Lectures 20, 21 – Renal Handling of NaCl – Block 5 Cell Biology and Physiology
Basic Renal Processes
 Filtration
o Nonselective  useful and waste substances are filtered
 Proteins are the exception  not filtered
 Reabsorption and Secretion
o Very selective
 Needs transporters/channels
 Occurs in specific locations
 Glucose reabsorbed in the proximal tubule
 Excretion
o Filtered + Secreted - Reabsorbed
Note:
 Inulin  filtered but not secreted or reabsorbed
 PAH  filtered and secreted but not reabsorbed
 Glucose  filtered and reabsorbed but not secreted
Net Reabsorption/Secretion
 Filtered load = GFR × [P]x
 Excretion rate = V × [U]x
 Reabsorption or secretion rate = Filtered load – Excretion rate
o If filtered load is 100 and excretion is 0  net reabsorption
o If filtered load is 100 and excretion is 120  net secretion
Reabsorption Mechanisms
 Diffusion  moves down its concentration gradient
 Facilitated Diffusion  can be regulated
o Ion channel  Epithelial sodium channel (ENaC)
o Transporter
 Uniporter: e.g. GLUT transporter
 Secondary Active Transport  couples the movement of an ion (typically Na+ or H+) down its
electrochemical gradient to the uphill movement of another molecule against a
concentration/electrochemical gradient
o Transporter
 Symporter: e.g. Na+-glucose symporter (SGLT)
 Antiporter: e.g. Na+/H+ exchanger (NHE3)
 Active Transport  uses ATP
o Na+-K+ ATPase  on basolateral membrane of tubular epithelial cells
 Maintains low intracellular sodium levels
 Receptor-Mediated Endocytosis  proteins get reabsorbed this way
Aspects Common to Most Regions of the Nephron
 Tubule lumen separated from capillary by a single epithelial cell layer
o Two ways by which solutes can transverse the epithelium to get into the capillary
 Paracellular  through tight junctions found between cells
 Diffusion and/or solvent drag
Lectures 20, 21 – Renal Handling of NaCl – Block 5 Cell Biology and Physiology
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Transcellular  across both apical and basolateral membrane and through the
cytoplasm of an epithelial cell
+ +
Na -K -ATPase localized ONLY to basolateral membrane of epithelial cell
 Develops an Na+ electrochemical potential gradient across the apical membrane
 drives reabsorptive and secretion processes
Sodium
 Minimum requirement in the diet  500 mg per day (less than one-sixth as much as many diets)
 Low sodium diet  no more than 1,500 to 2,400 mg/sodium per day
o One teaspoon of salt has about 2,300 mg sodium
 Moderate to severe heart failure  limit sodium intake to 2,000 mg per day
Extracellular Fluid Osmolality and Volume
 Na+ is the most abundant cation in the ECF
o Major determinant of ECF osmolality
 Regulated by maintaining water balance (ADH, thirst)
o Major determinant of ECF volume
 Regulated by maintaining Na+ balance
Distribution and Balance of Na+ Throughout the Body
 Kidneys maintain ECF volume by regulating the amount of sodium
o Daily urinary excretion of Na+ is only a tiny fraction of the total Na+ filtered
 Sodium  most important contributor to the overall osmolarity of the ECF
o Where sodium goes, water follows (Na active transport: water is passive)
 Water flows from low Osm to High Osm (if allowable*)
o *If ADH is present  water reabsorption channels present
Na+ and water reabsorption along the nephron
 Na+ freely filtered at the glomerulus
 Luminal membrane  Na+ movement via diffusion
 Basolateral membrane  Na+ movement via Na+-K+-ATPase
 Proximal tubule
o Isosmotic reabsorption (67% of both water and Na+ reabsorption)
 Loop of Henle
o Thin Descending Limb  20% of water reabsorption (no transporters for Na+)
o Thick Ascending Limb  25% of sodium reabsorption (impermeable to water)
 NKCC transporter
o After Loop of Henle, fluid is hyposmotic to the fluid in the proximal tubule
 Distal tubule
o 5% of sodium reabsorption
 Na+-Cl- transporter
 Epithelial Na+ channel (ENaC)  also found in the proximal collecting duct
 Collecting Duct
o Mostly water reabsorption
 If dehydrated, vasopressin present and retain as much water as possible
 Hydrated  no vasopressin, less reabsorption
Lectures 20, 21 – Renal Handling of NaCl – Block 5 Cell Biology and Physiology
Mechanisms of Reabsorption
 Reabsorption
o Transfer of water and solutes from tubular lumen to interstitium (*ISF) to peritubular
capillary (*PV)
 Motive force
o Transepithelial transport is powered by ATP  almost all for Na+-K+ ATPase activity
 Na+-K+ ATPase  pumps Na+ from the epithelial cell across the basolateral
membrane and into the interstitium
 Electrochemical gradient
o Keeps intracellular Na+ low, while luminal Na+ concentration is
very high
o 3 Na+ out and 2 K+ in at pump  charge (3 mV) between
lumen (negative) and interstitium (positive)
o Step 1 – Ion gradient
 Na+-K+ ATPase creates a Na+ gradient between outside and inside of cell
o Step 2 – Voltage gradient
 Ion transport creates a voltage gradient between tubule lumen and
interstitium
o Step 3 – Osmotic gradient
 Water follows Na+ by osmosis
o Step 4 – Solvent drag
 Solvents are carried along in the water flow by solvent drag
o Step 5 – Water reabsorption concentrates solutes remaining in the lumen
 Note: ISF and Plasma (volume) concentrations will equilibrate with each other
Proximal Tubule
 Reabsorbs 67% of Na+ and water
o Isosmotic reabsorption (coupling or Na+ and water reabsorption)
o Critical to maintain ECF volume
 Glomerulotubular balance
 Na+ reabsorption mechanisms in the early and late proximal tubule are different
o Early proximal tubule
 Na+-K+ ATPase sets up gradient (-4 mv in lumen, and 0 mv in blood)
 Glucose, HCO3-, and amino acids  almost completely reabsorbed
here
o HCO3- reabsorbed by the release of H+ into the lumen in the
Na+/H+ transporter  converts HCO3- to H2CO3 and then to
H20 and C02 which can be absorbed through the lumen
o Late proximal tubule
 All filtered glucose, amino acids and most HCO3- have been reabsorbed
 Cl- concentration is higher in late PT than it was in the glomerular filtrate
 Transported into cell via either via Cl-/Formate transporter or
diffusion via tight junctions pulling Na+ along after it has changed the
electrical potential enough
 Lumen has positive voltage now (+4 mv) and blood is 0 mv still
 Due to loss of Cl Isosmotic Reabsorption
Lectures 20, 21 – Renal Handling of NaCl – Block 5 Cell Biology and Physiology
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67% Na+ recovered then 67% water recovered
The major solutes are:
 Early proximal tubule
 Major cation = Na+
 Major anion = HCO3 Late proximal tubule
 Major cation = Na+
 Major anion = Cl Minor anions are PO4-, lactate, and citrate
Quantitatively most of solute reabsorbed = NaHCO3 or NaCl with water following
The [TF/P]osmolarity and [TF/P]Na+= 1.0 along the proximal tubule b/c of water coupling
and proportionality
Note: the major Starling force driving reabsorption is the high oncotic pressure (πc) of
peritubular capillary blood due to the presence of proteins
TF/P < 1.0
o Means that the solute is absorbed early at a rapid rate
All glucose, lacatate, and much Hc03 are reabsorbed in the PT
o Note: in diabetic patients  glucose will be extremely high, and will not all be able to
be reabsorbed  a high concentration will remain in the lumen causing reduced
osmotic reabsorption of water (causing osmotic diuresis)
 The glucose curve will be straight or slightly positive
Na+  same conc in tubular fluid and plasma along length of PT
Phosphate is taken up in PT  will decrease with parathyroid hormone (PTH)
Cl- concentration in PT  elevated compared to plasma because Na+ reabsorption is coupled
to HCO3 and phosphate
Solutes are secreted in the PT but quantitatively negligible
Glucose Handling in the Proximal Tubule
 Cell characteristics
Lectures 20, 21 – Renal Handling of NaCl – Block 5 Cell Biology and Physiology
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o Na,K-ATPase on basolateral membrane
o Luminal and basolateral membrane permeability to K >> Na
o Potential Difference of ICF is negative vs TF
o NaTF >> NaISF
Glucose is reabsorbed by secondary active transport
o Na+ moving down its electrochemical gradient  energy for glucose to move up its
chemical gradient
Transporters (co-transport)
o SGLT-2 in the EARLY PT
 Low affinity but high conc. of glucose available
o SGLT-1 in the LATE PT
 High affinity but low conc. of glucose available
Glucose Tm = 375 mg/min
o Splay means the excretion of a substance in small amounts before the Tm is reached
 Coincides w/ plasma (renal) threshold
Peptide Handling in the Proximal Tubule
 Proximal tubules reabsorb ∼99% of filtered oligopeptides
o Angiotensin II  freely filtered at corpuscle
 Catabolized to its amino acids in the PT lumen by peptidases on the luminal
plasma membrane
 Amino acids as well as di and tri peptides are reabsorbed
o Brush border enzymes hydrolyze the peptides (angiotensin, glutathione)
 Transporters
o Na+/Amino acid co-transporter  for those broken down by brush border enzymes
o H+/oligopeptide cotransporters  for those peptides (carnosine) that cannot be
broken down but the brush border enzymes
 PepT1  Early
 Low-affinity/high-capacity
 PepT2  Late
 High-affinity/low-capacity
Proximal Secretion of Organic Cations and Anions
 Secretion of these substance by the PT is important
 Many organic cations and anions secreted are end products of metabolism
 Proximal tubular secretion may be the only way the body can rid itself of these substances
o Travel bound to albumin so cannot be filtered
 Secretory mechanisms are very powerful
Secretion of Organic Cations in the Proximal Tubule
 Types of Organic Cations
o Endogenous
 Catecholamines
 Dopamine
 Histamine
 Thiamine
 Ach
Lectures 20, 21 – Renal Handling of NaCl – Block 5 Cell Biology and Physiology
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 Choline
 Guanidine
 CREATININE
o Exogenous (Drugs)
 Atropine
 Isoproterenol
 Cimetidine
 Morphine
 Procaine
 Quinine
 Antibiotics
Mechanisms of Secretion
o Transport on basolateral membrane electrogenic facilitated diffusion via organic cation
transporters (OCT)
 Due to negative charge within the cell and positive charge of cations
o Sodium hydrogen ion exchanger (NHE-3) provides luminal H+ to secrete organic cation
(OCTN)
o MDR1 transporter
o Demonstrates ubiquitous role of Na+-K+ ATPase in transport
Proximal secretion of organic anions
 Types
o Endogenous
 Bile Salts
 Fatty Acids
 Urate
 Prostaglandins
 Oxalate
 Hippurates (PAH)
 Hydroxybenzoates
o Exogenous (Drugs)
 Antibiotics
 Sulfonamides
 Penicillin
 Diuretics
 Furosemides
 Salicylates
 Chemotherapeutic Agents
 Estrogens
 Mechanisms of Secretion
o Negative charged anions must get into cell against electrochemical gradient
 Antiporter system for a-ketogluterate (powered by Na+-a-ketogluterate
symporter)
 OAT 1, 2, 3 (Powered by NaDC)
o Very non-specific therefore anions compete for the antiporter
and can leave more drug in the blood then would be expected
(ie. Increased Penicillin with addition of PAH)
Lectures 20, 21 – Renal Handling of NaCl – Block 5 Cell Biology and Physiology
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Then intracellular concentration flows out into lumen (unknown mechanism)
 Maybe OAT4 and MRP2 have role
PAH as a measure of renal plasma flow
 Fully secreted by the efferent arteriole at or below its Tm  no PAH will be present in the
peritubular capillary = RPF
o If above Tm  PAH will overflow into peritubular capillary
Protein Handling in the Proximal Tubule
 Palbumin = 5 g/dl
o 0.02% filtered in Bowmans Capsule  TFalbumin = 1 mg/dl
 Filtered Proteins and peptides reabsobed and enzymatically degraded
o Megalin and cubulin apical membrane bound proteins that mediate endocytosis of
proteins and peptides
o Degraded Products include AA, minerals and vitamins (Vitamin D!)
 Mitochondria in tubular epithelium activate are responsible for activating
(Vitamin D!)
 If becomes saturated, PT is only reabsorption area  albumin/proteins in urine  something
wrong with the kidney
Role of Starling Forces in Peritubular Capillary Reabsorption
 Reabsorption fueled by drop in the hyrdrostatic pressure in the peritubular capillaries and
while oncotic pressure remains the same (due primarily to plasma proteins)
o Oncotic pressure is now greater then capillary pressure  favors reabsorption
Glomerulotubular Balance
 A small increase in GFR would lead to excretion of more Na+ if there were no other changes
 What Happens
o Increase in GFR  more fluid filtered  oncotic pressure increases and capillary
pressure drops more than it did before the increase in GFR (due to more protein and
less fluid) in the peritubular capillary
 Maintains balance
Loop of Henle
 Composed of 3 segments  produce concentration and dilution of urine using countercurrent
multiplication
o Thin descending limb
 Permeable to water and small solutes (like NaCl and urea)
 Water moves out and solutes move in fluid becomes increasingly hyperosmotic
as it moves down the descending limb
o Thin ascending limb
 Permeable to NaCl/impermeable to water
 Solute moves out of thin ascending limb—progressively hyperosmotic
o Thick ascending limb
 Active reabsorption of 25% of filtered Na+/impermeable to water
Thick ascending limb
Lectures 20, 21 – Renal Handling of NaCl – Block 5 Cell Biology and Physiology
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Na+-K+-2Cl- co-transporter in the membrane
o Na+ reabsorption method is load-dependent  more Na+ delivered to thick loop the
more it reabsorbs
 Carbonic anhydrase  block Na+ reabsorption in the PT  more presented to
thick ascending limb
o Entering the blood
 Na+ extruded via the Na+-K+ ATPase
 K+ and Cl- diffuse through channels down their electrochemical gradients
 Some K+ backleaks into the lumen  maintains positive charge in the
lumen and aids in processing in divalent cations (Ca2+ and Mg2+)
o Loop diuretics  inhibit Na/K/Cl cotransport
 More Na sent to DT and CCD for reabsorption  greater K+ excretion
 May result in hypokalemia and metabolic alkalosis
Paracellular route provides the driving force for the diffusion of Na+ across the tight junctions
o Facilitated by the positive potential of lumen
Na+/H+ antiporter is also presnt
Cells are impermeable to water (called diluting segment b/c water kept in lumen)
Tubular fluid leaving limb is low Na+ and osmolarity  [TF/P]Na+ and [TF/P]osmolarity <1.0
Tamm-Horsfall glycoprotein (THP) (also known as uromodulin)
o Most abundant protein in normal urine  produced in the thick ascending limb (not in
the plasma) and since it is past the PT reabsorption point it is normal to find some
amount excreted in the urine
Early Distal Tubule
 Impermeable to water
o Fluid ends up more dilute than the fluid entering from thick ascending limb
 Na+ and Cl- enter cell via cotransporter (electrically neutral)  Thiazide diuretics target
o Na+ extruded via Na+-K+ pump
o Cl- diffuses via channels on blood (basolateral membrane) side
 Load-dependent  capacity to absorb Na+ delivered from proximal tubule
 ALL of Cl- reabsorption is transcellular
 Gitelman syndrome
o Causes the kidneys to pass sodium, magnesium, chloride, and potassium into the urine,
rather than allowing it to be resorbed into the bloodstream
o Inactivating mutations in the SLC12A3 gene resulting in a loss of function of the encoded
thiazide-sensitive sodium-chloride co-transporter (NCCT)
Late Distal Tubule and Collecting Duct (Na+ handling)
 Approx. 3% Na+ reabsorption
 Two major cell types:
o Principal cells  Na+ reabsorption, K+ secretion and water reabsorption
o α-intercalated cells  K+ reabsorption and H+ secretion
 Impermeable to water
o If ADH present  presence of aquaporins and reabsorption of water
 Aldosterone  secretion leads to increased expression of ENaC and the Na+-K+ pump
o Increased transport of Na+ into cell and pumping into blood
o Aldosterone is delivered via blood  endocrine control
Lectures 20, 21 – Renal Handling of NaCl – Block 5 Cell Biology and Physiology
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Effects
 Increases K+ secretion
 Increases Na+ reabsorption
 Increases H+ excretion
Lumen is VERY negative (-50 mv)
Amiloride  K+ sparing diuretic
o Any loop or thiazide diuretic will increase K+ excretion because Na+ will tend to get
reabsorbed in the principal cells due to increased Na+ in lumen
 K+ sparing diuretics target principal cells
o Directly blocks ENaC, thereby preventing sodium reabsorption and subsequent K+
secretion
Liddle’s symdrome
o Disregulation of an epithelial sodium channel (ENaC)  increased Na+ reabsorption 
hypertension
Late Distal Tubule and Collecting Duct (Cl- handling)
 Two major cell types:
o Principal cells – generates a transepithelial voltage that is favorable for paracellular
diffusion of Clo b-intercalated cells- reabsorb Cl- using a transcellular process that involves Cl-HCO3exchange across the apical membranes and Cl- channels in the basolateral membrane