Control of Potassium Flashcards
(34 cards)
What is the distribution of potassium in body fluids?
Total 3500mmol.
Mainly in the skeletal muscle cells, liver, RBC and bone.
98% in ICF (120-150mmol/L).
2% in ECF (3.5-5mmol/L).
How is the difference between ICF and ECF of [K+] maintained?
By Na-K-ATPase.
Why is maintaining ECF [K+] critical?
- K+ has a serious impact on resting membrane potentials.
- Subsequently K+ has an effect on the excitability of cardiac tissue.
- This means there is a risk of life threatening arrhythmias with hyperkalaemia and hypokalaemia.
What is the effect of [K+] on the resting membrane potential?
There is a concentration gradient for K+ to move out of the cell.
There is an electrical gradient driving K+ back into the cell.
This creates a resting membrane potential of around -90mV.
What happens to the resting membrane potential in different concentrations of K+?
Hypokalaemia: causes a decrease in RMP (more negative).
Hyperkalaemia: causes an increase in RMP (more positive).
How is ECF [K+] regulated?
- Immediately by the internal balance between ICF and ECF.
2. In the long term by overall K+ balance which is external and involves adjusting renal K+ excretion.
Describe the events of K+ following a meal.
The intestine and colon absorb dietary K+.
4/5ths of the K+ which has now been absorbed into the ECF will move into cells within minutes.
After a slight delay the kidneys begin to excrete K+. Excretion is complete in 6-12 hours.
Signals from the gut influence meal driven potassium excretion also.
How long does it take the kidney to excrete K?
6-12hours.
How is [K+] internally balanced?
- Movement of K+ from the ECF into cells via Na-K-ATPase.
2. Movement of K+ out of cells into the ECF via K+ channels (determine the K+ permeability of the cell membrane).
What factors increase K+ uptake into cells?
- Hormones via Na-K-ATPase: insulin, aldosterone and catecholamines.
- Increased [K+] in ECF, which creates a concentration gradient.
- Alkalosis: a low ECF [H+] causes H+ to leave the cell and thus K+ to enter the cells.
What factors promote K+ shift out of cells?
- Exercise.
- Cell lysis.
- Increase in ECF Osmolality, can cause water to come out of cells and K to follow.
- Decreased [K+] ECF.
- Acidosis: Increases in [H+] in the ECF causes H+ to move into cells and K+ to move out of cells.
How does insulin increase K+ cellular uptake?
K+ in the splanchnic blood stimulate insulin secretion by the pancreas.
Insulin increases Na-K-ATPase activity which increases K+ uptake into muscle cells and the liver.
What is the clinical use of insulin in terms of K+?
Insulin and dextrose can be used to treat hyperkalaemia.
How does aldosterone increase K+ cellular uptake?
K+ in blood stimulates aldosterone secretion.
This stimulates the uptake of K+ via Na-K-ATPase.
How does Catecholamines increase K+ cellular uptake?
Act via B2 adrenoreceptors.
This stimulates Na-K-ATPase and cellular uptake of K+.
How does exercise increase K+ back into the ECF?
Net release of K+ during the recovery phase of action potential, K+ exits the cell.
There is also skeletal muscle damage during exercise releasing K+.
There is subsequent uptake b y non-contracting tissues to prevent dangerously high hyperkalaemia.
Exercise also increases catecholamines which offset ECF [K+] rise (increase K+ uptake by other cells).
Cessation of exercise results in a rapid plasma decrease in potassium.
What 2 factors stop exercise from causing severe hyperkalaemia?
- Catecholamine release.
2. Non-contracting tissues uptake some K+.
Why can acidosis cause hyperkalaemia?
H+ shift into cells (due to conc gradient). This causes K+ to leave the cell into ECF and thus cause hyperkalaemia.
Why can alkalosis cause hypokalaemia?
H+ shift out of cells (due to conc gradient). This causes K+ to enter cells and thus less K+ is in the ECF so there is hypokalaemia.
Why can hyperkalaemia cause acidosis?
K+ high in the ECF means it enters the cell. This causes H+ to leave cells and thus acidosis.
Why can hypokalaemia cause alkalosis?
K+ is low so K+ leaves the cells. This causes H+ to enter cells and thus there is low ECF H+ and so alkalosis.
Explain how external balance controls [K+]?
This process is slower. It takes 6-12 hours to excrete a load of K+. It does it by controlling total body potassium content over the longer term. It regulates K+ secretion in the late DT and cortical collecting duct principal cells/
It is all done via the Na-K-ATPase on the basolateral membrane. This transporter increases the intracellular K+ and decreases the intracellular Na+.
High intracellular K+ creates a chemical gradient for secretion.
Na+ moves into the lumen into the cell down its concentration gradient (via apical ENaC) which creates an electrical gradient.
Both these gradients allow a favourable movement of K+ secretion via apical K+ channels.
What are the factors that affect K+ secretion by principal cells?
Tubular Factors:
1. ECF [K+]: directly stimulates Na-K-ATPase, increasing the permeability of apical K+ channels and stimulating aldosterone secretion.
2. Aldosterone stimulated by high ECF [K+]. Increases transcription of relevant proteins (Na-K-ATPase, K and ENac)
3. Acid Base Status: Acidosis decreases K+ secretion (H+ move into cells and K+ move out of cells into ECF) by inhibiting Na-K-ATPase and alkalosis does the opposite.
Luminal Factors:
4. Increases distal tubular flow rate washes away the luminal K+ and so increases K+ loss.
5. Increased Na delivery to the distal tubule means more Na absorbed and so more K+ lost.
Describe the K+ absorption by intercalated cells the distal tubule and cortical collecting duct.
K+ absorbed by the intercalated cells by an active process mediated by H+-K+-ATPase in the apical membrane of the lumen.
