Control of Potassium Flashcards
What percentage of potasium is found in the ICF as opposed to the ECF?
98% of potassium Is found inside the cells at about 120-150mmol/l. The other 2% is found in the ECF.
Which cells of the body contain most of the potassium
The majority of potassium in the ICF is found in skeletal muscle cells but also in liver, red blood and bone. If there is a shift of just 1% into the ECF from the ICF the concentration will rise by 50%.
Why can we assume serum potassium is the same as ECF potassium?
Because potassium travels freely across capillary walls
Why is maintaining potassium concentrations important?
Maintaining [K] is important because of its effects on the resting membrane potential and hence its effects on excitability of cardiac tissues. Changes in [K] risk life threatening arrhythmias with both hyperkalaemia and hypokalaemia.
What would occur if the ECF concentration were to increase/decrease?
If ECF [K] increases then this would cause depolarisation of the resting membrane potential of the cell. This is vice versa for if the ECF were to decrease.
How are potassium levels controlled?
Immediate control of K+ is done through maintaining the internal balance by moving K+ between ECF and ICF. Longer term control takes place via adjusting K+ renal excretion.
What happens after eating a meal with high amounts of potassium in?
4/5ths of this K moves directly into cells within minutes. After a slight delay the kidneys begin to excrete more K+ and this is completed within 6-12 hours, this is stimulated by changes in potassium plasma levels but also by signals from the gut.
What increases the K+ uptake by cells?
- Hormones such as: insulin (directly stimulated by potassium in GI blood), aldosterone (directly stimulated by potassium) and catecholamines (particularly Beta 2 agonists helping to move K into cells during stress and exercise) these all act via the Na-K-ATPase.
- Increased [K] in the ECF will obviously increase K+ uptake
- Alkalosis – low ECF [H] causes K+ shift into cells.
What causes K+ movement out of cells?
- Exercise due to Net release of K during action potentials of muscle contraction also damage to muscle cells during exercise. Uptake by non-contracting tissues offsets dangerous rises in K. Also, exercise and trauma causes release of catecholamines which again off sets the loss of K. Cessation of exercise causes dramatic drop in ECF K
- Cell lysis (such as when giving anti-tumour drugs)
- Increase in ECF osmolality causes water to leave the cells, dragging potassium with it
- Low ECF [K] and acidosis – high [H] in the ECF.
What is the link between H+ and K+ ions
To maintain electroneutrality. If [H+ ] in ECF is high then some moves into the cells thus K+ moves out of the cells to maintain the charge. And this works both ways leading to hypo/hyperkalaemia. This also works if the primary problem is Potassium being high. It causes the movement of H+ ions to maintain neutrality and leads to acidosis or alkalosis.
What percentage of potassium ingested is excreted by the GI system?
5%-10% of potassium ingested is excreted from the GI system.
Where is Potassium excreted/absorbed in the nerphron?
K+ is freely excreted in the Glomerulus but the majority is reabsorbed. K+ is secreted in the distal tubule and Cortical collecting duct by the Principle cells. Note the intercalated cells reabsorbs K+ via a H+/K+ ATPase antiporter on the apical side.
K+ secretion in distal tubule and collecting duct is done by Principle cells. Intracellular [K] high and [Na] low. Movement of Na+ into the cells creates an electric gradient and so K+ moves into the lumen down an electrochemical gradient.
What percentages of potassium are absorbed/secreted in the different parts of the Nephron?
Proximal tubule = 67% passively by paracellular diffusion.
Thick ascending limb = 20% actively by Na-K-2Cl.
Principle cells of DCT and Cortical collecting system = 15-120% secreted or nothing if low K+ diet.
Intercalated cells of DCT and cortical collecting duct and medullary collecting duct = 10-12% reabsorbed
What effects the secretion of K+
- ECF [K] which directly simulates Na-K-ATPase and increases permeability of apical K+ channels.
- High ECF potassium stimulates aldosterone secretion which increases transcription of all three proteins (ENaC is the third).
- Acidosis decreases K+ secretion inhibiting Na-K-ATPase and decreasing K+ channel permeability. Alkalosis does the opposite.
- Increased tubular flow rate washes away luminal K+ which increases K+ loss and increases Na delivery to distal tubule will mean more Na absorbed which results in more K+ secretion.
How is K+ absorbed in the intercalated cells of the DCT and Cortical collecting duct?
This is done using a Potassium hydrogen pump which required ATP. A hydrogen channels moves hydrogen ions back into the lumen.
