Sodium transporters and aquaporins: Future renal biomarkers?

Renal sodium and water reabsorption is mediated by renal sodium transporters and water channels or aquaporins which are localized in the apical and basolateral membranes of tubular epithelial cells. The main apical sodium transporters and water channels located along the nephron are: sodium-proton exchanger subtype 3 (NHE-3) which reabsorbs most of the sodium coming from the glomerular filtrate, sodium-phosphate type II cotransporter (NaPiII) and aquaporin-1, all of which are located in the proximal tubule; sodium-potassium-2 chloride cotransporter (NKCC2) which plays a key role in sodium reabsorption in the thick ascending limb; the sodium-chloride cotransporter (NCC) in the distal tubule; and the epithelial sodium channel (ENaC) and aquaporin-2 located in the collecting tubule. There are some experimental studies in which the role of these proteins has been associated with the pathophysiology of several sodium and water balance disorders. In humans, urine is the perfect source to obtain biomarkers useful for the diagnosis of kidney diseases and the assessment of disease progression without the use of invasive procedures. Thus, some of the renal sodium transporters or the aquaporins located in the apical membrane which are excreted in the tubular lumen and detected in urine could become biomarkers of some sodium and water balance disorders. Nowadays there are many studies investigating the role of these proteins in humans in clinical settings.