Background-The adenosine diphosphate (ADP) receptor P2Y 12 plays a pivotal role in platelet aggregation, as demonstrated by the benefit conferred by its blockade in patients with cardiovascular disease. Some studies have shown interindividual differences in ADP-induced platelet aggregation responses ex vivo, but the mechanisms underlying this variability are unknown. Methods and Results-We examined ADP-induced platelet aggregation responses in 98 healthy volunteers, and we identified 2 phenotypic groups of subjects with high and low responsiveness to 2 mol/L ADP. This prompted us to screen the recently identified G i -coupled ADP receptor gene P2Y 12 for sequence variations. Among the 5 frequent polymorphisms thus identified, 4 were in total linkage disequilibrium, determining haplotypes H1 and H2, with respective allelic frequencies of 0.86 and 0.14. The number of H2 alleles was associated with the maximal aggregation response to ADP in the overall study population (Pϭ0.007). Downregulation of the platelet cAMP concentration by ADP was more marked in 10 selected H2 carriers than in 10 noncarriers. Conclusions-In

Rationale Sphingosine-1-phosphate (S1P) signaling is essential for vascular development and postnatal vascular homeostasis. The level of redundancy in S1P sources sustaining these processes remains unclear. Objective To address S1P source redundancy in the regulation of vascular development, integrity and tone. Methods and Results S1P production was selectively impaired in mouse platelets, erythrocytes, endothelium, and/or smooth muscle cells by targeted deletion of genes encoding sphingosine kinases (Sphks) 1&2. S1P deficiency blunted aggregation and spreading of washed platelets and profoundly impaired their capacity to promote endothelial barrier function ex vivo. However, and in contrast to recent reports, neither platelets nor any other source of S1P was essential for vascular development, vascular integrity, or hemostasis/thrombosis. Yet rapid and profound depletion of plasma S1P during systemic anaphylaxis rendered both platelet- and erythrocyte-derived S1P essential for survival, with a contribution from blood endothelium observed only in the absence of circulating sources. Recovery was sensitive to aspirin in mice with but not without platelet S1P, suggesting that platelet activation and stimulus-response coupling is needed. S1P2 mediated most of the survival benefit of S1P, while endothelial S1P1 was dispensable for survival despite its importance for maintaining vascular integrity. Accordingly, S1P deficiency aggravated vasoplegia, arguing a vital role for S1P in maintaining vascular resistance during recovery from circulatory shock. Conclusions While source redundancy secures essential roles of S1P in vascular development and integrity, profound depletion of plasma S1P during anaphylactic shock renders both erythrocyte and platelet S1P pools necessary for recovery and high basal plasma S1P levels protective.