Control of potassium Flashcards
Are most potassium ions in the body intracellular or extracellular?
Intracellular
What are the concentrations of intracellular and extracellular potassium ions?
Intracellular - 160mmol/L
Extracellular - 5mmol/L
What is the main source of potassium ions?
Diet
How are most potassium ions lost from the body?
Urine
then faeces, sweat
What is responsible for matching potassium ion intake to potassium ion loss from the body?
Kidneys excreting it into the urine
What mechanisms are responsible for maintaining a constant plasma potassium ion concentration? Which is the first-line mechanism?
Cells taking it up or releasing it - first line mechanism
Kidneys excreting it into urine
How are potassium ions taken up into cells?
Na+ K+ ATPase on cell membranes
actively transports 2 potassium ions into cells
What stimulates potassium ions to be taken up into cells?
Extracellular potassium ion concentration
Insulin
Noradrenaline
Aldosterone
What receptor does noradrenaline bind to when stimulating potassium ion uptake into cells?
B2 adrenoceptors
What stimulates potassium ion release from cells?
Acidosis
Cell damage, death
Increased extracellular fluid osmolality
How does increased extracellular fluid osmolality stimulate potassium ion release from cells?
Water moves out of cells into the extracellular fluid
increase in intracellular potassium ion concentration
potassium ions diffuse out of the cell
What inhibits potassium ion release from cells?
Alkalosis
What are the proportions of filtered potassium ions reabsorbed along the nephron?
Proximal convoluted tubule - 67%
Loop of Henle, mostly thick ascending limb - 20%
Collecting duct - variable
What does the proportion of filtered potassium ions reabsorbed in the collecting duct depend on?
Potassium ion intake
How are potassium ions reabsorbed in the proximal convoluted tubule?
By solvent drag
What is solvent drag?
Ions and molecules being moved by the flow of water
How are potassium ions secreted in principal cells of the collecting duct?
Na+ K+ ATPase on basolateral domain
actively transports 2 potassium ions from interstitial fluid into principal cell
ROMK channels and BK channels on apical domain
potassium ions move through them from the cell into the tubular lumen down their electrochemical gradient
What stimulates potassium ion secretion from the principal cells?
High extracellular fluid potassium ion concentration
Aldosterone
High tubular flow rate
How does high extracellular fluid potassium ion concentration stimulate potassium ion secretion from the principal cells?
Increased concentration gradient for potassium ion secretion
Stimulates aldosterone secretion
How does aldosterone stimulate potassium ion secretion from the principal cells?
Increases activity of Na+ K+ ATPase on basolateral domain
more potassium ions actively transported from interstitial fluid into cell
Increased expression of ENaC on apical domain
more sodium ions move through it from tubular lumen into cell
more negatively charged lumen
more potassium ions move through ROMK and BK channels from cell into lumen down their electrochemical gradient
Increased ROMK and BK channels on apical domain
What is the aldosterone paradox?
Refers to how the sodium ion reabsorption can be stimulated without increasing potassium ion secretion
And how potassium ion secretion can be stimulated without increased sodium ion reabsorption
How can sodium ion reabsorption be stimulated without increasing potassium ion secretion in the aldosterone paradox?
Aldosterone increases expression of ENaC on apical domain of epithelial cells on later distal convoluted tubule and principal cells of collecting duct
more sodium ions move through it from tubular lumen into cells
Angiotensin 2 inhibits ROMK on apical domain of epithelial cells in principal cells of collecting duct
fewer potassium ions move through it from cell into lumen
How can potassium ion secretion be stimulated without increasing sodium ion reabsorption in the aldosterone paradox?
High extracellular fluid potassium concentration stimulates aldosterone release
aldosterone gives increased expression of ENaC on apical domain of principal cells
more sodium ions move through it from tubular lumen into cells
more negatively charged lumen
more potassium ions move through ROMK channels on apical domain from cells into lumen down their electrical gradient
But other sodium transporters are inhibited
fewer sodium ions moved from lumen into epithelial cells
How does a high tubular flow rate stimulate potassium ion secretion from principal cells?
Removes secreted potassium ions from tubular lumen
reduced concentration of potassium ions in tubular lumen
maintains concentration gradient
Increased expression of BK channels on apical domain
more potassium ions move through them from principal cell into lumen
What inhibits potassium ion secretion from the principal cells?
Acidosis
What are the signs and symptoms of hypokalaemia?
Weakness
Muscle weakness, muscle pain, muscle tremors
Constipation
Cardiac arrythmias
Why does hypokalaemia cause muscle weakness, pain and tremors?
Disturbed function of skeletal muscle
Why does hypokalaemia cause constipation?
Disturbed function of smooth muscle
What does an ECG of a patient with hypokalaemia look like?
Shorter T waves
ST segment depression, T wave inversion
U wave
What are the causes of hypokalaemia?
Reduced dietary intake
Increased uptake into cells - metabolic alkalosis, stress
GI loss - vomiting, diarrhoea
Urinary loss - high aldosterone, high flow
How is hypokalaemia treated?
Oral potassium supplements
IV potassium
