SAQs 2012

Question 1

Outline the mechanisms by which the kidney maintains potassium homeostasis.

Answer 1

Structure:

• Normal physiological levels
• Filtration
• Reabsorption
• Secretion

Normal levels:

• 3.5-5
• Important for cellular function, esp cardiac cells.

filtration:

• Freely filtered by glomerulus
• glomerular filtrate 180L/day x ~4mmol = 720mmol k filtered

Reabsorption:

• Mostly reabsorbed in PCT (55%), via solute drag (paracellular)
• This proportion is fixed (note proportion, not amount)
• TAL ~30% via paracellular route and active Na/K/2Cl cotransporter.

Secretion:

• Variable, therefore this is the way that it’s altered.
• Principal cells of late distal tubules and cortical collecting ducts
• Depending on [K+], tubular cells either secrete or reabsorb K.
• 2 steps:
• –> 1. Uptake of K from interstitium via basolateral Na/K ATPase (ACTIVE)
• –> 2. K passively diffuses into tubular fluid via apical K channels (from cell into filtrate).

Determinants of K excretion:

1. Increased K in ECF:
   - This stimulates the Na/K ATPase, therefore increasing activity, promoting excretion of potassium.
2. Aldosterone
   - Aldosterone also stimulates Na/K ATPase activity, therefore causing increased potassium secretion.
   increased concentration of [K+] causes depolarisation of zona glomerulosa cells in the adrenal gland, causing release of Aldosterone into the blood.
3. Distal tubular flow rate:
   - increased flow results on increase potassium secretion.
   - Because movement of K outside the principal cells is passive (through the luminal K channel), if there is an increase in the flow rate in the lumen, then the gradient between them is increased, so more potassium leaves the cell.
4. Acidosis:
   - Decreases the effectiveness of the basal Na/K pump, therefore decreases potassium excretion.