Eleven patients with heparin-induced thrombocytopenia were studied. Thrombocytopenia appeared 3-16 days following the initiation of prophylactic or therapeutic doses of heparin. The mean lowest platelet count recorded was 48,000/mm3. When heparin was stopped, recovery from thrombocytopenia began within 24 hours and was complete by ten days. Two patients developed fatal thromboses, and two others had myocardial infarctions while thrombo-cytopenic. In the serum of seven patients, including three of the four with arterial thrombosis, a heparin-dependent platelet aggregating factor was present. The factor caused release of platelet 14C serotonin but did not lyse platelets. It was present in the globulin fraction of all positive sera, and in one serum studied it was isolated in the IgG/IgA immunoglobulin fraction. The factor was not present in 16 normal sera or in the sera of 15 nonthrombocytopenic patients receiving heparin. Our observations suggest that heparin-induced thrombocytopenia is common and that, in some patients it may be accompanied by severe arterial thrombosis. In vivo platelet aggregation is a possible explanation for the thrombocytopenia and the thrombosis in this disorder.

Two families are described with members who have both von Willebrand's disease and telangiectasias. Family A has four members in three consecutive generations that have both von Willebrand's disease and telangiectasias. von Willebrand's disease in this family is characterized by decreased ristocetin cofactor (FVIII-vWF), variably depressed factor VIII coagulant (FVIII-AHG), and factor VIII-related antigen (FVIII-AGN) levels. FVIII-AGN mobility on two-dimensional crossed immunoelectrophoresis was found to be normal. Four generations in Family B have von Willebrand's disease characterized by decreased FVIIII-AHG, FVIII-vWF, FVIII-AGN, and prolonged template bleeding times. Two members of this family also have telangiectasias and recurrent gastrointestinal bleeding. Results in these two families suggest an association between von Willebrand's disease and telangiectasia--perhaps a defect in vascular endothelial cell function.

Sudden, severe thrombocytopenia developed in each of three patients receiving diphenylhydantion, diazepam, and sulfisoxazole, respectively. Recovery followed discontinuance of the drugs. An antiplatelet antibody requiring the presence of an appropriate drug for interaction with platelets was deomonstrated in each case by the 51Cr platelet lysis test using normal, paroxysmal nocturnal hemoglobinuric, or enzyme-treated normal platelets as target cells. These antibodies could not be detected by techniques that depend on clot retraction inhibition, complement fixation, or platelet factor-3 activation. Quinine-and quinidine-dependent antiplatelet antibodies in the serum of 16 patients who developed acute thrombocytopenia while taking either quinine or quinidine could be demonstrated readily with the 51Cr platelet lysis test and could also be detected by other methods employed.

SummaryCytochalasin B alters the structure and functional properties of filamentous actin. Platelet-mediated clot retraction in dilute platelet-rich plasma (PRP) is inhibited progressively at cytochalasin B concentrations of 0.01 mg/ml, 0.05 mg/ml and 0.1 mg/ml. Dynamic rheological measurements of recalcified PRP in a Weissenberg Rheogoniometer indicate that platelet contractility (as reflected in measurements of elastic moduli) is reduced by 33%, 57% and 63% at cytochalasin B concentrations of 0.01, 0.05 and 0.1 mg/ml, respectively.In contrast, pre-incubation of human platelet-rich plasma (PRP) with 0.01 mg/ml or 0.05 mg/ml cytochalasin B does not inhibit collagen-induced [14C-]serotonin release or collagen-induced-platelet aggregation, which is dependent on the release of ADP from platelet dense granules. Even at a cytochalasin B concentration of 0.1 mg/ml, collagen-induced [14C-]serotonin release and aggregation are impaired only moderately. Cytochalasin B does not interfere with the uptake by platelets of [14C-]-serotonin, or with the kinetics and extent of clot formation in platelet-free plasma.Thus, concentrations of cytochalasin B which impair platelet contractility do not inhibit the release of platelet dense granule contents. It is concluded that neither the platelet release reaction nor platelet aggregation is dependent on platelet contractile mechanisms.

The effect of platelet type-specific transfusion in posttranfusion purpura is reported. Seven days after receiving 4 units of whole blood during total hip replacement a 69-year-old woman developed fulminant thrombocytopenic purpura. Her undiluted serum inhibited the clot retraction of PlAl-positive but not PlAl-negative blood. Anti-PlAl titer of her serum, determined by 51Cr platelet lysis technique, was 1:64. The serum had no lytic activity against platelet-rich plasma from two PlAl-negative donors. No anti-HLA antibody was detectable in the serum by lymphocytotoxicity technique, and serum obtained prior to transfusions had no platelet lytic activity. Four units of PlAl-negative platelet concentrate were administered, the first instance in which this treatment has been used. No rise in platelet count ensued, and the patient succumbed to purpura. Exchange transfusion or plasmapheresis remain the treatments of choice.

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SummaryIn dilute suspensions of platelet-rich plasma (PRP) or gel-separated platelets (GSP), dibutyryl-cAMP (DBcAMP) and monobutyryl-cAMP inhibited platelet-mediated fibrin clot retraction in concentrations of 2–3 × 10–6M, with complete inhibition at 1–3 × 10–4M. Prostaglandin E1 (PGE1), which inhibited fibrin clot retraction in concentrations greater than 1.5–3 × 10–8M, was a more effective inhibitor than either PGE2 or PGF2α. In the presence of theophylline (10–4M), concentrations of DBcAMP, PGE1 PGE2 and PGF2α necessary to inhibit fibrin clot retraction were reduced 50-fold for DBcAMP and 2.5 to 20-fold for the prostaglandins. In dilute PRP or GSP, inhibition of fibrin clot retraction does not result from inhibition of thrombin-induced platelet aggregation. Thus, compounds which increase platelet cAMP levels result in the inhibition of platelet-mediated fibrin clot retraction, and this inhibitory effect may be mediated, at least in part, through suppression of platelet contractility. Cyclic GMP, dibutyryl-cGMP and carbamylcholine-Cl (which stimulates guanylate cyclase) did not influence fibrin clot retraction, and did not prevent inhibition of fibrin clot retraction by DBcAMP and PGE?. Colchicine, in concentrations known to disrupt platelet microtubules (2.5 × 10–6M to 2.5 x 10–3M), had little inhibitory effect on either fibrin clot retraction or platelet (3H)-serotonin release.