(Adapted from Applicant's Abstract) The pathophysiology of sickle cell vasoocclusion is complex and multifactorial, and HbS polymerization and red cell sickling are essential but not sufficient. The premise is that in sickle cell anemia reversible sickling in vivo results in pleiotropic effects such as red cell heterogeneity, endothelial damage, red cell- endothelial interactions, and altered microvascular responses, contributing to vasoocclusive crisis and multiple organ damage. While considerable data has been obtained by us and others using in vitro approaches and microcirculatory preparations, studies in intact animals have been lacking and are indispensable to assess fully these phenomena. The availability of the sickle transgenic mouse (alphaH BetaS BetaS- Antilles[BetaMDD], MDD=mouse homozygous Beta major deletion) makes it possible to test the central hypothesis that the observed pathological manifestations in this model are the result of reversible in vivo sickling and persistent microvascular flow abnormalities. The investigators have recently established (JCI, 96;2845-2853, 1995), using a cremaser preparation in intact sickle transgenic mice, that adhesion intravascular sickling and vascular dynamic abnormalities do occur in vivo. This proposal builds on these novel findings. In the present proposal the objectives are: 1) to test strategies that help reduce red cell water content and thereby improve cell deformability and microcirculatory flow. In particular, the investigators will evaluate the effect of specific inhibitors of Gardos channel as well as test magnesium therapy; 2) to test the hypothesis that intravascular sickling, endothelial damage and red cell-endothelium interactions in transgenic mice would alter vascular reactivity by affecting the status of nitric oxide (NO) and endothelin. The preliminary observations indicate a blunted arteriolar diameter response to acetylcholine in the transgenic mouse despite an increase NO synthase activity. The investigators will investigate endothelium- dependent vascular responses and their relationship to the pathophysiology in these mice; 3) to test the hypothesis that the observed red cell endothelium interaction in the transgenic mouse is the consequence of increased expression of adhesion molecules expressed by damaged/activated endothelium and/or changes in the membrane surface of red cells undergoing sickle-encycl cycles. The investigators will focus on the role of von Willebrand factor (vWF), cytokines and associated expression of adhesion molecules. The proposed use of transgenic mice is expected to provide not only a greater insight into complex vasoocclusive mechanisms but also an efficient and rapid validation of therapeutic approaches.