Active Transport Flashcards
Why is the active transport system called the sodium pump?
Since the extrusion of Na+ takes place against a steep electrochemical gradient (concentration and electrical)
Structure of sodium pump:
ATP binding sites on intracellular surface
Cardiac glycoside binding sites on extracellular surface
2 alpha and 2 beta subunits
What do the subunits in the sodium pump form?
Tetratmer
Where do ions bind on sodium pump?
Na+ bind internally (3 per alpha subunit)
K+ bind externally (2 per alpha subunit)
What is the sodium pump equation?
3Na+ (inside) + 2K+ (outside)
Against gradient (backwards reaction) = requires energy from breakdown of ATP
Reversible reaction
In sodium pump what is ATP hydrolysed into?
Into ADP by enzyme (Na + K)ATPase
What’s the Km for Na+ in sodium pump?
20nM
What’s the Km for K+ in sodium pump?
1mM
How fast does sodium pump reaction occur?
100 times per second
What can sodium pump reaction be inhibited by?
Removing K+ concentration on outside
What can sodium pump reaction be stimulated by?
By increasing NA+ concentration on inside
What can sodium pump be inhibited by?
Cardiac glycosides (digoxin)
What does digoxin do to the sodium pump?
Competes with K+ on outside of pump
What happens if there is a decrease in K+ on outside?
There’s an increased affinity between cardiac glycosides and sodium pump
What’s a therapeutic index?
Dose producing toxicity in 50% of population
————————————————————————
Minimum effective dose for 50% of population
E.g. 2:1 is a narrow therapeutic index = toxic effect close to therapeutic effect
What’s hypokalaemia?
Low potassium blood concentration
Leads to an increase in digoxin binding
Result in abnormal heart rhythms, muscle weakness, spasms
What’s normokalaemia?
Normal blood potassium levels
What’s hyperkalaemia?
High blood potassium levels
Can result in nausea, palpitations, muscle weakness
How may a patient develop digoxin toxicity?
May become hypokalaemic if on digoxin and diuretics
This reduces competition between K+ and digoxin
This results in increased digoxin binding to sodium pump
Digoxin has very narrow therapeutic index
Emergency treatment for digoxin toxicity:
Administration of digitoxin binding antibody e.g. Digiband
Reverses toxicity by causing digoxin to dissociate from sodium pump
What are primary active transport systems?
They directly couple the hydrolysis of ATP to molecular movement
What are secondary active transport systems?
They use the energy stored in Na+ gradient to drive molecular transport against the electrochemical gradient
