We will study the sickle red cell membrane to evaluate its contribution to the pathophysiology of this complex disorder. For studies of RBC/endothelial interactions, our long term goal is to define the nature and pathophysiologic significance of sickle RBC adherence to endothelial cells. Four specific areas will be addressed. First, we will define the adherence-promoting role of certain factors in the RBC's environment. The effect of von Willebrand factor, thrombospondin, vitronectin, fibronectin and fibrinogen will be examined. Also we will examine the hypothesis that vasopressin effects on endothelium explain the precipitation of vasocclusive crisis by clinical dehydration. Second, we will determine whether the several identified mechanisms leading to abnormal RBC/endothelial interactions are related to each other or are truly unique. In particular, we will examine the possible interrelationships between RBC surface charge distribution, cellular hydration, oxidative phenomena, and adhesivity. Third, we will examine the endothelial cell's participation in sickle RBC/endothelial interactions by examining a number of endothelial surface components including glycosaminoglycans and the endothelial cell's GpIIbIIIa equivalent. In addition, the effect of physiologically relevant endothelial injury will be examined (e.g., that induced by activated PMN, or that induces by NO2). Fourth, we will use a flow model, perfusion of the isolated rat lung, to dissect out the relative contributions of RBC polymer content versus endothelial adhesivity in abnormal sickle rheology. For studies of RBC autoxidation, our long term goal is to define the nature and extent of autoxidative processes in sickle RBC, the contribution of these to development of sickle RBC membrane defects, and the relationship between these different membrane defects. First, we will examine the instability of HbS as a potential factor in sickle disease pathobiology. We will compare hemoglobins A, F, S, and C for oxidative instability in aqueous solution and upon contact with phospholipid (simple model of possible membrane interaction). Second, we will examine the nature and significance of free iron associated with sickle membrane phospholipids, with particular emphasis upon its chelatability as a potential therapeutic option. Third, we will attempt to utilize data regarding HbS autoxidation and membrane iron to assemble a hypothesis explaining sickle membrane defects and clinical severity. To tentatively identify causal relationships, we will assess a variety of cellular parameters in RBC from different density subpopulations and in RBC from different sickling disorders. Related studies will examine the susceptibility of membrane thiols to oxidation and the specific role of this in spectrin dysfunction. Fourth, we will examine the role of cellular antioxidants, in particular emphasizing the potential harmful effect of these in Fenton chemistry, deposition of iron on sickle membranes, and stimulation of lipid peroxidation. Finally, using the exchange transfused rat, we will examine the hypothesis that sickle disease vascular pathobiology has critical features in common with reperfusion injury physiology.