The mechanism by which Na+ is reabsorbed in coupled exchange with H+ and K+ in the collecting duct has been discussed previously; that is, Na+-driven K+ secretion is partially under mineralocorticoid control. Aldosterone and other compounds with mineralocorti-coid activity bind to a specific mineralocorticoid receptor in the cytoplasm of late distal tubule cells and of principal cells of the collecting ducts. This hormone-receptor complex is transported to the cell nucleus, where it induces synthesis of multiple proteins that are collectively called aldosterone-induced proteins. The precise mechanisms by which these proteins enhance Na+ transport are incompletely understood. However, the net effect is to increase Na+ entry across apical cell membranes and to increase basolateral membrane Na+-K+-ATPase activity and synthesis.
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