-specific domain on GPIIb/IIIa. Studies of the insertion into an arteriovenous shunt of de-endothelialised vessel segments’’ or ’Dacron’ grafts21 and the use of antibodies to GPIIb/IIIa also confirm that high shear and vWf are required for platelet deposition.
Several studies that used different techniques show that shear activates a unique domain9 on GPIIb/IIIa and, in the presence of vWf as ligand, platelets can aggregate, and also stick to collagen25 and pile up to form thrombi. This mechanism is not inhibited by aspirin or ticlopidine and is independent of platelet aggregation as studied in the aggregometer when initiated by ADP, thrombin, adrenaline, or thromboxane A2. The contribution of other molecules and other mechanisms to these shear events is less certain but presumably all these processes may come into play, irrespective of antecedent events such as plaque rupture.26 However, the importance of this shear-induced mechanism will not be established unless inhibition of this mechanism is shown to convey clinical benefit. The intravenous injection of antibodies or even small peptide sequences containing the Arg-Gly-Asp sequence will probably never be extensively used. This sequence, common to vWf and fibrinogen, is involved in the binding of these ligands to GPIIb/IIIa. Small peptides containing the Arg-Gly-Asp sequence can compete with vWf and prevent shear-induced platelet aggregation27 and thrombus formation. ",229 Perhaps efforts should be made specifically to "toughen" the platelet membrane to inhibit shearinduced platelet activation with GPIIb/IIIa exposure. In the filter test platelet deposition and filter blockage can be prevented in vitro by adding "membrane stabilising compounds" such as cocaine, chlorpromazine, quinidine, and imipramine. However, at the concentrations required for platelet inhibition (1-10 mmol/1), these drugs will also inhibit other membranes and be lethal. The search for drugs and other methods to inhibit shear-induced activation of the platelets should continue. I thank Dr P. J. Green and Mr G. P. Salmon for Ms M. Bastable for secretarial help.
helpful discussions and
REFERENCES 1.
Trialists’ Collaboration.
Antiplatelet Secondary prevention of vascular disease by prolonged antiplatelet treatment. Br Med J 1988; 296:
320-31. 2. O’Brien JR, Etherington MD, Weir P. Platelet agregation inhibitors: a 5 µ nuclepore filter "bleeding time". Monte Carlo: Sixth international on Meditterranean thrombosis, congress league against thromboembolic diseases, 1980: 307T. 3. O’Brien JR, Heywood JB. Some interactions between human platelets and glass: von Willebrand’s disease compared with normal. J Clin Pathol 1967; 20: 56-64. 4. Badimon L, Badimon JJ, Rand J,Turritto VT, Fuster V. Platelet deposition on von Willebrand factor-deficient vessels: extracorporeal perfusion studies in swine with von Willebrand’s disease using native and heparinized blood. J Lab Clin Med 1987; 110: 634-47.
713
5.
Wurzinger LJ, Opitz R, Blasberg P, Schmid-Schongein H. Platelet and coagulation parameters following millisecond exposure to laminar
shear stress. Thromb Haemostat 1985; 54: 381-86. 6. Moake JL, Turner NA, Stathopoulos NA, Nolasco L, Hellums JD, Shear-induced platelet aggregation can be mediated by vWf released from platelets, as well as by exogenous large or unusually large vWf multimers, requires adenosine diphosphate, and is resistant to aspirin. Blood 1988; 71: 1366-74. 7. Ikeda Y, Murata M, Araki Y, et al. Importance of fibrinogen and platelet membrane glycoprotein IIb/IIIa in shear-induced platelet aggregation. Thromb Res 1988; 51: 157-63. 8. O’Bnen JR, Salmon GP. Shear stress activation of platelet glycoprotein IIb/IIIa plus von Willebrand factor causes aggregation: filter blockage and the long bleeding time in von Willebrand’s disease. Blood 1987; 70: 1354-61. 9. Berliner S, Niinga K, Roberts JR, Houghton BA, Ruggeri ZM. Generation and characterization of peptide-specific antibodies that inhibit vW factor binding to glycoprotein IIb/IIIa without interacting with other adhesive molecules. J Biol Chem 1988; 263: 7500-05. 10. Baumgartner HR. The role of blood flow in platelet adhesion, fibrin deposition, and formation of mural thrombi. Microvasc Res 1973; 5: 167-79. 11. Weiss HJ, Hawiger J, Ruggeri ZM, Turitto VT, Thiagarajan P, Hoffman T. Fibrinogen-independent platelet adhesion and thrombus formation on subendothelium mediated by glycoprotein IIb/IIIa complex at high shear rate. J Clin Invest 1989; 83: 288-97. 12. Badimon L, Badimon JJ, Turitto VT, Fuster V. Role of von Willebrand factor in mediating platelet-vessel wall interaction at low shear rate: the importance of perfusion conditions. Blood 1989; 73: 961-67. 13. Sakariassen KS, Nievelstein PFEM, Coller BS, Sizma JJ. The role of platelet membrane glycoproteins Ib and IIb-IIIa in platelet adherence to human artery subendothelium. Br J Haematol 1986; 63: 681-91. 14. Baumgartner HR, Tschopp TB, Meyer D. Shear rate dependent inhibition of platelet adhesion and aggregation on collagenous surfaces by antibodies to human factor VIII/von Willebrand factor. Br J Haematol 1980; 44: 127-39. 15. Mannucci PM. Desmopressin: a nontransfusional form of treatment for congenital and acquired bleeding disorders. Blood 1988; 72: 1449-55. 16. Fuster V, Stein B, Badimon L, Chesebro JH. Antithrombotic therapy after myocardial reperfusion in acute myocardial infarction. J Am Coll Cardiol 1988; 12: 78A-84A. 17. O’Brien JR, Salmon GP. Heat treatment in von Willebrand’s disease. Br Med J 1985; 291: 409. 18. O’Brien JR, Green PJ, Salmon GP. Desmopressin and sheared platelets: a test. Lancet 1988; i: 655. 19. Coller BS, Folts JD, Scudder LE, Smith SR. Antithrombotic effect of a monoclonal antibody to the platelet glycoprotein IIb/IIIa receptor in an experimental animal model. Blood 1986; 68: 783-86. 20. Hanson SR, Pareti FI, Ruggeri ZM, et al. Effects of monoclonal antibodies against the platelet glycoprotein IIb/IIIa complex on thrombosis and hemostasis in the baboon. J Clin Invest 1988; 81: 149-58. 21. Yasuda T, Gold HK, Fallon JT, et al. Monoclonal antibody against the platelet glycoprotein (GP) IIb/IIIa receptor prevents coronary artery reocclusion after reperfusion with recombinent tissue-type plasminogen activator in dogs. J Clin Invest 1988; 81: 1284-91. 22. Bellinger DA, Nichols TC, Read MS, et al. Prevention of occlusive coronary artery thrombus by a mural monoclonal antibody to porcine von Willebrand factor. Proc Natl Acad Sci USA 1987; 84: 8100-04. 23. Lambrecht LK, Young BR, Stafford RE, et al. The influence of preadsorbed canine von Willebrand factor, fibronectin and fibrinogen on ex vivo artificial surface-induced thrombosis. Throm Res 1986; 41: 99-117. 24. Davies MJ, Thomas
AC, Knapman PA, Hangartner JR. Intramyocardial platelet aggregation in patients with unstable angina suffering sudden
ischemic cardiac death. Circulation 1986; 73: 418-27. 25. Coller BS, Beer JH, Scudder, Steinberg MH. Collagen-platelet interactions: evidence for a direct interaction of collagen with platelet GPIa/IIa and an indirect interaction with platelet GPIIb-IIIa mediated by adhesive proteins. Blood 1989; 72: 182-92. 26. Fuster V, Chesebro JH. Mechanisms of unstable angina. N Engl J Med 1986; 315: 1023-25. 27. Parker RI, Graenick HR. Inhibition of platelet-von Willebrand factor binding to platelets by adhesion site peptides. Blood 1989; 74: 1226-30. 28. Plow EF, Pierschbacher MD, Ruoslahti E, Marguerie GA, Ginsberg MH. The effect of arg-gly-asp-containing peptides on fibrinogen and von Willebrand factor binding to platelets. Proc Natl Acad Sci USA 1985; 82: 8057-61. 29. Cadroy Y, Houghton RA, Hanson SR. RGDV peptide selectively inhibits platelets-dependent thrombus formation in vivo: studies using a baboon model. J Clin Invest 1989; 84: 939-44.
VIEWPOINT Judicial electrocution and the prison doctor
Execution in the electric chair passed into law in New York State on Jan 1, 1889. Despite falling into disuse in the late 1960s and 1970s, this method of capital punishment is again becoming widely used in the United States. Further executions became possible following a ruling by the Supreme Court on June 26, 1989, which announced that nothing barring cruel and unusual punishment prevents mentally retarded individuals and those under the age of 18 from paying the ultimate penalty. Some 2000 people are now on "death row" waiting the results of protracted appeals. Most will have years to wait, a form of mental strain akin to torture. Acceptance of capital punishment remains enshrined in the traditions of the United States, and the morality of execution and the physiological responses to electrocution are seldom questioned. Nor is the role of the
prison surgeon.
History of the electric
chair
In New York State botched public hangings in the mid-19th century provided the impetus for a more humane method of execution. In the 1870s the electric power industry was in its infancy. Voltages were much higher than they are today, and in cities power was transmitted by overhead cables.1 This led to several accidental deaths from electrocution. The opponents of hanging soon made the connection. The advice of Thomas Edison was sought. A key factor in overwhelming opposition to electrocution was the authority of his confident claim that death would be instantaneous. This was combined with a shameless "dirty tricks" campaign/ in which Westinghouse’s alternating current, then competing with the direct current preferred by Edison, was to be denigrated. One of Edison’s assistants installed Westinghouse AC generators for electric chairs at Auburn, Sing Sing, and Clinton prisons.i-3 Agents in Rio de Janeiro were instructed to purchase generators which had been used at Oneonta in New York State4 so as not to alert Westinghouse to the plan. Meanwhile articles emphasising the deadliness of alternating current in causing accidents and its eminent suitability for the purpose of execution appeared in the trade press. It was hoped that AC generators would be stigmatised by this debased use.2 Two provisions of the New York State Electrical Execution Law are noteworthy. First, the killing and burial were to be kept private, and second, the corpse was to be dissected by doctors.1
The electric chair in
practice
The first electrocution took place on Aug 6, 1890.1.5 The victim was William Kemmler, who had murdered his mistress. In violation of the law the State Governor admitted correspondents from Associated Press and United Press;1 26 ADDRESS: 30 Poplar
PhD).
Walk,
London SE24 0BU, UK
(G. R. N. Jones,
REVIEW ARTICLE Shear-induced
platelet aggregation
Two platelet mechanisms contribute to haemostasis and thrombosis. (1) Compounds such thrombin activate glycoprotein as IIb/IIIa; fibrinogen is the ligand. The cyclooxygenase pathway is involved and so this process is aspirin sensitive. (2) Shearing forces alone activate a different domain on glycoprotein IIb/IIIa; von Willebrand’s factor is the ligand. This process is probably non-enzymatic and is aspirin insensitive. The prevention of shear-induced platelet activation may prove to be more rewarding therapeutically than inhibition of aspirin sensitive
pathways. Introduction Platelet activation induced by agonists such as thrombin and adrenaline is one mechanism involved in arterial thrombosis. Activation of the cyclooxygenase pathway results in the exposure of a domain on platelet glycoprotein IIb/IIIa (GPIIb/IIla); fibrinogen is then the ligand binding platelet to platelet or platelet to collagen. Thus aspirin is clinically beneficial because it inhibits the cyclooxygenase pathway. Shear-induced platelet activation is another mechanism, which is not affected by aspirin. Atherosclerotic stenosis, which induces high shearing forces, is almost invariably present in patients with arterial thrombi in the heart or head. Therefore this mechanism may be of great importance in thrombogenesis. The effect of shear cannot be investigated fully in man. We review the various models and discuss the mechanisms involved in shear-induced platelet activation.
Sheared
platelets
Early studies When blood is forced through small holes platelets aggregate and block the holes without the addition of any aggregating agent.2 This method is poorly reproducible, and the importance of shearing forces was little appreciated; and any contribution from the walls of the hole is uncontrolled. Forcing blood through a column of glass beads and measuring platelet retentioncan discriminate between patients with von Willebrand’s disease and controls, and is unaffected by aspirin. The blood must be forced rapidly through the column. However, these results from 1967 were similar to those obtained 20 years later,4 when shear forces and von Willebrand’s factor (vWf) were shown to determine the deposition of platelets on pig de-endothelialised aorta. Thus platelet-to-platelet aggregation induced by shear and platelet adhesion to subintima apparently share basic mechanisms.
Viscometers With a rotational viscometer precisely measured shearing forces can be applied. Large but practically inert surfaces are involved. Usually, but not always,5 the blood is sheared for many seconds or minutes and adenosine diphosphate may be released from platelets and/or red cells. Such release may occur before or after platelet aggregation. So is it cause or effect? In a study6 of patients with congenital deficiencies; for normal aggregation to occur, high shearing forces were required together with GPIIb/IIIa, a normal calcium ion concentration, and vWf, particularly the unusually large multimers. GPIbband fibrinogen7 may also be involved in this system. Aspirin does not inhibit viscometer shearinduced platelet aggregation.6 Filter
technique
Blood is forced at a predetermined pressure through a filter made of fine fibres.8 Particles (eg, platelet aggregates) larger than 10 pm are retained. As blood is forced through the tortuous, capillary sized channels the platelets are activated; they aggregate and the aggregates block the filter. The time for blood to pass through the filter is initially 8 ms so this is a very fast reaction, as is that required for haemostasis and thrombogenesis. Filters made of positively and negatively charged hydrophilic glass fibres and lipophilic polypropylene fibres give similar results. These and electron microscopy studies indicate that the filter surface plays little or no part. Normally platelet retention in the filter and the time and number of platelets required to achieve blockage are monitored. Only at high shear (40 mm Hg) does the filter block. Platelet retention is decreased and filter blockage is delayed in samples from patients with von Willebrand’s disease, and when all six antibodies to vWf so far tested are added to normal blood. Thus vWf is required for normal blocking. Retention is also abnormal in specimens from patients with thrombasthaenia, in which GPIIb/IIIa is decreased or abnormal, and on the addition of five antibodies to GPIIb/IIIa to normal blood. A critical concentration of anti-GPIIb/IIIa was less inhibitory at higher pressure (100 mm Hg), which suggests that the degree of shear determines the degree of exposure of GPIIb/IIIa. One patient with Bernard-Soulier syndrome (abnormal GPIb) and one antibody to GPIb had normal results, so GPIb may not be involved. Removal of calcium ions by edetic acid prevents platelet retention in normal blood. All these abnormal results were obtained in blood with normal fibrinogen, thus ligands such as fibrinogen and ADDRESS: Central Laboratory, St Mary’s Hospital, Portsmouth, Hampshire P03 6AG, UK (Dr J. R. O’Brien, FRCP).
712
fibrinectin cannot replace vWf as the adhesive protein in this model. In this and other systems9 the receptor site on GPIIb/IIIa for vWf differs from that required for fibrinogen. Thus in this system high shearing forces, vWf, GPIIb/IIIa, and calcium ions, but not fibrinogen, are essential for normal platelet aggregation and filter blockage. Other molecules may contribute but since enzyme inhibitors (dinitrophenol, sodium azide, and iodoacetic acid) and lowering the temperature to 15°C have little effect, it seems that the shearing forces induce a physical nonenzymatic exposure of GPIIb/IIIa. Aspirin and ticlopidine have no effect on this system. The filter technique can be used to study native blood passing through capillary sized holes under controlled shear stress. It is simple, quick, and cheap, with good
reproducibility.
Lambrecht et al23 reported that biomaterials coated with vWf in arteriovenous shunts promote platelet deposition and thrombus anchoring. The effect of shear in disease of large vessels such as the carotid and coronary arteries has been considered. vWf is also involved in small vessel haemostasis, as indicated by the long bleeding time in von Willebrand’s disease and its correction by cryoprecipitate and desmopressin. Since the bleeding time in von Willebrand’s disease is more prolonged than that after taking aspirin, in vivo the von Willebrand dependent mechanism may be the more important. Unstable angina is almost always associated with coronary stenosis and Davies et a124 found intramyocardial microthrombi in such patients who die suddenly, these microthrombi may also depend on the same sheardependent mechanism.
Conclusion
Perfusion chambers The Baumgartner technique involves mounting an everted, de-endothelialised ring of aorta on a glass rod. This is placed in an annular chamber and native or anticoagulated blood is forced at known shear rates through the chamber.
Platelet adhesion to the sub-endothelium and thrombus formation is monitored. This method is cumbersome, expensive, and has poor reproducibility; however, it accurately mimics in-vivo events. At low shear rates other mechanisms, probably involving fibrinogen and its appropriate domain on GPIIb/IIIa, may be involved" as well as vWf.12 At high shear vWf and GPIIb/IIIa have repeatedly4,1l,13,14 been shown to be important in platelet deposition and thrombus formation. Again GPIb may be involved. 13 In-vivo studies Patients with von Willebrand’s disease who have normal fibrinogen and platelets have a standard skin bleeding time that is long when the tourniquet is inflated to 40 mm Hg. Cryoprecipitate, which raises the vWf plasma level (and fibrinogen), or desmopressin, which raises both vWf15 and fibrinolytic precursors, shorten the bleeding time. Thus vWf must be involved in some aspect of small vessel haemostasis. These in-vivo observations are supported by Fuster et al.16 They passed heparinised blood at high shear over collagen I in a perfusion chamber. Platelet deposition, relative to controls, was decreased by 30 % in the presence of aspirin, by 90% in blood from patients with von Willebrands disease, and by 80% on adding an antibody to vWf. Incidentally, in the absence of collagen or a damaged vessel wall (ie, in the filter test), similar results can be obtained and therapeutic manoeuvres, giving cryoprecipitate or desmopressin, can be monitoredp.18 Coller et all and others2021 have exposed the carotid and the coronary artery in animals and found that if the endothelium is slightly damaged and a 70-90% stenosis is created by external constriction of the vessel, then platelet thrombi form distal to the stenosis. High shearing forces through the sternotic area are essential for the formation of these in-vivo platelet thrombi. Injecting antibodies to GPIIb/IIla19-21 and to vWf22 inhibits thrombus formation. Thus vWf must act as ligand. It also follows that shear forces induced by the stenosis must activate the vWf-specific domain on GPIIb/IIIa. Studies of the insertion into an arteriovenous shunt of de-endothelialised vessel segments’’ or ’Dacron’ grafts21 and the use of antibodies to GPIIb/IIIa also confirm that high shear and vWf are required for platelet deposition.
Several studies that used different techniques show that shear activates a unique domain9 on GPIIb/IIIa and, in the presence of vWf as ligand, platelets can aggregate, and also stick to collagen25 and pile up to form thrombi. This mechanism is not inhibited by aspirin or ticlopidine and is independent of platelet aggregation as studied in the aggregometer when initiated by ADP, thrombin, adrenaline, or thromboxane A2. The contribution of other molecules and other mechanisms to these shear events is less certain but presumably all these processes may come into play, irrespective of antecedent events such as plaque rupture.26 However, the importance of this shear-induced mechanism will not be established unless inhibition of this mechanism is shown to convey clinical benefit. The intravenous injection of antibodies or even small peptide sequences containing the Arg-Gly-Asp sequence will probably never be extensively used. This sequence, common to vWf and fibrinogen, is involved in the binding of these ligands to GPIIb/IIIa. Small peptides containing the Arg-Gly-Asp sequence can compete with vWf and prevent shear-induced platelet aggregation27 and thrombus formation. ",229 Perhaps efforts should be made specifically to "toughen" the platelet membrane to inhibit shearinduced platelet activation with GPIIb/IIIa exposure. In the filter test platelet deposition and filter blockage can be prevented in vitro by adding "membrane stabilising compounds" such as cocaine, chlorpromazine, quinidine, and imipramine. However, at the concentrations required for platelet inhibition (1-10 mmol/1), these drugs will also inhibit other membranes and be lethal. The search for drugs and other methods to inhibit shear-induced activation of the platelets should continue. I thank Dr P. J. Green and Mr G. P. Salmon for Ms M. Bastable for secretarial help.
helpful discussions and
REFERENCES 1.
Trialists’ Collaboration.
Antiplatelet Secondary prevention of vascular disease by prolonged antiplatelet treatment. Br Med J 1988; 296:
320-31. 2. O’Brien JR, Etherington MD, Weir P. Platelet agregation inhibitors: a 5 µ nuclepore filter "bleeding time". Monte Carlo: Sixth international on Meditterranean thrombosis, congress league against thromboembolic diseases, 1980: 307T. 3. O’Brien JR, Heywood JB. Some interactions between human platelets and glass: von Willebrand’s disease compared with normal. J Clin Pathol 1967; 20: 56-64. 4. Badimon L, Badimon JJ, Rand J,Turritto VT, Fuster V. Platelet deposition on von Willebrand factor-deficient vessels: extracorporeal perfusion studies in swine with von Willebrand’s disease using native and heparinized blood. J Lab Clin Med 1987; 110: 634-47.
713
5.
Wurzinger LJ, Opitz R, Blasberg P, Schmid-Schongein H. Platelet and coagulation parameters following millisecond exposure to laminar
shear stress. Thromb Haemostat 1985; 54: 381-86. 6. Moake JL, Turner NA, Stathopoulos NA, Nolasco L, Hellums JD, Shear-induced platelet aggregation can be mediated by vWf released from platelets, as well as by exogenous large or unusually large vWf multimers, requires adenosine diphosphate, and is resistant to aspirin. Blood 1988; 71: 1366-74. 7. Ikeda Y, Murata M, Araki Y, et al. Importance of fibrinogen and platelet membrane glycoprotein IIb/IIIa in shear-induced platelet aggregation. Thromb Res 1988; 51: 157-63. 8. O’Bnen JR, Salmon GP. Shear stress activation of platelet glycoprotein IIb/IIIa plus von Willebrand factor causes aggregation: filter blockage and the long bleeding time in von Willebrand’s disease. Blood 1987; 70: 1354-61. 9. Berliner S, Niinga K, Roberts JR, Houghton BA, Ruggeri ZM. Generation and characterization of peptide-specific antibodies that inhibit vW factor binding to glycoprotein IIb/IIIa without interacting with other adhesive molecules. J Biol Chem 1988; 263: 7500-05. 10. Baumgartner HR. The role of blood flow in platelet adhesion, fibrin deposition, and formation of mural thrombi. Microvasc Res 1973; 5: 167-79. 11. Weiss HJ, Hawiger J, Ruggeri ZM, Turitto VT, Thiagarajan P, Hoffman T. Fibrinogen-independent platelet adhesion and thrombus formation on subendothelium mediated by glycoprotein IIb/IIIa complex at high shear rate. J Clin Invest 1989; 83: 288-97. 12. Badimon L, Badimon JJ, Turitto VT, Fuster V. Role of von Willebrand factor in mediating platelet-vessel wall interaction at low shear rate: the importance of perfusion conditions. Blood 1989; 73: 961-67. 13. Sakariassen KS, Nievelstein PFEM, Coller BS, Sizma JJ. The role of platelet membrane glycoproteins Ib and IIb-IIIa in platelet adherence to human artery subendothelium. Br J Haematol 1986; 63: 681-91. 14. Baumgartner HR, Tschopp TB, Meyer D. Shear rate dependent inhibition of platelet adhesion and aggregation on collagenous surfaces by antibodies to human factor VIII/von Willebrand factor. Br J Haematol 1980; 44: 127-39. 15. Mannucci PM. Desmopressin: a nontransfusional form of treatment for congenital and acquired bleeding disorders. Blood 1988; 72: 1449-55. 16. Fuster V, Stein B, Badimon L, Chesebro JH. Antithrombotic therapy after myocardial reperfusion in acute myocardial infarction. J Am Coll Cardiol 1988; 12: 78A-84A. 17. O’Brien JR, Salmon GP. Heat treatment in von Willebrand’s disease. Br Med J 1985; 291: 409. 18. O’Brien JR, Green PJ, Salmon GP. Desmopressin and sheared platelets: a test. Lancet 1988; i: 655. 19. Coller BS, Folts JD, Scudder LE, Smith SR. Antithrombotic effect of a monoclonal antibody to the platelet glycoprotein IIb/IIIa receptor in an experimental animal model. Blood 1986; 68: 783-86. 20. Hanson SR, Pareti FI, Ruggeri ZM, et al. Effects of monoclonal antibodies against the platelet glycoprotein IIb/IIIa complex on thrombosis and hemostasis in the baboon. J Clin Invest 1988; 81: 149-58. 21. Yasuda T, Gold HK, Fallon JT, et al. Monoclonal antibody against the platelet glycoprotein (GP) IIb/IIIa receptor prevents coronary artery reocclusion after reperfusion with recombinent tissue-type plasminogen activator in dogs. J Clin Invest 1988; 81: 1284-91. 22. Bellinger DA, Nichols TC, Read MS, et al. Prevention of occlusive coronary artery thrombus by a mural monoclonal antibody to porcine von Willebrand factor. Proc Natl Acad Sci USA 1987; 84: 8100-04. 23. Lambrecht LK, Young BR, Stafford RE, et al. The influence of preadsorbed canine von Willebrand factor, fibronectin and fibrinogen on ex vivo artificial surface-induced thrombosis. Throm Res 1986; 41: 99-117. 24. Davies MJ, Thomas
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ischemic cardiac death. Circulation 1986; 73: 418-27. 25. Coller BS, Beer JH, Scudder, Steinberg MH. Collagen-platelet interactions: evidence for a direct interaction of collagen with platelet GPIa/IIa and an indirect interaction with platelet GPIIb-IIIa mediated by adhesive proteins. Blood 1989; 72: 182-92. 26. Fuster V, Chesebro JH. Mechanisms of unstable angina. N Engl J Med 1986; 315: 1023-25. 27. Parker RI, Graenick HR. Inhibition of platelet-von Willebrand factor binding to platelets by adhesion site peptides. Blood 1989; 74: 1226-30. 28. Plow EF, Pierschbacher MD, Ruoslahti E, Marguerie GA, Ginsberg MH. The effect of arg-gly-asp-containing peptides on fibrinogen and von Willebrand factor binding to platelets. Proc Natl Acad Sci USA 1985; 82: 8057-61. 29. Cadroy Y, Houghton RA, Hanson SR. RGDV peptide selectively inhibits platelets-dependent thrombus formation in vivo: studies using a baboon model. J Clin Invest 1989; 84: 939-44.
VIEWPOINT Judicial electrocution and the prison doctor
Execution in the electric chair passed into law in New York State on Jan 1, 1889. Despite falling into disuse in the late 1960s and 1970s, this method of capital punishment is again becoming widely used in the United States. Further executions became possible following a ruling by the Supreme Court on June 26, 1989, which announced that nothing barring cruel and unusual punishment prevents mentally retarded individuals and those under the age of 18 from paying the ultimate penalty. Some 2000 people are now on "death row" waiting the results of protracted appeals. Most will have years to wait, a form of mental strain akin to torture. Acceptance of capital punishment remains enshrined in the traditions of the United States, and the morality of execution and the physiological responses to electrocution are seldom questioned. Nor is the role of the
prison surgeon.
History of the electric
chair
In New York State botched public hangings in the mid-19th century provided the impetus for a more humane method of execution. In the 1870s the electric power industry was in its infancy. Voltages were much higher than they are today, and in cities power was transmitted by overhead cables.1 This led to several accidental deaths from electrocution. The opponents of hanging soon made the connection. The advice of Thomas Edison was sought. A key factor in overwhelming opposition to electrocution was the authority of his confident claim that death would be instantaneous. This was combined with a shameless "dirty tricks" campaign/ in which Westinghouse’s alternating current, then competing with the direct current preferred by Edison, was to be denigrated. One of Edison’s assistants installed Westinghouse AC generators for electric chairs at Auburn, Sing Sing, and Clinton prisons.i-3 Agents in Rio de Janeiro were instructed to purchase generators which had been used at Oneonta in New York State4 so as not to alert Westinghouse to the plan. Meanwhile articles emphasising the deadliness of alternating current in causing accidents and its eminent suitability for the purpose of execution appeared in the trade press. It was hoped that AC generators would be stigmatised by this debased use.2 Two provisions of the New York State Electrical Execution Law are noteworthy. First, the killing and burial were to be kept private, and second, the corpse was to be dissected by doctors.1
The electric chair in
practice
The first electrocution took place on Aug 6, 1890.1.5 The victim was William Kemmler, who had murdered his mistress. In violation of the law the State Governor admitted correspondents from Associated Press and United Press;1 26 ADDRESS: 30 Poplar
PhD).
Walk,
London SE24 0BU, UK
(G. R. N. Jones,
