# Lecture 3: Tubular Function - Reabsorption and Secretion ## Unit 2.1: Renal System --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the mechanisms of tubular reabsorption and secretion 2. Explain sodium reabsorption along the nephron 3. Describe glucose and amino acid reabsorption and transport maximum 4. Explain the handling of organic acids and bases 5. Describe water reabsorption along the nephron 6. Apply tubular function concepts to understanding diuretic action --- ## Overview of Tubular Transport The tubular epithelium transforms the large volume of protein-free glomerular filtrate into a comparatively small volume of urine with carefully regulated composition. This transformation occurs through two fundamental processes: reabsorption, the movement of substances from the tubular lumen back into the peritubular blood, and secretion, the transfer of substances from peritubular blood into the tubular lumen. Both processes operate simultaneously along the nephron, with the net effect determining the final urinary excretion of each substance. Transport across the tubular epithelium occurs via several distinct mechanisms. Passive transport moves substances down their concentration or electrical gradients without energy expenditure, as exemplified by water movement through aquaporins and chloride diffusion through the paracellular pathway. Primary active transport directly uses ATP to move substances against their gradients, with the sodium-potassium ATPase serving as the archetypal example. Secondary active transport couples the movement of one substance against its gradient to the movement of another substance down its gradient, as seen with the sodium-glucose cotransporters SGLT1 and SGLT2. Facilitated diffusion employs carrier proteins to accelerate movement down gradients, exemplified by the GLUT transporters that carry glucose across the basolateral membrane. Substances can cross the tubular epithelium by two anatomical routes. The transcellular route involves transport across the apical membrane, through the cell cytoplasm, and across the basolateral membrane. This pathway allows for precise regulation through channel and transporter expression. The paracellular route passes between adjacent cells through tight junctions, which vary in permeability along different nephron segments. In the proximal tubule, tight junctions are relatively leaky, permitting substantial paracellular transport, while in the collecting duct they form tight barriers essential for generating concentrated or dilute urine. The sodium-potassium ATPase located on the basolateral membrane of all tubular epithelial cells establishes the fundamental driving force for most secondary active transport. This pump extrudes three sodium ions from the cell while importing two potassium ions, consuming one ATP molecule per cycle. The result is a low intracellular sodium concentration (approximately 10 to 20 mEq/L compared to 140 mEq/L in filtrate) and a negative intracellular potential. This sodium gradient provides the energy source for numerous apical transporters that couple sodium entry to the movement of other solutes. Panel A: Tubular epithelial cell with basolateral Na+/K+-ATPase pumping 3 Na+ out and 2 K+ in with ATP→ADP, establishing low intracellular Na+ (~10-20 mEq/L). Panel B: Apical membrane showing secondary active transport (SGLT2 coupling Na+ entry with glucose) and ion channel (ENaC allowing Na+ entry) with tubular Na+ at 140 mEq/L. Panel C: Paracellular pathway between cells for Cl- movement, with arrows indicating transcellular versus paracellular routes. Panel D: Color-coded transport types showing blue for passive, red for primary active, and green for secondary active transport. --- ## Proximal Tubule Function The proximal tubule accomplishes the bulk of nephron reabsorption, recovering 65 to 70 percent of the filtered sodium, water, bicarbonate, glucose, amino acids, phosphate, and numerous other substances. The proximal tubular epithelium is specialized for high-capacity transport, featuring a prominent brush border that dramatically increases apical surface area, abundant mitochondria to power the sodium-potassium ATPase, and leaky tight junctions that permit substantial paracellular transport. Reabsorption in the proximal tubule is isosmotic, meaning water follows solute proportionally such that the tubular fluid osmolality remains equal to plasma. The early proximal tubule, designated the S1 segment, focuses on reabsorption of sodium coupled to organic solutes and bicarbonate. The apical sodium-hydrogen exchanger NHE3 represents the predominant sodium entry mechanism, coupling sodium reabsorption to hydrogen ion secretion and thereby driving bicarbonate reclamation. The sodium-glucose cotransporter SGLT2 couples sodium entry to glucose reabsorption with high capacity but relatively low affinity. Sodium-coupled cotransporters also recover amino acids and phosphate. Bicarbonate reabsorption is effectively complete in the early proximal tubule through the action of carbonic anhydrase both in the brush border (type IV) and cytoplasm (type II). The late proximal tubule, comprising the S2 and S3 segments, continues sodium reabsorption but with different characteristics. As glucose, amino acids, and bicarbonate are largely removed from the tubular fluid in the early proximal tubule, chloride becomes the predominant anion. The relatively high chloride concentration in late proximal tubular fluid drives paracellular chloride reabsorption down its concentration gradient. This chloride movement creates a lumen-positive potential that drives paracellular sodium reabsorption. The late proximal tubule is also the principal site for secretion of organic anions and cations, including many drugs. Glomerulotubular balance describes the proximal tubule's ability to reabsorb a constant fraction of the filtered load regardless of GFR. When GFR increases, the filtered load of sodium and water increases proportionally, but the proximal tubule reabsorbs approximately 65 percent regardless of the absolute amount. This regulatory mechanism involves changes in peritubular capillary Starling forces: increased filtration fraction raises peritubular oncotic pressure, enhancing reabsorption. Glomerulotubular balance prevents massive sodium losses when GFR rises transiently. Panel A: Early PCT (S1) showing apical NHE3 (Na+/H+ exchanger), SGLT2 (Na+/glucose), Na+/amino acid cotransporters, and Na+/phosphate cotransporter with bicarbonate reabsorption via carbonic anhydrase. Panel B: Late PCT (S2-S3) showing paracellular Cl- movement with concentration gradient, lumen-positive potential, and organic anion/cation secretion pathways (OAT, OCT transporters). Panel C: Glomerulotubular balance showing two nephrons with different GFR values but same 65% fractional reabsorption. Panel D: Color-coded solutes with green for glucose, yellow for amino acids, orange for chloride, and pink for bicarbonate. --- ## Glucose Handling The kidney filters approximately 180 grams of glucose daily, yet under normal circumstances not a single gram appears in the urine. This remarkable efficiency reflects the high-capacity transport systems of the proximal tubule. Glucose reabsorption is an example of saturable transport with a well-defined transport maximum (Tm), concepts that illuminate fundamental principles of nephron physiology. Two sodium-glucose cotransporters arranged in series accomplish glucose reabsorption. SGLT2, located in the early proximal tubule S1 segment, has low affinity but high capacity, handling approximately 90 percent of filtered glucose. This transporter couples one sodium ion to one glucose molecule. SGLT1, located in the late proximal tubule S3 segment, has high affinity but low capacity, serving as a "safety net" to capture the remaining 10 percent of glucose. SGLT1 couples two sodium ions to each glucose molecule, providing additional driving force. On the basolateral membrane, GLUT2 in the early proximal tubule and GLUT1 in the late proximal tubule carry glucose into the interstitium via facilitated diffusion down the concentration gradient created by SGLT-mediated apical uptake. The transport maximum for glucose averages approximately 375 mg/min in healthy adults. Below this capacity, all filtered glucose is reabsorbed. The renal threshold, the plasma glucose concentration at which glucose first appears in urine, averages 180 to 200 mg/dL. The discrepancy between the theoretical threshold (where filtered load first exceeds Tm) and the observed threshold reflects splay, the variation in transport capacity among individual nephrons and the kinetics of transporter saturation. Between threshold and complete Tm saturation, glucosuria increases progressively. Above approximately 350 mg/dL plasma glucose, the transporters are fully saturated and glucosuria increases linearly with plasma glucose. These principles have direct clinical applications. In diabetes mellitus, chronic hyperglycemia exceeds the transport maximum, resulting in glucosuria with osmotic diuresis, polyuria, and volume depletion. SGLT2 inhibitors represent a therapeutic approach that deliberately blocks glucose reabsorption to lower blood glucose in diabetes; these medications also provide cardiovascular and renal protection through mechanisms beyond glucose lowering. Familial renal glucosuria is a benign genetic condition caused by SGLT2 mutations, resulting in glucosuria at normal plasma glucose concentrations. Panel A: Glucose titration curve with plasma glucose (0-500 mg/dL) versus filtered, reabsorbed, and excreted glucose (mg/min), showing filtered load as straight line and reabsorption curve reaching Tm plateau (~375 mg/min). Panel B: Excretion beginning at threshold (~180 mg/dL) with splay region between threshold and complete saturation. Panel C: Proximal tubule showing SGLT2 (90%, low affinity, high capacity) in early segment and SGLT1 (10%, high affinity, low capacity) in late segment with GLUT2 and GLUT1 on basolateral membranes. Panel D: Clinical applications showing diabetes (exceeds Tm), SGLT2 inhibitors (therapeutic glucosuria), and familial glucosuria (SGLT2 mutation). --- ## Amino Acid and Protein Handling The kidney filters approximately 50 grams of amino acids daily and reabsorbs greater than 99 percent through sodium-coupled cotransport in the proximal tubule. Multiple distinct transporter systems handle different amino acid classes, reflecting the chemical diversity of amino acids. Neutral amino acid transporters handle the largest group including alanine, serine, and glutamine. Basic amino acid transporters carry arginine, lysine, and ornithine. Acidic amino acid transporters handle glutamate and aspartate. A separate imino acid transporter handles proline and hydroxyproline. The multiplicity of transporters explains why genetic defects typically affect specific amino acid classes rather than all amino acids. Cystinuria exemplifies a clinically significant genetic disorder of amino acid transport. This autosomal recessive condition results from mutations in the rBAT/b0,+AT transporter complex that handles cystine and the dibasic amino acids (ornithine, lysine, arginine, remembered by the mnemonic COLA). Affected individuals lose these amino acids in urine. While most amino acid losses are clinically inconsequential, cystine poses a particular problem: it is poorly soluble, especially at acidic pH, and precipitates to form cystine kidney stones. These distinctive hexagonal crystals can cause significant morbidity. Treatment involves high fluid intake to dilute urinary cystine, alkalinization to increase solubility, and sometimes thiol-containing drugs like penicillamine that form more soluble mixed disulfides. Protein handling differs fundamentally from amino acid transport. Large plasma proteins do not cross the glomerular filtration barrier and therefore are not filtered. Small proteins and peptides that do filter are reclaimed by receptor-mediated endocytosis rather than carrier-mediated transport. The multi-ligand receptors megalin and cubilin on the proximal tubular brush border bind filtered proteins, triggering clathrin-mediated endocytosis. Internalized proteins undergo lysosomal degradation, with amino acids returned to the circulation. Even albumin, the majority of which is retained by the filtration barrier, appears in tiny amounts in the filtrate and is reclaimed by this endocytic pathway. The minimal protein normally appearing in urine consists mainly of Tamm-Horsfall protein secreted by the thick ascending limb. Panel A: Multiple amino acid transporter systems on proximal tubule apical membrane labeled by class (neutral, basic/dibasic, acidic, imino) with dibasic transporter handling COLA (cystine, ornithine, lysine, arginine). Panel B: Cystinuria callout with hexagonal cystine crystal illustration and note about stone formation at low pH. Panel C: Protein endocytosis mechanism with megalin/cubilin receptors on brush border binding small proteins, clathrin-coated pit formation, and endosome with lysosomal degradation. Panel D: Amino acid release to circulation and size exclusion at glomerulus for large proteins. --- ## Loop of Henle Function The loop of Henle creates the conditions necessary for urine concentration while also performing significant sodium reabsorption. Its distinct segments, each with unique permeability properties and transport characteristics, work together to establish and maintain the medullary osmotic gradient. The thin descending limb possesses high water permeability through constitutive aquaporin-1 expression but very low permeability to sodium chloride. As tubular fluid descends into the progressively hyperosmotic medullary interstitium, water exits down its osmotic gradient while solutes remain trapped. The tubular fluid becomes increasingly concentrated, reaching osmolalities of 1200 mOsm/kg at the hairpin turn in juxtamedullary nephrons. The thin ascending limb reverses this process. It has essentially no water permeability but moderate permeability to sodium chloride. As the now-concentrated tubular fluid ascends toward the cortex through decreasingly hyperosmotic interstitium, sodium chloride diffuses passively into the interstitium while water cannot follow. The tubular fluid becomes progressively more dilute. The thick ascending limb dramatically amplifies this dilution through active transport. The nickname "diluting segment" reflects its critical role in generating dilute tubular fluid. The apical Na-K-2Cl cotransporter NKCC2 represents the key transport protein, moving one sodium, one potassium, and two chloride ions into the cell together. The sodium-potassium ATPase on the basolateral membrane maintains the sodium gradient, while potassium and chloride exit through basolateral channels. Importantly, potassium entering via NKCC2 must recycle back to the lumen through the ROMK potassium channel to sustain NKCC2 activity, as luminal potassium would otherwise become limiting. This potassium recycling generates a lumen-positive transepithelial potential that drives paracellular reabsorption of calcium and magnesium. Loop diuretics such as furosemide and bumetanide specifically block NKCC2. This inhibition reduces sodium chloride reabsorption, producing profound diuresis. By impairing the thick ascending limb's contribution to the medullary gradient, loop diuretics also diminish concentrating ability. The loss of lumen-positive potential increases urinary calcium and magnesium excretion, causing hypocalcemia and hypomagnesemia with chronic use. Panel A: Thin descending limb with aquaporin-1 allowing water exit (blue arrows) toward hyperosmotic interstitium with osmolality scale 300→1200 mOsm/kg. Panel B: Thin ascending limb with no water permeability (barrier symbol) but NaCl exit (orange arrows) with dilution. Panel C: Thick ascending limb cell showing NKCC2 on apical membrane (1Na+, 1K+, 2Cl-), ROMK recycling K+ to lumen, basolateral Na+/K+-ATPase and K+/Cl- channels, with lumen-positive potential driving paracellular Ca2+ and Mg2+ movement. Panel D: Loop diuretic blocking NKCC2 with resulting effects: ↓NaCl reabsorption, ↓medullary gradient, ↓Ca2+/Mg2+ reabsorption. --- ## Distal Convoluted Tubule Function The distal convoluted tubule continues the dilution process begun in the thick ascending limb, earning its designation as the "cortical diluting segment." Though reabsorbing only approximately 5 percent of filtered sodium, this segment plays important roles in fine-tuning sodium balance and regulating divalent cation excretion. The apical sodium-chloride cotransporter NCC represents the principal sodium entry mechanism in the distal convoluted tubule. This electroneutral transporter couples one sodium with one chloride ion, driven by the favorable sodium gradient. Sodium exits across the basolateral membrane via the sodium-potassium ATPase, while chloride exits through chloride channels. Like the thick ascending limb, the distal convoluted tubule is essentially water-impermeable, so sodium chloride reabsorption without water further dilutes the tubular fluid. Calcium handling in the distal convoluted tubule is distinctive and highly regulated. Unlike the paracellular, voltage-driven calcium reabsorption in the thick ascending limb, distal tubular calcium transport is transcellular and subject to hormonal control. Calcium enters the cell through the apical TRPV5 channel, binds to intracellular calbindin that buffers cytosolic calcium and facilitates diffusion, and exits across the basolateral membrane via the sodium-calcium exchanger NCX and the calcium ATPase. Parathyroid hormone stimulates each step of this pathway, increasing calcium reabsorption in response to hypocalcemia. Thiazide diuretics block NCC, producing natriuresis and diuresis. An interesting and clinically useful consequence is enhanced calcium reabsorption. The mechanism involves the sodium-calcium exchanger on the basolateral membrane, which couples sodium entry to calcium extrusion. When NCC blockade lowers intracellular sodium, NCX activity increases, driving more calcium reabsorption. This property makes thiazides valuable for patients with hypercalciuria and calcium nephrolithiasis. Panel A: DCT cell showing apical NCC cotransporter (Na+ and Cl- entering together) with thiazide blocking indicated, and TRPV5 calcium channel with Ca2+ entry and PTH stimulation arrow. Panel B: Intracellular calbindin binding calcium for buffered transport, with basolateral Na+/K+-ATPase, chloride channels, NCX, and Ca2+-ATPase. Panel C: Thiazide effect callout showing blocking NCC → ↓intracellular Na+ → ↑NCX activity → ↑Ca2+ reabsorption. Panel D: Water impermeability label with color-coded ions: green for sodium, orange for chloride, purple for calcium. --- ## Collecting Duct Function The collecting duct represents the final site for regulating urine composition, determining the ultimate sodium, potassium, and water content through processes subject to tight hormonal control. Two distinct cell types populate the collecting duct epithelium, each with specialized functions. Principal cells constitute the majority of collecting duct epithelium and mediate sodium reabsorption, potassium secretion, and water reabsorption. The apical epithelial sodium channel ENaC provides the pathway for sodium entry, driven by the favorable electrochemical gradient. This sodium entry generates a lumen-negative transepithelial potential that favors potassium secretion through the apical ROMK channel. Water permeability is regulated by aquaporin-2 on the apical membrane; ADH stimulates translocation of AQP2-containing vesicles to the apical surface, while AQP3 and AQP4 provide constitutive basolateral water permeability. Aldosterone, the principal regulator of collecting duct sodium transport, exerts its effects over hours through genomic mechanisms. After diffusing into principal cells and binding the mineralocorticoid receptor, aldosterone stimulates transcription of genes encoding ENaC subunits, the sodium-potassium ATPase, and ROMK. The net effect is enhanced sodium reabsorption coupled to increased potassium secretion. Multiple factors modulate potassium secretion beyond aldosterone: high dietary potassium intake increases secretion, high tubular flow rate washes away secreted potassium thereby maintaining a favorable gradient for continued secretion, and alkalosis enhances potassium secretion through intracellular potassium shift. Intercalated cells manage acid-base balance. Type A intercalated cells secrete hydrogen ions via an apical H+-ATPase and H+/K+-ATPase, with bicarbonate exiting the basolateral membrane through the AE1 chloride-bicarbonate exchanger. These cells predominate during acidosis. Type B intercalated cells do the opposite, secreting bicarbonate via apical pendrin and reabsorbing hydrogen ions, predominating during alkalosis. The ratio of type A to type B cells shifts with chronic acid-base disturbances. Potassium-sparing diuretics target the collecting duct. Amiloride and triamterene directly block ENaC, reducing sodium reabsorption and diminishing the electrical gradient for potassium secretion. Spironolactone and eplerenone block the mineralocorticoid receptor, preventing aldosterone's stimulatory effects. Both drug classes cause potassium retention and can produce hyperkalemia, particularly in patients with impaired renal function or concurrent use of other potassium-retaining agents. Panel A: Principal cell showing apical ENaC (Na+ entry), ROMK (K+ secretion), and AQP2 regulated by ADH vesicle insertion, with basolateral Na+/K+-ATPase and AQP3/4, and lumen-negative potential from Na+ entry. Panel B: Type A intercalated cell with apical H+-ATPase secreting H+ and basolateral AE1 (Cl-/HCO3- exchanger); Type B with apical pendrin secreting HCO3-. Panel C: Aldosterone mechanism inset showing hormone entering cell, binding MR, and increasing ENaC/ROMK/Na+K+ATPase expression. Panel D: Drug targets showing amiloride blocking ENaC and spironolactone blocking MR with distinct cell shapes and consistent ion colors. --- ## Organic Anion and Cation Secretion The proximal tubule actively secretes a remarkable variety of organic anions and cations, including endogenous waste products, drugs, and toxins. This secretory capacity provides an important mechanism for eliminating substances that are protein-bound and therefore not filtered, as well as accelerating clearance of filtered compounds. Organic anion transport exemplifies a well-characterized secretory system. Organic anion transporters OAT1 and OAT3 on the basolateral membrane take up organic anions from peritubular blood in exchange for intracellular dicarboxylates, effectively using the dicarboxylate gradient to power organic anion uptake. On the apical membrane, multidrug resistance-associated proteins MRP2 and MRP4 export organic anions into the tubular lumen. Substrates include para-aminohippuric acid (the basis for RPF measurement), uric acid, numerous antibiotics including penicillins and cephalosporins, loop diuretics, and nonsteroidal anti-inflammatory drugs. Organic cation transport operates through a parallel system. The organic cation transporter OCT2 on the basolateral membrane mediates cation uptake driven by the inside-negative membrane potential. Multidrug and toxin extrusion proteins MATE1 and MATE2-K on the apical membrane export organic cations into the lumen in exchange for hydrogen ions. Substrates include creatinine (contributing to the slight overestimation of GFR by creatinine clearance), dopamine, epinephrine, morphine, metformin, and many other drugs. Drug interactions commonly arise from competition for these transporters. The classic example involves probenecid, which competes with penicillin for organic anion secretion. Before penicillin became abundant, probenecid was co-administered to slow penicillin excretion and maintain therapeutic levels. Today, understanding transporter interactions remains important for predicting drug-drug interactions affecting renal elimination. Uric acid handling is particularly complex, involving both reabsorption and secretion. After free filtration, approximately 90 percent of filtered urate is reabsorbed, primarily through URAT1 on the proximal tubule apical membrane. Some secretion also occurs. Net urate excretion averages about 10 percent of the filtered load. Uricosuric drugs like probenecid block URAT1, increasing urate excretion for treatment of gout. Conversely, low-dose aspirin paradoxically increases serum urate by inhibiting secretion more than reabsorption. Panel A: Proximal tubule cell showing organic anion transport with basolateral OAT1/OAT3 exchanging organic anions for dicarboxylates, and apical MRP2/MRP4 exporting to lumen; substrates include PAH, penicillins, furosemide, NSAIDs, uric acid. Panel B: Organic cation transport with basolateral OCT2 importing organic cations and apical MATE1/MATE2 exporting in exchange for H+; substrates include creatinine, metformin, morphine, dopamine. Panel C: Urate handling diagram showing filtration, 90% URAT1 reabsorption, and net 10% excretion with probenecid blocking URAT1. Panel D: Color coding with red for anionic and blue for cationic compounds. --- ## Water Reabsorption Along the Nephron Water reabsorption follows predictable patterns along the nephron, with each segment's water permeability determining how much fluid is recovered. Understanding these patterns explains how the kidney produces urine ranging from maximally dilute to maximally concentrated. The proximal tubule reabsorbs approximately 65 percent of filtered water through constitutively expressed aquaporin-1. This reabsorption is obligatory, meaning it occurs automatically following solute reabsorption regardless of water balance status. The isosmotic nature of proximal reabsorption maintains tubular fluid at plasma osmolality. The thin descending limb also expresses aquaporin-1, allowing water to exit into the hyperosmotic medullary interstitium; the amount varies with medullary tonicity and nephron length. The thick ascending limb and distal convoluted tubule are essentially water-impermeable. This impermeability is crucial for generating dilute urine: as sodium chloride is reabsorbed without water, the tubular fluid becomes hypotonic. By the end of the distal convoluted tubule, tubular fluid osmolality has fallen to approximately 100 mOsm/kg. The collecting duct determines final urine concentration through ADH-regulated water permeability. Without ADH, the collecting duct remains water-impermeable, and the dilute fluid from the distal tubule passes through largely unchanged, producing urine as dilute as 50 mOsm/kg. With maximal ADH, aquaporin-2 insertion renders the collecting duct highly water-permeable. As the collecting duct traverses the hyperosmotic medulla, water exits down its osmotic gradient, concentrating the urine to equilibrate with the surrounding interstitium at up to 1200 mOsm/kg. This represents facultative water reabsorption, occurring only when the body requires water conservation. Aquaporins are the molecular mediators of water permeability. AQP1 provides constitutive permeability in the proximal tubule and thin descending limb. AQP2 is the regulated channel in the collecting duct apical membrane, cycling between intracellular vesicles and the plasma membrane under ADH control. AQP3 and AQP4 provide basolateral water exit from collecting duct cells. ADH binding to the V2 receptor triggers a cAMP-dependent signaling cascade that culminates in AQP2 vesicle fusion with the apical membrane. Long-term ADH exposure also increases AQP2 gene transcription. Panel A: Nephron from glomerulus to collecting duct showing PCT 65% water reabsorption (obligatory, AQP1, isosmotic) and thin descending limb (variable, AQP1, follows medullary gradient). Panel B: TAL 0% (water impermeable, dilutes fluid) and DCT 0% (water impermeable, continues dilution) with tubular fluid osmolality falling to ~100 mOsm/kg. Panel C: Collecting duct 5-24% (facultative, AQP2, ADH-dependent) with detailed inset of principal cell showing V2 receptor → cAMP → PKA → AQP2 vesicle insertion. Panel D: ADH presence/absence states with resulting urine osmolality (50-1200 mOsm/kg) and osmolality gradient indicated by color. --- ## Summary of Nephron Transport and Diuretics Integrating the transport properties of each nephron segment provides a coherent picture of solute and water handling. The proximal tubule performs bulk reabsorption, recovering 65 percent of sodium and water isosmotically through NHE3, SGLT2, and aquaporin-1. The thick ascending limb reabsorbs 25 percent of sodium via NKCC2 without water, diluting the tubular fluid and contributing to the medullary gradient. The distal convoluted tubule reabsorbs 5 percent of sodium via NCC without water, continuing dilution. The collecting duct reabsorbs 1 to 2 percent of sodium via ENaC and controls final water excretion through ADH-regulated AQP2. Each segment's principal transporter represents a diuretic target. Carbonic anhydrase inhibitors such as acetazolamide impair proximal bicarbonate reabsorption, causing bicarbonaturia and modest diuresis. Osmotic diuretics like mannitol remain in the tubular fluid and obligate water retention, primarily affecting the proximal tubule. Loop diuretics block NKCC2 in the thick ascending limb, producing the most powerful diuretic effect and impairing concentrating ability. Thiazides block NCC in the distal tubule, causing moderate diuresis with hypocalciuria. Potassium-sparing diuretics target ENaC or the mineralocorticoid receptor in the collecting duct, producing mild diuresis with potassium retention. Vasopressin receptor antagonists (vaptans) block V2 receptors, causing aquaresis without natriuresis. Inherited tubulopathies illustrate the importance of specific transporters. Fanconi syndrome involves generalized proximal tubule dysfunction with losses of glucose, amino acids, phosphate, and bicarbonate. Bartter syndrome results from genetic defects in NKCC2, ROMK, or related proteins, mimicking chronic loop diuretic administration with salt wasting, hypokalemia, and metabolic alkalosis. Gitelman syndrome involves defective NCC, resembling chronic thiazide use with salt wasting, hypokalemia, hypomagnesemia, and hypocalciuria. Liddle syndrome is caused by gain-of-function mutations in ENaC, producing sodium retention, hypertension, and hypokalemia without elevated aldosterone. Panel A: Nephron showing PCT (NHE3/SGLT2, 65% Na reabsorption) with CA inhibitors, and TAL (NKCC2, 25%) with loop diuretics. Panel B: DCT (NCC, 5%) with thiazides and CD (ENaC, 1-2%) with K+-sparing diuretics and vaptans (V2 receptor). Panel C: Table of inherited tubulopathies by segment: Fanconi (PCT), Bartter (TAL/NKCC2), Gitelman (DCT/NCC), Liddle (CD/ENaC) with clinical features. Panel D: Consistent color coding for each segment with clear organization. --- ## Summary - The sodium-potassium ATPase on the basolateral membrane drives all secondary active transport by maintaining low intracellular sodium - The proximal tubule performs bulk reabsorption (65%), coupling sodium to glucose, amino acids, and bicarbonate - Glucose is reabsorbed by SGLT2 (90%) then SGLT1 (10%) with transport maximum approximately 375 mg/min and threshold approximately 180 mg/dL - The thick ascending limb reabsorbs 25% of sodium via NKCC2 while remaining water-impermeable, serving as the diluting segment - The distal convoluted tubule reabsorbs 5% of sodium via NCC with active PTH-regulated calcium reabsorption - The collecting duct fine-tunes sodium through aldosterone-regulated ENaC and water through ADH-regulated AQP2 - Diuretics target specific transporters: loop diuretics inhibit NKCC2, thiazides inhibit NCC, potassium-sparing agents block ENaC or the mineralocorticoid receptor - Water reabsorption is 65% obligatory in the proximal tubule with final concentration determined by ADH-regulated AQP2 in the collecting duct --- ## Key Terms | Term | Definition | |------|------------| | Transport maximum (Tm) | Maximum rate of carrier-mediated transport for a given substance | | SGLT2 | Sodium-glucose cotransporter 2, responsible for 90% of renal glucose reabsorption | | NKCC2 | Na+-K+-2Cl- cotransporter in the thick ascending limb, target of loop diuretics | | NCC | Na+-Cl- cotransporter in the distal convoluted tubule, target of thiazides | | ENaC | Epithelial sodium channel in the collecting duct, target of amiloride | | Aquaporin | Water channel protein; AQP2 is ADH-regulated in collecting duct | | Glomerulotubular balance | Maintenance of constant fractional reabsorption regardless of GFR changes | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*