Discuss the regulation of body potassium by the kidney.
A number of candidates discussed the distribution of potassium in the body and its role in
membrane potentials. This was not asked for. Common omissions were a lack of comment on
glomerular filtration, a lack of detail regards mechanisms of potassium transport in various parts of
the glomeruli and failing to discuss control mechanisms other than aldosterone.
Syllabus: E1,2b, D1,2f
References: Guyton and Hall Textbook of Medical Physiology, Chp 29
Surely it must be hard to hold yourselves up as experts when you confuse glomeruli and nephrons in an official training document. Anyway, we can be sure that when they asked for "potassium transport in various parts of the glomeruli", surely they meant the various segments of the nephron. Thus:
- Potassium is freely filtered in the glomerulus
- 50-60% of potassium is reabsorbed in the proximal tubule:
- Several mechanisms are involved, but the most important is solute drag
- Potassium is carried across the epithelium by moving together with the reabsorbed water
- This is not under any specific regulatory control
- In the thin limbs of the loop of Henle, potassium undergoes countercurrent exchange
- Potassium is added to the tubular fluid in the thin descending limb
- It then diffuses out again in the ascending limb
- The net effect of this is a conservation of potassium in the inner medulla
- 30% of filtered potassium is reabsorbed in the thick ascending limb
- This is due to the NKCC2 co-transporter, the drug target of frusemide
- Potassium is secreted into the tubular lumen in the distal convoluted tubule and the collecting duct
- This happens because of ENac activity, which reclaims sodium from the tubular fluid
- As the result of this, potassium leaks out of the tubule cells to maintain electroneutrality
- With increased sodium delivery to the distal nephron, sodium reabsorption increases and potassium loss increases.
- As the result of this potassium secretion, urinary potassium increases to 15-40 mmol/L
- Apical ROMK channels are responsible for most of the potassium secretion in the distal nephron
- The regulatory influences that play a role in the renal regulation of potassium are:
- Aldosterone (increases the activity of ENaC channels
- Vasopressin (increases the availability of ROMK channels)
- High potassium intake: leads to the increased expression of ROMK channels
- High distal sodium delivery: compensatory increase in potassium secretion to maintain electroneutrality.
- Acid-base disturbances: metabolic acidosis causes distal potassium secretion to decrease, probably because of the extreme sensitivity of sodium and potassium channels to intracellular pH.