Sodium reabsorption continues in the distal convoluted tubule, which accounts for some 6 to 8% of the transport of sodium. The entry of Na+ across the apical cell membrane is mediated by Na+-Cl~ cotransport (Fig. 21.4). This protein is a distinct gene product that differs from the Na+-K+-2C1~ cotransporter in thick ascending limbs.
The permeability properties of the distal convoluted tubule are regulated by antidiuretic hormone (ADH, or vasopressin). In hypotonic conditions, ADH secretion by the posterior pituitary is suppressed and the distal convoluted tubule is impermeant to water. Conversely, in hypertonic or volume-contracted states, ADH is released by the posterior pituitary and increases the permeability and water reabsorption by the distal convoluted tubule.
The distal convoluted tubule, along with the collecting duct, is an important site of K+ transport. The direc-
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