Urate is the major excretory end product of nitrogen metabolism in birds, most reptiles, and some amphibians and undergoes net secretion by the renal tubules. Secretion has been studied in isolated renal proximal tubules from reptiles and birds. Net secretion is influenced by passive backflux between the tubule cells that varies with perfusion rate. Transepithelial transport from bath to lumen involves uptake into the cells against an electrochemical gradient at the basolateral membrane and movement from the cells to the lumen down an electrochemical gradient. However, the apparent permeability of the basolateral membrane to urate is much higher than that of the luminal membrane. Transport into the cells at the basolateral membrane is dependent, in part, on the presence of filtrate (or an appropriate substitute) in the lumen. Because reptilian and avian nephrons filter intermittently, passive back-diffusion between the cells, high permeability of the basolateral membrane, and dependence of basolateral transport on luminal perfusion may prevent accumulation of urate in the cells or lumens of nonfiltering nephrons. In reptiles, basolateral transport is Na-independent but K-dependent and may involve countertransport for some unknown anion. In birds, this transport step, which is Na- and K-dependent, may occur by two separate systems, one involving countertransport for an unknown anion and the other involving the general tertiary transport process for other organic anions such as PAH. It has not yet been possible to demonstrate that transport from the cells to the lumen down an electrochemical gradient is carrier-mediated in either reptiles or birds and some other process may be involved.
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