What area of the nephron is responsible for the reabsorption of most of the water

Water Follows the Osmotic Pressure Gradient Through Water Channels

Water reabsorption is by osmosis through water channels in the membrane. These water channels consist of a family of proteins called aquaporin. At least seven different aquaporin isoforms are expressed in the kidney. The proximal tubule has abundant AQP1 on the apical and basolateral membranes and AQP7 on the apical membrane of the late proximal tubule.

The blood that flows through the peritubular capillaries that surround the proximal tubules originates from the efferent arterioles of cortical nephrons. This blood has passed through the glomerulus and has had a protein-free filtrate abstracted from it. Thus the protein concentration in the efferent arterioles is increased by removal of 20% of the plasma volume while leaving the proteins behind. Therefore, the peritubular capillaries contain plasma with a higher oncotic pressure. As Na+ is reabsorbed with other solutes, the concentration of osmolytes in the spaces between the cells increases, causing a local increase in the osmotic pressure in this space. Water moves in response to the high oncotic pressure of the peritubular capillaries and the slight hyperosmolarity of the lateral intracellular space, so that water flows across the basolateral membrane into the lateral intracellular space and into the interstitial space surrounding the capillaries, and from there into the peritubular capillaries. As water moves from the cell, it concentrates the cell contents so that the osmotic gradient is transferred to the apical membrane. Water moves from the tubular fluid into the cell in response to this gradient. The net effect is water reabsorption from the tubular fluid into the peritubular capillaries, caused by the increased oncotic pressure of the capillary blood and the active reabsorption of Na+ and other solutes (see Figure 7.4.11).

Regulation of Renal Aquaporins

Water reabsorption along the renal tubule is generally passive (does not directly require ATP/energy) to occur, but rather follows a concentration gradient created by the active (ATP utilizing) reabsorption of NaCl. Therefore, a primary determinant of water reabsorption by the renal tubule is the tubule’s permeability. One might expect a lipid bilayer, which constitutes the outer membrane of epithelial cells lining the renal tubule, to be fairly hydrophobic, and not allow efficient water entry/exit. This is useful, for example, in the TAL, for generating the cortico-medullary osmotic gradient to allow for eventual fine-tuning of water reabsorption. However, in the PT and distal tubule (including CD), numerous intramembrane hydrophilic proteins known as water channels or “aquaporins” increase the porosity of the lipid bilayer allowing for H2O molecules to be reabsorbed from the luminal fluid across the apical membrane, into the cell, and then across the basolateral membrane and back into the circulation. Aquaporins (AQPs), as a class, have six-membrane-spanning regions, with charged amino acids (hydrophilic) forming the actual water channel and intracellular amino and carboxy tails.

AQP1 is the isoform found expressed in PT and thin limbs. Most reports suggest that this water channel is constitutively expressed at high levels allowing for reabsorption of a large number of water molecules in an unregulated fashion. A recent study by Herak-Kramberger et al. [26] demonstrated that male rats had greater expression of AQP1 in kidney (80% protein, 40% higher mRNA) as compared to females. Gonadectomy appeared to reduce expression in both sexes. It is unclear from this study if the number of channels is higher in males due to the greater length or size of the PT in male animals (as discussed in the early portion of this chapter), or if the number of channels per cell is different.

AQP2 is the apical water channel expressed in the late DCT, CNT, and CD (cortical through inner medullary). AQP2 appears to be the most highly expressed mRNA transcript in the CCD, as assessed by deep-sequencing in rat [22]. Recent studies also show AQP2 expression in bladder, reproductive tissues, and brain. AQP2 transcription is upregulated by AVP [27]; therefore this provides one mode for increased permeability of the CD during periods of high AVP. A second mechanism involves translocation (trafficking) of existing AQP2 molecules from cytosolic sites into the apical membrane [27]. This mode of regulation, which may involve phosphorylation, can rapidly (in a matter of minutes) alter the water permeability of the membrane and increase water reabsorption [28]. A handful of studies have reported sex or sex hormone differences in the regulation of AQP2. Prepubertal levels of urinary AQP2 (a marker for renal levels) have been shown to be higher than postpubertal levels in a study conducted on healthy children of different ages [29]. Sharma et al. [30] reported a greater baseline protein expression of AQP2 in the kidney of female mice. However, in this study, females also experienced a greater downregulation of AQP2 on a high-fructose diet, as compared to male mice [30]. Kim et al. [31] demonstrated increased AQP2 expression in rat bladder in response to estradiol repletion in ovariectomized rats. Moreover, in support of greater expression in females, and with estradiol, Zou et al. [32] recently identified an estrogen-response element in the AQP2 gene promoter region.

AQP3 and AQP4 are found on the basolateral membranes of the renal CD, and mediate conductance of water molecules across this membrane. Similar to AQP2, both of these channels are expressed in a number of other tissues, in addition to kidney. Similar to AQP2, AQP3, but not AQP4, has been demonstrated to be upregulated transcriptionally by vasopressin [33]. Less has been reported regarding sex differences in the expression of these channels. Similar to what was observed for AQP2, estradiol repletion of ovariectomized rats led to increased expression of AQP3 in bladder [31]. AQP4 is the most highly expressed water channel in brain tissue and has an important role in protecting against edema. Estradiol has been shown to increase the expression of AQP4 in brain [34].

Effects of ANP on the Cortical Collecting Duct

Water reabsorption takes place in the cortical collecting duct (CCD) cells, induced by the peptide hormone vasopressin. An increase in 3’5’-cyclic adenosine monophosphate (cAMP) levels in these cells leads to an increasing permeability of the CCD apical membrane to water. The ANP-induced diuresis probably results from the direct inhibition of renal tubular epithelial water transport.8 The results that ANP leads to an inhibition of the hydraulic conductivity response to the hormone vasopressin but not to either cAMP or forskolin suggests that ANP acts proximally to cAMP formation. A similar result was found for aquaporin (AQP-1),23 which is also localized in the kidney but predominantly in the proximal tubule, descending limb of Henle’s loop, and vasa recta. AQP-1 is upregulated when expressed in Xenopus laevis oocytes and treated with AVP. Water permeability increases and could be inhibited by ANP or incubation with 8-Br-cGMP.23 A competing phosphorylation mechanism between protein kinase A (PKA) and PKG has been shown previously for dopamine and ANP in regulating the Na+/K+-ATPase of the proximal tubule1 and might be a mechanism in the CCD as well. However, in several cell types, natriuretic peptides decrease cAMP concentration via the activation of the NPR-C receptor to which all natriuretic peptides can bind. This effect can either be mediated by the activation of the receptor’s coupling to G-protein or by regular cGMP increase and subsequent stimulation of cAMP-sensitive phosphodiesterases. What is the effect of ANP on ion transport processes at this nephron site? A few groups were able to convincingly show that ANP neither has an effect on Na+ transport in the CCD26 nor does it regulate electrogenic electrolyte transport in principal cells of rat CCD.27 With the localization of the NPR-A/GC-A receptor in the luminal membrane of intercalated cells, the authors show that ANP, urodilatin, and its membrane-permeable second-messenger 8-Br-cGMP inhibit Na+/H+ exchange activity,11 thus regulating the acid-base equilibrium of these cells (Fig. 3).

4.8 Water Sorption

Water absorption by composite materials is a diffusion-controlled process, and the uptake of water occurs largely in the resin matrix [14]. The water sorbed by the polymer matrix can cause filler/matrix debonding (Fig. 4.26) or even hydrolytic degradation of the fillers [15] and may reduce the mechanical properties of the composite materials [16]. Water sorption (WS) also results in increased overall weight.

Solubility, leaching, and hydrolytic degradation are results of either the breaking of chemical bonds in the resin or softening through the plasticizing action of water [17]. When resin samples are immersed in water, some of the components, such as unreacted monomers or fillers, dissolve and leach out of the sample. The release of these components may influence the initial dimensional changes of the composite, the clinical performance, the esthetic aspect of the restoration, or the biocompatibility of the material [17]. Solubility therefore results in weight reduction.

WS and water solubility (WSL) measurements were performed as described by Oysaed and Ruyter [18]. Ten disk specimens were used for each material. The diameter and the thickness of the specimens were measured and the volume (V) was calculated. The discs were conditioned in a desiccator for 3 days, containing calcium sulfate at 37°C until a constant weight had been achieved (w0). The disks were placed in a glass vial containing 100 mL of distilled water. The vials were wrapped by aluminum foil to protect from light and placed in an incubator at 37°C and at intervals removed, blot dried, and weighed. This was continued until the weight change/week was less than 0.32 μg (constant weight—w1). The discs were removed from the water and replaced in a desiccator for 24 h and then reweighed for the last time (w2). These steps were carried out to evaluate WS and WSL, in μg/cm3.

WS=w1−w 2/VWSL=w0−w2=V

where w0 is the sample weight before immersion, w1 is the sample weight after immersion, and w2 is the sample weight after immersion and desiccation.

For WS and WSL, statistical differences were recorded when nanofilled composites were compared. The exclusive nanofilled composite, Supreme XT, was more susceptible to water uptake. This can be explained by the higher contact surface of nanosized fillers. Samples cured with halogen device were less sensitive in water (Figs. 4.12 and 4.13).

Absorption of Ions and Water

Most water absorption takes place in the distal third of the small intestine, but the bulk of intestinal water is absorbed by the large intestine.18 However, Na+ and water absorption in the small intestine is important in absorption of nutrients and other ions.

The routes for transepithelial movement of ions and water (see Figure 36-1) are through the cells (transcellular) and through the paracellular space (extracellular).13,14 Transcellular movement of Na+ involves entry from the lumen into the cell, down an electrochemical gradient. However, exit of Na+ from the cell is against an electrochemical gradient and is therefore an active process that requires energy from the Na+ pump (Na+/K+–ATPase), located along the basal and lateral membranes.14,18 The active transport of Na+ across the cell creates a transmucosal electrical potential difference.

The electrochemical gradient for Na+ across the apical membrane has sufficient potential energy to move Cl− into the cell against its electrical gradient. The cotransport of both ions is brought about by a carrier mechanism on the luminal membrane that is specific for Na+ and Cl− (see Figure 36-1). The net movement of Cl− by this mechanism is called secondary active transport.24 The Na+ gradient also energizes uptake of hexoses (glucose), amino acids, and most B vitamins against their chemical gradients into the cell.24

Water transport is passive, closely coupled to solute movement, and is primarily paracellular (see Figure 36-1).14 As absorbed Na+ is pumped across the basolateral membrane, it creates an osmotic gradient that draws water into the intercellular space.14,24 Water accumulation in this space increases the hydrostatic pressure, and this pressure forces the water across the basolateral membrane toward the capillary bed. Although the tight junction restricts backflow of absorbed water and electrolytes into the lumen, paracellular permeability and back-leak through this route in the jejunum is high. Therefore Starling forces have a considerable influence on ion and fluid transport in the proximal bowel, just as in the proximal tubule of the kidney.13,18

Net water movement from lumen to plasma through the paracellular route will “drag” permeant ions and low-molecular-weight substances with it (sugars, amino acids, Ca2+, and Mg2+), and this mechanism is called solvent drag, or convection.13,18 Fluid absorbed by the epithelium moves into the central lacteal of the villus, from which it moves into the deeper lymphatics.14 There is little proof that the intestinal villus has a countercurrent multiplier that could enhance water and solute absorption, at least in the dog.25

Luminal and Basal Membrane Domains

Transcellular water reabsorption in the proximal tubule is mediated by the constitutive water channel, aquaporin-1 (AQP1), located in both membrane domains (408, 452, 455, 530, 560). In mice another water channel, AQP4, is located in the basolateral membranes of S3 segments and associated with orthogonal arrays of intramembrane particles, as revealed by freeze fracture studies (632).

The plasma membrane of the microvilli is covered by a glycocalix containing hydrolases (phosphatases, peptidases, nucleotidases), which cleave their substrates in the tubular fluid (ectoenzymes). The microvillus membrane holds a large variety of transport proteins for uptake of solutes from the tubular fluid. Many of the transport proteins are anchored by adaptor proteins, such as PDZ-proteins and NHERF-1/-2, to the underlying apical scaffold (64, 159, 211, 649, 659).

The density of a given sodium cotransport protein in the microvillus membrane can be dissimilar along the segments of the proximal tubule and among nephron generations and can vary with the functional conditions. For instance, the expression of the sodium phosphate cotransporter NaPi-IIa usually decreases in nephron generations from S1 to S3. In the three proximal tubule segments of juxtamedullary nephrons, it is much more abundant than in those of superficial nephrons (388). These patterns may be profoundly modified by the functional conditions (132, 359, 376, 383, 514, 613). Inversely, the sodium glucose cotransporter SGLT1 has its highest expression in the brush-border membrane (BBM) of S3 and is almost undetectable in S1. In females SGLT1 is higher expressed than in males (529).

The sodium/hydrogen exchanger NHE3 is responsible for most, if not all, apical membrane Na+-H+ exchange in the proximal tubule and the reabsorption of the bulk of filtered sodium (17, 238). The N–H exchanger is enriched in the intermicrovillar microdomain (12), where it interacts with the scavenger receptor megalin (10, 65, 414). Changes in the NHE3-mediated sodium transport rates might involve rapid and reversible redistribution between the two microdomains (61, 302, 414, 415, 419, 685).

Secretion of organic amphiphilic electrolytes from the blood into the tubular fluid is a pathway for clearance and detoxification of xenobiotics and drugs, including diuretics (305, 351, 502, 680, 681, 690). The uptake into the proximal tubule epithelium proceeds via multispecific organic anion transporters (OATs) and organic cation transporters (OCTs) in the basolateral membrane domain. Most members of the OAT and OCT families have been immunolocalized to the basolateral cell membrane of S3 proximal tubule (281, 626, 627), yet OAT 1 has been detected mainly in S2 (612), a few of them also in S1. The expression of the OATs and OCTs is strongly regulated by sex hormones (97, 284, 285, 366, 625, 628).

The export into the tubular lumen of conjugated and unconjugated lipophilic anionic substrates involves various OATs and primarily active transporters with ATP-binding cassette motifs, belonging to the MRP family (574) and located in the brush border membrane of S1, S2, and S3 proximal tubule segments (544, 574).

11. What causes hyponatremia with concentrated (less than maximally dilute) urine?

Electrolyte-free water reabsorption. Water reabsorption is mediated by binding of vasopressin, or antidiuretic hormone (ADH—same stuff, 2 names), to vasopressin 2 (V2) receptors on the basolateral membranes of principal cells in the collecting ducts of nephrons. Vasopressin binding in turn stimulates insertion of aquaporin water channels into the luminal membrane. Water then flows through these aquaporin channels into cells and onward to the hypertonic medullary interstitium via constitutively active basolateral aquaporin channels. ADH release is regulated by osmoreceptors in the hypothalamus. Normally, vasopressin release is inhibited when the serum sodium concentration falls below 135 mEq/L. However, poor effective circulating volume—as sensed by carotid baroreceptors as a decrease in arterial pressure—serves as a potent stimulus for ADH secretion despite osmoregulatory inhibition. Hyponatremia in the setting of hypovolemia arises from hemodynamically driven ADH release and should improve with volume repletion. ADH secretion in the setting of arterial underfilling, due either to a low output state in the case of heart failure or to splanchnic vasodilation in cirrhosis, occurs by the same hemodynamic mechanism. These patients, conversely, will appear hypervolemic on exam. They are hyponatremic and hypotonic despite a clear excess in total body sodium content.

The Role of Aquaporins in the Kidneys

The majority of water reabsorption that occurs in the nephron is facilitated by the AQPs. Most of the fluid that is filtered at the glomerulus is then reabsorbed in the proximal tubule and the descending limb of the loop of Henle. AQP1, which is expressed in the apical and basolateral segment of the renal tubular epithelial cell plasma membrane, is primarily responsible for this water transport.8 Additionally, in the outer medullary descending vasa recta, which is rich in AQP1 channels, water resorption occurs despite the existence of hydrostatic forces that favor influx. This observation suggests that in this portion of the vasa recta, water transport involves the water only AQP1 pathway, facilitated by transtubular sodium and urea concentration gradients creating the osmotic driving force for water movement.9

In the distal tubule and collecting duct, other AQPs serve to regulate water resorption with a dominant role played by AQP2 and its interaction with arginine vasopressin (AVP). Although only 15% of the filtrate reaches the distal nephron, regulation of water resorption in this segment allows the kidneys to “fine tune” water balance to accommodate the needs of the body. The osmotic driving force for the water movement through the collecting duct epithelia is the hypertonic milieu of the medullary portion of the kidneys created by active transport of sodium and urea from the lumen of the thick ascending loop of Henle into the interstitial space surrounding the collecting ducts. AVP secretion by the pituitary gland, in response to central volume and osmoreceptors, upregulates AQP2 expression to match the fluid needs of the body. This occurs when AVP binds to the V2 receptor in the collecting duct of the nephron, increasing intracellular cyclic adenosine monophosphate (cAMP) production. The cAMP in turn stimulates protein kinase A–dependent phosphorylation of the AQP2 protein. Vesicles carrying the phosphorylated AQP then fuse with the nephron epithelial cells, implanting the AQP in the wall of the apical plasma membrane. This results in dramatically increased water permeability of the collecting duct epithelial cell. After entering the duct cell from the collecting duct lumen through the AQP2 channels, the water exits the cell through the AQP3 and AQP4 channels located in the cell basolateral membrane, causing the water to enter the interstitial space of the nephron. AVP is also responsible for upregulating the AQP4 channels, but not the AQP3 channels.8,10

Medullary Hypertonicity

To allow osmotically driven water absorption, the osmotic concentration in the medullary interstitium must be slightly higher than that in the collecting duct lumen. For example, when a final urine with an osmolality of 1200 mOsm/kg is excreted, the medullary interstitium at the tips of the papillae must be a little higher than 1200 mOsm/kg. The generation of such a unique extracellular environment is achieved by the countercurrent multiplication system of the renal medulla, which consists of the countercurrent arrangement of descending and ascending limbs of the loops of Henle.

Mechanism of Action

Osmotic diuretics primarily inhibit water reabsorption in the proximal convoluted tubule and the thin descending loop of Henle and collecting duct, regions of the kidney that are highly permeable to water. As Na+ is reabsorbed in the proximal tubule, water normally follows and is reabsorbed by passive diffusion. In the presence of an osmotic diuretic, reabsorption of water is reduced relative to Na+. In other words, despite the actions of transporters to generate a Na+ concentration gradient favorable for osmosis, mannitol and urea negate this driving force. Osmotic diuretics also extract water from intracellular compartments, increasing extracellular fluid volume. Overall, urine flow increases with a relatively small loss of Na+. In fact, urine osmolarity actually decreases.