03-11-21 - Active Transport and The Sodium Pump Flashcards
Learning outcomes
- Explain the role that the sodium pump plays in regulating the intracellular ionic concentrations
- Briefly explain the clinical significance of the interaction between the binding of the cardiac glycosides such as ouabain, and the extracellular potassium concentration
- Explain the difference between primary active transport and secondary active transport
- Explain sodium dependent calcium transport
- Explain the mechanisms involved in the transepithelial transport of glucose
What is the sodium pump?
What is it capable of doing?
What does it require to function?
What does extrusion of Na+ from cells take place against?
- The sodium pump (aka Na+/K+ ATPase) is an integral membrane protein enzyme
- It is capable of transport of Na+ and K+ across the membrane in opposite directions
- It requires energy in the form of ATP
- The extrusion of Na+ from a cell takes place against a steep electrochemical gradient (concentration and electrical) in the Na/K ATPase system
What are the subunits of the Na/K ATPase?
What are the functions of the subunits
What is this structure called?
What is the pumps molecular weight?
What does it require to function?
Where are the binding sites present on the ATPase?
How many times do the subunits pass through the membrane?
Where can these ATPases be found?
How many Na+ and K+ ions bind?
What does this result in?
Where do inhibitors bind?
- The Na/K ATPase consists of 2 alpha subunits (which do a majority of the work) and 2 beta subunits (function not clear) that are heavily glycosylated on the extracellular surface
- This is known as an α2β2 tetramer
- The molecular weight is 270,000 (α – 95,000 β – 40,000)
- ATP must be available intracellularly for the Na/K ATPase to function
- ATP and Sodium binding sites are on the Alpha subunits intracellularly
- Potassium and cardiac glycoside (inhibitor) binding sites are on the Alpha subunits extracellularly
- The alpha subunits pass through the membrane multiple times, whereas the beta subunits only pass through the membrane once
- Na/K ATPase can be found on the surface of every cell, with approximately 1 million pump sites per cell
- 3 Na+ ions bind internally (3 per α) and 2 K+ ions bond externally (2 per α)
- When sodium goes out and potassium comes in, this results in a net ion loss
- Cardia glycosides are inhibitors that only bind on the extracellular surface
What is the sodium pump equation?
What enzyme hydrolyses ATP?
What is Km value?
What does a high Km value indicated?
What are Km values for Na+ and K+ for the Na/K ATPase reaction?
What is the rate limiting factor of this reaction?
How many times does this reaction occur a second?
How can it be stimulated?
How can it be limited?
- ATP is hydrolysed to ADP + Pi by the enzyme (Na+ + K+) ATPase
- Km value is half the substrate concentration required to reach half Vmax of an enzyme reaction
- An enzyme with a high Km value has a low affinity for its substrate, meaning it requires greater substrate concentration to achieve Vmax
- Na+ has a Km value of 20mM and K+ has Km value of 1mM (millimolar)
- Intracellular sodium ion concentration is the rate limiting factor, as there is only usually about 10mM inside the cell
- The Na/K ATPase reaction occurs 100 times per seconds
- It can be stimulated by increasing [Na+]i
- It can be limited by removing [K+]o
What is an example of cardiac glycosides (ATPase inhibitor)?
What is it used to treat?
How does it do this?
Is this reaction reversible?
- Digoxin is the most commonly prescribed cardiac glycosides
- It is used to treat heart failure and arrhythmias
- It does this by inhibiting the Na+/K+ pump by competing with K+ and binding to the K+ binding site, which leads to an increase in intracellular Na+
- This inhibition is reversible
- This will drive an influx of calcium in the heart, causing an increase in contractility (more powerful contractions)
How does the concentration of extracellular K+ influence digoxin efficacy?
Why is this?
What is hypokalaemia?
What does it lead to?
- An increase in extracellular K+ concentration leads to a decrease in the efficacy of digoxin
- The more potassium present, the less digoxin can bind to the pump
- Hypokalaemia is low extracellular potassium concentrations
- This will lead to increase digoxin binding
What is the formula for therapeutic index (TI)?
Why is it important?
What is the TI for digoxin?
What can increase/decrease potassium toxicity?
- Therapeutic index is important as drug doses need to be enough to have a therapeutic effect, but not so much that we observe the toxic effect
- The TI for digoxin is 2:1, meaning twice the effective dose will produce a toxic effect
- Digoxin toxicity is increase by hypokalaemia, as low potassium levels will increase digoxin binding to the K+ binding site of the Na/K ATPase, leading to increased intracellular sodium levels
- Conversely, hyperkalaemia will diminish digoxin’s effectiveness
What is the typical homeostatic range for potassium?
What qualifies as hypokalaemia and hyperkalaemia?
What can hypokalaemia and hyperkalaemia cause?
- Normokalaemia – 3.6-5.2 mEq/L (milliequivalents per litre)
- Hypokalaemia – less than 3.5 mEq/L
- Hyperkalaemia – More than 5.5 mEq/L
What does the sodium-potassium pump transport?
What does this maintain?
What does this establish?
What is this vital for?
How does the pump control cell volume?
What would happen if it did not do this?
- The sodium potassium pump transports Na+ outside of cells and K+ into cells
- This maintains the Na+ and K+ differences across cell membranes
- This can establish a negative voltage inside the cell (compared with outside)
- This is vital for nerve function and signal transmission
- The net movement of ions outside of the cell dilute the cytosol
- Without this, too much water would enter the cell due to osmotic pressure (due to high concentration of osmotically active Na+) resulting in the cell bursting
Describe the 3 steps in the functioning of the Na+/K+ ATPase
1) 2xK+ bind extracellularly and 3xNa+ bind intracellularly, which triggers the activation of the ATPase
2) This causes the ATPase to catalyse the hydrolysis of ATP to ADP+Pi, which results in the phosphorylation of the pump
3) This phosphorylation causes a chemical and conformation change to the carrier protein, causing Na+ to be extruded across the membrane, and K+ to be introduced into the cell
What is the sodium calcium exchanger?
What is it an example of?
What does it transport?
How is this process enabled by the Sodium potassium pump?
- The sodium calcium exchanger is a secondary active transport protein
- It is an example of an antiport
- Sodium is pushed into the cell with its concentration gradient, and calcium is pushed out of the cell against its concentration gradient
- This process is enabled by the sodium potassium pump as the pump pushes sodium out of the cell, which can be used in this process
What are the 5 different types of glucose transporters?
Where are they found?
What is glucose transport an example of?
• Glucose transport is an example of facilitate transport, as it uses these transporter proteins
GLUT
What is another example of glucose transporters?
What kind of transport is this?
What are the 2 different types of this transporter?
How do they differ in: • Km • Affinity for glucose • Na+/glucose ratio • Accumulation ratio
- Another example of a glucose transporter is the sodium-dependent glucose transporter (SGLT)
- This is an example of secondary active co-transport, where sodium and glucose move across the membrane in symport (same direction)
How is glucose taken up in the small intestine?
How is this process made possible by sodium potassium pumps?
How does this glucose then get into blood vessels?
- In the apical compartment of epithelial cells in the small intestine there are SGLT 1 transporters which transport sodium and glucose in symport into the cells
- There are also sodium/potassium pumps present that pump sodium extracellularly, which allows this process to happen
- There are GLUT 2 glucose transporters on the basal surface of the epithelial cells, which transports glucose into the blood vessels
What do diuretics cause?
What can this be a treatment for?
What do loop diuretics also do?
How can this cause digoxin toxicity?
What is the emergency treatment for this?
- Diuretics cause the increase in urine output by the kidneys (promote diuresis)
- Diuretics can be a treatment for high blood pressure and excessive fluid retention
- Loop diuretics, such as Furosemide, increases urinary secretion of potassium
- Patients on digoxin (narrow therapeutic index) who start diuretics (furosemide) may become hypokalaemia
- The reduction in K+ competition on digoxin will cause more digoxin to bind to Na/K pumps
- Because of the narrow therapeutic index of digoxin, the patient will develop digoxin toxicity
- K+ is pumped by active transport, which is an example of carrier mediated transport (aka facilitated diffusion?), which can saturate due to limited binding sites and time taken to transport
- A digoxin binding antibody can be administered, which will cause the digoxin to dissociate from the pump,
- This reverses the toxicity associated with increases pump inhibition caused by hypokalaemia
