Whole Blood Platelet Aggregation Based on Cell Counting Procedures

Activation of leukocytes in whole blood samples by N-formyl-methionyl-leucyl-phenylalanine (FMLP) enhances platelet aggregability but not platelet P-selectin exposure and adhesion to leukocytes

Adhesion of platelets to neutrophils and monocytes is believed to play an important role in intercellular communication. Evidence has been provided that such heterotypic cell-cell contacts via adhesion molecules may be directly involved in intercellular signal transduction as well as facilitate the action of soluble signal transmitters, e.g. cathepsin G, PAF or nitric oxide. With respect to platelet activation, stimulatory and inhibitory effects of leukocytes have been reported, and the results obtained seem to be influenced by the experimental conditions. We investigated the effect of leukocyte stimulation on platelet behaviour in samples of human citrated whole blood. Adding the chemotactic peptide FM LP, which stimulates neutrophils and monocytes but not lymphocytes and platelets, to stirred whole blood samples resulted in a significant enhancement ( P < 0.01) of spontaneous as well as ADP-induced platelet aggregation (25 vs 33% and 66 vs 69% , respectively). In contrast stirring-induced as well as ADP-induced increase of P-selectin exposure (33 and 107% , respectively) was not affected by FMLP. In unstirred whole blood samples, about 10 to 20% of neutrophils and monocytes had bound platelets to their surfaces, and the number of these heterotypic conjugates was enhanced about twofold during spontaneous platelet aggregation. Addition of FMLP significantly reduced the stirring-induced formation of platelet-neutrophil conjugates but not of platelet-monocyte conjugates. These results indicate that neutrophil and/or monocyte activation in whole blood may enhance platelet aggregation, but not secretion (CD62P exposure) and formation of heterotypic platelet-leukocyte conjugates.

Platelet responses to several agonists and combinations of agonists in whole blood: a placebo controlled comparison of the effects of a once daily dose of plain aspirin 300 mg, plain aspirin 75 mg and enteric coated aspirin 300 mg, in man

Platelet responses to several agonists and combinations of agonists have been measured in whole blood from healthy volunteers. We have determined the effects of once daily treatment for five days with plain aspirin 300 mg, plain aspirin 75 mg, enteric coated aspirin 300 mg or placebo. Measurements were made of platelet aggregation (using a platelet counting technique) and the release reaction (14C-5HT release from pre-labelled platelets). The extents of these responses before aspirin administration depended on the agonist used. ADP, adrenaline and PAF failed to induce any 14C-5HT release in most subjects, but combinations of these agonists acted synergistically to produce extensive 14C-5HT release. All three aspirin preparations reduced the extent of the platelet responses to most agonists: platelet aggregation induced by collagen, ristocetin and arachidonate and 14C-5HT release induced by collagen, streptokinase, and various combinations of ADP, adrenaline and PAF. None of the preparations had any effect on the aggregation that occurred in the absence of an agonist (spontaneous aggregation), but they all reduced streptokinase-induced aggregation to control (spontaneous) levels, and abolished the 14C-5HT release induced by arachidonate and by ristocetin. All three aspirin preparations were equally effective after two daily doses. No further inhibition of platelet responses was obtained after five daily doses. Plain aspirin 300 mg achieved its maximal effect after only a single dose, but enteric coated aspirin 300 mg (and sometimes plain aspirin 75 mg) produced sub-maximal inhibition after only a single dose. Parallel investigations on the effects of these aspirin regimes on gastric mucosal prostaglandin E2 synthesis and gastroduodenal mucosal injury were performed. These results will be reported separately.

'Spontaneous' platelet aggregation in whole blood in diabetic patients with and without microvascular disease

Consistent abnormalities of agonist-induced platelet aggregation, in either whole blood or platelet rich plasma, have not been demonstrated in diabetic patients without microvascular disease. In the present study platelet aggregation in the absence of exogenous agonists ('spontaneous' aggregation) was compared between 22 non-diabetic subjects and 23 Type 1 diabetic patients with (n = 12) and without (n = 11) microvascular disease. 'Spontaneous' aggregation was determined by measuring the percentage fall in single platelet number in aliquots of whole blood shaken for 60 min. Diabetic patients without microvascular disease had fewer single platelets remaining (greater aggregation) than non-diabetic subjects at all time-points (69.7 +/- 6.6 vs 82.3 +/- 7.3% at 60 min p less than 0.001), but more platelets remaining than in diabetic patients with microvascular disease at all time-points (69.7 +/- 6.6 vs 61.0 +/- 7.8% at 60 min p less than 0.02). No significant correlations were observed between platelet aggregation and plasma glucose, blood cell counts, or glycated haemoglobin levels. The study suggests that platelet abnormalities antedate the appearance of microvascular disease in diabetic patients.