Potassium balance

Question 1

Explain distribution of potassium

Answer 1

Intracellular [K+] ~ 150mmol/L

Extracellular [K+] ~ 4.5mmol/L

Question 2

How is day to day/minute to minute regulation controlled?

Answer 2

Through insulin, aldosterone, pH, adrenaline etc. (internal balance)

Question 3

How is external balance controlled?

Answer 3

Diet

Question 4

What is acute regulation?

Answer 4

Distribution of K+ through ICF and ECF compartments

Question 5

What is chronic regulation?

Answer 5

Achieved by the kidney adjusting K+ excretion and reabsorption

Question 6

What are the functions of potassium?

Answer 6

• Determines ICF osmolality → cell volume
• Determines resting membrane potential (RMP) → very important for normal functioning of excitable cells
  i. e. repolarisation of cell  myocardial, skeletal muscle & nerve cells
• Affects vascular resistance

Question 7

What is Hyperkalaemia?

Answer 7

Plasma [K+] > 5.5mM

Question 8

What is Hypokalaemia?

Answer 8

Plasma [K+] < 3.5mM

Question 9

What normally determines resting membrane potential?

Answer 9

The dynamic balance between membrane conductance to Na+ and K+. Can be measured using Nernst equation.

Question 10

What causes hypokalaemia?

Answer 10

Hypokalaemia caused by renal or extra-renal loss of K+ or by restricted intake
e.g
Long-standing use of diuretics w/out KCl compensation
Hyperaldosteronism/Conn’s Syndrome
( aldosterone secretion)
Prolonged vomiting → Na+ loss → aldosterone secretion → K+ excretion in kidneys
Profuse diarrhoea (diarrhoea fluid contains 50mM K+)

Question 11

How does hypokalaemia affect hormones?

Answer 11

Hypokalaemia results in ↓release of adrenaline, aldosterone & insulin

Question 12

What causes hyperkalaemia?

Answer 12

Acute hyperkalaemia normal following prolonged exercise → normal kidneys excrete K+ easily
Disease states:
Insufficient renal excretion
Increased release from damaged body cells eg. during chemotherapy, long-lasting hunger, prolonged exercise or severe burns
Long-term use of Potassium-sparing diuretics
Addison’s disease (adrenal insufficiency)

Question 13

What can result from hyperkalaemia?

Answer 13

Plasma [K+] > 7mM life-threatening → asystolic cardiac arrest

Question 14

How to treat Hyperkalaemia?

Answer 14

Insulin/Glucose infusion used to drive K+ back into cells 
Other hormones (aldosterone, adrenaline) stimulate Na+-K+ pump  = increase in cellular K+ influx

Question 15

How is normal K homeostasis a limiting factor in the therapy of CVD?

Answer 15

Drugs like β-blockers, ACE inhibitors etc raise serum [K] →risk of hyperkalaemia
Conversely, loop diuretics used to treat heart failure, enhance the risk of hypokalaemia

Question 16

Where does most K get reabsorbed?

Answer 16

PCT and loop of Henle. 90%

Question 17

Explain K movement in PCT

Answer 17

• Sodium-potassium pump between tubular cells and ECF
• K can diffuse out into ECF through conc gradient
• Ions can leave from the tubular lumen to ECF through passive and paracellular movements.

Question 18

Explain Na/K movement in LoH

Answer 18

• Sodium-potassium pump between tubular cells and ECF
• K can diffuse out into ECF through conc gradient
• Sodium clacum potassium pump.

Question 19

What happens which lots of K in diet?

Answer 19

Excretion of K into urine by overload is controlled by secretion in principal cells of late DCT & CD.

ENaC=epithelial Na channel (sodium from lumen to cell )(aldosterone sensitive) causes k to move from cell to lumen

Question 20

What determines K+ secretion in DCT?

Answer 20

Increased K+ intake
Changes in blood pH
Alkalosis ⇒ ↑excretion of K+ ⇒ ↓serum [K+]
*Acute Acidosis ⇒ ↓excretion of K+ ⇒ ↑serum [K+]

Question 21

How is K secretion in DCT achieved?

Answer 21

activity of Na-K-ATPase pump
electrochemical gradient
permeability of luminal membrane channel

Question 22

Aldosterone is a major regulator of K balance in the body. How does it work?

Answer 22

• ↑activity of Na+/K+ pump ↑K+ influx  ↑[K+]i  cell-lumen concentration gradient
• ↑ENaC channels ↑Na+ reabsorption  ↓cell negativity and ↑lumen negativity  voltage gradient
• Redistributes ENaC from intracellular localization to membrane
• ↑permeability of luminal membrane to K+

Question 23

How does High potassium conc cause increase in potassiums secretion?

Answer 23

slows exit from basolateral membrane ↑[K+]i  cell-lumen concentration gradient

↑activity of Na+/K+ ATPase ↑[K+] within the cell

↑Plasma [K] stimulates aldosterone secretion

Question 24

How does Addison’s disease affect potassium?

Answer 24

Deficiency in aldosterone means body secrets lots of Na, low serum Na levels. Causes body to retain K - hyperkalaemia.

Question 25

What is Conn’s syndrome?

Answer 25

↑↑↑ Plasma Aldosterone kidneys to stimulate Na+ reabsorption & K+ excretion. develop hypertension* and ↑fluid volume and hypokalaemia (↓[K+]), hypernatremia and alkalosis
*↑bp & Na delivery to macula densa leads to ↓↓release of renin  renin-independent cause of hypertension (very difficult to control)