Potassium Control

Question 0

What maintains the potassium concentrations?

Answer 0

3Na-2K-ATPase

Question 1

Normal [K+] in ICF and ECF?

Answer 1

ICF = 120-150 mmol/L
ECF = 3.5-5 mmol/L

Question 2

Why is high potassium inside cells and mitochondria essential?

Answer 2

Maintains cell volume
Regulates intracellular pH
Controls cell-enzyme function
DNA and protein synthesis
Cell growth 
Low potassium outside cells is largely responsible for the membrane potential of excitable and non-excitable cells

Question 3

How does hyper and hypokalaemia affect the membrane potential?

Answer 3

Hyperkalaemia: depolarises it
Hypokalaemia: hyperpolarises it

Question 4

What can hypokalaemia cause?

Answer 4

Inability of kidney to form concentrated urine
Metabolic alkalosis
Enhancement of renal ammonium secretion

Question 5

What can hyperkalaemia cause?

Answer 5

Cardiac arrhythmias

Question 6

Where in the nephron is potassium reabsorbed?

Answer 6

PCT
Thick ascending limb
DCT
Cortical collecting duct

Question 7

Where in the nephron is potassium secreted?

Answer 7

Distal tubule
Cortical collecting duct

Both by principal cells

Question 8

What percentage of potassium is reabsorbed in the PCT and by what processes?

Answer 8

67%

Paracellular diffusion

Question 9

What percentage of potassium is reabsorbed in the thick ascending limb and by what processes?

Answer 9

20%
Na-K-2Cl transporter in apical membrane
(Driven by Na-ATPase)

Question 10

How much potassium is secreted in principal cells of the DCT and cortical CD?

Answer 10

15-20%

Question 11

What percentage of potassium is reabsorbed by intercalated cells of the DCT and CD? By which transporter?

Answer 11

10-12%

Hydrogen-potassium transporter

Question 12

Describe process of secretion of potassium

Answer 12

Na-K-ATPase on basolateral membrane increases intracellular K conc and decreases Na conc
High intracellular K creates gradient for potassium secretion
Na moves from lumen to cell via ENaC making lumen negatively charged
Favourable electrochemical gradient for K+ secretion via apical K+ channels

Question 13

How does high ECF potassium concentration stimulate more potassium secretion?

Answer 13

Directly stimulates Na-K-ATPase and increases permeability of apical K+ channels
Also stimulates aldosterone secretion

Question 14

How does aldosterone stimulate K secretion?

Answer 14

Increases transcription of Na-K-ATPase in basolateral membrane and ENaC/K+ channels in apical membrane

Question 15

What luminal factors affect potassium secretion?

Answer 15

Increased distal tubular flow rate - washes away luminal K+, increasing K+ loss
Increased Na+ delivery to distal tubule - more sodium can the. Be absorbed so more potassium is lost

Question 16

How does external balance control extracellular potassium concentration?

Answer 16

Adjusts renal potassium excretion to match intake - regulates total body content
Depends on dietary intake and excretion by GI and kidneys
Responsible for long term control
Slow acting - 6-12 hours to secrete a load

Question 17

How does internal balance regulate potassium concentration?

Answer 17

Immediate effect
Shift potassium between extracellular fluid and intracellular fluid
-If ECF [K] increases - K+ into cells via Na-K-ATPase
-If ECF [K] decreases - K+ out of cells via ROMK

Question 18

What are the main things that promote potassium uptake into cells?

Answer 18

Hormones

• insulin
• aldosterone
• catecholamines

Increased [K+] in ECF

Alkalosis (low ECF H+ concentration)

Question 19

What are the general factors that promote potassium shift into ECF out of cells?

Answer 19

Low [K+] ECF
Exercise
Cell lysis
Increased plasma/ECF osmolarity
Acididosis

Question 20

How does insulin promote uptake of potassium into cells (causing hypokalaemia)

Answer 20

K+ in splanchnic blood stimulates insulin secretion by pancreas
Insulin stimulates K+ uptake by muscle and liver via increase in Na-K-ATPase

Question 21

How does aldosterone reduce ECF [K+]

Answer 21

K+ in blood stimulates aldosterone secretion
Stimulates Na-K-ATPase
Stimulates K+ secretion in kidneys

Question 22

How do catecholamines reduce [K+] in ECF?

Answer 22

Act via β2 adrenoreceptors

Stimulate Na-K-ATPase

Question 23

How does alkalosis cause hypokalaemia?

Answer 23

H+ excreted from cells to correct alkalosis

Get reciprocal K+ shift into cells because the inside becomes electronegative

Question 24

How does exercise increase ECF potassium conc?

Answer 24

Skeletal muscle contraction causes net release of potassium during recovery phase of action potential so K+ exits cells Skeletal muscle damage also releases K+

Question 25

How is hyperkalaemia prevented in exercise?

Answer 25

Uptake of potassium by non-contacting tissues | Release of catecholamines

Question 26

How can cessation of exercise lead to hypokalaemia?

Answer 26

Non-respiring tissues still taking up potassium Can drop to <3mmol Cause sudden death

Question 27

What can cause cell lysis and release of potassium into ECF?

Answer 27

Trauma in skeletal muscle causing muscle fell necrosis Intravascular haemolysis due to eg incompatible blood transfusion or G6PD deficiency Chemotherapy - release of potassium from tumour cell lysis

Question 28

Why can diabetics become hyperkalaemic?

Answer 28

If they go into diabetic ketoacidosis Increase in plasma and ECF toxicity causes water to move from ICF to ECF Increases [K+] in cells so K+ leaves down conc gradient Also acidosis causes increased uptake of H+ increased loss of K+ from cells.

Question 29

Can hypokalaemia cause acidosis or alkalosis?

Answer 29

Alkalosis

Question 30

How does hypokalaemia cause cardiac arrhythmias?

Answer 30

Hyperpolarises cardiac cells More fast Na+ channels available in active form Heart more excitable

Question 31

Causes of hypokalaemia?

Answer 31

``` Alkalosis Inadequate intake Excessive loss -vomiting -diarrhoea -diuretics -osmotic diuresis (diabetes) -high aldosterone ```

Question 32

Clinical features of hypokalaemia?

Answer 32

Heart and neuromuscular dysfunction - paralytic ileus - muscle weakened - dysfunction of CD cells - unresponsive to ADH - nephrogenic diabetes

Question 33

Treatment of hypokalaemia?

Answer 33

Treat cause K+ replacement (IV/oral) If due to high aldosterone - K+ sparing diuretics blocking action of aldosterone on principal cells eg Amiloride Aldosterone antagonist - spironolactone

Question 34

How does hyperkalaemia cause heart arrhythmias?

Answer 34

Depolarises cardiac cells More fast Na+ channels inactive Heart less excitable

Question 35

Causes of hyperkalaemia?

Answer 35

``` Inadequate renal excretion Acute/chronic kidney injury Reduced mineralocorticoid effect K+ sparing diuretics ACE inhibitors Adrenal insufficiency Acidaemia Cell lysis K+ shift ```

Question 36

Clinical features of hyperkalaemia?

Answer 36

Heart and neuromuscular dysfunction - paralytic ileus - acidosis

Question 37

Treatment of hyperkalaemia?

Answer 37

Emergency - Ca2+ gluconate to reduce effect on heart - IV insulin and glucose - dialysis to remove excess K+ Long term - dialysis - oral potassium binding resins - bind in the gut - reduce intake - treat cause