Renal Oxalate Secretion

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DESCRIPTION (provided by applicant): Hyperoxaluria is one of the principal risk factors in the etiology of renal stone disease and in hyperoxaluric diseases like Primary Hyperoxaluria the renal excretory pathways are continuously challenged to excrete the oxalate load. While it has long been agreed that the kidney actively secretes oxalate and a number of membrane transport proteins have been identified that may transport the oxalate anion, it is remarkable that no vectorial transport mechanism for secretion has emerged from these studies. The research aims described in this proposal are directed at extending our understanding of the mechanisms and regulation of transepithelial oxalate secretion by the renal tubule in hyperoxaluric rats where oxalate excretion is elevated. Hyperoxaluria generated by oxalate-loaded osmotic pumps implanted in rats will be used to assess (using differential gene expression techniques) the nature and degree of up-regulation of transport systems that may participate in oxalate transport along the nephron in the hyperoxaluric state. Our previous studies concerning intestinal oxalate secretion, together with new findings regarding chloride secretion in the nephron, have led us to test the hypothesis that mechanistically similar, cAMP-dependent pathways can mediate oxalate secretion along the renal tubule. This hypothesis will be evaluated by measuring camp stimulated, radioisotope-labeled oxalate fluxes across confluent monolayers of cell lines derived from the proximal tubule and the inner medullary collecting duct. Additionally, a specific aspect of the cAMPdependent secretory mechanism (i.e., presence of an apical membrane conductive pathway for oxalate) will be assessed using membrane vesicles derived from the renal cortex of the hyperoxaluric animals. Finally, the hypothesis that oxalate permeates chloride channels will be explicitly tested by using the heterologous expression of CFTR gene product in Xenopus laevis oocytes. Oxalate flux through CFTR can be examined in the nominal absence of other oxalate transport pathways using efflux of labeled oxalate together with electrophysiological measurements in control and CFTR expressing oocytes. The hyperoxaluric/osmotic pump animal model, the novel transport perspectives, and the new experimental approaches that will be developed and explored in this proposal will provide specific answers regarding renal oxalate secretion and they will offer a foundation and a direction for future studies of renal oxalate.