Tubular Transport and Diuretics

Question 1

The Proximal Convoluted Tubules and their three main functions

Answer 1

• Highly permeable to water
  
  • this is because the PCT cell membranes express AQP1 (AQP = aquaporin)
• Water reabsorption:
  
  • 65% of sodium and water reabsorption occurs here
• Many mitochondria to power active transport
• Extensive brush border → ⇡SA for rapid transport
• Sodium carrier proteins bring sodium into the cell (facilitated diffusion)
  
  • cotransport
    
    • first half of proximal tubule with glucose, amino acids, and HCO3-
    • second half pf proximal tubule with chloride ions thru intercellular jxns
  • counter-transport with H+ going out
• Reabsorption:
  
  • 60 to 80% of UF (isotonic)
  • 90% of bicarbonate
  • Glucose, AA, urea, uric acid
  • K, Ca, PO4, Mg
• Secretion:
  
  • Organic anions (uric acid)
  • Organic Cations (creatinine)
  • Protein-bound drugs
• Major site of NH3 production

Question 2

PCT Structure leading to Function

Answer 2

• Pars convoluta
  
  • Where most of reabsorption takes place due to rich basolateral space
  • S1 cells (upper 2/3 of convoluta)
    
    • prominent brush border
    • Well-developed endoplasmic-lysosomal apparatus
    • Extensive basolateral invaginations
  • S2 cells (lower 1/3 of convoluta)
    
    • Small brush border
• Pars recta
  
  • Where most of the secretion of organic acids and bases occurs
  • S2 cells (beginning of recta)
  • S3 cells (rest of recta)
    
    • Tall brush border
    • Less developed endoplasmic-lysosomal apparatus
    • Sparse basolateral invaginations

Question 3

Loop of Henle

Answer 3

• Descending Loop of Henle (DLoH):
  
  • Highly permeable to water (again, AQP1)
  • Tubular fluid becomes hyperosmotic as it moves in toward the renal medulla
    
    • this is because the DLoH is not permeable to Na+ and Cl-
• THIN ascending Loop of Henle (TnALoH)
  
  • water permeability = 0 (almost no AQPs)
  • Na+, Cl-, and K+ are reabsorbed????
• THICK ascending Loop of Henle (TcALoH)
  
  • water permeability = 0 (almost no AQPs)
  • Na+, Cl-, and K+ are reabsorbed
    
    • 1Na+ and 2Cl- via 2° active transport (transcellularly)
      
      • imbalance b/w ⊕ and ⊖ → formation of electro⊖ gradient inside the cell
    • Ca+ , HCO3-, Mg2+ are also reabsorbed using established electro⊖ gradient (paracellularly)
  • Site of action of loop diuretics (e.g. furosemide), which inhibit the 1-sodium, 2-chloride, 1-potassium co-transporter

Question 4

Distal Convoluted Tubule (DCT)

Answer 4

• Early DCT
  
  • Juxtaglomerular complex
    
    • First portion of the DCT
    • Provides feedback control of RBF and GFR
  • Diluting segment
    
    • Second portion of the DCT
    • Reabsorbs many ions (Na+, Cl, K+, etc.), but is impermeable to water and urea
    • NaCl co-transporter moves sodium from lumen intp epithelial cells of DCT
      
      • Thiazide diuretics inhibit this transporter
    • Ca reabsorbed via a calcium channel on the luminal side of the DCT cell

Question 5

Collecting Duct

Answer 5

• What happens to water is dependent on the presence of ADH
  
  • with ADH: principal cells express AQP2, water is reabsorbed (if medullary osmotic gradient is maintained)
  • w/o ADH: no AQP2 expression → no water reabsorption
    
    • some is reabsorbed via paracellular pathways
• Na+, Cl-, and other ions are also reabsorbed
• Usually water and ion reabsorption are balanced so that the tubular fluid does not undergo any change is osmolarity (remains at 60 mOsm)
• Principal Cells
  
  • ADH-dependent water permeability
    
    • ADH binds to V2 GPCR (a Gs)
  • Na reabsorption via ENaC
    
    • it is in slight excess to Cl- secretion, which creates an electro⊖ gradient that helps K+ get secreted into the lumen
• Intercalated Cells
  
  • Secrete H+ ions into the lumen and generate a HCO3- molecule that is reabsorbed
    
    • ***Note: a few do the reverse, and are important in metabolic alkalosis
  • ​No ENaCs so no Na reabsorption

Question 6

Control Centers at the Vascular Pole of the Glomerulus

Answer 6

• Juxtaglomerular Apparatus (JGA)
  
  • Baroreceptor
  • Increased stretch suppresses renin
  • Decreased stretch stimulates renin
• Macula Densa (MD)
  
  • Chemoreceptor
  • Decreased tubular flow or NaCl
    
    • Increased Cl flow → afferent arteriole vasoconstriction
    • Decreased Cl flow → afferent arteriole relaxation

Question 7

PCT Reabsorption

Answer 7

• Na+ reabsorption via:
  
  • Na+-glucose, Na+-phosphate, Na+-AA, or Na+-citrate cotransporters
  • Na+-H+ exchanger (countertransport)
  • Selective Na+ channel
• All powered by the favorable electrochemical gradient established by the Na+-K+ ATPase
• Eventually, all this solute reabsorption creates an osmotic gradient → water reabsorption via many AQPs and “leaky” tight junctions
  
  • Leaky/permeable epithelium prevents the maintenance of concentration or osmotic gradients → [Na+] and osmolality of tubular fluid is equal to plasma [Na+] and osmolality
• Note: urea, potassium, and calcium reabsorption is passively linked to that of sodium, so these solutes would be affected as well.

Question 8

LoH Reabsorption

Answer 8

• Descending LoH is permeable to water, but not to ions
• Ascending LoH is permeable to ions, but not to water
• Apical Na+-K+-Cl- cotransporter powered by basolateral Na+-K+ ATPase
  
  • Cl- binding is the rate-limiting step for cotransporter activity
  • Loop diuretics (furosemide) inhibit co-transporter by competing for Cl- binding site
• [K+] lumen << [Na+] or [Cl-] lumen
  
  • K+ leaves tubular cell and enters lumen via apical selective K+ channel
  • Generates electropositivity of lumen → passive paracellular reabsorption of Na+, Ca+, and Mg2+
    
    • cortical TcALoH = major site of Mg2+ reabsorption

Question 9

DCT Reabsorption

Answer 9

• Apical Na+-Cl- cotransporter
  
  • Also powered by Na+-K+ ATPase
  • NaCl reabsorption → ⇣tubular fluid [NaCl] → ⇣Cl- in the tubular lumen → ⇣Na+-Cl- contransporter activity (this is how NaCl reabsorption is limited in the LoH and DCT)
  • ⇣tubular fluid [NaCl] also → ⇣lumen [Na+] → backflux of Na+ from peritubular interstitium into the lumen across tight junctions
  • For these reasons, transport in LoH and DCT is flow dependent.
• Calcium Reabsorption
  
  • Ca2+ enters the tubular cell down a favorable electrochemical gradient through apical Ca2+ channels and a Vit. D-dependent calcium-binding protein (CBP)
  • Ca2+ enters peritubular interstitium via basolateral 3Na+-1Ca2+ exchanger or Ca2+-ATPase (more rare)

Question 10

Collecting Duct Reabsorption

Answer 10

• Na+ reabsorption via ENaCs
  
  • Number of open ENaCs is under hormonal control by aldosterone and atrial natriuretic peptide (ANP)
    
    • Aldosterone ⇡ the # of open ENaCs when volume depletion activates RAAS
    • ANP ⇣ the # of open ENaCs (acts primarily in the inner medulla)
• Aldosterone-induced entry of Na+ inside the tubular cell promotes K+ secretion from the cell into the lumen due to:
  
  • increasing luminal negativity (since Na+ is reabsorbed without Cl-, which stays in the lumen except when undergoing passive paracellular transport)
  • Na+-K+ ATPase activity at basolateral mb
• Water reabsorption
  
  • Under the control of ADH
    
    • ADH binds V2-vasopressin-R (G_s GPCR) in the basolateral mb → activation of adenylyl cyclase → ⇡cAMP →insertion of cytosolic vesicles that contain preformed AQP2 channels in the apical mb
  • AQP3 and AQP4 on basolateral mb are constitutively expressed