Tubular Reabsorption and Secretion - MLS 419 PDF

| Topic #2 | [MLS 419-LEC] Analysis of Urine and Other Body Fluids P2: Tubular Reabsorption and Secretion Part 1 Professor: Mr. Christian Villahermosa, RMT, MSMT Date: January 22, 2024 INTR...

| Topic #2 | [MLS 419-LEC] Analysis of Urine and Other Body Fluids P2: Tubular Reabsorption and Secretion Part 1 Professor: Mr. Christian Villahermosa, RMT, MSMT Date: January 22, 2024 INTRODUCTION ○ Against its gradient → it will not diffuse, it Once the blood enters the glomerulus from the is forcibly transported afferent arteriole, it undergoes filtration. ○ A substance will move from an area of low The filtrate goes into the bowman’s space and will concentration to an area that is already proceed to the proximal convoluted tubule. present in high concentrations The filtrate will undergo reabsorption and secretion Needs a carrier protein to transport substance until it will form urine. and requires energy (e.g., hydrolysis of ATP) ○ The filtrate will be called urine if it no PASSIVE TRANSPORT longer undergoes reabsorption and Movement is due to differences in the concentration secretion. or electrical potentials (may be TRANSCELLULAR TUBULAR REABSORPTION or PARACELLULAR) The body cannot lose 120-125 mL of water every Diffusion → a substance will naturally move from minute an area of higher concentration to an area of lower Therefore, some substances (e.g., water, concentration to achieve equilibrium electrolytes, glucose) in the urine need to be Does not need carrier protein and does not REABSORBED require energy Tubular reabsorption refers to the return of most 2 PATHS THAT INVOLVE THE MOVEMENT OF of the filtered water and many solutes from the SUBSTANCES INTO THE TUBULAR CELL tubules to the bloodstream TRANSCELLULAR ○ About 99% of filtered water reabsorbed The substance will enter the cell and leave via that If the kidney will not reabsorb → same cell until it reaches the peritubular capillaries urination every minute PARACELLULAR ○ Proximal convoluted tubule cells make The substance will move from the lumen of the the largest contribution tubule into the blood vessels via the tight junctions Reabsorption is highly selective (space between the cells) ○ Only essential substances are reabsorbed FILTRATION, REABSORPTION, AND EXCRETION RATES OF DIFFERENT SUBSTANCES BY THE KIDNEYS Substances Amount Amount Amount % of filtered filtered reabsorbed excreted load reabsorbed Glucose 180 180 0 100 (g/day) Bicarbonate 4320 4318 2 >99.9 BULK FLOW/ULTRAFILTRATION (mEq/day) Movement of substances from the tubular cell into the peritubular capillaries Sodium 25,560 25,410 150 99.4 ○ The substance crosses the interstitial fluid (mEq/day) BRUSH BORDERS Fingerlike projections which contain microvilli ○ The presence of brush borders increases Chloride 19,440 19,260 180 99.1 (mEq/day) surface area = increases reabsorption of substances present in the lumen Potassium 756 664 92 87.8 REABSORPTION OF SODIUM (mEq/day) Sodium diffuses across the luminal membrane (also called the apical membrane) into the cell down Urea 46.8 23.4 23.4 50 an electrochemical gradient (passive transport) (g/day) ○ Luminal membrane → membrane of the tubular cells facing the lumen Creatinine 1.8 0 1.8 0 (g/day) Sodium is transported across the basolateral membrane against an electrochemical gradient by ACTIVE TRANSPORT the Na+-K+ ATPase pump (Sodium-Potassium Movement of substance across a cell membrane ATPase pump) and against an osmotic gradient (always ○ Basolateral membrane → membrane TRANSCELLULAR) facing the interstitial fluid @mlstranses | 1  SODIUM-POTASSIUM ATPASE PUMP Smaller proteins that goes in the proximal Uses ATP convoluted tubule → the kidney will reabsorb them, Its action is to release sodium since they are essential ○ Since sodium is extracellular, it should not PROTEINS be present PCT reabsorbs proteins via pinocytosis ○ In return for releasing sodium → ○ Digestion into amino acids in vesicles potassium enters the cell MEGALIN CUBILIN COMPLEX In the presence of the pump in the tubular cells → Found on the surface of the tubular cell (labelled as expels sodium from the tubular cells into the M and C at the diagram below) peritubular capillaries → back to the circulation Binds to any protein present in the filtrate (reabsorbed) ○ When the protein binds to the complex → undergoes pinocytosis → leads to the entry of megalin and cubilin + protein inside a vesicle ○ The vesicle will merge with a lysosome → the proteins will be degraded into a simpler amino acid ○ The amino acid will be sent back to the REABSORPTION OF GLUCOSE circulation for recycling An example of secondary active transport Glucose is increased inside the cell Naturally, glucose would not enter the cell → tubules will use transporters in order for it to enter the cell SGLT (SODIUM-GLUCOSE LINKED TRANSPORTER) “Entry point” WATER Carries glucose into the tubular cell cytoplasm Water reabsorption: Since glucose is elevated intracellularly, glucose ○ Due to aquaporins (found on the surface of would not be attracted to the cell hence SGLT the tubular cells) and tight junctions should attract glucose. water can either pass through ○ In order for sodium to diffuse into the cell, intracellularly via aquaporin or it has to pass through SGLT tight junctions ○ When sodium enters the SGLT → It will go ○ Dependent on membrane permeability out directly through the Na+-K+ ATPase the different parts of our tubule of pump our nephron have their different ○ Since the pump would utilize ATP→ the affinity to water ATP released is enough to attract glucose PCT/DLOH: always high due to into the cell abundant aquaporins Entry of glucose into the cell → active transport always reabsorb water GLUT (GLUCOSE TRANSPORTER) ALOH: always low (low surface Helps glucose diffuse out of the cell (facilitated area and less permeable tight diffusion) junctions Glucose is in the cell, however, it must leave the cell impermeable to water to go to the circulation (reabsorption) → it will not reabsorb ○ Glucose is increased intracellularly and water decreased extracellularly → diffusion the tight junctions are occurs tighter compare to PCT ○ Glucose will leave the cell via GLUT Has less surface area Exit of glucose from the cell → diffusion because there are less brush borders REABSORPTION OF PROTEINS AND WATER Albumin is the protein prioritised in the glomerulus Other smaller protein such as vitamins, hormones, immune system products → they are still filtered by the glomerulus @mlstranses | 2  REABSORPTION OF UREA AND CHLORIDE LATE PCT UREA Pars recta Half of the urea is reabsorbed sodium and chloride reabsorption Passive reabsorption via urea transporters LOOP OF HENLE CHLORIDE THIN DESCENDING LOOP OF HENLE Transcellular passive diffusion Thin epithelial membranes with no brush Transported along with sodium due to lumen borders and few mitochondria negative potential ○ Reabsorptive capacity is low Chloride will be reabsorbed only if there is a Simple diffusion of water and some solutes negative potential in the lumen THICK ASCENDING LOOP OF HENLE ○ Negative potential: if sodium (cation) will Thick epithelial cells with high metabolic activity easily diffuse into the cell → the lumen will Active reabsorption of sodium, potassium, and become negative chloride via NKCC2 cotransporter ○ negatively charged lumen is not normal → ○ 1 sodium, 1 potassium, and 2 chloride at a chloride will be reabsorbed time since chloride is anion, the Note: Loop diuretics (furosemide, ethacrynic acid, negative potential will decrease bumetanide) are inhibitors of NKCC2 ACTIVE TRANSPORT ○ diuretics can prevent sodium reabsorption MAXIMAL REABSORPTIVE CAPACITY (Tm) because they want sodium to Highest rate of reabsorption of a substance remain in the lumen and water before it appears in urine; amount of solute would also remain → both of reabsorbed per minute them will be urinated Unit: time/rate of reabsorption of all the nephrons in ALOH is impermeable to water the kidney ○ As filtrate moves through the ascending ○ “How fast can the nephrons process a limb, NaCl moves out making the filtrate solute?” more and more dilute Example: Glucose is at 350 mg/min. (once this is Significance: Keeping the renal medulla region of reached, all nephrons have reached maximal the kidney at high osmolarity so water can move capacity to reabsorb glucose) passively out of the filtrate in the CT ○ if beyond 350 mg/min → it will not be DISTAL CONVOLUTED TUBULE processed → seen in the urine EARLY DISTAL TUBULE RENAL THRESHOLD FIRST PORTION the plasma concentration of substances at which forms the macula densa active transport of reabsorption stops SECOND PORTION ○ “What is the plasma concentration?” Reabsorber part of the DCT because of the Refers to the blood level of the substance presence of the Sodium-chloride co-transporter ○ Example: Glucose is at 160-180 mg/dL same reabsorptive characteristics of the thick ALOH ○ Note: When the plasma glucose ○ impermeable to water except for concentration exceeds the transport electrolytes maximum, reabsorption stops, and the Avid reabsorption of most ions substance is excreted in urine. ○ Sodium-chloride co-transporter It is important because you have to correlate the ○ Note: Thiazide diuretics inhibit positive urine glucose to the blood glucose of the sodium-chloride co-transporter patient Virtually impermeable to water (requires ADH for PARTS OF THE NEPHRON water reabsorption) and urea PROXIMAL CONVOLUTED TUBULE ○ In special conditions, it can be permeable Major contributor of tubular reabsorption because it to water if there is ADH (vasopressin) is highly metabolic (high number of mitochondria) and contains many brush borders Substances reabsorbed (U.S.W.A.G): ○ Urea ○ Salt (Sodium and Chloride) ○ Water ○ Amino acid ○ Glucose EARLY PCT Closest to the bowman’s capsule sodium reabsorption and co-transport with glucose, amino acids, other solutes @mlstranses | 3  | Topic # | [MLS 419-LEC] Analysis of Urine and Other Body Fluids P1: DISTAL CONVOLUTED TUBULE, COLLECTING DUCT, AND CLEARANCE TESTS Professor: Mr. Christian Villahermosa, RMT, MSMT Date: Jan 28, 2024 DISTAL CONVOLUTED TUBULE AND THE COLLECTING DUCT The collecting duct is divided into two: ○ Cortical collecting duct → a part of the collecting duct that is located in cortex and functions similarly to the late distal tubule ○ Medullary collecting duct → a part of the collecting duct that is located in the renal medulla LATE DISTAL TUBULE AND CORTICAL COLLECTING TUBULES Both parts of the nephron contain Principal cells MEDULLARY COLLECTING DUCT PRINCIPAL CELLS Final site for urine processing Reabsorb sodium (controlled by aldosterone) via Permeability to water and urea is controlled by Epithelial Sodium Channels (ENaC) and NKA ADH pump ○ Presence of aquaporins and urea ○ In normal cases, sodium reabsorption is transporters (urea goes back to the DLOH) minimal, however, if there is the presence ○ Urea is reabsorbed because the kidney’s of aldosterone → reabsorption of sodium renal medulla has to be preserved in a ○ Sodium to the blood vessels → hypertonic environment → water can be Sodium-Potassium ATPase pump attracted and leave the collecting duct Secrete potassium via the Renal Outer Medullary Potassium channel (ROMK) ○ When sodium is reabsorbed → negative potential in the lumen → secretion of potassium from the principal cells into the lumen Note: Target of Potassium-Sparing Diuretics → inhibits the activity of the principal cells ○ Needed by patients who need diuretic and also experiencing hypokalemia ○ Two types: COUNTERCURRENT MECHANISM Mineralocorticoid receptor What happens to the osmolarity of the filtrate as it antagonists(e.g. Spironolactone) travels along the different parts of the nephron The drug will bind to the The renal medulla is hypertonic → water can easily receptor of aldosterone be reabsorbed → Inhibits binding to the principal cells Sodium channel blockers (e. g. Amiloride) Directly block epithelial sodium channel Permeability to water depends on ADH (binds only to aquaporin-2) ○ The late distal tubule and cortical collecting tubules do not normally reabsorb water ○ If ADH is present → the tubular cells will manifest aquaporin-2, which is the only aquaporin that will respond to ADH → The filtrate that leaves the bowman’s space is 300 Water reabsorption mosm/L and enters the proximal convoluted tubule As it travels down the descending loop of Henle, osmolarity increases up to 900 mosm/L ○ Since the DLOH can reabsorb water → water is removed from the filtrate → what @mlstranses | 1  remains inside is a very concentrated TUBULAR REABSORPTION filtrate → increased osmolarity CONTROL OF WATER LOSS The highest increased level of osmolarity is found at How concentrated or diluted your urine is, depends the “hairpin turn” since the movement/absorption is upon the body’s state of hydration: slowed down ○ Dehydration = hypertonic urine (more The osmolarity is decreased at the ascending loop concentrated) of henle since sodium/chloride is reabsorbed High blood osmolarity in a ○ The NaCl removed from the ALOH dehydrated person → stim. contributes to the osmolarity of the medulla Pituitary gland → release ADH → ○ When NaCl is removed → the osmolality of Increased aquaporin channels in the medulla is increased → reabsorbs cell membrane → increased CD’s water water permeability → less urine is The osmolality is also increased at the collecting formed ducts since water is reabsorbed due to the ○ Drinking lots of water = hypotonic urine presence of ADH (called water diuresis) Summary of the concurrent mechanism More water excretion Renal concentration begins in the loop of Henle, Continuous urination → water where filtrate is exposed to high gradient of the diuresis renal medulla. TUBULAR SECRETION Water is reabsorbed through osmosis in the Passage of substances from the blood in the descending loop of Henle peritubular capillaries into the tubules. Na and Cl are reabsorbed in the ascending loop Secretion happens due to: of Henle Dilution of the highly concentrated medulla is ○ Elimination of waste products not filtered prevented by the impermeable walls of the by the glomerulus (size and charge) ascending loop to water. ○ Regulation of acid-base balance in the Therefore, most of the water delivered to this body through the secretion of hydrogen segment remains in the tubule, despite Many foreign substances, such as medications, are reabsorption of large amounts of solute. not filtered by the glomerulus because they are Thus, the tubular fluid becomes very dilute and results into diluted urine. bound to proteins, but when they travel to the peritubular capillaries → loses their affinity to their protein carriers→ they develop affinity to the tubules, which causes them to dissociate from their protein carriers = SECRETED ACID-BASE BALANCE Regulates acid-base balance by eliminating either hydrogen ions or bicarbonate ions Normal blood pH: 7.4 (excess acids must be eliminated) BICARBONATE IONS If alkalosis → the body will eliminate bicarbonate ions, in exchange for the reabsorption of hydrogen Responsible for buffering capacity ○ Readily filtered and reabsorbed HYDROGEN IONS If acidosis → the body will eliminate hydrogen ions, in exchange for the reabsorption of bicarbonate Secreted by RTE cells into the filtrate (sodium-hydrogen exchanger) ○ Prevents the filtered bicarbonate from being excreted Hydrogen cannot be eliminated by itself, it has to bind to a larger substance for it to be eliminated Excreted via: ○ Filtered phosphate ion Hydrogen goes out the cell → binds with hydrogen phosphate → to form dihydrogen phosphate → urine ○ Ammonia Deamination of glutamine → forms ammonia → hydrogen will @mlstranses | 2  bind to ammonia → to form Neither reabsorbed or ○ requires infusion at ammonium ion secreted a constant rate (3-4 hours) because it is not a normal body constituent Impractical RADIONUCLIDES Safe radioisotopes that is infused to the patient Sample: Plasma tested by their disappearance from the plasma, thereby eliminating the need for urine collection. CELLS THAT PERFORM ACID-BASE BALANCE ○ If 100mg of 99mTc-DTPA is infused to the INTERCALATED CELLS patient, the same value should be seen Located in the Late distal tubules and cortical after collection collecting tubules along with the principal cells Examples: Late distal tubules and cortical collecting tubules ○ 125I-iothalamate and 99mTc-DTPA ALPHA-INTERCALATED CELLS (diethylene-triamine –pentaacetate) secrete hydrogen ions and reabsorb bicarbonate ○ 51Cr-EDTA BETA-INTERCALATED CELLS ADVANTAGES DISADVANTAGE secrete bicarbonate and reabsorb hydrogen ions RENAL FUNCTION TESTS - GLOMERULAR FILTRATION Enables visualization of the More labor intensive and RATE filtration in the kidneys costly About 1200 ml of blood (650 ml plasma) passes BETA-2-MICROGLOBULIN through the kidneys every minute. forms part of the class I MHC present in leukocytes About 120-125 ml is filtered per minute by the (11,800 kda) kidneys & this is referred to as glomerular ○ REVIEW: MHC class I will connect filtration rate (GFR). antigen-presenting cells with cytotoxic t GLOMERULAR FILTRATION TESTS cell CLEARANCE TESTS Dissociates at a constant rate from WBCs and is Gold standard rapidly removed from the plasma by the kidneys measures the rate by which the kidneys remove a filterable substance from the blood. Sample used is 24-hour urine CHARACTERISTICS OF AN IDEAL CLEARANCE TEST SUBSTANCE The substance must be neither reabsorbed nor secreted by the tubules The substance must be stable during the 24-hour collection Substance’s availability in the body DISADVANTAGE Consistency of plasma level Availability of the test in the lab However, the test is not reliable in patients with UREA immunologic disease or malignancy earliest glomerular filtration test ○ Patients with immunologic diseases are prone to antigen presentation → more ADVANTAGES DISADVANTAGE beta-2-microglobulin production Also degrades in an acidic environment Present in all urine 50% of the filtered specimens urea is reabsorbed by BETA TRACE PROTEIN Available lab methods the tubules Normally produced (endogenous) Endogenous ○ hydration needs to ○ From the normal be done A low-molecular weight protein isolated in the CSF waste product of DISADVANTAGE protein metabolism Freely filtered and reabsorbed by the PCT INULIN TRYPTOPHAN GLYCOCONJUGATE polymer of fructose that is a prebiotic fiber Filtered freely and not reabsorbed considered as the gold standard in measuring Strong linear correlation with inulin clearance GFR DISADVANTAGE ADVANTAGES DISADVANTAGE Highly stable Exogenous @mlstranses | 3  Measured ONLY using HPLC (expensive and time-consuming) CYSTATIN C A small protein produced by all nucleated cells Sample: Serum (blood) Endogenous Readily filtered, reabsorbed, and broken down by renal tubules ○ Normal levels in plasma: Low Since the renal tubules can break it down → content in blood is low/ zero → good GFR ○ Plasma level is inversely proportional to GFR ○ Changes in serum concentration are used as indirect estimate of GFR Potential marker for long-term monitoring of renal function ADVANTAGES DISADVANTAGE Constant in serum Higher analysis cost levels Independent of age, gender, and muscle mass More sensitive to GFR changes than serum creatinine CREATININE most widely used endogenous procedure Used in the philippines Not reabsorbed/secreted If the serum creatinine is increased → Kidney failure (indirect testing) DISADVANTAGE Secreted by tubules and secretion increases as blood level increases Chromogens in the plasma can react in the chemical analysis for creatinine ○ The machine might detect chromogens as creatinine (false increase) Bacteria will break down urinary creatinine if specimen is kept at room temp for extended period A diet heavy in meat consumed during collection of a 24hr urine will influence the creatinine level Not reliable in patients with muscle wasting diseases ○ Increases creatinine Interference by medication (salicylate, trimethoprim, cimetidine) @mlstranses | 4