Uremia is associated with a bleeding diathesis. We investigated platelet adhesion as a cause for the impaired primary hemostasis and the role of von Willebrand factor (vWF) in this process in uremic patients. Perfusions with blood with standardized hematocrit, platelet count, and free Ca2+ ions were performed over inverted and deendothelialized artery segments from human umbilical cords in a modified Baumgartner perfusion chamber. Platelet adhesion in patient perfusates was comparable with control adhesion. However, the high vWF levels present in uremic whole blood did not increase adhesion above the adhesion in control blood with lower vWF levels. These results suggested that a relative adhesion defect was present in patient blood. Control blood in which vWF levels were raised to uremic levels showed the high adhesion that uremic whole blood failed to show. Additionally, in perfusions with uremic plasma in which the initially high vWF level was normalized by dilution with vWF-depleted uremic plasma, adhesion was clearly lower than in normal plasma. Washed patient platelets did not differ from normal platelets in their association with purified vWF, via their adhesion receptors glycoprotein Ib and IIb-IIIa. Patient platelets present in patient plasma showed a similar adhesion defect as control platelets, which were resuspended in the uremic plasma. Therefore, primary defects of uremic platelets were of minor importance for the observed adhesion defect in uremic whole blood. The adhesion defect was not dependent on the presence of uremic vWF; plasma of uremic patients depleted of vWF also inhibited adhesion, and the defect remained present when purified control vWF was added to vWF-depleted uremic plasma. The binding of uremic vWF to the vessel wall and its support of subsequent adhesion were not impaired. These results indicate that the observed adhesion defect was not due to abnormal vWF. Our current results suggest an unknown component present in uremic plasma that directly inhibits platelet interaction with artery segments; however, it has no effect on vWF binding to the vessel wall. High vWF levels in uremic plasma are able to compensate for the defect.

The risk of acute cardiovascular events is highest during morning hours, and platelet activity peaks during morning hours. The effect of timing of aspirin intake on circadian rhythm and morning peak of platelet reactivity is not known. It was our objective to evaluate the effect of timing of aspirin intake on circadian rhythm and morning peak of platelet reactivity. A randomised open-label cross-over trial in healthy subjects (n=14) was conducted. Participants used acetylsalicylic acid (80 mg) on awakening or at bedtime for two periods of two weeks, separated by a four-week wash-out period. At the end of both periods blood was drawn every 3 hours to measure COX-1-dependent (VerifyNow-Aspirin; Serum Thromboxane B2 [STxB2]) and COX-1-independent (flow cytometry surface CD62p expression; microaggregation) platelet activity. VerifyNow platelet reactivity over the whole day was similar with intake on awakening and at bedtime (mean difference: -9 [95 % confidence interval (CI) -21 to 4]). However, the morning increase in COX-1-dependent platelet activity was reduced by intake of aspirin at bedtime compared with on awakening (mean difference VerifyNow: -23 Aspirin Reaction Units [CI -50 to 4]; STxB2: -1.7 ng/ml [CI -2.7 to -0.8]). COX-1-independent assays were not affected by aspirin intake or its timing. Low-dose aspirin taken at bedtime compared with intake on awakening reduces COX-1-dependent platelet reactivity during morning hours in healthy subjects. Future clinical trials are required to investigate whether simply switching to aspirin intake at bedtime reduces the risk of cardiovascular events during the high risk morning hours.

Platelet activation in diabetes mellitus may precede vasculopathy. We have studied platelet adhesion and thrombus formation in flowing blood of diabetic patients without macro-and microvascular complications. Platelet function and release, bleeding time, fibrinogen, FVIII:RAg, Ri:Co, Triglycerides, Cholesterol and HDL Cholesterol levels, Apo A, Apo B and TK values were in the normal range. Glucose levels and HBA1 were increased. The matrix of cultured human umbilical vein endothelial cells (ECM) was used for adhesion studies.In an adhesion model with citrated blood (3.1%) and an untreated ECM, three male and three female diabetics (mean age 29.6 y., range 20-32 y., average duration of diabetes 10.6 y., range 2-23 y.) were compared with six age and sex-matched corf-trols. No differences in platelet coverage (en face) was obtained at low (300 s™1 ) and high (1300 s™1 ) wall shear rates, and the time dependence (1-20 min) was the same.We also used a thrombosis model, consisting of the matrix of PMA-treated endothelial cells which induces tissue factor production, perfused with blood anticoagulated with low molecular weight heparin, which does not inhibit local thrombus formation. Four male and two female patients (mean age 25.4 y., range 22-40 y., average duration of diabetes 10 y., range 2-28 y.) were compared with six age and sex matched controls. Cross sections showed equal total adhesion at 300 s™1 and 1300 s™1 after 1, 3, 5 or 10 minutes perfusion. In the diabetics, spread platelets were significantly decreased at 300 s™1 after 1 and 3 min, and at 1300 s™1 after 5 min; small aggregates (< 5μM) were increased after 1 min at 300 s™1 . At 300 s™1 some patients (2 of 6) showed a faster and more extensive fibrin formation.We conclude that _ in this group of diabetics there is no difference in primary adhesion but that there is an enhanced tendency in thrombus formation and fibrin deposition.

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Uremia is associated with a bleeding diathesis. We investigated platelet adhesion as a cause for the impaired primary hemostasis and the role of von Willebrand factor (vWF) in this process in uremic patients. Perfusions with blood with standardized hematocrit, platelet count, and free Ca2+ ions were performed over inverted and deendothelialized artery segments from human umbilical cords in a modified Baumgartner perfusion chamber. Platelet adhesion in patient perfusates was comparable with control adhesion. However, the high vWF levels present in uremic whole blood did not increase adhesion above the adhesion in control blood with lower vWF levels. These results suggested that a relative adhesion defect was present in patient blood. Control blood in which vWF levels were raised to uremic levels showed the high adhesion that uremic whole blood failed to show. Additionally, in perfusions with uremic plasma in which the initially high vWF level was normalized by dilution with vWF-depleted uremic plasma, adhesion was clearly lower than in normal plasma. Washed patient platelets did not differ from normal platelets in their association with purified vWF, via their adhesion receptors glycoprotein Ib and IIb-IIIa. Patient platelets present in patient plasma showed a similar adhesion defect as control platelets, which were resuspended in the uremic plasma. Therefore, primary defects of uremic platelets were of minor importance for the observed adhesion defect in uremic whole blood. The adhesion defect was not dependent on the presence of uremic vWF; plasma of uremic patients depleted of vWF also inhibited adhesion, and the defect remained present when purified control vWF was added to vWF-depleted uremic plasma. The binding of uremic vWF to the vessel wall and its support of subsequent adhesion were not impaired. These results indicate that the observed adhesion defect was not due to abnormal vWF. Our current results suggest an unknown component present in uremic plasma that directly inhibits platelet interaction with artery segments; however, it has no effect on vWF binding to the vessel wall. High vWF levels in uremic plasma are able to compensate for the defect.