 Na/K-ATPase pumps, transmembrane proteins that are localized in the lateral folds of the plasma membrane. They are responsible for the reabsorption of Na, which is the major driving force for reabsorption of water in the proximal convoluted tubule. As in the intestinal and gallbladder epithelia, this process is driven by active transport of Na into the lateral intercellular space. Active transport of Na is followed by passive diffusion of Cl to maintain electrochemical neutrality. The accumulation of NaCl in the lateral intercellular spaces creates an osmotic gradient that draws water from the lumen into the intercellular compartment. This compartment distends as the amount of fluid in it increases; the lateral folds separate to allow this distension.
 AQP-1, a small (30 kilodaltons) transmembrane protein that functions as a molecular water channel in the cell membrane of proximal convoluted tubules. Movement of water through these membrane channels does not require the high energy of Na/K-ATPase pumps. Immunocytochemical methods can be used to demonstrate the presence of these proteins.
 The hydrostatic pressure that builds up in the distended intercellular compartment, presumably aided by contractile activity of the actin filaments in the base of the tubule cells, drives an essentially isosmotic fluid across the tubule basement membrane into the renal connective tissue. Here, the fluid is reabsorbed into the vessels of the peritubular capillary network.
The proximal convoluted tubule also reabsorbs amino acids, sugars, and polypeptides.
As in the intestine, the microvilli of proximal convoluted tubule cells are covered with a well-developed glycocalyx that contains several ATPases, peptidases, and high concentrations of disaccharidases. In addition to amino acids and monosaccharides, the ultrafiltrate also contains small peptides and disaccharides. The latter adsorb on the glycocalyx for further digestion before internalization of the resulting amino acids and monosaccharides (including glucose). Also, as in the gut, amino acid and glucose resorption in the proximal convoluted tubule depends on active Na transport.
Proteins and large peptides are endocytosed in the proximal convoluted tubule.
Deep tubular invaginations are present between the microvilli of the proximal convoluted tubule cells. Proteins in the ultra-filtrate, on reaching the tubule lumen, bind to the glycocalyx that covers the plasma membrane of the invaginations. Then endocytotic vesicles containing the bound protein bud from the invaginations and fuse in the apical cytoplasm to form large protein-containing early endosomes (see Fig. 20.18). These early endosomes are destined to become lysosomes, and the endocytosed proteins are degraded by acid hydro-lases. The amino acids produced in the lysosomal degradation are recycled into the circulation via the intercellular compartment and the interstitial connective tissue.
Also, the pH of the ultrafiltrate is modified in the proximal convoluted tubule by the reabsorption of bicarbonate and by the specific secretion into the lumen of exogenous organic acids and organic bases derived from the peritubular capillary circulation.
The cells of the proximal straight tubule (i.e., the thick descending limb of the loop of Henle) are not as specialized for absorption as are those of the proximal convoluted tubule.
 FOLDER 20.5 Functional Considerations: Structure and Function of Aquaporin Water Channels