USMLE Digoxin and parasympathetic tone? {UW Q155}

[worldbeater]

Answer: Digoxin, increases parasympathetic tone. Why?

Vignette:
-45 y/o Caucasian male
-CC: palpations and fatigue
-BP: 110/70 --> Resistance (-10 from normal)/Volume (-10 than normal); HR 120/min (20 over normal)
-Rhythm irregularly irregular -> AFib
-IV administration of Digoxin causes HR of 82

Answer pts:
Digoxin MOA:
-Inhibits Na+/K+ ATPase ->> which will not allow Na+ to leave the cell -> Na+/Ca+ exchange can't occur, so Ca+ is trapped --> Increased intracellular Ca+

At this point, I would think the HR would increase due to increased intracellular levels of Ca+ , but it's decreasing, apparently due to increased parasympathetic tone on action of vagus nerve. (not getting this point)

I understand vagus nerve activation will causes the HR to drop. I know parasympathetic means means "rest and digest"/DUMBBELLS, they are taking bradycardia out of that.

But how does an increased level of intracellular calcium causes parasympathetic activation?

Thanks in advance, I am new to this section of the forum, but I am really enjoying it so far.

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[mbMD]

digoxin also influences AV node and SA node...making them slow, also digoxin increase strength of contraction...increase calcium increases contractility which doesnt hv to translate into increased heart rate

 

[worldbeater]

"increase calcium increases contractility which doesnt hv to translate into increased heart rate" --> did not know this, thanks

I realize now it's parasympathetic because it involves the vagus nerve -> which is a cranial nerve, head and below the belt is in the parasympathetic area.

 

[suarad32]

I vaguely recall hearing somewhere that digoxin also inhibits neuronal Na/K ATPase. I have no idea if this is relevant or why it would selectively enhance vagal activity, but this effect is distinct from and unrelated to digoxin's calcium sequestering ability by enhancing Na/Ca exchange.

 

[DrEnderW]

You got it with the vagal tone part.

My vague understanding off the MOA is that the increased contractility is received by the vagus (it has baroreceptor activity) and that increases reflexive PNS firing in the region. Not 100% sure on this. Obviously the vagus is firing PNS though.