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Old 05-24-2015
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Default Q. class I antiarrhythmics MOA

In Kaplan and in U-world, it's mentioned that Class 1B antiarrhythmics shortens phase 3 repolarization by blocking slow Na "window" currents. My understanding is phase 3 is associated with k+ efflux, then how come Na+ in phase 3? Can anybody explain- what is this Na "window" current and how will phase 3 shortens by blocking it?
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Old 05-26-2015
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It's confusing to think at first but the "slow" Na+ "window" current is a current which occurs during phase 3, this Na+ current enters the cell resulting into depolarisation while K+ is exiting the cell during phase3 causing repolarization,obviously K+ leaving the cell is major so now if we can block this Na+ current then K+ will exit the cell resulting into rapid repolarization without the slight depolarisation which was being caused by this Na+ current and thus phase 3 will come to an end quicker.

Just a thought
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Old 05-26-2015
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Good logic. Thanks cleverfox.
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Old 05-26-2015
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Here is one more related question:

Q. how it make sense that class 1A being a Na channel blocker also blocks K+ channel in phase 3,as mentioned in kaplan ( in addition to blocking Na channel in phase 0 which completely makes sense), and prolong repolarization, thus contributing to increased AP duration?
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Quote:
Originally Posted by Aumsai View Post
Here is one more related question:

Q. how it make sense that class 1A being a Na channel blocker also blocks K+ channel in phase 3,as mentioned in kaplan ( in addition to blocking Na channel in phase 0 which completely makes sense), and prolong repolarization, thus contributing to increased AP duration?
If you are blocking the fast Na+ channels you prevent depolarisation(sodium entry into the cell) thus delaying phase 0 and class 1a also blocks K+ channels so will prevent the outward delayed rectifier K+ current(potassium goes out) and slow it down so phase 3 will take longer to complete and thus as a whole the apd(action potential duration) will increase...

As a rule of thumb remember,
1a: apd increase (more time spent in phase 0 and 3)
1b: apd decrease (less time spent in phase 3)
1c: doesn't affect apd

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Old 05-26-2015
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I know what you mentioned . I mean to say, It's somewhat weird to imagine Na channel blockers (class 1A) blocking K+ channels??? Is there any explanation -how Na+ channel blockes K+ channel?
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Quote:
Originally Posted by Aumsai View Post
I know what you mentioned . I mean to say, It's somewhat weird to imagine Na channel blockers (class 1A) blocking K+ channels??? Is there any explanation -how Na+ channel blockes K+ channel?
Thats chemistry at molecular level brother...
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Old 05-26-2015
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em...ok, better remember and understand as it is. thanks man.
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