Regulation of ECF Potassium Flashcards Preview

CVPR: Renal > Regulation of ECF Potassium > Flashcards

Flashcards in Regulation of ECF Potassium Deck (8)
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1

Normal ECF potassium

  • 3.5 - 5.0 mM
  • < 3.5 mM = hypokalemia
  • > 5.0 mM = hyperkalemia

2

Potassium reabsorption & secretion @ kidney

  • filtration = 30 g K+, but excretion = 0 - 45 g K+
  • @ proximal segments: 80% of filtered load is reabsorped via passive, peracellular pathway
  • @ LOH: transcellular reabsorption
    • Na/K/2Cl co-transporter @ apical then through channels @ basolateral (10-15% of filtered load)
  • @ collecting duct: K+ reabsorbed by principal cells
  • ~ all of filter K+ reabsorbed; regulated K+ secretion @ fine tuning segments = potassium excretion/balance

3

Cellular mechanism of potassium secretion

  • occurs @ principal cells @ fine tuning segments
  1. basolateral entry of K+ (via Na/K ATPase)
  2. apical secretion of K+ into tubular lumen via ion channels (passive)

4

Regulation of potassium secretion: mass-action effect

  • change in [ECF K+] ==> change in secretion via law of mass action
  • K+ = rate-limiting step in ATPase pump cycle ==> [K+] determines rate of pump into cells
  • [K+] w/in cells determines electrochemical gradient 
  • works best for large changes in [K+

 

5

Regulation of potassium secretion: hormonal regulation

  • [K+] changes stimulate adrenal zona glomerulosa ==> increase aldosterone 
  1. aldosterone ==> increased # of Na/K/ATPase pumps
    1. increased rate of K+ entry
  2. aldosterone ==> increased # of apical sodium channels 
    1. increased outward K+ movement in exchange for inward flow of Na+
  3. aldosterone ==> increased # of apical potassium channels

6

Regulation of potassium secretion: tubular flow

  • slow tubular flow ==> build up of K+ w/in lumen
    • lumenal [K+] increases = electrochemical gradient decreases = secretion decreases
  • fast tubular flow ==> secreted K+ is "washed away" before gradient falls
    • maintains steep gradient for apical K+ flow
  • e.g. Loop diuretics =
    • 1. inhibit reabsorption of 15% of K+
    • 2. increased water w/in tubule @ fine-tuning segments ==> increased flow ==> enhanced K+ secretion

7

Regulation of potassium secretion: hormonal vs. tubular flow

  • e.g. primary hyperaldosteronism:
    • aldosterone ==> increased potassium secretion + increased sodium reabsorption (+ water = decreased tubular flow)
    • ultimately: potassium secretion increases bc elevated ECF volume/MAP maintains tubular flow
  • e.g. cardiac insufficiency ==> RAS ==> secondary hyperaldosteronism
    • ultimately: potassium secretion decreases bc decreased MAP = decreased GFR ==> decreased tubular flow

8

Regulation of potassium secretion: pH

  • alkalosis increaseds potassium secretion ==> hypokalemia
  • high pH ==> K+ into cells (including tubular cells) = more K+ crosses apical membrane
  • K+ ion channels inhibited by increased [H+]
    • w/alkalosis + low [H+] ==> inhibition is released ==> faster K+ flow into lumen
  • acidosis
    • mild/moderate ==> inhibition of K+ channels ==> decreased K+ secretion
    • severe ==> decreases Na reabsorp = increased water and tubular flow ==> increased K+ secretion
    • overall slightly unpredictable