Lack of effect of supplementation with EPA or DHA on platelet-monocyte aggregates and vascular function in healthy men

Platelet surface P-selectin is considered the "gold standard" marker of platelet activation. Degranulated, P-selectin-positive platelets, however, aggregate with leukocytes in vitro and rapidly lose surface P-selectin in vivo. Flow cytometric tracking of autologous, biotinylated platelets in baboons enabled us to directly demonstrate for the first time in vivo that (1) infused degranulated platelets very rapidly form circulating aggregates with monocytes and neutrophils, and (2) 30 minutes after infusion of the degranulated platelets, the percentage of circulating monocytes aggregated with infused platelets persist at high levels, whereas the percentage of circulating neutrophils aggregated with infused platelets and the platelet surface P-selectin of nonaggregated infused platelets return to baseline. We therefore performed 2 clinical studies in patients with acute coronary syndromes. First, after percutaneous coronary intervention (n=10), there was an increased number of circulating monocyte-platelet (and to a lesser extent, neutrophil-platelet) aggregates but not P-selectin-positive platelets. Second, of 93 patients presenting to an Emergency Department with chest pain, patients with acute myocardial infarction (AMI) (n=9) had more circulating monocyte-platelet aggregates (34.2+/-10.3% [mean+/-SEM]) than patients with no AMI (n=84, 19.3+/-1.4%, P<0.05) and normal control subjects (n=10, 11.5+/-0.8%, P<0.001). Circulating P-selectin-positive platelets, however, were not increased in chest pain patients with or without AMI. As demonstrated by 3 independent means (in vivo tracking of activated platelets in baboons, human coronary intervention, and human AMI), circulating monocyte-platelet aggregates are a more sensitive marker of in vivo platelet activation than platelet surface P-selectin.

Recent evidence supports differential effects of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), the 2 major omega3 fatty acids of marine origin, on blood pressure in humans and vascular reactivity in adult spontaneously hypertensive rats. We investigated possible differences in the effects of purified EPA or DHA on forearm vascular reactivity in overweight hyperlipidemic men that might contribute to the blood pressure-lowering effects of fish oils. With a double-blind, placebo-controlled trial of parallel design, 59 overweight, mildly hyperlipidemic men were randomized to receive 4 g/d purified EPA, DHA, or olive oil (placebo) capsules while continuing their usual diets for 6 weeks. Forearm blood flow (FBF) was measured with venous occlusion, strain-gauge plethysmography during the sequential intra-arterial administration of acetylcholine (7.5, 15, and 30 microg/min), sodium nitroprusside (1.5, 3, and 10 microg/min), norepinephrine (10, 20, and 40 ng/min), a single-dose infusion of N:(G)-monomethyl-L-arginine (L-NMMA) (1 mg/min), and coinfusion of acetylcholine (7.5, 15, and 30 microg/min) and L-NMMA. Forty of the 56 subjects who completed the study underwent FBF measurements. Plasma phospholipid EPA levels increased significantly (P:<0.0001) after supplementation with EPA, and DHA composition increased with DHA supplementation (P:<0.0001). Relative to placebo, DHA, but not EPA, supplementation significantly improved FBF in response to acetylcholine infusion (P:=0.040) and coinfusion of acetylcholine with L-NMMA (P:=0.040). Infusion of L-NMMA alone showed no group differences. DHA significantly enhanced dilatory responses to sodium nitroprusside (P:<0.0001) and attenuated constrictor responses to norepinephrine (P:=0.017). Relative to placebo, DHA, but not EPA, enhances vasodilator mechanisms and attenuates constrictor responses in the forearm microcirculation. Improvements in endothelium-independent mechanisms appear to be predominant and may contribute to the selective blood pressure-lowering effect observed with DHA compared with EPA in humans.

Compelling evidence exists for the cardioprotective benefits resulting from consumption of fatty acids from fish oils, EPA (20:5n-3) and DHA (22:6n-3). EPA and DHA alter membrane fluidity, interact with transcription factors such as PPAR and sterol regulatory element binding protein, and are substrates for enzymes including cyclooxygenase, lipoxygenase and cytochrome P450. As a result, fish oils may improve cardiovascular health by altering lipid metabolism, inducing haemodynamic changes, decreasing arrhythmias, modulating platelet function, improving endothelial function and inhibiting inflammatory pathways. The independent effects of EPA and DHA are poorly understood. While both EPA and DHA decrease TAG levels, only DHA appears to increase HDL and LDL particle size. Evidence to date suggests that DHA is more efficient in decreasing blood pressure, heart rate and platelet aggregation compared to EPA. Fish oil consumption appears to improve arterial compliance and endothelial function; it is not yet clear as to whether differences exist between EPA and DHA in their vascular effects. In contrast, the beneficial effect of fish oils on inflammation and insulin sensitivity observed in vitro and in animal studies has not been confirmed in human subjects. Further investigation to clarify the relative effects of consuming EPA and DHA at a range of doses would enable elaboration of current understanding regarding cardioprotective effects of consuming oily fish and algal sources of long chain n-3 PUFA, and provide clearer evidence for the clinical therapeutic potential of consuming either EPA or DHA-rich oils.