03-11-21 - Active Transport and The Sodium Pump Flashcards
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 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?
what happens to calcium?
Here’s a clearer explanation of how digoxin works:
What is Digoxin?
- Digoxin is a commonly prescribed medication from the class of drugs called cardiac glycosides.
- It is used to treat:
- Heart failure: Helps the heart pump more effectively.
- Arrhythmias: Helps regulate abnormal heart rhythms.
How Does Digoxin Work?
1. Inhibits the Na⁺/K⁺ Pump:
- Digoxin blocks the Na⁺/K⁺ pump on heart cell membranes.
- It competes with potassium (K⁺) for the pump’s K⁺ binding site, stopping the pump from working.
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Results of Pump Inhibition:
-
Intracellular Sodium (Na⁺) Increases:
- When the pump is blocked, Na⁺ cannot leave the cell, so its levels build up inside.
-
Calcium (Ca²⁺) Influx Increases:
- The rise in intracellular Na⁺ affects another mechanism (the Na⁺/Ca²⁺ exchanger), leading to more Ca²⁺ entering the cell.
-
Intracellular Sodium (Na⁺) Increases:
-
Increased Contractility:
- Higher levels of intracellular Ca²⁺ make heart muscles contract more strongly, improving the heart’s pumping ability.
Key Points:
- Reversible Inhibition: Digoxin’s action on the Na⁺/K⁺ pump is not permanent; the pump can work again after the drug is cleared.
- Stronger Heart Contractions: By increasing intracellular calcium, digoxin makes heart contractions more powerful, improving blood flow in heart failure.
Summary:
Digoxin helps treat heart failure and arrhythmias by blocking the Na⁺/K⁺ pump, leading to more calcium in heart cells. This boosts the heart’s ability to pump blood effectively.
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?
Here’s a clearer explanation of the sodium-potassium pump and its importance:
What Does the Sodium-Potassium Pump Do?
- The sodium-potassium pump moves:
- Sodium (Na⁺) out of cells.
- Potassium (K⁺) into cells.
- This process keeps sodium levels high outside the cell and potassium levels high inside the cell.
Why is the Sodium-Potassium Pump Important?
1. Maintains Ion Balance:
- By moving Na⁺ and K⁺ in opposite directions, the pump maintains a balance of these ions across the cell membrane.
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Creates a Negative Voltage:
- More positive ions (Na⁺) are moved out than are brought in (K⁺), which makes the inside of the cell slightly negative compared to the outside.
- This electrical charge difference is essential for nerve function and sending signals in the body.
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Prevents Cell Swelling:
- The pump removes sodium from the cell, which helps prevent too much osmotic pressure (water flowing into the cell).
- Without the pump:
- Sodium would build up inside the cell.
- Water would follow sodium into the cell, causing it to swell and possibly burst.
Summary:
- The sodium-potassium pump is essential for:
1. Maintaining ion balance and electrical charge in cells (important for nerves and signals).
2. Regulating water movement to prevent cells from swelling and 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?
Here’s a clearer explanation of the sodium-calcium exchanger and how it works:
What is the Sodium-Calcium Exchanger?
- The sodium-calcium exchanger is a transport protein found in cell membranes.
- It is a type of antiport (moves two ions in opposite directions).
- Its role is to move:
- Sodium (Na⁺) into the cell (following its concentration gradient).
- Calcium (Ca²⁺) out of the cell (against its concentration gradient).
How Does it Work?
1. Powered by the Sodium Gradient:
- Sodium naturally moves into the cell because there is less sodium inside the cell than outside.
- This movement of sodium provides the energy needed to push calcium out of the cell, even though it goes against calcium’s gradient.
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Depends on the Sodium-Potassium Pump:
- The sodium-potassium pump maintains a low sodium level inside the cell by actively pumping sodium out of the cell.
- This creates the sodium gradient that powers the sodium-calcium exchanger.
Summary:
- The sodium-calcium exchanger uses the flow of sodium into the cell to push calcium out, helping keep calcium levels low.
- This process relies on the sodium gradient created by the sodium-potassium pump.
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, like furosemide, 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
Digibind rapidly binds to the digoxin causing it to dissociate from the sodium pump reversing the toxicity associated with the increased sodium pump inhibition caused by the hypokalaemia
