Diabetes mellitus (DM) and hyperglycaemia are associated with platelet activation. The present study was designed to investigate how high glucose levels influence platelet function. Fasting human blood was incubated with different concentrations of D-glucose (5, 15 and 30 mmol/l) and other sugars without or with in vitro stimuli. Platelet activation was monitored by whole blood flow cytometry. High glucose levels enhanced adenosine diphosphate (ADP)- and thrombin receptor-activating peptide (TRAP)-induced platelet P-selectin expression, and TRAP-induced platelet fibrinogen binding. Similar effects were seen with 30 mmol/l L-glucose, sucrose and galactose. Hyperglycaemia also increased TRAP-induced platelet-leucocyte aggregation. Protein kinase C (PKC) blockade did not counteract the enhancement of platelet P-selectin expression, but abolished the enhancement of TRAP-induced platelet fibrinogen binding by hyperglycaemia. Superoxide anion scavenging by superoxide dismutase (SOD) attenuated the hyperglycaemic enhancement of platelet P-selectin expression, but did not counteract the enhancement of TRAP-induced platelet fibrinogen binding. Hyperglycaemia did not alter platelet intracellular calcium responses to agonist stimulation. Blockade of cyclo-oxygenase (COX), phosphotidylinositol-3 (PI3) kinase, or nitric oxide synthase, or the addition of insulin did not influence the effect of hyperglycaemia. In conclusion, high glucose levels enhanced platelet reactivity to agonist stimulation through elevated osmolality. This occurred via superoxide anion production, which enhanced platelet P-selectin expression (secretion), and PKC signalling, which enhanced TRAP-induced fibrinogen binding (aggregablity).

Summary.  Background: Platelets can bind to, and thereby influence, lymphocyte function. Objective: The propensities of different lymphocyte subpopulations to form platelet–lymphocyte conjugates/aggregates (P‐Lym) was investigated using four‐color whole blood flow cytometry. Results: P‐Lym constituted approximately 3% of circulating lymphocytes. Platelet conjugation was most common among large (monocyte‐sized) lymphocytes. Platelet activation by ADP slightly increased platelet–T‐cell conjugation, mainly to T‐cytolytic (Tc) cells, but markedly elevated platelet–natural killer (NK)‐cell conjugation. T‐cell activation by phytohemagglutinin increased heterotypic conjugation among both T‐helper (TH) and Tc cells, whilst NK‐cell activation by interleukin‐2 affected platelet–NK‐cell aggregation little. Neither platelet activation nor lipopolysaccharides‐induced B‐cell activation enhanced platelet–B‐cell aggregation. Activation‐dependent heterotypic conjugation was mainly found among large cells, with increased percentages of conjugated cells and more platelets bound per lymphocyte. P‐Lym formation initiated by platelet activation was abolished by P‐selectin blockade, and tended to be reduced by inhibition of GPIIb/IIIa, CD11b, or CD40L. P‐Lym formation initiated by lymphocyte activation was partially inhibited by each of these blocking agents, but more markedly inhibited when the blocking agents were combined. Conclusions: Platelets selectively bind to larger and activated lymphocytes. T‐lymphocyte activation enhances platelet–T‐cell aggregation. Platelet activation enhances platelet–Tc aggregation slightly and platelet–NK‐cell aggregation markedly, while cellular activation affects platelet–B‐cell aggregation little. P‐selectin ligation is essential, but GPIIb/IIIa, CD40L, and CD11b also contribute to the heterotypic conjugation.

Lymphocytes are present in atherosclerotic lesion. We hypothesise that platelets may facilitate lymphocyte infiltration into the arterial wall. Reconstituted human blood or whole blood was perfused through a collagen-coated parallel-plate flow chamber at different shear rates. Adhered platelets markedly enhanced lymphocyte adhesion that increased lymphocyte deposition from 10 ± 3 cells/mm2 of platelet-depleted blood to 38 ± 11 cells/mm2 of platelet-containing blood at the arterial shear rate of 500 s-1. Platelet-dependent lymphocyte adhesion was inhibited by P-selectin, CD40L, and GPIIb/IIIa-blocking agents, suggesting the involvement of multiple adhesion molecules in this heterotypic interaction. Lymphocyte deposition was more marked among T cells, and seen in both small and large cells. B and natural killer cell adhesion was, however, mainly seen in small cells. Platelet-conjugation facilitated lymphocyte adhesion, as suggested by the selective deposition of platelet-conjugated lymphocytes. In a mouse model of arterial thrombosis, FeCl3-induced thrombus formation markedly enhanced lymphocyte adhesion and infiltration into platelet thrombi, which was abolished by GPIIb/IIIa inhibition. In conclusion, platelets support lymphocyte adhesion under arterial flow conditions, which is selective among T cells and involves multiple adhesion molecules. Our data imply that platelets may facilitate the recruitment of circulating lymphocytes at the arterial injured sites.