Tubular reabsorption and secretion Flashcards
Describe the nephron and list what is reabsorbed and secreted at different portions of the nephron
- The nephron is lined with tubule brush border cells
- the apical face faces the tubular lumen, with is lined with microvilli
- the microvilli increases the surface area for reabsorption
- the basolateral surface faces the interstitium (between tubule and peritubular capillaries)
Proximal tubule:
NOTE: most reabsorption occurs here
- reabsorbed: sodium, chloride, bicarbonate, potassium, water, glucose, amino acids
- secreted: hydrogen ions, organic acids and organic bases
Thin descending loop of Henle:
- water is reabsorbed
Thick ascending loop of Henle:
- reabsorbed: sodium, chloride, porassium, calcium, bicarbonate ions, magnesium
- hydrogen ions are secreted
Early distal tubule:
- reabsorbed: sodium, chloride, calcium, magnesium
Late distal tubule and collecting duct:
- reabsorbed: sodium, chloride, water and ADH, bicarbonate, potassium
- secreted: potassium, hydrogen ions
Describe what occurs at the PCT
- PCT is where most reabsorption occurs
- Water and solutes e.g. sodium are reabsorbed
- metabolic byproducts e.g. organic acids, bases, and hydrogen ions, and medication
- tubular fluid contain increased sodium concentration, therefore it generates a concentration gradient
Paracellular sodium transport
- sodium moves freely through intercellular space between adjacent tubule cells due to leaky tight junctions
Free diffusion
- urea, water etc flow down concentration gradient from tubule into blood (50% of urea is reabsorbed in this way)
- ammonium (produced from breakdown of glutamine into ammonium and bicarbonate ions) diffuses from tubular cells into tubular fluid
- organic acids, medications (e.g. penicillin) diffuse out of peritubular capillary then secreted into tubular fluid for excretion
Apical surface
- sodium/glucose co-transporter: moves sodium and glucose into tubular cell (glucose then transported passively through GLUT1/2 into interstitium): lactate, AAs, phosphate, and citrate all reabsorbed this way
- sodium/hydrogen exchanger: allows 1 Na and 1 H out –> helps with bicarbonate reabsorption
- bicarbonare binds to hydrogen ions to form carbonic acid, carbonic anhydrase 4 splits this into water and carbon dioxide, which can freely diffuse into tubular cell
- within cell, carbonic anhydrase 2 combines water and carbon dioxide to H2CO3, this then dissolves into HCO3 and H+
Basolateral surface
- Na/K ATPase: pumps 3 Na out of tubular cells into interstitium in exchange for 2K in (67% of Na reabsorbed in PCT)
- Na/HCO3- co-transporter: pushes both sodium and bicarbonate into blood (85% of HCO3- reabsorbed this way)
PST stuff/
Describe what occurs at the Loop of Henle
- Surrounded by vasa recta
- Tubular fluid leaving the PCT has osmolarity around 300mOsm/L (= interstitial
fluid osmolarity) - Tubular fluid mainly consists of H2O, K+, Cl-, Na+, Ca2+ and urea
Thin descending limb: osmolarity in medullary interstitium increases rapidly
to 1200mOsm/L at bottom of loop
* Na+ and urea reabsorption causes concentration
* Squamous epithelial cells contain many aquaporins on apical and
basolateral sides but few channels for solutes
* Water diffuses from the lumen into vasa recta
Thin ascending limb: squamous epithelial cells don’t have aquaporins ∴
impermeable to water
* Many Na+ and Cl- channels that allow ions to diffuse from tubular fluid into interstitium down concentration gradients
* Interstitial fluid decreases in osmolarity to ~600mOsm/L at top of thin ascending limb
Thick ascending limb: cuboidal epithelial cells - larger than squamous cells (thick)
* No aquaporins ∴ impermeable to water
* Na+/K+/Cl- cotransporter: on apical surface that shuttles 1 Na+, 1 K+ and 2Cl- into the cell
* Target of loop diuretics
* Na+/K+ ATPase: on basolateral surface pumps 3 Na+ into interstitial fluid and 2K+ into the cell
* Na+ and Cl- channels: allow ions to move down gradients
* Volume in tubular fluid < volume in interstitium
* Interstitial fluid becomes more concentrated in medulla
* Countercurrent multiplication: tubular fluid decreases osmolarity up the loop - 325mOsm/L at top of thick ascending loop
Describe what occurs at the DCT
- Tubule cells don’t have microvilli
- Regulated by aldosterone
- Diffuses into nucleus of principal cells and triggers increased synthesis of ENaC, K+ ATPase and Na+/K+ ATPase transporters ∴ ↑
Na+ reabsorption and K+ secretion - In α intercalated cells, ↑ H+/K+ ATPase to ↑ H+ secretion
Early DCT: impermeable to water but reabsorbs ~5% of Na+ (load dependent)
* Tubular fluid contains more Na+ than tubule cells ∴ Na+ flows down gradient
into tubular cells
* Na+/Cl- cotransporter: apical surface of early DCT moving 1Na+ and 1 Cl-
into the cell
* Calcium channels: apical surface - diffusion down concentration gradient into
cell
* Na+/Ca2+ antiporter: basolateral surface - 1 Na+ from interstitium IN and 1
Ca2+ from cell OUT
* Ca2+ reabsorption regulated by PTH → more Na+/Ca2+ channels on
basolateral membrane
* Na+/K+ ATPase: basolateral surface - pumps Na+ out into interstitium and K+ in
Late DCT/collecting duct: net movement of Na+ and Cl- into blood and H+ into tubule
* Principal cells
* Epithelial Na+ channel (ENaC): apical surface - allows Na+ into the cell
to drive K+ out of the cell
* K+ channel: apical surface - allows K+ into the lumen
* When H2O ↓: ADH hormone released from pituitary → prompts
aquaporins to be released from vesicles inside principal cells ∴ water
reabsorbed from tubule
* α intercalated cells
* H+ ATPase: apical surface - uses ATP to pump hydrogen into the tubule
against its gradient
* H+/K+ ATPase: apical surface - uses ATP to pump 1 H+ into the tubule in
exchange for 1 K+
Describe how sodium is handled in the kidney
Sodium transport in the proximal convoluted tubule (67%)
* [Na+] remains the same because water also reabsorbed from plasma via osmosis
* Reabsorption driven by Na+/K+ ATPase (3 Na+ out, 2K+ in) - generates ↓ internal [Na+]
* Osmotic gradient draws water paracellularly (solvent drag)
* Moving net positive charge leaves the lumen negative ∴ some sodium drawn back
* Na+/H+ exchanger: apical membrane - H+ derived from water generates hydroxyl (OH-), which
reacts with CO2 to make bicarbonate (HCO3-)
* 3 HCO3-/Na+ symporter on basolateral membrane - bicarbonate gradient drives sodium into
interstitial space across basolateral membrane
Sodium transport in the thick ascending limb
* Na+/K+/2Cl- symporter: driven by Na+ and Cl- gradients to bring potassium in
* Transporter targeted by loop diuretics (fuorsemide or bumetanide) - retains sodium in
lumen of tubule (so water stays as well ∴ diuretic effect)
* Na+/H+ exchanger
* Na+/K+ ATPase: generates ↓ [Na+] within the cell, allowing symporters to work
* Potassium leaks back out into lumen through potassium channels - moves positive charge with
it, making apical membrane more -ve than basolateral membrane
* ∴ allows more Na+ to move via paracellular diffusion from tubule into interstitial space
(accounts for 50% of Na+ absorption in TAL)
* No water permeability ∴ ‘diluting segment’ since osmolarity of solutes decreases
Sodium transport in distal convoluted tubule
* Transport only via transcellular route
* Na+/Cl- symporter inhibited by thiazide diuretics (e.g. chlorothiazide)
* Uses gradient generated by Na+/K+ ATPase
* Chloride that enters the cel can then diffuse across the basolateral membrane via chloride
channels
Sodium transport in the proximal convoluted tubule (67%)
* [Na+] remains the same because water also reabsorbed from plasma via osmosis
* Reabsorption driven by Na+/K+ ATPase (3 Na+ out, 2K+ in) - generates ↓ internal [Na+]
* Osmotic gradient draws water paracellularly (solvent drag)
* Moving net positive charge leaves the lumen negative ∴ some sodium drawn back
* Na+/H+ exchanger: apical membrane - H+ derived from water generates hydroxyl (OH-), which
reacts with CO2 to make bicarbonate (HCO3-)
* 3 HCO3-/Na+ symporter on basolateral membrane - bicarbonate gradient drives sodium into
interstitial space across basolateral membrane
Sodium transport in the thick ascending limb
* Na+/K+/2Cl- symporter: driven by Na+ and Cl- gradients to bring potassium in
* Transporter targeted by loop diuretics (fuorsemide or bumetanide) - retains sodium in
lumen of tubule (so water stays as well ∴ diuretic effect)
* Na+/H+ exchanger
* Na+/K+ ATPase: generates ↓ [Na+] within the cell, allowing symporters to work
* Potassium leaks back out into lumen through potassium channels - moves positive charge with
it, making apical membrane more -ve than basolateral membrane
* ∴ allows more Na+ to move via paracellular diffusion from tubule into interstitial space
(accounts for 50% of Na+ absorption in TAL)
* No water permeability ∴ ‘diluting segment’ since osmolarity of solutes decreases
Sodium transport in distal convoluted tubule
* Transport only via transcellular route
* Na+/Cl- symporter inhibited by thiazide diuretics (e.g. chlorothiazide)
* Uses gradient generated by Na+/K+ ATPase
* Chloride that enters the cel can then diffuse across the basolateral membrane via chloride
channels
Describe how sodium transport in the kidney is regulated
Sodium regulation
* Achieved through water homeostasis - often triggered by hypovolaemia
* Juxtaglomerular apparatus: paracrine system for renin, angiotensin and aldosterone
* Macula densa (MD): modified cells which are sensitive to salt levels in tubular fluid
* Hypovolaemia:↓ GFR → ↓ Na+ and H2O delivered to distal tubule → MD releases NO
and prostaglandins → ↑ GFR (maintain normal filtered load) + ↑ renin secretion
(conserves body Na+)
* Hypervolaemia: ↑ GFR → ↑ Na+ delivered to distal tubule → MD releases ATP
(adenosine) → ↓ GFR (maintains filtered load) → ↓ renin secretion (allows more Na+
excretion)
* Juxtaglomerular cells: modified smooth muscle cells within afferent arterioles
* Juxtaglomerular cells receive signals from sympathetic neurons, decreased afferent
arteriole pressure and macula densa → hypovolaemia activates juxtaglomerular cells
to produce renin
Mediators that increase Na+ excretion
* Dopamine
* ECF (Na+)
* ECF volume
* Natriuretic peptides
* Intrarenal messengers
Mediators that decrease Na+ excretion
* RAAS system → aldosterone
* Sympathetic stimulation
* ADH
Describe the regulation of potassium transport in the kidney
Potassium regulation
* Narrow safety limits have necessitated an independent potassium regulation pathway
* K+ follows Na+, except distal nephron where it flows in the opposite direction to Na+ ∴ distal nephron critical for potassium homeostasis
* [K+] < 4mmol: adrenal gland ↓ aldosterone production
* Low K+ diet
* Angiotensin II increases Na+ reabsorption proximally ∴ less Na+ delivered distally, so K+ excretion ↓
* In hypovolaemic and hyperkalaemic state, kidney struggles to excrete excess K+ since not enough Na+ delivered to distal
tubule
* Glucocorticoid hypertension: genetic cause of hypertension where there is a defect in the enzyme required to convert
glucocorticoid to inactive products → works like aldosterone to drive hypertension (↑ salt reabsorption, ↑ K+ excretion)
* [K+] > 5mmol: ↑ Na+ reabsorption to maintain electronegative environment in tubular
cell ∴ ↑ K+ excretion through ROMK
* ROMK: major channel through which potassium is excreted - kept intracellularly
when [K+] ↓, but secrete K+ in normal and high [K+] conditions
* High K+ diet
* High Na+ delivery to principal cells
* Aldosterone production from adrenal gland
Describe how potassium is handled in the kidney
Potassium
* Potassium absorption depends on potassium in diet
* 98% of K+ found in intracellular fluid, remaining 2% in extracellular fluid (plasma, interstitial fluid)
* ↓ dietary K+: ~2% of filtered potassium is eliminated in the urine
* K+ reabsorbed mostly in the PCT (80%), ascending limb (10%), DCT (3%) and collecting duct (6%)
* Normal/↑ dietary K+: amount reabsorbed by PCT remains at 80%, but more is
absorbed in the DCT and collecting duct
Potassium transport
* Kidneys not as good at restricting potassium loss as they are at restricting sodium loss
PCT
exclusively paracellular reabsorption of K+
* Some K+ secreted into the lumen, but solvent drag from Na+ reabsorption drives
potassium via paracellular route
* Lumen becomes more positive further down the tubule ∴ adds to repulsion of K+
via paracellular route
* There are K+ transporters at this point, but they cancel each other out ∴ net
movement only paracellular
Thick ascending limb
paracellular and transcellular reabsorption
* 50% of K+ reabsorption occurs paracellularly as K+ follows Na+
* Na+/K+/2Cl- symporter contributes remaining 50%
Cortical collecting tubule
↓ K+: reabsorption
* K+/H+ ATPase: uses ATP to transport protons into tubule in exchange for K+
* K+ channels then allow K+ to move across the other membrane
* Overexpression or increased activity of this transporter can cause
hypokalaemia alkalosis - protons pumped into urine making urine acidic ∴
patient loses acid, causing blood pH to become alkalotic
↑ K+: secretion
* Passive process via K+ channels moving K+ into tubule lumen
* Na+ channels enable reabsorption of sodium due to Na+ gradient - leaves
-ve charge that can be used by K+ channels (RAAS - aldosterone)
* K+/Cl- symporter allows K+ secretion into tubule
* Na+/K+ ATpase is active process - effluxes Na+ and influxes K+
* Although some K+ moves back across interstitial space, it is more favourable to go across apical membrane into tubule lumen
Describe the glucose reabsorption curve
- Urine clearance is proportional to GFR at high plasma [glucose]
- If transporters are saturated, then too much glucose is filtered in the tubule
- Occurs in DM - glucose concentration in plasma becomes very high ∴ transporters
saturated and cannot reabsorb all glucose present (glycosuria) - Grey column = physiological range of plasma [glucose]
- Filtration: Amount of glucose filtered is proportional plasma [glucose] (linear straight
line) - Reabsorption: all filtered glucose initially reabsorbed (matches filtration rate), but GLUT
saturated - Tmax at approx 14-15mM of glucose - Splay: gradual effect, because there might be differences in number of transporters (heterogeneity)
in some nephrons compared to others i.e. gradual tapering off of amount of glucose
that can be reabsorbed - Excretion: when [glucose] reaches threshold so excess need to be excreted
- Difference between filtration and absorption
- Similar principles apply to handling of AAs, phosphate, citrate, etc
Describe how glucose is handled in the kidney
Glucose in absorbed in the PCT
Glucose handling
* Glucose is 100% reabsorbed (body needs glucose)
* 2 main transporters for glucose reabsorption:
* Sodium-glucose linked transporter 2 (SGLT2): 98% of sodium reabsorption in early PCT
* Moves 1 Na+ and 1 glucose - driven by Na+/K+ ATPase-derived Na+ gradient
* Low affinity (doesn’t bind well to glucose) ∴ requires very high [glucose] to saturate
* High capacity - since it doesn’t saturate, it can keep increasing rate of transport as
[glucose] increases
* Glucose crosses the basolateral membrane via GLUT2 transporter (uniporter)
* Facilitated diffusion
* Sodium-glucose linked transporter 1 (SGLT1): remaining 2% of sodium reabsorption in
late PCT
* 2 Na+ ions in conjunction with single glucose molecule
* High affinity but low capacity
* Binds to glucose very tightly at very low concentrations, but easily saturated
* Mops up any remaining glucose that wasn’t absorbed by lower affinity SGLT2
* Can concentrate glucose to much higher levels in the cell because it exchanges 2 Na+
for 1 glucose
Describe PAH and how it is handled in the kidney
Para-aminohippuric acid (PAH)
* Organic acid not normally present in body - useful for measuring renal plasma flow
* Plasma concentration (mg/mL) x renal plasma flow (600mL/min) = how much passing
through kidney/min
* Many organic anions and drugs are handled in a similar way by the kidney
* Endogenous: oxalate, bile salts
* Exogenous: penicillin, furosemide
* Uncharged molecules conjugated to anions e.g. glutathione conjugates
* Some filtering by Bowman’s capsule, but mostly secreted into the tubule directly ∴ via a
single pass through the kidney, there is no more PAH remaining in circulation
* Complex - several transporters need to work together (tertiary active transport)
* Na+/dicarboxylate cotransporter: Na+ gradient (from Na+/K+ ATPase) into cell
drives sodium in with dicarboxylate (DC2-)
* DC2- exchanged for PAH via another transporter (OAT - Organic Anion
Transporter)
* PAH transported across the apical membrane via another exchanger for an anion
(e.g. urate, uric acid)
* PAH transporters can be saturated - not all reabsorbed via a single pass if plasma [PAH]
too high
Describe the PAH reabsorption curve
PAH reabsorption curve
* Filtration: as unbound [PAH] ↑, filtered load also ↑ linearly
* Secretion: higher blood [PAH] = more secretion, but carrier proteins in tubular cells get
saturated so Tmax reached beyond which secretion is constant
* Excretion: amount excreted proportional to amount filtered + amount secreted into tubule
before transporter saturation
* Below Tmax, small increases in plasma [PAH] gives rise to steep increases in excreted
PAH (both filtration and excretion rising)
* Above Tmax, secretion gets saturated ∴ excretion rises less sharply and becomes
parallel to filtration curve
Notes:
- similar pattern to glucose transport – swap secretion for reabsorption
- at Tm: transporters become saturated
- peak of secretion curve: reaches a point where some PAH remains in blood leaving kidney
Describe how urea is handled in kidney
- Urea is filtered, reabsorbed and secreted - 40% ends up in urine
- Urea recycling establishes corticopapillary gradient for water reabsorption
- Urea continually produced in body as product of AA breakdown
- 100% filtered through Bowman’s capsule
- Urea is reabsorbed alongside water in the PCT - more H2O reabsorbed than urea so [urea] in
tubule lumen > blood [urea] → 50% reabsorbed - Difference in handling between superficial nephrons and juxtamedullary nephrons
- Urea that is secreted into juxtamedullary nephrons is reabsorbed from collecting tubule
- Deep inner medulla has higher [urea] in interstitium than in tubule ∴ urea secreted back
into the lumen (more urea secreted than was present in initial filtrate ~110%) - Urea accounts for ~½ the osmolarity in the inner medulla
- Thick ascending limb and DCT are impermeable to urea ∴ no reabsorption or secretion
- Collecting duct: ADH increases urea transporter on apical surface allowing urea to
diffuse into interstitium ∴ 40% remains to be excreted - Urea reabsorbed from superficial nephrons comes from vasa recta
Blood urea nitrogen (BUN)
* Changes in urine flow affect renal urea handling
* Renal disease: ↓ GFR → ↓ urine flow → ↑ urea retention → ↑ BUN
* Nitrogen in blood is in the form of urea (CONH2NH2) that carries 2 nitrogen groups
* Amount of urea excreted influenced by urine flow rate - as flow increases, amount of urine excreted increases to a plateau
* If person is dehydrated, ↓ rate of urine production so urine flow ↓ ∴ amount of urea excreted decreases, resulting in ↑ in BUN
* BUN can be measured as an indication of kidney function and dehydration
Describe how amino acids and proteins are handled in the kidney
Amino acids
* Plasma [amino acid] = 2.4mM
* Very similar to glucose - 0% of filtered AAs are eliminated in urine
* Non-Na+ dependent transporters on the basolateral membrane
* Sodium-dependent AA transporters move AAs into cell from apical side
Proteins
* GF barrier prevents passage of large quantities of protein into tubule
* ∴ Protein shouldn’t be in urine - if present, can indicate pathology
* Proteins are filtered into the tubule, but these are efficiently reabsorbed
* Reabsorption in PCT via clathrin-coated endocytosis of secreted proteins
* Engulfed, then broken down by lysosomes into AAs - AAs transported
across basolateral membrane into interstitial space
List some genetic disorders of kidney tubule transport
- **Liddle’s syndrome: severe hypertension due to overactivity of ENaC
- Nephrogenic diabetes insipidus: dilute urine, dehydration, thirst, deficient
aquaporin 2 - Renal glucosuria: high glucose in urine even with normal plasma glucose levels,defective SGLT2
- Cystinuria: high cysteine (lysine, arginine and ornithine) in urine - kidney stones***
- Bartter’s syndrome: salt in urine, metabolic alkalosis, short stature, defective Na+/
K+/2Cl- transporter - Gitelman’s syndrome: defective NaCl transporter
