The medulla of the kidney is a hypoxic environment that is exclusively perfused by descending vasa recta (DVR). Salt transporting nephrons and collecting ducts in the medulla are vulnerable to injury. In addition to vasodilatory paracrine signaling, the DVR wall is a syncytium wherein myo- endothelial gap junctions provide a conduit for endothelium dependent hyperpolarization. Moreover, DVR endothelial syncytia can transmit signals along the DVR axis to dilate feeding vessels and enhance nutrient blood flow. We show, for the first time, that renal ischemia preconditions DVR endothelia and pericytes. They become hyperpolarized, conduct hyperpolarizing currents more effectively and increase generation of NO. DVR vasoconstriction is blunted. We will investigate these changes and their mechanisms with the following aims. Aim 1 will test the roles of NOS isoforms and other endothelium dependent relaxing factors in the blunted vasoconstriction observed after ischemic preconditioning. We will test whether the endothelium is needed to sustain pericyte hyperpolarization. We will test whether mural conduction of pericyte depolarization and cytoplasmic calcium responses is inhibited. Aim 2 will test for modification of gap junction coupling and ion channel activity after ischemic preconditioning. We will test mural conductance by electrophysiology and intercellular tracer diffusion. We will examine expression of connexin proteins and determine whether selective blockade with Gap mimetic peptides alters mural conduction. We will measure monovalent ion channel activity that determines membrane potential and test whether reduced contraction is accompanied by inhibition of voltage gated Ca2+ and Na+ channel activity. Aim 3 will focus on the pathways involved in ischemic and toxic DVR preconditioning. We will test whether direct ischemia is mimicked by remote hindlimb ischemia or erythropoietin treatment. We will test whether recovery from toxic injury from gentamicin modifies DVR. We will test whether increasing the activity of hypoxia inducible factors prevents endothelial dysfunction and mimics the DVR adaptations conferred by ischemic preconditioning.