Autosomal-dominant polycystic kidney disease (ADPKD): mechanisms of intracystic fluid accumulation [13,14]. The primary mech anism of intracystic fluid accumulation seems to be a net transfer of chloride into the lumen. This secretion is mediated by a bumetanide-sensitive Na+-K+-2Cl- cotransporter on the basolateral side and cystic fibrosis transmembrane regulator (CFTR) chloride channel on the apical side. The activity of the two transporters is regulated by protein kinase A (PKA) under the control of cyclic adenosine monophosphate (AMP). The chloride secretion drives movement of sodium and water into the cyst lumen through electrical and osmotic coupling, respectively. The pathway for transep-ithelial Na+ movement has been debated. In some experimental conditions, part of the Na+ could be secreted into the lumen via a mispolarized apical Na+-K+-ATPase ("sodium pump"); however, it is currently admitted that most of the Na+ movement is paracellu-lar and that the Na+-K+-ATPase is located at the basolateral side. The movement of water is probably transcellular in the cells that express aquaporins on both sides and paracellular in others [13, 14]. AQP—aquaporine; DPC—diphenylamine carboxylic acid.
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