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General physiology MCQ (B7-PHYS1-2)

Q: What is the answer to this Physiology question?

The best answer is C, Na+-K+ ATPase inhibition raises intracellular Na+, slowing Na+-Ca2+ exchange and leaving more Ca2+ for contraction. The Na+-K+ ATPase pumps 3 Na+ out and 2 K+ in per ATP.

Q: What concept does this question test?

It tests General physiology: Na+-K+ ATPase and digoxin in Physiology. Digoxin inhibits it, so intracellular Na+ rises and the Na+ gradient that drives the Na+-Ca2+ exchanger falls. Less Ca2+ is extruded, the sarcoplasmic reticulum stores more Ca2+, and contractility increases.

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Physiology · General physiology: Na+-K+ ATPase and digoxin

Sample item

A 68-year-old man with chronic heart failure is started on digoxin. Which sequence best explains its positive inotropic effect on the myocardium?

ABeta-1 receptor stimulation raises cAMP and phosphorylates the L-type Ca2+ channelBDirect opening of L-type Ca2+ channels increases Ca2+ entry with each action potentialCNa+-K+ ATPase inhibition raises intracellular Na+, slowing Na+-Ca2+ exchange and leaving more Ca2+ for contractionDPhosphodiesterase 3 inhibition raises cAMP and increases Ca2+ uptake into the sarcoplasmic reticulum

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Na+-K+ ATPase inhibition raises intracellular Na+, slowing Na+-Ca2+ exchange and leaving more Ca2+ for contraction

The Na+-K+ ATPase pumps 3 Na+ out and 2 K+ in per ATP. Digoxin inhibits it, so intracellular Na+ rises and the Na+ gradient that drives the Na+-Ca2+ exchanger falls. Less Ca2+ is extruded, the sarcoplasmic reticulum stores more Ca2+, and contractility increases.

Option by option

A. Beta-1 receptor stimulation raises cAMP and phosphorylates the L-type Ca2+ channel
Beta-1 stimulation is the mechanism of dobutamine and adrenaline, not digoxin.
B. Direct opening of L-type Ca2+ channels increases Ca2+ entry with each action potential
Digoxin does not open Ca2+ channels directly; agents such as Bay K8644 do this experimentally.
C. Na+-K+ ATPase inhibition raises intracellular Na+, slowing Na+-Ca2+ exchange and leaving more Ca2+ for contraction (correct)
Reduced Ca2+ extrusion through the exchanger raises cytosolic and stored Ca2+, which strengthens contraction.
D. Phosphodiesterase 3 inhibition raises cAMP and increases Ca2+ uptake into the sarcoplasmic reticulum
Phosphodiesterase 3 inhibition is the mechanism of milrinone, not digoxin.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

General physiology: Na+-K+ ATPase and digoxin: practice question

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The answer and the reasoning

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