What is the main mechanism of action of digoxin?

Digoxin primarily acts by inhibiting the Na+/K+ ATPase enzyme in cardiac myocytes. This inhibition leads to an increase in intracellular sodium concentration, which ultimately results in increased intracellular calcium levels through the sodium-calcium exchanger. The rise in calcium enhances myocardial contractility, a phenomenon known as positive inotropy, which is beneficial in conditions such as congestive heart failure.

In addition to its positive inotropic effects, digoxin also has a vagomimetic effect, which helps to control heart rate by increasing parasympathetic tone and decreasing conduction through the atrioventricular (AV) node. This dual action makes digoxin effective not only in strengthening cardiac contractions but also in managing heart rate in patients with atrial fibrillation.

The other mechanisms mentioned in the alternatives do not describe the action of digoxin. For example, blocking calcium channels pertains more to other classes of drugs, while stimulating adrenergic receptors relates to sympathomimetics. Inhibiting phosphodiesterase is characteristic of another group of medications that would elevate cyclic adenosine monophosphate (cAMP) levels, leading to effects on cardiac contractility but through a different pathway than digoxin. Hence, the main mechanism of action of digoxin is