Sodium and chlorine transport
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1 Kidney physiology 2
2 Sodium and chlorine transport The kidneys help to maintain the body's extracellular fluid (ECF) volume by regulating the amount of Na+ in the urine. Sodium salts (predominantly NaCl) are the most important contributor to the osmolality of the ECF; hence, where Na+ goes, water follows. Na+ and Cl reabsorption decreases from proximal tubules to Henle's loops to classic distal tubules to collecting tubules and ducts
3 The proximal tubule reabsorbs the largest fraction of filtered Na+ (~67%). Because [Na+] in tubule fluid (or TFNa) remains almost the same as that in plasma (i.e., TF Na /P Na = 1.0) throughout the length of the proximal tubule, it follows that the [Na + ] in the reabsorbate is virtually the same as that in plasma. Because Na + salts are the dominant osmotically active solutes in the filtrate, reabsorption must be a nearly isosmotic process.
4 The tubule can reabsorb Na+ and Cl via both transcellular and paracellular pathways. In the transcellular pathway, Na+ and Cl sequentially traverse the apical and basolateral membranes before entering the blood. In the paracellular pathway, these ions move entirely by an extracellular route, through the tight junctions between cells. In the transcellular pathway, transport rates depend on the electrochemical gradients, ion channels, and transporters at the apical and basolateral membranes. However, in the paracellular pathway, transepithelial electrochemical driving forces and permeability properties of the tight junctions govern ion movements.
5 Transcellular and paracellular mechanisms of Na+ and Cl reabsorption. The example in B illustrates the electrochemical driving forces for Na+ in the early proximal tubule. The equivalent circuit demonstrates that the flow of positive charge across the apical membrane slightly depolarizes the apical membrane ( 67 mv) relative to the basolateral membrane ( 70 mv).
6 Transcellular Na+ Reabsorption The basic mechanism of transcellular Na+ reabsorption is similar in all nephron segments and is a variation on the classic two-membrane model of epithelial transport. The first step is the passive entry of Na+ into the cell across the apical membrane. -Because the intracellular Na+ concentration ([Na+]i) is low and the cell voltage is negative with respect to the lumen, the electrochemical gradient is favorable for passive Na+ entry across the apical membrane. -However, different tubule segments use different mechanisms of passive Na+ entry across the apical membrane. The proximal tubule, the TAL, and the DCT all use a combination of Na+coupled cotransporters and exchangers to move Na+ across the apical membrane; however, in the cortical and medullary collecting ducts, Na+ enters the cell through epithelial Na+ channels (ENaCs). The second step of transcellular Na+ reabsorption is the active extrusion of Na+ out of the cell across the basolateral membrane. -This Na+ extrusion is mediated by the Na-K pump, which keeps [Na+]i low (~15 mm) and [K+]i high (~120 mm). Because the basolateral membrane is primarily permeable to K+, it develops a voltage of ~70 mv, with the cell interior negative with respect to the interstitial space. Across the apical membrane, the cell is negative with respect to the lumen. The magnitude of the apical membrane voltage may be either lower or higher than that of the basolateral membrane, depending on the nephron segment and its transport activity.
7 Paracellular Na+ Reabsorption The basic mechanism of paracellular Na+ transport is similar among nephron segments: the transepithelial electrochemical gradient for Na+ drives transport. However, both the transepithelial voltage (Vte) and luminal [Na+] vary along the nephron.as a result, the net driving force for Na+ is positive favoring passive Na+ reabsorption only in the S2 and S3 segments of the proximal tubule and in the TAL. In the other segments, the net driving force is negative favoring passive Na+ diffusion from blood to lumen ( backleak ). In addition to undergoing purely passive, paracellular reabsorption in the S2 and S3 segments and TAL, Na+ can move uphill from lumen to blood via solvent drag across the tight junctions. In this case, the movement of H2O from the lumen to the lateral intercellular space energized by the active transport of Na+ into the lateral intercellular space also sweeps Na+ and Cl in the same direction. Nephron segments also vary in their leakiness to Na+ ions. This leakiness is largely a function of the varying ionic conductance of the paracellular pathway between cells across the tight junction, due to the expression of different claudins. In general, the leakiness of the paracellular pathway decreases along the nephron from the proximal tubule (the most leaky) to the papillary collecting ducts.
8 An important consequence of a highly leaky paracellular pathway is that it provides a mechanism by which the basolateral membrane voltage can generate a current that flows through the tight junctions and charges up the apical membrane, and vice versa. For example, hyperpolarization of the basolateral membrane leads to a hyperpolarization of the apical membrane. A consequence of this paracellular electrical coupling is that the apical membrane of a leaky epithelium, such as the proximal tubule, has a membrane voltage that is negative ( 67 mv) and close to that of the basolateral membrane ( 70 mv), whereas one would expect that, based on the complement of channels and ion gradients at the apical membrane, the apical membrane would have a far less negative voltage. A practical benefit of this crosstalk is that it helps couple the activity of the basolateral electrogenic Na-K pump to the passive entry of Na+ across the apical membrane. If the Na-K pump rate increases, not only does [Na+]i decrease, enhancing the chemical Na+ gradient across the apical membrane, but also the basolateral membrane hyperpolarizes (i.e., the cell becomes more negative with respect to the blood). Electrical coupling translates this basolateral hyperpolarization to a concomitant apical hyperpolarization, thus also enhancing the electrical gradient favoring apical Na+ entry.
9 Na+ and Cl, and Water Transport at the Cellular and Molecular Level
10 Na+ and Cl, and Water Transport at the Cellular and Molecular Level Proximal Tubule Along the first half of the tubule, a variety of cotransporters in the apical membrane couples the downhill uptake of Na+ to the uphill uptake of solutes such as glucose, amino acids, phosphate, sulfate, lactate, and citrate. Both cotransporters and exchangers exploit the downhill Na+ gradient across the apical cell membrane that is established by the Na-K pump in the basolateral membrane. The Na-K pump and, to a lesser extent, the electrogenic Na/HCO3 cotransporter 1 (NBCe1) are also responsible for the second step in Na+ reabsorption, moving Na+ from cell to blood. The presence of K+ channels in the basolateral membrane is important for two reasons. 1- these channels establish the negative voltage across the basolateral membrane and establish a similar negative voltage across the apical membrane via paracellular electrical coupling. 2- these channels permit the recycling of K+ that had been transported into the cell by the Na-K pump. Because of a lumen-negative Vte in the early proximal tubule, as well as a paracellular pathway that is permeable to Na+, approximately one third of the Na+ that is transported from lumen to blood by the transcellular pathway diffuses back to the lumen by the paracellular pathway ( backleak ).
11 Thin Limbs of Henle's Loop Na+ transport by the thin descending and thin ascending limbs of Henle's loop is almost entirely passive and paracellular. Thick Ascending Limb Two major pathways contribute to Na+ reabsorption in the TAL: transcellular and paracellular. The transcellular pathway includes two major mechanisms for taking up Na+ across the apical membrane: -Na/K/Cl cotransporter 2 (NKCC2) couples the inward movement of 1 Na+, 1 K+, and 2 Cl ions in an electroneutral process driven by the downhill concentration gradients of Na+ and Cl. -The second entry pathway for Na+ is an NHE3. As in the proximal tubule, the basolateral Na-K pump keeps [Na+]i low and moves Na+ to the blood.
12 Distal Convoluted Tubule Na+ reabsorption in the DCT occurs almost exclusively by the transcellular route. The apical step of Na+ uptake is mediated by an electroneutral Na/Cl cotransporter that belongs to the same family as NKCC2 in the TAL. Initial and Cortical Collecting Tubules Na+ reabsorption in the connecting tubule, initial collecting tubule (ICT), and CCT is transcellular and mediated by the majority cell type, the principal cell. The neighbouring β-intercalated cells are important for reabsorbing Cl, as discussed below. Na+ crosses the apical membrane of the principal cell via the epithelial Na+ channel (ENaC), which is distinct from the voltage-gated Na+ channels expressed by excitable tissues. Medullary Collecting Duct The inner and outer medullary collecting ducts reabsorb only a minute amount of Na+, ~3% of the filtered load. It is likely that ENaC mediates the apical entry of Na+ in these segments and that the Na-K pump extrudes Na+ from the cell across the basolateral membrane
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14 Proximal Tubule The proximal tubule reabsorbs Cl by both the transcellular and the paracellular routes, with the paracellular believed to be the dominant one in the early proximal tubule Thick Ascending Limb Cl reabsorption in the TAL takes place largely by Na/K/Cl cotransport across the apical membrane Distal Convoluted Tubule Cl reabsorption by the DCT occurs by a mechanism that is somewhat similar to that in the TAL. Collecting Ducts The ICT and the CCT reabsorb Cl by two mechanisms. First, the principal cell generates a Vte (~40 mv, lumen negative) that is favorable for paracellular diffusion of Cl. Second, the β-type intercalated cells reabsorb Cl via a transcellular process in which pendrin (SLC26A4) mediates Cl uptake across the apical membrane in exchange for HCO3-, and Cl exits via channels in the basolateral membrane Water reabsorption is passive and secondary to solute transport
15 The body regulates Na+ excretion by three major mechanisms: 1. Changes in renal hemodynamics alter the Na+ load presented to the kidney and regulate Na+ reabsorption in the proximal tubule and distal nephron. 2. Three factors that respond to decreases in effective circulating volume the renin-angiotensin-aldosterone axis, renal sympathetic nerve activity, and AVP - do so in part by increasing Na+ reabsorption in various nephron segments. 3. Several factors that respond to increases in effective circulating volume including atrial natriuretic peptide and dopamine do so in part by reducing Na+ reabsorption in various segments of the nephron. That is, they produce a natriuresis.
16 Transport of Urea, Glucose, Phosphate, Calcium, Magnesium, and Organic Solutes The kidney plays a central role in controlling the plasma levels of a wide range of solutes that are present at low concentrations in the body. The renal excretion of a solute depends on three processes: -filtration -reabsorption -secretion
17 Urea The kidney filters, reabsorbs, and secretes urea The liver generates urea from, the primary nitrogenous end product of amino-acid catabolism. The primary route for urea excretion is the urine, although some urea exits the body through the stool and sweat. The normal plasma concentration of urea is 2.5 to 6 mm. Clinical laboratories report plasma urea levels as blood urea nitrogen (BUN) in the units (mg of elemental nitrogen)/(dl plasma); normal values are 7 to 18 mg/dl. For a 70-kg human ingesting a typical Western diet and producing 1.5 to 2 L/day of urine, the urinary excretion of urea is ~450 mmol/day. The kidney freely filters urea at the glomerulus, and then it both reabsorbs and secretes it. Because the tubules reabsorb more urea than they secrete, the amount of urea excreted in the urine is less than the quantity filtered. In the example shown in Figure 36-1A (i.e., average urine flow), the kidneys excrete ~40% of the filtered urea. The primary sites for urea reabsorption are the proximal tubule and the medullary collecting duct, whereas the primary sites for secretion are the thin limbs of the loop of Henle.
18 Urea handling by the kidney: In A, we assume a normal urine flow and thus a urea excretion of 40% of the filtered load. The numbered yellow boxes indicate the fraction of the filtered load that various nephron segments reabsorb. The talh and the tip of the tdlh in juxtamedullary nephrons secrete urea. In superficial nephrons, the entire tdlh may secrete urea. The red box indicates the fraction of the filtered load jointly secreted by both nephron types. The green boxes indicate the fraction of the filtered load that remains in the lumen at various sites. The values in the boxes are approximations. PCT, proximal convoluted tubule; PST, proximal straight tubule.
19 Glucose The proximal tubule reabsorbs glucose via apical, electrogenic Na/glucose cotransport and basolateral facilitated diffusion The fasting plasma glucose concentration is normally 4 to 5 mm (70 to 100 mg/dl) and is regulated by insulin and other hormones. The kidneys freely filter glucose at the glomerulus and then reabsorb it, so that only trace amounts normally appear in the urine. The proximal tubule reabsorbs nearly all the filtered load of glucose, mostly along the first third of this segment. More distal segments reabsorb almost all of the remainder. In the proximal tubule, luminal [glucose] is initially equal to plasma [glucose]. As the early proximal tubule reabsorbs glucose, luminal [glucose] drops sharply, falling to levels far lower than those in the interstitium. Accordingly, glucose reabsorption occurs against a concentration gradient and must, therefore, be active. Glucose reabsorption is transcellular; glucose moves from the lumen to the proximal tubule cell via Na/glucose cotransport, and from cytoplasm to blood via facilitated diffusion Glucose excretion in the urine occurs only when the plasma concentration exceeds a threshold
20 Glucose handling by the kidney. The yellow box indicates the fraction of the filtered load that the proximal tubule reabsorbs. The green boxes indicate the fraction of the filtered load that remains in the lumen at various sites. The values in the boxes are approximations. PCT, proximal convoluted tubule; PST, proximal straight tubule.
21 Other organic solutes The proximal tubule reabsorbs amino acids using a wide variety of apical and basolateral transporters. The total concentration of amino acids in the blood is ~2.4 mm. These L-amino acids are largely those absorbed by the gastrointestinal tract, although they also may be the products of protein catabolism or of the de novo synthesis of nonessential amino acids. The glomeruli freely filter amino acids. Because amino acids are important nutrients, it is advantageous to retrieve them from the filtrate. The proximal tubule reabsorbs >98% of these amino acids via a transcellular route, using a wide variety of amino-acid transporters, some of which have overlapping substrate specificity. At the apical membrane, amino acids enter the cell via Na+-driven or H+-driven transporters as well as amino-acid exchangers. At the basolateral membrane, amino acids exit the cell via amino-acid exchangers some of which are Na+ dependent and also by facilitated diffusion. Particularly in the late proximal tubule and postproximal nephron segments, where the availability of luminal amino acids is low, SLC38A3 mediates the Na+-dependent uptake of amino acids across the basolateral membrane. This process is important for cellular nutrition or for metabolism.
22 Phosphate The metabolism of inorganic phosphate (Pi) depends on bone, the gastrointestinal tract, and the kidneys. About half of total plasma phosphate is in an ionized form, and the rest is either complexed to small solutes (~40%) or bound to protein (10% to 15%). The plasma concentration of total Pi varies rather widely, between 0.8 and 1.5 mm (2.5 to 4.5 mg/dl of elemental phosphorus). Thus, the filterable phosphate (i.e., both the ionized and complexed) varies between ~0.7 and 1.3 mm. At a normal blood ph of 7.4, 80% of the ionized plasma phosphate is HPO4 2- and the rest is HPO4-. Assuming that the total plasma phosphate concentration is 4.2 mg/dl, that only the free and complexed phosphate is filterable, and that the GFR is 180 L/day, each day the kidneys filter ~7000 mg of phosphate. Because this amount is more than an order of magnitude greater than the total extracellular pool of phosphate, it is clear that the kidney must reabsorb most of the phosphate filtered in the glomerulus.
23 Calcium Binding to plasma proteins and formation of Ca2+-anion complexes influence the filtration and reabsorption of Ca2+ The filterable portion, ~60% of total plasma calcium, consists of two moieties. The first, ~15% of the total, complexes with small anions such as carbonate, citrate, phosphate, and sulphate. The second, ~45% of total calcium, is the ionized calcium (Ca2+) that one may measure with Ca2+sensitive electrodes or dyes. It is the concentration of this free, ionized calcium that the body tightly regulates; plasma [Ca2+] normally is 1.0 to 1.3 mm (4.0 to 5.2 mg/dl). The most important regulator of renal Ca2+ reabsorption is PTH, which stimulates Ca2+ reabsorption in the DCT and the connecting tubule. Regarding TRPV5, PTH increases transcription and open probability, and inhibits endocytosis, thereby stimulating Ca2+ reabsorption. In addition to its effects to stimulate apical Ca2+ entry, PTH also upregulates expression of calbindin and NCX1. Acting on gene transcription, vitamin D increases Ca2+ reabsorption in the distal nephron; this renal reabsorption complements the major Ca2+-retaining action of vitamin D, Ca2+ absorption in the gastrointestinal tract In renal tubule cells, vitamin D upregulates TRPV5 Ca2+-binding proteins, which contribute to enhanced Ca2+ reabsorption by keeping [Ca2+]i low during increased Ca2+ traffic through the cell.
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25 Magnesium Most of magnesium reabsorbtion takes place along the TAL Approximately 99% of the total body stores of magnesium reside either within bone (~54%) or within the intracellular compartment (~45%), mostly muscle. Renal magnesium excretion plays an important role in maintaining physiological plasma magnesium levels. The body maintains the total magnesium concentration in blood plasma within narrow limits, 0.8 to 1.0 mm (1.8 to 2.2 mg/dl).
26 Transport of potassium 98% of the total-body K+ content (~50 mmol/kg body weight) is inside cells; only 2% is in the extracellular fluid (ECF). The body tightly maintains the plasma [K+] at 3.5 to 5.0 mm.
27 Distribution and balance of K+ throughout the body. Intracellular K+ concentrations are similar in all tissues in the four purple boxes. The values in the boxes are approximations. RBC, red blood cell.
28 The proximal tubule reabsorbs most of the filtered K+, whereas the distal nephron reabsorbs or secretes K+, depending on K+ intake
29 Passive K+ reabsorption along the proximal tubule follows Na+ and fluid movements K+ reabsorption along the TAL occurs predominantly via a transcellular route that exploits secondary active Na/K/Cl cotransport K+ secretion by principal and intercalated cells of the ICT and CCT involves active K+ uptake across the basolateral membrane K+ reabsorption by intercalated cells involves apical uptake via an H-K pump K+ reabsorption along the MCD is both passive and active
30
31 References for kidney physiology lectures, from Boron 3 rd Edition Lecture 1 Chapter 33: p Chapter 34: p Lecture 2 Chapter 35: p Chapter 36: p , e1 Chapter 37: p
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