Electrical aggregometry in whole blood from human, pig and rabbit

The objective of this study was to characterize and standardize whole blood electrical aggregometry (WBEA) in the pig and rabbit, animal models extensively used in atherosclerosis research, and to compare their platelet response with that of man. Platelet aggregation was studied in blood (WBEA) and platelet rich plasma (optical aggregometry, OA). Dose response curves were obtained for ADP and collagen. The effect of hematocrit on WBEA was also evaluated. Aggregation with ADP and collagen using WBEA was more extensive with human than with pig or rabbit platelets. OA revealed similar differences among species but the time to reach maximal aggregation was markedly shorter. Using WBEA, the extent of aggregation was inversely related to the hematocrit. We conclude that WBEA is a useful technique that may be of particular importance in situations where hyperlipidemic plasma prevents the use of OA, as occurs in some atherosclerosis research animal models.     

Reduced sensitivity of platelets from type 2 diabetic patients to acetylsalicylic acid (aspirin)-its relation to metabolic control

Aspirin (acetylsalicylic acid, ASA), which is recommended for primary and secondary prevention in diabetes mellitus (DM), has been shown to have a lower antiplatelet activity in diabetic patients. We conducted a crossover designed observational study to evaluate whether there is an association between the parameters relevant to metabolic control of diabetes and platelet sensitivity to aspirin in type 2 diabetic patients. Platelets' ability to adhere and aggregate was monitored with the use of platelet function analyser (PFA-100 collagen/epinephrine closure time, CT(CEPI) or collagen/ADP closure time, CT(CADP)), classical turbidimetric aggregometry and whole blood electrical aggregometry (WBEA), using collagen (WBEA(coll)), ADP (WBEA(ADP)) and arachidonic acid (WBEA(AA)) as platelet agonists, in 48 control healthy volunteers (mean age+/-S.D., 49+/-9 years) and 31 type 2 DM patients (50+/-9 years; HbA(1c) 9.4+/-1.6%). In majority of control subjects (69%) and minority of diabetic patients (29%, p=0.0006), the use of 150 mg aspirin daily for 1 week significantly reduced platelet adhesiveness and reactivity (by 14.1% in diabetes vs. 78.6% in control, p(np)=0.0035, as expressed by the relative changes in CT(CEPI)). Aspirin reduced WBEA(coll) and WBEA(AA) to a lesser extent in diabetic patients (by 2.1% vs. 8.3% in controls, p(np)=0.0397, and by 97.3+/-12.8% vs. 100% in controls, p(np)=0.0383, respectively), which corresponded to ASA-mediated decreased aggregation in platelet-rich plasma (PRP, r(S)=0.45 and r(S)=0.78 for collagen- or arachidonate-agonized platelets, p<0.01 or lower). The maximal inhibition of platelet aggregation was lower and IC(50) higher in diabetic compared to control subjects, both in the presence of arachidonic acid (71% vs. 39%, p(np)0.0001; 0.5 microg/ml vs. 1.3 microg/ml, p<0.0001) and collagen (52% vs. 35%, p<0.0004; 1.6 microg/ml vs. 2.1 microg/ml, p<0.01). The reduced response of platelets from diabetic subjects to aspirin was associated with a higher level of HbA(1c), lower concentration of HDL-cholesterol and a higher total cholesterol concentration. Overall, there is evidence that reduced platelets response to aspirin may occur more often in diabetic patients. Poor metabolic control may play a role in the reduced platelet sensitivity to aspirin in DM patients. Thus, our findings strongly support the requirements for an excellent near-normal metabolic control and may suggest a need for alternative ASA dosing schedules in DM patients.     

Is platelet aggregation a more important contributor than platelet adhesion to the overall platelet-related primary haemostasis measured by PFA-100?

Background: Platelet-related primary haemostasis (PRPH), measured in PFA-100™ as a closure time (CT), reflects platelets' combined ability to adhere and aggregate under higher shear stress. The inputs of platelet aggregation and platelet adhesion into the real values of CT remain unknown, and this poor discrimination results in the complexity of the PFA-100™ measurement. Objective: To estimate the particular contributions of two physiological phenomena, platelet aggregation and adhesion, and the importance of various membrane receptors underlying platelets' capability of the plug formation in PFA-100™ cartridges. Materials and methods: Effects of various blockers antagonizing ligands binding to platelet surface membrane receptors (antagonists of GPIIb-IIIa complex, collagen receptors and purinoreceptors), and aurintricarboxylic acid (ATA), the antagonist of GPIb–von Willebrand factor (vWF) interaction, were monitored in 47 healthy donors with the use of PFA-100™ and whole blood electrical aggregometry (WBEA). Results: PFA-100™ collagen/ADP CT was the most sensitive in probing the effect of platelet membrane receptor antagonists acting via the blockade of GPIIb-IIIa complex and those antagonizing GPIb–vWF interaction (GR144053F, Integrilin, ATA), whereas the other blockers, acting on collagen receptors or purinoreceptors, remained much less efficient. For the examined GPIIb-IIIa and GPIb antagonists, the overall variability in WBEA explained a very significant part (30–60%) of the overall variability in PFA-100™ CT. Conclusions: GPIIb-IIIa-mediated platelet aggregation and von Willebrand factor interactions with GPIb and/or GPIIb-IIIa seem to be the major determinants of PFA-100™ CT. On the contrary, other platelet receptors participating in platelet aggregation and/or platelet adhesion are of secondary importance and minor significance in blood flow at higher shear stress monitored in PFA-100™.     

Multiple electrode aggregometry: a new device to measure platelet aggregation in whole blood

Several methods are used to analyse platelet function in whole blood. A new device to measure whole blood platelet aggregation has been developed, called multiple electrode platelet aggregometry (MEA). Our aim was to evaluate MEA in comparison with the single platelet counting (SPC) method for the measurement of platelet aggregation and platelet inhibition by aspirin or apyrase in diluted whole blood. Platelet aggregation induced by different concentrations of ADP, collagen and TRAP-6 and platelet inhibition by apyrase or aspirin were determined in citrateor hirudin-anticoagulated blood by MEA and SPC. MEA indicated that spontaneous platelet aggregation was lower, and stimulated platelet aggregation was higher in hirudin- than citrate-anticoagulated blood. In hirudin-anticoagulated, but not citrate-anticoagulated blood, spontaneous platelet aggregation measured by MEA was inhibited by apyrase. For MEA compared with SPC the dose response-curves of agonist-induced platelet aggregation in citrate- and hirudin-blood showed similar EC50 values for TRAP, and higher EC50 values for ADP (non-significant) and collagen (p < 0.05). MEA and the SPC method gave similar results concerning platelet-inhibition by apyrase and aspirin. MEA was more sensitive than SPC to the inhibitory effect of aspirin in collagen-induced aggregation. In conclusion, MEA is an easy, reproducible and sensitive method for measuring spontaneous and stimulated platelet aggregation, and evaluating antiplatelet drugs in diluted whole blood. The use of hirudin as an anticoagulant is preferable to the use of citrate. MEA is a promising technique for experimental and clinical applications.     

Early detection of preeclampsia by determination of platelet aggregability

Preeclampsia is still a leading cause of maternal and fetal morbidity and mortality. There is evidence for the involvement of platelets. Therefore, we investigated the suitability of corrected whole blood impedance aggregometry as an early predictor of preeclampsia in 71 consecutive, high-risk pregnancies. According to the occurrence of preeclampsia, defined postpartum by an independent investigator, and the stage of pregnancy (early and late, cutoff: 25 weeks of gestation), four study groups were defined. Platelet aggregation data were corrected for the influence of hematocrit and platelet count by a special purpose software package. Women developing preeclampsia showed significantly higher platelet aggregation response compared to controls in early and late pregnancy. In early pregnancy, all women developing preeclampsia had aggregation responses to collagen higher than the highest responses among the controls. Hence, this test had a 100% positive predictive value of subsequent preeclampsia. Despite being significantly increased, platelet aggregability was of minor predictive value in late pregnancy. We conclude that preeclampsia is accompanied by exaggerated platelet aggregability, particularly perceptible early in the course of pregnancy. We propose collagen-induced whole blood platelet aggregation with correction for the influence of hematocrit and platelet count for early detection of preeclampsia.     

Age, cardiovascular risk factors and coronary heart disease as determinants of platelet function in men. A multivariate approach

Older age, the cardiovascular risk factors and arteriosclerosis have been reported to be associated with stimulated platelet function. To evaluate the relative importance of these factors in determining platelet function, a cross-sectional multivariate study in 191 men, 113 healthy subjects and 78 patients with angiographically documented coronary heart disease, was performed. In healthy subjects, stepwise multiple linear regression identified age to be a major determinant of platelet aggregability. After induction with both ADP and collagen the platelet aggregatory response markedly increased with age. In the patients, platelet function was not age dependent. In multivariate analysis of variance, neither smoking status nor hypercholesterolemia (>- 240 mg/dl) were determinants of platelet function in either group. An increase in systolic blood pressure was associated with slightly more inhibited ADP induced aggregation in both healthy subjects and patients with coronary heart disease. In patients compared to healthy subjects, aggregation after induction with ADP and collagen was markedly enhanced and the in vitro formation of thromboxane after collagen stimulation increased. Thus, by multivariate analysis, age and the presence or absence of coronary heart disease were found to be major determinants of platelet function. In contrast, the cardiovascular risk factors smoking, hypercholesterolemia and hypertension were associated with only minor or no alterations of platelet function.     

Collagen-mediated platelet aggregation: the role of multiple interactions between the platelet surface and collagen

Although the requirement for collagen fibrils to initiate platelet aggregation is well established, there has been no satisfactory explanation for this requirement. One possibility is that multiple simultaneous and linked interactions between collagen and the platelet surface must occur to initiate the release reaction and subsequent aggregation. Direct evidence in support of this proposal was obtained by examination of the ability of collagen crosslinked in a random manner with glutaraldehyde to initiate platelet aggregation. Collagen crosslinked with 0.25% glutaraldehyde is only a slightly less effective aggregating agent than native fibrillar collagen. Further studies revealed that whereas native triple helical cross-linked collagen is an effective aggregating agent, denatured crosslinked collagen is ineffective. It thus appears that crosslinking of platelet receptor sites by multiple simultaneous and linked interactions with a rigid collagen matrix is required to initiate platelet aggregation. The precise steric relationship of the collagen sites does not appear to be of great importance.     

Evaluation of a quantitative platelet-collagen adhesiveness test system

A method is described for the quantitative measurement of the blood platelet-collagen reaction, in which the adhesiveness of blood platelets is measured with a standard collagen suspension. Though glass has been used in many blood platelet adhesiveness tests, collagen fibers are a more biological substrate. The test, as developed, involves the mixing of a pre-determined volume of a standard collagen suspension (that which produces ～ 50% adhesion in normal PRP) to an aliquot of platelet-rich plasma (platelet count adjusted to 400,000/mm³), shaking with a vortex shaker for 5 seconds, and fixing immediately with glutaraldehyde fixative. The mixture is then centrifuged at 100 × g and a platelet count determined on the supernatant. Percent decrease in platelet count over blank is used as the platelet adhesiveness index. If the PRP-collagen mixture is shaken for 5 seconds but not fixed with glutaraldehyde for 30 seconds, platelet aggregation also occurs. This decreases the platelet count even further; such a measurement can be used to distinguish conditions or agents that affect aggregation but not adhesion. The method is illustrated by determinations of the effects of chlorpromazine and EDTA $\text{in vitro}$ and aspirin $\text{in vivo}$, and the effect of dialysis on the adhesiveness index and platelet aggregation in uremic patients. The results indicated that chlorpromzine inhibited adhesion while EDTA did not, but EDTA inhibited the subsequent aggregation. Aspirin did not inhibit adhesion, but inhibited aggregation.The uremic patients had a very low adhesiveness index before dialysis. Dialysis improved the adhesiveness index, but it still was lower than normal. This method could be valuable both clinically and preclinically to test compounds for pharmacologic activity.     

A method of testing platelet aggregation in native whole blood

A method of testing collagen induced platelet aggregation and ATP release in native (= non anticoagulated) whole blood by monitoring the electrical impedance in the Chrono Log Whole Blood Aggregometer is reported. It is the first simple method by which aggregation of human platelets can be measured in their natural environment. In normal individuals lower threshold collagen concentrations could induce platelet aggregation as determined with this method than in the other tested methods (impedance method with citrated blood, optical method in platelet rich plasma). The aggregation response was not inhibited by hirudin or heparin in therapeutic dose. The luminescence channel of the Whole Blood Aggregometer permits measurements of ATP release in native whole blood.     

Enhancement of collagen-induced aggregation of platelets in whole blood

In order to elucidate the features of platelet aggregation in whole blood, studies were carried out on human blood. The platelet aggregation reaction was monitored by counting the residual free platelet number with an electronic particle counter (Coulter). Platelets in citrated whole blood were aggregated by a very small amount of collagen which did not aggregate platelets in citrated plasma. Such enhancement was not observed if ADP or epinephrine was used. By addition of isolated erythrocytes to platelet rich plasma, enhancement of the platelet response to collagen was obtained. The erythrocyte membrane stabilizer, Dilazep, abolished the enhancement effect of erythrocytes. Those results indicated that erythrocytes enhanced the platelet response to collagen. Although participation of ADP from the erythrocytes in the enhancement was suggested, the results of ADP determinations on suspensions of erythrocytes indicated that other factors of the erythrocytes might be involved in the enhancement.     

Exercise-induced changes in platelet aggregation; a comparison of whole blood and platelet rich plasma techniques

Studies have been performed to assess the effect of exercise on spontaneous platelet aggregation in shaken whole blood, and on agonist-induced platelet aggregation in whole blood and platelet rich plasma (PRP). Spontaneous platelet aggregation in shaken whole blood was increased following exercise compared to pre-exercise values. The increase in spontaneous aggregation after exercise correlated inversely with the increase in white cell count in whole blood. Platelet sensitivity in whole blood to adrenaline, collagen and adenosine diphosphate (ADP) was increased following exercise. Changes in platelet sensitivity to adrenaline following exercise correlated with increases in plasma noradrenaline levels but not with changes in blood cell counts. In PRP, platelet sensitivity to ADP and to collagen was increased following exercise when the pre and post-exercise PRP platelet counts were not corrected to allow for the increase in platelet count which occurred with exercise. When the PRP platelet counts were corrected, no changes in platelet sensitivity to any agonist after exercise were observed.     

Platelet adenine nucleotides and arachidonic acid metabolism in the myeloproliferative disorders

Platelet aggregation to collagen, adenosine diphosphate and arachidonic acid has been investigated in 17 patients with various myeloproliferative states. Ten patients who had abnormalities of aggregation to collagen and/or ADP were also all found to have diminished intracellular and releasable adenine nucleotides but aggregation to arachidonic acid was normal. Seven other patients who had normal aggregation responses had normal platelet adenine nucleotides. In the ten patients with abnormal platelet function platelet cyclo-oxygenase activity was normal but in two patients platelet lipoxygenase activity was reduced. Thromboxane B₂ production during collagen stimulation was found to be normal suggesting normal release of endogenous arachidonic acid. These findings suggest that the platelet defect in myeloproliferative states is due to an acquired storage pool disease.     

Heparin enhances platelet aggregation irrespective of anticoagulation with citrate or with hirudin.

The effects of anticoagulation with citrate or hirudin on heparin effects on platelet aggregation was studied with whole blood aggregometry on blood from healthy volunteers. Platelet aggregation was initiated by collagen. The heparin effect was also studied with filtragometry where hirudin was used as the anticoagulant. In citrated blood, a mean collagen dose of 0.42 +/- 0.04 micrograms/ml resulted in an impedance change of 1.1 +/- 0.3 Ohm. Preincubation with heparin doses of 0.5, 2.5 and 5 IU/ml enhanced the impedance induced by the same dose of collagen by 2.9 +/- 1.4, 11.4 +/- 1.6 and 9.9 +/- 2.3 times, respectively (p less than 0.0001, ANOVA). In hirudinized blood a similar degree of change in impedance (1.5 +/- 0.2 Ohm) was achieved at significantly lower concentration of collagen (0.08 +/- 0.006 micrograms/ml, p less than 0.0001). Preincubation with heparin in doses of 0.5, 2.5 and 5 IU/ml increased impedance by 1.5 +/- 0.5, 3.9 +/- 1.6 and 1.3 +/- 0.5 times, respectively (p less than 0.0001, ANOVA). The dose-related increments were smaller in hirudinized blood as compared to citrated blood (p less than 0.04). With filtragometry, heparin dose-dependently shortened the aggregation time (p less than 0.0007). Compared to hirudin alone as anticoagulant, heparin in an equipotent dose in these experiments shortened aggregation time (p less than 0.05). In conclusion, heparin enhanced platelet aggregation both in calcium-chelated blood and in blood anticoagulated with hirudin. Heparin also dose-dependently increased platelet aggregation in filtragometry. Thus the heparin potentiating effect on platelet aggregation seems to be independent of extracellular ionized calcium and be operative at physiological calcium concentrations.     

Effect of postprandial lipaemia on platelet function in man evaluated in whole blood

The effect of a fatty meal (100 g of fat) on platelet function is evaluated. Two hours after the fat intake (dairy cream) there is a significant reduction in the initial stages of platelet activation by collagen (1 and 0.5 micrograms/ml) as measured by a new analytical method, the BASIC wave, and by the decrease in the beta-Thromboglobulin released by stimulated platelets. This effect is greater in platelet rich plasma (PRP) than in whole blood. Red blood cells (RBC) have a potentiating effect on platelet activation by collagen both before and after the fat intake which is indicated by an increase in the BASIC wave intensity. No significant differences were found, however, in platelet aggregation in PRP or whole blood evaluated by impedance aggregometry. These results suggest that the increase of chylomicrons after fat intake has an inhibitory effect on platelet activation but does not modify platelet aggregation. In addition, it seems that lipaemia does not modify RBC interactions with platelets in collagen stimulated samples.     

Platelet aggregation during abdominal surgery in an experimental pig model: the effects of presurgical antibiotic protocols and volume replacement with hydroxyethyl starch

The effect of presurgical antibiotic protocols in combination with hemodilution on platelet aggregation was studied. Thirty pigs were randomly assigned to three groups. Group 1 received amoxicillin/clavulanic acid, group 2 metronidazole+cefuroxime, and group 3, as a control, sodium chloride. They underwent laparotomy, massive blood loss, and volume replacement with hydroxyethyl starch 200, followed by an anaphylactoid reaction. Platelet aggregation was measured by the turbidometric method. Neither antibiotic protocols had any effect on platelet aggregation as compared with the control group. In all three groups, aggregation to ADP and collagen was significantly reduced after volume replacement with hydroxyethyl starch. In contrast, the sensitivity to the aggregating effects of collagen was increased as assessed by a higher frequency of responses to low concentrations of collagen and a shortened latency of the aggregation response after collagen addition. Further in vitro studies revealed that dilution of plasma with hydroxyethyl starch specifically induced the changes seen after in vivo volume replacement. The results suggest that the plasma substitute hydroxyethyl starch 200 increases the sensitivity to low doses of collagen, an effect never described before and considered of clinical relevance.     

Studies on the stimulation of human blood platelets by semi-synthetic platelet-activating factor

"Saturated" and "unsaturated" platelet-activating factor (PAF) obtained from ratfish liver oil were proved to exert potent stimulation on human blood platelets. Using 0.025 to 1.0 mumol/1 PAF a dose-dependent platelet aggregation in platelet-rich plasma was observed. During PAF-induced irreversible aggregation a 9 to 40% release of platelet bound serotonin occurred. The specific effect of PAF, however, seems to be limited to induce reversible aggregation since second wave of aggregation and serotonin release were suppressed by a combination of acetylsalicylic acid and an ADP scavenging system. Incubation of PAF for 30 min in plasma resulted in a 90% loss of its platelet aggregating power. Subthreshold concentrations of PAF enhanced the platelet aggregation triggered by suboptimal concentrations of ADP, epinephrine, or collagen. Vice versa non-aggregating concentrations of ADP, epinephrine, collagen, Ca-ionophore A 23,187, or arachidonic acid amplified PAF-induced platelet aggregation. The synergistic effect of PAF and other stimuli of blood platelet activation can be partly interpreted as a stimulating effect of PAF on the metabolization of arachidonic acid.     

The effects of the second generation antipsychotics and a typical neuroleptic on collagen-induced platelet aggregation in vitro

Objective. Antipsychotics are widely used in psychiatry, and consequently a lot of their side effects have been reported. One of them is cardiovascular disease leading to increased risk of stroke, thrombosis, pulmonary, embolism, in which hyperactivation of blood platelets is involved. The purpose of the present study was to examine the effects of the second generation antipsychotics (SGAs) such as clozapine, risperidone, and olanzapine, and a typical neuroleptic – haloperidol – on the one step of platelet activation–platelet aggregation induced by collagen in vitro. Blood was collected into buffered sodium citrate (3.8%) and centrifuged to get platelet-rich plasma (PRP). In PRP (2×10⁸ platelets/ml) obtained from healthy volunteers that was incubated with antipsychotics (clozapine, risperidone, olanzapine, haloperidol; 30 min) aggregation of blood platelets was measured using a Chrono-Log Lumi-aggregometer. Aggregation of platelets was measured after stimulation of platelets with 1 µl of collagen (2 µg/ml). Results. Clozapine, like haloperidol reduced platelet aggregation induced by collagen (inhibition of platelet aggregation reached about 20%) (P=1×10^?5 and P=0.003, respectively). Risperidone had also a weak antiaggregatory effect (P=0.05). Among tested antipsychotics only olanzapine had no effect on collagen-stimulated platelet aggregation (P>0.05). Conclusion. The obtained results indicate that the difference in action of tested drugs on platelet aggregation may dependent on the various chemical structures of these drugs. Clozapine, risperidone and haloperidol are structurally diverse, and they all significantly reduce platelet aggregability induced by collagen. On the other hand, a close structural analog of clozapine – olanzapine – did not inhibit platelet aggregation. However, mechanism of antipsychotics action on blood platelets is not clear. Moreover, it seems that clozapine, risperidone and haloperidol treatment due to antiaggregatory action may have even some antithrombotic effects.     

Effects of verapamil, diltiazem and nifedipine on some parameters of hemostasis in the rat

Together with a mechanism such as activation of the coagulation system, vascular smooth muscle contraction and the activation of blood platelets are important processes in the hemostasis. Various calcium channel blockers besides those found to be vasodilators (1) have been shown to influence the functioning of blood platelets (2). They inhibit the aggregation induced by ADP, adrenaline, arachidonic acid or collagen (3). From these data it has been suggested that significant differences exist in the ability of calcium channel blockers to inhibit aggregatory responses (4). In order to elucidate further the possible role of calcium channel blockers in hemostasis we analyzed the effect of verapamil, diltiazem and nifedipine on the bleeding time, the amount of initial blood loss, platelet aggregation and some coagulation parameters in rats.     

Glycosaminoglycan inhibition of collagen induced platelet aggregation

Chondroitin 6-sulfate (Ch6-S), a glycosaminoglycan (GAG), has been shown to inhibit collagen fibrillogenesis and collagen induced platelet aggregation. Complexation of soluble microfibrillar collagen with Ch6-S at low pH followed by saline dialysis results in the stabilization of 300Å wide microfibrillar aggregates with no banding in the electron microscope. These structures, which may be intermediates in collagen fibrillogenesis, do not aggregate platelets or cause serotonin release, whereas fibrils formed from uncomplexed microfibrillar collagen induce platelet aggregation and serotonin release. Neutral complexation of microfibrillar collagen with Ch6-S does not inhibit fibril formation or platelet aggregation. Soluble Ch6-S does not interfere with platelet aggregation in response to soluble microfibrillar collagen, indicating that Ch6-S does not block sites on the platelet membrane or on collagen fibrils which may be specifically involved in the collagen-platelet interaction. These results imply that GAG complexes with collagen may be suitable as blood compatible materials.     

Platelet interaction with vessel wall and collagen in pigs with homozygous von Willebrand's disease associated with abnormal collagen aggregation

A subgroup of pigs with von Willebrand's disease from the Mayo Clinic stock shows abnormal platelet aggregation in response to collagen [vWD-Homo(-)], in contrast to the normal aggregation responses observed in the main colony of pigs with homozygous vWD [vWD-Homo(+)]. This subgroup has been characterized at Mayo as a storage pool deficiency due to the reduced levels of ADP and Serotonin in the platelet dense granules. In the present studies, an ex-vivo perfusion chamber was utilized to investigate the deposition of 111In-labeled platelets on aortic subendothelium and collagen type I exposed to blood from vWD-Homo(-), vWD-Homo(+) and normal animals. Both non-anticoagulated and heparinized blood were exposed for wall shear rates ranging from 212 sec-1 to 3380 sec-1 and exposure times as long as 30 min. An enhanced decrease in platelet deposition in the vWD-Homo(-) animals was observed compared to vWD-Homo(+) animals. The decrease was observed primarily at the higher shear rates and was more pronounced in the absence of heparin and on the collagenous substrate. Thus, the abnormality in collagen-induced aggregation, which has been characterized as a storage-pool type defect, results in a decreased platelet deposition compared with that produced by severe vWD alone.