SODIUM-POTASSIUM PUMP

• ICF – High K⁺ concentration.

• ECF – High Na⁺ concentration.

• Both Na⁺ & K⁺ transported against concentration gradient.

• Na⁺-K⁺ pump extrudes –

– 3 Na⁺ out from cell.

– Pumps 2 K⁺ into cell.

• Coupling ratio of Na⁺-K⁺pump – 3:2.

• Na⁺-K⁺pump-Carrier protein – Heterodimer.

• Made of two separate globular proteins:

– Larger α-subunit – 

• Molecular weight -100,000

– Smaller β-subunit – 

• Molecular weight – About 55,000.

• Na⁺ & K⁺ transport occurs through α-subunits.

• Both α- & β- subunits span across membrane with an intracellular & extracellular site.

• Intracellular site has,

– 3 Na²⁺binding/receptor sites for Na⁺.

– An ATP binding site.

– A phosphorylating site.

– ATPase activity.

• Extracellular site has, 

– 2 K⁺ binding/receptor sites for K⁺.

– An Ouabain binding site.

FUNCTION:

• Acts as an electrogenic pump.

– Due to net movement of positive charge out of cell.

– Maintains intracellular fluid ionic composition.

– Maintains electrochemical potential difference across cell membrane.

• Contributes only 4 millivolts to total resting membrane potential of 90 volts which is almost negligible.

• Active transport of Na+ & K+ – One of major energy-using process in body.

• Average, energy utilized, 

– In cell – Accounts about 24% of energy.

– In neurons – 70% of energy.

FACTORS INFLUENCING PUMP ACTIVITY:

• Factors stimulating pump activity:

– Increased intracellular Na⁺ concentration 

– Thyroid hormones

– Aldosterone

– Insulin.

• Factors inhibiting pump activity:

– CHF

– CKF

– Digoxin toxicity

– Dopamine.