Regulation of renal proximal and distal tubule transport: sodium, chloride and organic anions.
Renal tubular transport and its regulation are reviewed for Na(+) (and Cl(-)), and for fluid and organic anions (including urate). Filtered Na(+) (and Cl(-)) is reabsorbed along the tubules but only in mammals and birds does most reabsorption occur in the proximal tubules. Reabsorption involves active transport of Na(+) and passive reabsorption of Cl(-). The active Na(+) step always involves Na-K-ATPase at the basolateral membrane, but the entry step at luminal membrane varies among tubule segments and among vertebrate classes (except for Na(+)-2Cl(-)-K(+) cotransporter in diluting segment). Regulation can involve intrinsic, neural and endocrine factors. Proximal tubule fluid reabsorption is dependent on Na(+) reabsorption in all vertebrates studied, except ophidian reptiles. Fluid secretion occurs in glomerular and aglomerular fishes, reptiles and even mammals, but its significance is not always clear. A non-specific transport system for net secretion of organic anions (OAs) exists in the proximal renal tubules of almost all vertebrates. Net transepithelial secretion involves: (1) transport into the cells at the basolateral side against an electrochemical gradient by a tertiary active transport process, in which the final step involves OA/alpha-ketoglutarate exchange and (2) movement out of the cells across the luminal membrane down an electrochemical gradient by unknown carrier-mediated process(es). Regulation may involve protein kinase C and mitogen-activated protein kinase. Urate is net secreted in the proximal tubules of birds and reptiles. This process is urate-specific in reptiles but in birds, it may involve both a urate-specific system and the general OA system.
Dantzler
William H
WH
Department of Physiology, College of Medicine, University of Arizona, Tucson, AR 85724-5051, USA. dantzler@u.arizona.edu
D000818:Animals; D000838:Anions / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism*; D017136:Ion Transport; D007686:Kidney Tubules, Distal / Q000502:physiology*; D007687:Kidney Tubules, Proximal / Q000502:physiology*; D027361:Organic Anion Transporters / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*
D000838:Anions; D002712:Chlorides; D027361:Organic Anion Transporters; D012964:Sodium