Examkrackers Exam 1 B/B #168

[djsbaseball2014]

Can anybody explain the reasoning behind this? The passage stated that the SA node has a higher than normal permeability to Na+ and that sodium fluxes in during a cardia action potential through a VG Na+/Ca2+ channel. But my understanding is that both the Na+/Ca2+ exchanger (NCX) and Na/K pump work to restore membrane potential. Normally the NCX pumps 3 Na+ into the cell, down its conc. gradient to pump 1 Ca2+ out of the cell against its conc. gradient. (secondary active transport) where as the Na/K pump pumps 3Na+ outside and 2K+ inside both going against the gradient and uses ATP to fuel it. Why is the NCX all of a sudden functioning differently in the heart?

Any information would help.

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[PhilzCoffeeAddict]

NA/K pump works to restore membrane potential, as you stated. To restore that potential it has to INVEST energy, hence it will cost it ATP to restore said potential. Normally in an action potential Na+ comes rushing in to depolarize membrane. So to restore the potential, Na+ has to be kicked out and K+ is moved in to restore that potential. "3 Na+ out, 2K+ in" That's what you should remember everytime you hear Na/K ATPase pump.

 

[aldol16]

Can you post the rest of the passage? You're generally correct in your logic. Energy must be used to established transmembrane gradients and the Na+ and K+ gradients are established using energy from ATP hydrolysis. The Na+/Ca2+ exchanger just uses the energy from Na+ import (down its gradient) to export Ca2+ (up its gradient). That's how it usually works - the passage probably says something about the exception for the heart.

 

[djsbaseball2014]

Lol guys this was 3 months ago, I've already taken my MCAT (Jan. 20th) and got a 512 (130/125/130/127). I appreciate the help though! @aldol16 @PhilzCoffeeAddict

 

[PhilzCoffeeAddict]

hey congrats!!