Fig. 4. Model of net transepithelial urate transport in avian (chicken) proximal tubules, indicating both independent transport and transport via the tertiary active transport process for most other organic anions (e.g. PAH). The independent process may involve countertransport for some unknown anion. The PAH transport system, which may be shared by urate, involves countertransport of PAH (or urate) for -ketoglutarate (-KG^2-) that is produced by intracellular metabolism or that has entered the cells via the sodium dicarboxylate cotransport system. Symbols denoting these processes have same meanings as in Fig. 3 (Dantzler, 1996).