Hyperoxaluria is considered a major risk factor in the genesis of calcium oxalate stone disease which occurs in about 12% of the U. S. population costing an estimated $2 billion annually. Oxalate homeostasis is governed by the amount of dietary oxalate absorbed in the intestine and that produced by the liver and is offset by both renal and enteric excretion. Thus, the epithelial membranes of the intestinal tract and the kidneys are the principal interfaces for the exchange of oxalate between the organism and its environment and the transport systems that are poised and coordinated to move oxalate across epithelia have significant roles in oxalate homeostasis. Anion exchange proteins have long been considered to be involved in oxalate movements in both intestine and kidney with proteins encoded by members of the SLC26 and possibly the SLC4 gene families attracting the most attention. One gene (SLC26A6), termed PAT1 is abundantly expressed in the apical membrane of small intestine and proximal tubule and is presumed to be involved in exchanging oxalate between the lumen and the cytosol, yet the relative importance of PAT1 has been difficult to resolve given the presence of other exchangers. Our initial studies, using PAT1 knockout (KO) mice show that these animals are significantly hyperoxaluric and support a dramatic increase in oxalate absorption by the distal ileum compared to their wild type (WT) littermates. Thus, we have already demonstrated that a one-gene deletion results in dramatic changes in oxalate handling. Based upon our preliminary data, we have proposed that PAT1 mediates apical oxalate efflux and we hypothesize that DRA (SLC26A3) is the major apical transporter responsible for oxalate uptake. In Aim 1, we will directly address the physiological significance of the PAT1 and DRA transporters to oxalate homeostasis by a comparison of renal and intestinal oxalate handling in WT and 3 KO models, namely PAT1, DRA, and combined PAT1/DRA null mice. In Aim 2, possible compensatory adaptations in the expression patterns of other members of the SLC26 and SLC4 families will be determined by real-time PCR and quantitative immunohistochemistry in all KO mice. In Aim 3, we will use knockdown and functional expression approaches in a renal and intestinal cell culture model to evaluate the relative importance of individual anion exchangers. Understanding oxalate handling in health and disease is essential for the development of potential pharmacological therapies.