Potassium balance
24 important questions on Potassium balance
What are the functions of potassium?
- Intracellular K+
- Cell-volume maintenance
- intracellular pH regulation
- cell enzyme functions
- DNA/protein synthesis, growth
- Transmembrane [K+] ratio
- resting cell membrane potential
- neuromuscular activity
- cardiac activity
- vascular resistance
What is the amount of K+ (mmol) per kg body weight?
--> 98% is intracellular
--> 2% is extracellular
When do you speak of hyperkalemia? And of hypokalemia?
Hypo = [K+] < 3.5 mmol/L in ECF
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What is the effect of hypo- and hyperkalemia on the resting potential and action potential?
High =higher resting potential, threshold is sooner reached
What can you see on the ECGs from persons with varying plasma concentrations?
Hypo = low T wave and U wave
What is the distribution of potassium along the body?
10% is excreted in feces
90% excreted via urine
How is rise in potassium concentration in the plasma controlled?
Which hormones are released in response to a rise in plasma K+?
- Insulin secretion by the pancreas
- Aldosterone secretion by adrenal cortex
- Epinephrine secretion by adrenal medulla
--> insulin is the most important hormone that shifts K+ into the cells
What effects the distribution of K+ between ICF and ECF?
Pathology (acid-base disorders, osmolality, cell lysis)
Drugs that induce hyperkalemia
What are drugs that induce hyperkalemia?
Angiotensin converting enzyme inhibitors
K+ sparing diuretics
Heparin
How do you calculate the filtered K + load?
-> if there was only filtration and no reabsorption, total K+ pool will be lost in 2 hours
Where in the kidney is K + reabsorbed?
Thick ascending limb of Henle's loop (10%)
Initial collecting duct (2%)
Inner medullary collecting duct (6%)
2% is excreted
--> difference with normal to high dietary intake is that initial collecting duct can secrete more K+ (20-180%) & inner medullary collecting duct absorbs more (20-40%)
What are the renal potassium "handling" segments in the kidney?
Inner medullary collecting duct (MCT =
How is K+ reabsorbed in the proximal tubule?
- Solvent drag (with water) = paracellular
- Electrodiffusion = paracellular
- Used for K+/N+ in basolateral membrane
How is K + reabsorbed in the thick ascending limb?
- Active transcellular transport (Na/
K /2Cl) - Diffusion via chemogradient
- At basolateral membrane channel and
K /Cl pump which are linked to ATP
How is K + reabsorbed in the cortical collecting tubule?
- H-K pump
- Passive efflux across basolateral membrane
- Passive at basolateral membrane
How is K + secreted in the cortical collecting tubule?
- K+ taken up by K+/Na+ pump (active)
- High apical K+ membrane permeability
- Favorable electrochemical gradient (passive diffusion)
--> aldosterone is important regulator
What is the function of aldosterone on K+ secretion? And in which segment of the tubule?
- Aldosterone stimulates the Na/K pump (basolateral) -> more sodium absorbed and K+ secreted
- Amplification of the area of principal cells
- Increase in number of Na/K pumps
How do you calculate potassium excretion? And of what is this dependent?
--> depends on flow and [K+] in CCT
What is the function of amiloride?
What is the effect of licorice on potassium and blood pressure?
Licorice inhibits 11ß-HSD2 and thus the conversion of cortisol to cortisone. Cortisol preferentially occupies MR and GR over aldosterone and aldosterone function is not visible.
What are two mechanisms that cause hypokalemia in diarrhea?
- More excreted via feces (external balance)
- A lot
of water is lost in feces and the kidney reabsorbs more water and sodium (and more potassium is expelled)
What are two mechanisms that cause hyperkalemia in patients with ESRD?
- The kidney does not excrete potassium
- Acid-base balance is lost (acid cannot be removed), this results in metabolic acidosis -> more potassium out of cell and H+ taken up by cell (H/K pump)
What are three mechanisms that cause hypokalemia when vomiting?
- RAAS is activated because a lot of fluid is lost by vomiting -> more potassium is excreted to reabsorb water and Na+ (in urine)
- Acid is lost because the gastric juice is thrown out. This highers the pH in the body (bicarbonate concentration goes up, which is the base of acid). H+ + bicarbonate => H2O + CO2. The total amount of filtered bicarbonate increases, which is negatively charged and potassium compensates (in urine)
- Lot of bicarbonate (alkalosis) -> potassium goes down, hydrogen goes out of cell and potassium into the cell
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