DISTAL CONVOLUTED TUBULE
Only about 10% of the filtered NaCl is reabsorbed in the distal convoluted tubule (DCT). Like the TAL of Henle’s loop, this seg-ment is relatively impermeable to water, and NaCl reabsorption further dilutes the tubular fluid. The mechanism of NaCl trans-port in the DCT is an electrically neutral thiazide-sensitive Na+ and Cl− cotransporter (NCC, Figure 15–4).
Because K+ does not recycle across the apical membrane of the DCT as it does in the TAL, there is no lumen-positive potential in this segment, and Ca2+ and Mg2+ are not driven out of the tubular lumen by electrical forces. Instead, Ca2+ is actively reab-sorbed by the DCT epithelial cell via an apical Ca2+ channel and basolateral Na+/Ca2+ exchanger (Figure 15–4). This process is regulated by parathyroid hormone.