Potassium Homeostasis (7.6)

Question 1

To explain the body distribution of K+, the importance of K+ homeostasis and to describe physiological consequences of plasma K+ imbalance

Answer 1

Body distribution of K⁺:

• Most abundant ICF ion
• 98 % intracellular

Importance of K⁺ homeostasis:

• Maintains the resting potential of cells (Na⁺/K⁺ ATPase)
• Acid-base regulation
• Enzymatic reactions
• Volume regulation

Physiological consequence of plasma K⁺ imbalance:

• Hypokalaemia: < 3.5 mmol/L
• Hyperkalaemia: > 5.5 mmol/L
• Changes in gastric mobility, bradycardia, ECG changes (peaked T wave, loss of P wave and QRS elongation), muscle weakness

Question 2

Describe sites for K+ reabsorption and secretion along the nephron and explain the cellular means by which this occurs, with particular reference to mechanisms employed by the proximal convoluted tubule and the distal nephron (principal cells and α-intercalated cells)

Answer 2

PCT: Paracellular via solvent drag

CT: Principle cells for excretion via 3 mechanisms.

Intercalated cells for reabsorption via K⁺/H⁺ ATPase → K⁺ channel

Question 3

Describe physiological and pathological factors that influence K+ shifts between the intracellular and extracellular compartments

Answer 3

Question 4

Describe factors e.g. hormones, that influence K+ reabsorption and secretion

Answer 4

Increased tubular flow:

• Increased urinary flow (such as stimulated by diuretics) prompts increased sodium reabsorption, this in turn increases potassium secretion.
• Maxi K⁺ channels are activated. Allow for high flow excretion of potassium
• Leads to hypokalaemia (hence, diuretics are strongly associated with hypokalaemia)

Increased dietary intake:

• Directly stimulates aldosterone secretion: Prompts Na⁺ reabsorption and K⁺ secretion
  
  • NOTE - Aldosterone paradox: Activation of RAAS, due to Na⁺ depletion, does not lead to increased K⁺ secretion. As urinary flow will be decreased (sympathetic mediated afferent arteriole constriction) the maxi K⁺ channels are not activated.
• Stimulates principle cell activity (ROMK, maxi K⁺ and K⁺/Cl^- ATPase)

Question 5

Explain the relationship between K+ levels and plasma pH

Answer 5

• As H⁺ levels increase in the ECF this prompts exchange of H⁺ ions for K⁺ ions. More extracellular K⁺ is seen, leading to hyperkalaemia
• Increased H⁺ levels are cause increased K⁺ reabsorption via alpha-intercalated cells in the CD (K⁺/H⁺ ATPase antiporter)
• Alkolosis → hypokalaemia

Question 6

State molecules/ions which are reabsorbed and secreted by the kidneys

Answer 6

• Glucose
• HCO₃^-
• Urea
• Na⁺
• Cl^-
• Water
• Ca²⁺
• Secretion of organic acids and bases by the PCT: PAH (allows measurement of renal plasma flow)