Week 6 - Potassium control Flashcards
Why does small shifts of K result in such a big increase in ECF concentrations?
-The ECF is kept at such a low conc (approx 4.5mmol) that a shift of 1% of ICF results in a 50% rise in ECF
What maintains the difference between ECF and ICF?
-NaKATPase
What effect does K have on the resting membrane potential during normal, decreased ECF and increased ECF?
-K+ establishes a gradient across the cell membrane which sets up the RMP
normal -> steady RMP
decreased ->increases the gradient across the cell and hyperpolarises
increased -> decreases the gradient across the cell and depolarises
By what two mechanisms is potassium regulated?
-Internal and external balance
What is the purpose of internal balance?
-To have an immediate effect by shifting K between ECF and ICF -> does not solve the problem only helps cope with it
What is the function of external balance?
-To adjust renal K excretion to correct K imbalance
Describe the events which occur to deal with K after ingestion of a meal
1)Intestine and colon absorb dietary K -> large amount of K enters ECF
2)4/5ths of ingested K moved into cells within minutes
3)After a slight delay kidneys begin to excrete K and K is slowly released from cells
Excretion is complete within 6-12 hours.
By what mechanism is ECF shifted to ICF in internal balance?
-Mediated via NaKATPase
By what mechanism is K shifted from ICF to ECF in internal balance?
-Through ROMK
What are the three factors which increase K uptake by cells?
- Hormones
- Increased [K] in ECF
- Alkalosis
What hormones cause cellular uptake of K?
- Insulin
- Aldosterone
- Catecholamines
How does alkalosis cause cellular uptake of K?
-ECF K is exchanged for H+ in order to correct alkalosis
How do hormones promote cellular uptake of K?
-Increase NaKATPase activity
What factors influence K to be shifted out of cells?
- Exercise
- Cell Lysis
- Increased ECF osmolarity
- Low ECF [K]
- Acidosis
How does exercise promote K to be released from cells? How is this corrected?
- Skeletal muscle contraction releases K in proportion to intensity of exercise
- Surrounding non-contracting tissues uptake K and prevent hyperkalaemia
- Exercise also increases catecholamines to offset increase in ECF [K]
Give 3 examples of clinical conditions/treatments which cause cell lysis
- Rhabdomyolysis
- Intravascular haemolysis
- Cancer chemotherapy
How does increase in plasma osmolarity effect K?
- Water moves from ICF to ECF to decrease osmolarity
- Increases [K] in ICF so K leaves down conc gradient
Describe the renal handling of potassium
- K is freely filtered at the glomerulus
- Most reabsorbed in PCT and LoH
- Control occurs in DCT and CD where potassium secretion/absorbtion is altered to match ingestion
From which cells in the nephron is potassium secreted?
-Principal cells of DCT and cortical CD
Describe K secretion in principal cels off DCT/CD
- NaKATPase basolateral activity maintains high ICF [K]
- this creates a chemical gradient for secretion
- Na influx produces a high electrical gradient
- K leaves the cell down the electro-chemical gradient via apical K channels
How does [K] ECF effect K secretion by principle cell
-high [K] ECF stimulates NaKATPase, increases permeability of apical K channels and stimulates aldosterone
How does aldosterone affect K secretion by principle cells?
-Aldosterone increases expression of NaKATPase, K channels and ENaC thus increasing secretion
How does acid base status influence K secretion by principle cells?
- Acid base status -> acidosis decreases secretion via inhibition of NaKATPase and decreased K channel permeability
- > alkalosis increases secretion via stimulating NaKATPase and increasing K channel permeability
What luminal factors affect K secretion by principal cells?
- Increased DCT flow rate washes away luminal K -> Increased gradient = increased secretion
- Increased Na delivery to distal tubule -> more Na resorbed, more K secreted
