H1v: Renal handling of potassium

  • K+ is the major intracellular cation
  • Key role in RMP → ∴regulation is v. important
  • Plasma K = 3.5 – 5mmol/L

Renal Handling

K excreted = K filtered – K reabsorbed + K secreted

  • K freely filtered by glomerulus
  • 5 – 15% filtered load excreted
  • ∴must be reabsorbed
  • Most segments of nephron reabsorb K
  • The distal segments are the important ones tweaking K secretion

PCT

  • Reabsorbs 55%
  • Paracellular diffusion due to SOLVENT DRAG

Thick ASC LoH

  • Reabsorbs 30%
  • Co-transporter Na/K/2Cl → driven by Na/K/ATPase
  • Paracellular diffusion → due to +ve trans-tubular potential

Collecting Duct

Principle Cells

  • 70% of cells
  • Secrete K+ depending on dietary load
  • Active transport Na/K/ATPase
Principle Cell

Type A Intercalated Cells

  • The majority of the remaining 30% of cells
  • Reabsorbs K+
  • Active transport via K/H/ATPase
Type A Intercalated Cells

Regulation

1) Plasma K Level

  • Na/K/ATPase is highly sensitive to plasma K
  • ↑↑↑upregulated with hyperkalaemia
  • ↑K secretion in collecting duct

2) Aldosterone

  • Stimulated by hyperkalaemia
  • Directly stimulates adrenal cortex
  • ↑ no. of Na/K/ATPase channels on distal nephron

3) Na+ Delivery to Distal Nephron

  • K so strongly linked to Na via Na/K/ATPase
  • ∴↑Na delivery → attempt to reabsorb Na =↑K secretion

4) Flow Rate to Distal Nephron

  • K secretion ∝flow rate
  • ↑fluid delivery = ↑K+ secretion