RBCs regulate platelet function and hemostasis under shear conditions through biophysical and biochemical means.

ABSTRACT: Red blood cells (RBCs) have been hypothesized to support hemostasis by facilitating platelet margination and releasing platelet-activating factors such as adenosine 5'-diphosphate (ADP). Significant knowledge gaps remain regarding how RBCs influence platelet function, especially in (patho)physiologically relevant hemodynamic conditions. Here, we present results showing how RBCs affect platelet function and hemostasis in conditions of anemia, thrombocytopenia, and pancytopenia and how the biochemical and biophysical properties of RBCs regulate platelet function at the blood and vessel wall interface and in the fluid phase under flow conditions. We found that RBCs promoted platelet deposition to collagen under flow conditions in moderate (50 × 103/µL) but not severe (10 × 103/µL) thrombocytopenia in vitro. Reduction in hematocrit by 45% increased bleeding in mice with hemolytic anemia. In contrast, bleeding diathesis was observed in mice with a 90% but not with a 60% reduction in platelet counts. RBC transfusion improved hemostasis by enhancing fibrin clot formation at the site of vascular injury in mice with severe pancytopenia induced by total body irradiation. Altering membrane deformability changed the ability of RBCs to promote shear-induced platelet aggregation. RBC-derived ADP contributed to platelet activation and aggregation in vitro under pathologically high shear stresses, as observed in patients supported by left ventricular assist devices. These findings demonstrate that RBCs support platelet function and hemostasis through multiple mechanisms, both at the blood and vessel wall interface and in the fluidic phase of circulation.

State of the Art Review: Aortic Valve Repair in Infants and Children.

BACKGROUND: Congenital aortic and truncal valve disease is challenging in infants and children given the lack of available prostheses in very small sizes and the limited durability of homograft aortic valve replacement. METHODS: A comprehensive literature search was performed using the PubMed database. Studies were included either if the report included patients less than 1 year of age or if the technique was tailored to accommodate for somatic growth. RESULTS: Techniques for aortic and truncal valve repair addressing each aspect of the aortic valve complex - the aorta, aortic annulus, commissures, and cusps - were reviewed. The incidence of reoperation following aortic or truncal valve repair is significant at 10-years ranging from 30% to 70% depending on the underlying diagnosis and the repair technique utilized. A significant challenge in interpreting the published literature relates to the lack of anatomic data available in the publications limiting the ability to make direct comparisons between operative techniques and also limits the ability to draw conclusions regarding these techniques as applied to varied etiologies. CONCLUSIONS: A comprehensive understanding of the aortic valve complex is necessary to achieve adequate results in pediatric aortic valve repair given the high variability in these valves.

Dynamic and functional linkage between von Willebrand factor and ADAMTS-13 with aging: an Atherosclerosis Risk in Community study.

BACKGROUND: von Willebrand factor (VWF) is a multimeric glycoprotein critically involved in hemostasis, thrombosis, and inflammation. VWF function is regulated by its antigen levels, multimeric structures, and the state of enzymatic cleavage. Population studies in the past have focused almost exclusively on VWF antigen levels in cross-sectional study designs. OBJECTIVE: To identify subjects in the Atherosclerosis Risk in Community study who had persistently low and high VWF antigen over 10 years and to quantify longitudinal changes in the biological activities and cleavage of VWF in these subjects. METHODS: We measured VWF antigen, propeptide, adhesive activities, and cleavage by ADAMTS-13 quantified using a mass spectrometry method that detected the cleaved VWF peptide EQAPNLVY, as well as coagulation factor VIII activity. RESULTS: We determined the mean subject-specific increase in VWF to be 22.0 International Units (IU)/dL over 10 years, with 95% between -0.3 and 59.7 IU/dL. This aging-related increase was also detected in VWF propeptide levels, ristocetin cofactor activity, and VWF binding to collagen. We identified 4.1% and 25.0% of subjects as having persistently low (<50 IU/dL) and high (>200 IU/dL) VWF antigen, respectively. Subjects with persistently low VWF had enhanced ristocetin cofactor activity, whereas those with persistently high VWF had elevated levels of ADAMTS-13, resulting in a comparable rate of VWF cleavage between the 2 groups. CONCLUSIONS: These results provide new information about the effects of aging on VWF antigens and adhesive activity and identify a functional coordination between VWF and the rate of its cleavage by ADAMTS-13.