reabsorbed by the DCT epithelial cell via an apical Ca2 channel and basolateral Na'/Ca2+exchanger(Figure 4).This process is regulated by parathyroid hormone. Distal convoluted Lumen- tubule Interstitium- urine blood NCC Na Na ATP Ca2+ Ca Na Ca2 Figure 4.Ion transport pathways across the luminal and basolateral membranes of the distal convoluted tubule cell.As in all tubular cells,Na"/K ATPase is present in the basolateral membrane.NCC is the primary sodium and chloride transporter in the luminal membrane.(R,parathyroid hormone [PTH]receptor.) COLLECTING TUBULE The collecting tubule(CCT)is responsible for only 2-5%of NaCl reabsorption by the kidney.Despite this small contribution,the CCT plays an important role in renal physiology and in diuretic action.As the final site of NaCl reabsorption,the collecting tubule is responsible for tight regulation of body fluid volume and for determining the final Na"concentration of the urine.Furthermore,the collecting tubule is a site at which mineralocorticoids exert a significant influence.Lastly,the collecting tubule is the most important site of K*secretion by the kidney and the site at which virtually all diuretic-induced changes in K*balance occur. The mechanism of NaCl reabsorption in the CCT is distinct from the mechanisms found in other tubule segments.The principal cells are the major sites of Na',K, 66 reabsorbed by the DCT epithelial cell via an apical Ca2+ channel and basolateral Na+ /Ca2+ exchanger (Figure 4). This process is regulated by parathyroid hormone. Figure 4. Ion transport pathways across the luminal and basolateral membranes of the distal convoluted tubule cell. As in all tubular cells, Na+ /K+ ATPase is present in the basolateral membrane. NCC is the primary sodium and chloride transporter in the luminal membrane. (R, parathyroid hormone [PTH] receptor.) COLLECTING TUBULE The collecting tubule (CCT) is responsible for only 2-5% of NaCl reabsorption by the kidney. Despite this small contribution, the CCT plays an important role in renal physiology and in diuretic action. As the final site of NaCl reabsorption, the collecting tubule is responsible for tight regulation of body fluid volume and for determining the final Na+ concentration of the urine. Furthermore, the collecting tubule is a site at which mineralocorticoids exert a significant influence. Lastly, the collecting tubule is the most important site of K+ secretion by the kidney and the site at which virtually all diuretic-induced changes in K+ balance occur. The mechanism of NaCl reabsorption in the CCT is distinct from the mechanisms found in other tubule segments. The principal cells are the major sites of Na+ , K+