12 - Countercurrent Mechanism of Distal Tubule and Collecting Duct

countercurrent mechanism

mechanism for kidney concentrating urine

• kidney can concentrate urine 4-fold (1200 mOsm/L) or dilute urine 7-fold (40mOsm/L) with respect to plasma

• plasma in osmotic equilibrium at 290 mOsm/L

countercurrent components

• glomerulus → filtrate is iso-osmolar (290 mOsm/L)

• proximal tubule → 2/3 reabsorption of water and solutes

• descending limb → filtrate becomes hyper-osmolar (1200 mOsm/L)

  • water reabsorption via aquaporin 1 and paracellular transport

  • impermeable to solutes

• ascending limb 

  • NaCl reabsorbed paracellularly

  • impermeable to water

• thick ascending limb → filtrate becomes hypo-osmolar (100mOsm/L)

  • solute reabsorption of Na⁺, K⁺, and Cl^- via Na⁺-K⁺-2Cl^- transporter

  • impermeable to water

• distal tubule and collecting duct → solute and water reabsorption is tightly regulated 

  • osmolality of final urine is a function of the degree to which the fluid in collecting ducts is allowed to equilibrate with interstitium 

hyperosmotic gradient

as filtrate moves from cortex to medulla, filtrate becomes more concentrated

• 50% of gradient due to urea

  • urea recycling (reabsorbed and secreted) between collecting ducts and loop enhances gradient

• 50% of gradient due to NaCl

thick ascending limb

cells do not have brush border, so reabsorption is not as abundant

• Na⁺/2Cl^-/K⁺ co-transporter (apical)→ macula densa sensor that detects NaCl in filtrate for TGF response

  • responsible for 20-30% of hyperosmotic gradient

  • K⁺ back leak is key to keep the transporter working, but also creates a negative charge in lumen to push Na⁺ and K⁺ paracellularly

  • ADH increases activity of transporter

  • diuretic: furosemid (Lasix)

• Na⁺/K⁺-ATPase (basolateral) → pumps Na⁺ out and K⁺ in

• Cl^-/K⁺ co-transporter (basolateral) → pumps Cl^- and K⁺ out

• K⁺ and Cl^- pores (basolateral) → leaks K⁺ and Cl^-

convoluted distal tubule

contain type 1 cells closer to macula densa, and type 2 cells closer to collecting duct

• type 1 cells:

  • Na⁺/Cl^- co-transporter (NCC, apical) → pumps Na⁺ and Cl^- into cell

    • upregulated by AngII, aldosterone, ADH

    • diuretic: thiazide derivatives

  • Na⁺/K⁺-ATPase (basolateral) → pumps Na⁺ out and K⁺ in

  • Cl^- pores (basolateral) → leaks Cl^- into blood

  • K⁺ leaky channels (apical/basolateral) → leaks K⁺ into blood and lumen

• type 2 cells:

  • NCC (apical)

  • epithelial Na⁺ channel (ENaC, apical) → pumps Na⁺ into cell

    • upregulated by aldosterone and increased tubular flow

    • downregulated by ANP

    • diuretic: spironolactone and amiloride

  • Na⁺/K⁺-ATPase (basolateral) → pumps Na⁺ out and K⁺ in

  • Cl^- pores (basolateral) → leaks Cl^- into blood

  • K⁺ leaky channels (apical/basolateral) → leaks K⁺ into blood and lumen

collecting duct

contain principal cells and intercalated cells, with more principal cells at 2:1 to 3:1 ratio

• principal cells

  • ENaC (apical)

  • K⁺ excretion (apical) → Big K⁺ (BK) channel and renal outer medulla K⁺ (ROMK) channels

  • aquaporins (apical/basolateral)

  • Na⁺/K⁺-ATPase (basolateral)

• ⍺-intercalated cells (99% of intercalated cells)

  • reabsorbs HCO₃^-

  • excretes H⁺ (apical) → H⁺-ATPase and H⁺/K⁺-ATPase (counter-transporter)

  • reabsorbs K⁺ (basolateral) → leaky channels to compensate for high K⁺ excretion from principal cells

• β-intercalated cells (1% of intercalated cells)

  • opposite polarization → secretes HCO₃^- and reabsorbs H⁺

diuretics

drugs that increase urine production by blocking solute reabsorption, increasing amount of sodium and water in filtrate to be excreted as urine

• Na⁺/2Cl^-/K⁺ → loop diuretics (furosemide/Lasix)

• NCC → thiazide derivatives (chlorothiazide, hydrochlorothiazide)

• ENaC → amiloride, spironolactone

• osmotic → mannitol

• carbonic anhydrase → blocks HCO₃^- reabsorption in proximal tubule

aldosterone

increases Na⁺ reabsorption and K⁺ secretion

• aldosterone activates intracellular receptor

  • increases ROMK expression

  • increases Na⁺/K⁺-ATPase expression

  • SGK1 expression

    • active SGK1 phosphorylates NEDD4

    • NEDD4 phosphorylation prevents ENaC ubiquitination for degradation

• released from adrenal gland in cortex

• AngII activates its release during volume depletion and lowered BP

• increased K⁺ activates release

aldosterone pathologies

• hyperaldosteronism → Conn’s syndrome

  • 33% caused by adrenal adenoma, 66-67% caused by enlarged hyperplasia of adrenal gland

  • increased Na⁺ reabsorption, leading to increased BP

• hypoaldosteronism → Addison’s syndrome, diabetic nephropathy

how would increase in Na⁺ intake affect aldosterone levels?

aldosterone would decrease → to decrease Na⁺ reabsorption

antidiuretic hormone (ADH)

increases water reabsorption in kidneys

• ADH binds V₂ receptor on basolateral membrane

  • activates adenyl cyclase to convert ATP to cAMP

  • cAMP activates PKA

  • PKA phosphorylates aquaporin-2 vesicle, allowing aquaporin-2 to insert into apical membrane

• aquaporin-3/4 (basolateral) → constitutively present

• increases activity of NCC, ENaC, Na⁺/K⁺-ATPase

• released from posterior pituitary

  • sensed by baroreceptors during volume depletion

  • sensed by osmoreceptors when plasma osmolarity increases

ADH pathologies

• central diabetes insipidus 

  • ADH not around and not released from pituitary gland

  • from head injury

  • causes hypo-osmotic urine and increased tubular flow rate

• nephrogenic diabetes insipidus

  • ADH released from pituitary but kidneys do not respond to it

  • from genetics, kidney disease, lithium

  • causes hypo-osmotic urine and increased tubular flow rate

ADH and extracellular fluid

normally → as plasma osmolality goes up, ADH goes up

• body prioritizes correcting plasma volume over plasma osmolality

  • volume contraction → ADH released more significantly to retain volume

  • volume expansion → ADH release limited to correct osmolality 

• morphine and nicotine increase ADH release, alcohol decreases ADH release