Renal secretory systems for organic anions (OA) and cations (OC) govern the elimination of most small (less than or equal to 500mol.wt.) foreign chemicals. We have previously shown that OA transport is indirectly coupled to metabolic energy through Na/alpha-ketoglutarate (alphaKG) co- transport and OA/alphaKG exchange. We are currently examining a) plasma membrane and intracellular events associated with secretory transport (in collaboration with D.S. Miller), b) molecular biology of these systems, c) energetics and mechanisms of extrarenal OA and OC transport, and d) toxicology of OA and OC in kidney and extrarenal sites. We have developed a monolayer cultured preparations for both endothelial and epithelial (choroid plexus) components of the blood-brain barrier. These systems actively transport OA and OC in vitro. In addition, once within the cells OC are taken up into membrane vesicles which subsequently release their contents on the blood side of the epithelium, a process disrupted by depolimerization of microtubules with nocodazole. A similar process appears to participate in renal secretion of charged organic xenobiotics as well. Fractionation of rat renal tubules has shown that renal alphaKG is largely confined to the interior of the mitochondria, and that cytoplasmic alphaKG is the primary determinant of the efficacy of renal OA transport and elimination. Finally, expression cloning and PCR probes have been utilized to screen a CDNA library prepared from rat kidney mRNA for the genes coding for these two important excretory transport systems. We have identified a clone for a pH-driven, potential-sensitive OC carrier. It is now being sequenced. A positive signal is also present in our library for an OA transporter and screening is under way to isolate it as well.
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