Cardiac Glycosides (Digoxin) Action on Electrolyte Pump! - USMLE Forums
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Old 12-27-2012
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Heart Cardiac Glycosides (Digoxin) Action on Electrolyte Pump!

I know cardiac glycosides inhibit the Na-K ATPase. Since it's blocked, the intracellular Na is increased. Now we have the Ca2+-Na+ exchanger. According to Costanzo Physiology..."When the intracellular Na+ concentration increases, the inwardly directed Na+ gradient decreases. As a result, Ca2+-Na+ exchange decreases because it depends on the Na+ gradient for its energy source."

Can someone explain to me why the inwardly directed Na+ gradient decreases? Sorry I'm sure this is a stupid question. I'm kind of confused. I thought since the intracellular Na concentration is increased, more Na would be pushed back in and more Ca will go back out.

I appreciate anyone's help. Thank you!
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Old 12-28-2012
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anyone, correct me if im wrong,

if u call Na/Ca exchange as Ca/Na exchange it all becomes simple.
bear with me.

Na/K pump = the 1st thing (Na) is going out and 2nd thing (K) is going in
Ca/Na exchange = same story 1st thing out (Ca) and 2nd thing (Na) going in
in other words
Na/K removes Na from the cell
Ca/Na removes Ca from the cell

now how does blocking Na/K block Ca/Na too?

if u block Na/K, ull hv a lot of Na left inside coz its not going out
this will block Ca/Na exchange coz Na cant get in, u already hv too much inside. This also means that the Ca that was going out will be blocked and left inside = More Ca inside = +Inotropy

this also explains the Hyperkalemia of digoxin, coz K is left outside coz its not getting in.

the end result is that u hv alot of Na and Ca inside (the 1st things)

what Costanzo is trying to say is that : Na/K ATPase is the pump that is driving all ions that are being run by Secondary Active Transport (Ca/Na exchanger is one of em). Na/K ATPase creates the Na gradient that is then utilized to carry over other things by channel proteins that dont hv ATPase activity themselves. if u block Na/K pump the Na gradient is lost (the lots of Na outside the cell trying to get in) and hence the energy to run the Secondary Active Transporters is lost and they stop.

eg of Secondary Active Transporters

Both items going in = Symporter = Na/Glucose transporter
Items being exchanged = Antiporter = Ca/Na exchange transporter
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Old 12-28-2012
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Quote:
Originally Posted by pukhtoon123 View Post
anyone, correct me if im wrong,

if u call Na/Ca exchange as Ca/Na exchange it all becomes simple.
bear with me.

Na/K pump = the 1st thing (Na) is going out and 2nd thing (K) is going in
Ca/Na exchange = same story 1st thing out (Ca) and 2nd thing (Na) going in
in other words
Na/K removes Na from the cell
Ca/Na removes Ca from the cell

now how does blocking Na/K block Ca/Na too?

if u block Na/K, ull hv a lot of Na left inside coz its not going out
this will block Ca/Na exchange coz Na cant get in, u already hv too much inside. This also means that the Ca that was going out will be blocked and left inside = More Ca inside = +Inotropy

this also explains the Hyperkalemia of digoxin, coz K is left outside coz its not getting in.

the end result is that u hv alot of Na and Ca inside (the 1st things)

what Costanzo is trying to say is that : Na/K ATPase is the pump that is driving all ions that are being run by Secondary Active Transport (Ca/Na exchanger is one of em). Na/K ATPase creates the Na gradient that is then utilized to carry over other things by channel proteins that dont hv ATPase activity themselves. if u block Na/K pump the Na gradient is lost (the lots of Na outside the cell trying to get in) and hence the energy to run the Secondary Active Transporters is lost and they stop.

eg of Secondary Active Transporters

Both items going in = Symporter = Na/Glucose transporter
Items being exchanged = Antiporter = Ca/Na exchange transporter
Thank you that made a lot of sense! I really appreciate your time in explaining this to me!
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  #4  
Old 12-28-2012
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ur welcome
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