What happens to the intracellular calcium levels when digoxin acts on its target?

Digoxin primarily acts by inhibiting the Na+/K+ ATPase pump in cardiac myocytes. This inhibition leads to an increase in intracellular sodium levels because the pump is responsible for exporting sodium out of the cell and bringing potassium in.

As the sodium gradient across the cell membrane is disrupted, the activity of the sodium/calcium exchanger (NCX) is altered. Normally, this exchanger extrudes calcium from the cell in exchange for sodium influx. With higher intracellular sodium levels because of the inhibition of the Na+/K+ ATPase, the function of the NCX is compromised. This results in less calcium being exported from the cell, thereby increasing intracellular calcium levels.

The increased intracellular calcium enhances the contractility of the cardiac muscle, leading to a positive inotropic effect. This mechanism underlies digoxin’s therapeutic use in conditions like heart failure, where increased contractility is desired.

Thus, the choice stating that intracellular calcium levels increase due to Na+/K+ ATPase inhibition accurately describes the underlying mechanism by which digoxin exerts its effects.